socat - Multipurpose relay (SOcket CAT)
socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan
Socat is a command line based utility that establishes two bidirectional
byte streams and transfers data between them. Because the streams can be
constructed from a large set of different types of data sinks and sources (see
address types), and because lots of address options may be applied to the
streams, socat can be used for many different purposes.
Filan is a utility that prints information about its active file
descriptors to stdout. It has been written for debugging
socat, but
might be useful for other purposes too. Use the -h option to find more infos.
Procan is a utility that prints information about process parameters to
stdout. It has been written to better understand some UNIX process properties
and for debugging
socat, but might be useful for other purposes too.
The life cycle of a
socat instance typically consists of four phases.
In the
init phase, the command line options are parsed and logging is
initialized.
During the
open phase,
socat opens the first address and
afterwards the second address. These steps are usually blocking; thus,
especially for complex address types like socks, connection requests or
authentication dialogs must be completed before the next step is started.
In the
transfer phase,
socat watches both streams’ read and
write file descriptors via select() , and, when data is available on one side
and can be written to the other side, socat reads it, performs newline
character conversions if required, and writes the data to the write file
descriptor of the other stream, then continues waiting for more data in both
directions.
When one of the streams effectively reaches EOF, the
closing phase
begins.
Socat transfers the EOF condition to the other stream, i.e.
tries to shutdown only its write stream, giving it a chance to terminate
gracefully. For a defined time
socat continues to transfer data in the
other direction, but then closes all remaining channels and terminates.
Socat provides some command line options that modify the behaviour of the
program. They have nothing to do with so called address options that are used
as parts of address specifications.
-
-V
- Print version and available feature information to stdout,
and exit.
-
-h | -?
- Print a help text to stdout describing command line options
and available address types, and exit.
-
-hh | -??
- Like -h, plus a list of the short names of all available
address options. Some options are platform dependend, so this output is
helpful for checking the particular implementation.
-
-hhh | -???
- Like -hh, plus a list of all available address option
names.
-
-d
- Without this option, only fatal and error messages are
generated; applying this option also prints warning messages. See
DIAGNOSTICS for more information.
-
-d -d
- Prints fatal, error, warning, and notice messages.
-
-d -d -d
- Prints fatal, error, warning, notice, and info
messages.
-
-d -d -d -d
- Prints fatal, error, warning, notice, info, and debug
messages.
-
-D
- Logs information about file descriptors before starting the
transfer phase.
-
-ly[<facility>]
- Writes messages to syslog instead of stderr; severity as
defined with -d option. With optional <facility>, the syslog type
can be selected, default is "daemon". Third party libraries
might not obey this option.
-
-lf<logfile>
- Writes messages to <logfile> [filename] instead of
stderr. Some third party libraries, in particular libwrap, might not obey
this option.
-
-ls
- Writes messages to stderr (this is the default). Some third
party libraries might not obey this option, in particular libwrap appears
to only log to syslog.
-
-lp<progname>
- Overrides the program name printed in error messages and
used for constructing environment variable names.
-
-lu
- Extends the timestamp of error messages to microsecond
resolution. Does not work when logging to syslog.
-
-lm[<facility>]
- Mixed log mode. During startup messages are printed to
stderr; when socat starts the transfer phase loop or daemon mode
(i.e. after opening all streams and before starting data transfer, or,
with listening sockets with fork option, before the first accept call), it
switches logging to syslog. With optional <facility>, the syslog
type can be selected, default is "daemon".
-
-lh
- Adds hostname to log messages. Uses the value from
environment variable HOSTNAME or the value retrieved with uname() if
HOSTNAME is not set.
-
-v
- Writes the transferred data not only to their target
streams, but also to stderr. The output format is text with some
conversions for readability, and prefixed with "> " or
"< " indicating flow directions.
-
-x
- Writes the transferred data not only to their target
streams, but also to stderr. The output format is hexadecimal, prefixed
with "> " or "< " indicating flow directions.
Can be combined with -v .
-
-r <file>
- Dumps the raw (binary) data flowing from left to right
address to the given file.
-
-R <file>
- Dumps the raw (binary) data flowing from right to left
address to the given file.
-
-b<size>
- Sets the data transfer block <size> [size_t]. At most
<size> bytes are transferred per step. Default is 8192 bytes.
-
-s
- By default, socat terminates when an error occurred
to prevent the process from running when some option could not be applied.
With this option, socat is sloppy with errors and tries to
continue. Even with this option, socat will exit on fatals, and will abort
connection attempts when security checks failed.
-
-t<timeout>
- When one channel has reached EOF, the write part of the
other channel is shut down. Then, socat waits <timeout>
[timeval] seconds before terminating. Default is 0.5 seconds. This timeout
only applies to addresses where write and read part can be closed
independently. When during the timeout interval the read part gives EOF,
socat terminates without awaiting the timeout.
-
-T<timeout>
- Total inactivity timeout: when socat is already in the
transfer loop and nothing has happened for <timeout> [timeval]
seconds (no data arrived, no interrupt occurred...) then it terminates.
Useful with protocols like UDP that cannot transfer EOF.
-
-u
- Uses unidirectional mode. The first address is only used
for reading, and the second address is only used for writing
(example).
-
-U
- Uses unidirectional mode in reverse direction. The first
address is only used for writing, and the second address is only used for
reading.
-
-g
- During address option parsing, don’t check if the
option is considered useful in the given address environment. Use it if
you want to force, e.g., appliance of a socket option to a serial
device.
-
-L<lockfile>
- If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.
-
-W<lockfile>
- If lockfile exists, waits until it disappears. When
lockfile does not exist, creates it and continues, unlinks lockfile on
exit.
-
-4
- Use IP version 4 in case that the addresses do not
implicitly or explicitly specify a version; this is the default.
-
-6
- Use IP version 6 in case that the addresses do not
implicitly or explicitly specify a version.
With the address command line arguments, the user gives
socat
instructions and the necessary information for establishing the byte streams.
An address specification usually consists of an address type keyword, zero or
more required address parameters separated by ’:’ from the
keyword and from each other, and zero or more address options separated by
’,’.
The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some
keywords there exist synonyms (’-’ for STDIO, TCP for TCP4).
Keywords are case insensitive. For a few special address types, the keyword
may be omitted: Address specifications starting with a number are assumed to
be FD (raw file descriptor) addresses; if a ’/’ is found before
the first ’:’ or ’,’, GOPEN (generic file open) is
assumed.
The required number and type of address parameters depend on the address type.
E.g., TCP4 requires a server specification (name or address), and a port
specification (number or service name).
Zero or more address options may be given with each address. They influence the
address in some ways. Options consist of an option keyword or an option
keyword and a value, separated by ’=’. Option keywords are case
insensitive. For filtering the options that are useful with an address type,
each option is member of one option group. For each address type there is a
set of option groups allowed. Only options belonging to one of these address
groups may be used (except with option -g).
Address specifications following the above schema are also called
single
address specifications. Two single addresses can be combined with
"!!" to form a
dual type address for one channel. Here, the
first address is used by
socat for reading data, and the second address
for writing data. There is no way to specify an option only once for being
applied to both single addresses.
Usually, addresses are opened in read/write mode. When an address is part of a
dual address specification, or when option -u or -U is used, an address might
be used only for reading or for writing. Considering this is important with
some address types.
With socat version 1.5.0 and higher, the lexical analysis tries to handle quotes
and parenthesis meaningfully and allows escaping of special characters. If one
of the characters ( { [ ’ is found, the corresponding closing character
- ) } ] ’ - is looked for; they may also be nested. Within these
constructs, socats special characters and strings : , !! are not handled
specially. All those characters and strings can be escaped with \ or within
""
This section describes the available address types with their keywords,
parameters, and semantics.
-
CREATE:<filename>
- Opens <filename> with creat() and uses the file
descriptor for writing. This address type requires write-only context,
because a file opened with creat cannot be read from.
Flags like O_LARGEFILE cannot be applied. If you need them use OPEN with
options create,create.
<filename> must be a valid existing or not existing path. If
<filename> is a named pipe, creat() might block; if <filename>
refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options: mode, user, group, unlink-early, unlink-late, append
See also: OPEN, GOPEN
-
EXEC:<command-line>
- Forks a sub process that establishes communication with its
parent process and invokes the specified program with execvp() .
<command-line> is a simple command with arguments separated by
single spaces. If the program name contains a ’/’, the part
after the last ’/’ is taken as ARGV[0]. If the program name
is a relative path, the execvp() semantics for finding the program via
$PATH apply. After successful program start, socat writes data to
stdin of the process and reads from its stdout using a UNIX domain socket
generated by socketpair() per default. (example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr,
ctty, setsid, pipes, login, sigint, sigquit
See also: SYSTEM
-
FD:<fdnum>
- Uses the file descriptor <fdnum>. It must already
exist as valid UN*X file descriptor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR
-
GOPEN:<filename>
- (Generic open) This address type tries to handle any file
system entry except directories usefully. <filename> may be a
relative or absolute path. If it already exists, its type is checked. In
case of a UNIX domain socket, socat connects; if connecting fails,
socat assumes a datagram socket and uses sendto() calls. If the
entry is not a socket, socat opens it applying the O_APPEND flag.
If it does not exist, it is opened with flag O_CREAT as a regular file
(example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT
-
IP-SENDTO:<host>:<protocol>
- Opens a raw IP socket. Depending on host specification or
option pf, IP protocol version 4 or 6 is used. It uses <protocol> to
send packets to <host> [IP address] and receives packets from host,
ignores packets from other hosts. Protocol 255 uses the raw socket with
the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6
Useful options: pf, ttl
See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV, UDP-SENDTO,
UNIX-SENDTO
-
INTERFACE:<interface>
- Communicates with a network connected on an interface using
raw packets including link level data. <interface> is the name of
the network interface. Currently only available on Linux. Option groups:
FD,SOCKET
Useful options: pf, type
See also: ip-recv
-
IP4-SENDTO:<host>:<protocol>
- Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4
-
IP6-SENDTO:<host>:<protocol>
- Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6
-
IP-DATAGRAM:<address>:<protocol>
- Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets arriving on the
local socket are checked if their source addresses match RANGE or TCPWRAP
options. This address type can for example be used for implementing
symmetric or asymmetric broadcast or multicast communications.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop,
ip-multicast-ttl, ip-multicast-if, ip-add-membership,
ip-add-source-membership, ttl, tos, pf
See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO, IP-RECVFROM, IP-RECV,
UDP-DATAGRAM
-
IP4-DATAGRAM:<host>:<protocol>
- Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE
-
IP6-DATAGRAM:<host>:<protocol>
- Like IP-DATAGRAM, but always uses IPv6. Please note that
IPv6 does not know broadcasts.
Option groups: FD,SOCKET,IP6,RANGE
-
IP-RECVFROM:<protocol>
- Opens a raw IP socket of <protocol>. Depending on
option pf, IP protocol version 4 or 6 is used. It receives one packet from
an unspecified peer and may send one or more answer packets to that peer.
This mode is particularly useful with fork option where each arriving
packet - from arbitrary peers - is handled by its own sub process. This
allows a behaviour similar to typical UDP based servers like ntpd or
named.
Please note that the reply packets might be fetched as incoming traffic when
sender and receiver IP address are identical because there is no port
number to distinguish the sockets.
This address works well with IP-SENDTO address peers (see above). Protocol
255 uses the raw socket with the IP header being part of the data.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: pf, fork, range, ttl, broadcast
See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV, UDP-RECVFROM,
UNIX-RECVFROM
-
IP4-RECVFROM:<protocol>
- Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
-
IP6-RECVFROM:<protocol>
- Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
-
IP-RECV:<protocol>
- Opens a raw IP socket of <protocol>. Depending on
option pf, IP protocol version 4 or 6 is used. It receives packets from
multiple unspecified peers and merges the data. No replies are possible.
It can be, e.g., addressed by socat IP-SENDTO address peers. Protocol 255
uses the raw socket with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, range
See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM, UDP-RECV,
UNIX-RECV
-
IP4-RECV:<protocol>
- Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE
-
IP6-RECV:<protocol>
- Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE
-
OPEN:<filename>
- Opens <filename> using the open() system call
(example). This operation fails on UNIX domain sockets.
Note: This address type is rarely useful in bidirectional mode.
Option groups: FD,REG,NAMED,OPEN
Useful options: creat, excl, noatime, nofollow, append, rdonly, wronly,
lock, readbytes, ignoreeof
See also: CREATE, GOPEN, UNIX-CONNECT
-
OPENSSL:<host>:<port>
- Tries to establish a SSL connection to <port> [TCP
service] on <host> [IP address] using TCP/IP version 4 or 6
depending on address specification, name resolution, or option pf.
NOTE: Up to version 1.7.2.4 the server certificate was only checked for
validity against the system certificate store or cafile or capath, but not
for match with the server’s name or its IP address. Since version
1.7.3.0 socat checks the peer certificate for match with the <host>
parameter or the value of the openssl-commonname option. Socat tries to
match it against the certificates subject commonName, and the certificates
extension subjectAltName DNS names. Wildcards in the certificate are
supported.
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify, commonname, cafile,
capath, certificate, key, compress, bind, pf, connect-timeout, sourceport,
retry
See also: OPENSSL-LISTEN, TCP
-
OPENSSL-LISTEN:<port>
- Listens on tcp <port> [TCP service]. The IP version
is 4 or the one specified with pf. When a connection is accepted, this
address behaves as SSL server.
Note: You probably want to use the certificate option with this address.
NOTE: The client certificate is only checked for validity against cafile or
capath, but not for match with the client’s name or its IP address!
Option groups: FD,SOCKET,IP4,IP6,TCP,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify, commonname, cafile,
capath, certificate, key, compress, fork, bind, range, tcpwrap, su,
reuseaddr, retry
See also: OPENSSL, TCP-LISTEN
-
OPENSSL-DTLS-CLIENT:<host>:<port>
- Tries to establish a DTLS connection to <port> [UDP
service] on <host> [IP address] using UDP/IP version 4 or 6
depending on address specification, name resolution, or option pf.
Socat checks the peer certificates subjectAltName or commonName
against the addresses option openssl-commonname or the host name.
Wildcards in the certificate are supported.
Use socat option -b to make datagrams small enough to fit with
overhead on the network. Use option -T to prevent indefinite hanging when
peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify, commonname, cafile,
capath, certificate, key, compress, bind, pf, sourceport, retry
See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT
-
OPENSSL-DTLS-SERVER:<port>
- Listens on UDP <port> [UDP service]. The IP version
is 4 or the one specified with pf. When a connection is accepted, this
address behaves as DTLS server.
Note: You probably want to use the certificate option with this address.
NOTE: The client certificate is only checked for validity against cafile or
capath, but not for match with the client’s name or its IP address!
Use socat option -b to make datagrams small enough to fit with
overhead on the network. Use option -T to prevent indefinite hanging when
peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify, commonname, cafile,
capath, certificate, key, compress, fork, bind, range, tcpwrap, su,
reuseaddr, retry
See also: OPENSSL-DTLS-CLIENT, OPENSSL-LISTEN, UDP-LISTEN
-
PIPE:<filename>
- If <filename> already exists, it is opened. If it
does not exist, a named pipe is created and opened. Beginning with socat
version 1.4.3, the named pipe is removed when the address is closed (but
see option unlink-close
Note: When a pipe is used for both reading and writing, it works as echo
service.
Note: When a pipe is used for both reading and writing, and socat tries to
write more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat
might block. Consider using socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode, unlink-early
See also: unnamed pipe
-
PIPE
- Creates an unnamed pipe and uses it for reading and
writing. It works as an echo, because everything written to it appeares
immediately as read data.
Note: When socat tries to write more bytes than the pipe can queue (Linux
2.4: 2048 bytes), socat might block. Consider, e.g., using option -b 2048
Option groups: FD
See also: named pipe
-
PROXY:<proxy>:<hostname>:<port>
- Connects to an HTTP proxy server on port 8080 using TCP/IP
version 4 or 6 depending on address specification, name resolution, or
option pf, and sends a CONNECT request for hostname:port. If the proxy
grants access and succeeds to connect to the target, data transfer between
socat and the target can start. Note that the traffic need not be HTTP but
can be an arbitrary protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options: proxyport, ignorecr, proxyauth, resolve, crnl, bind,
connect-timeout, mss, sourceport, retry
See also: SOCKS, TCP
-
PTY
- Generates a pseudo terminal (pty) and uses its master side.
Another process may open the pty’s slave side using it like a
serial line or terminal. (example). If both the ptmx and the openpty
mechanisms are available, ptmx is used (POSIX).
Option groups: FD,NAMED,PTY,TERMIOS
Useful options: link, openpty, wait-slave, mode, user, group
See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM
-
READLINE
- Uses GNU readline and history on stdio to allow editing and
reusing input lines (example).
Due to licensing restrictions the readline feature is disabled in Debian.
See BUGS.
You can use STDIO instead.
-
SCTP-CONNECT:<host>:<port>
- Establishes an SCTP stream connection to the specified
<host> [IP address] and <port> [TCP service] using IP version
4 or 6 depending on address specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, pf, connect-timeout, tos, mtudiscover, sctp-maxseg,
sctp-nodelay, nonblock, sourceport, retry, readbytes
See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN, TCP-CONNECT
-
SCTP4-CONNECT:<host>:<port>
- Like SCTP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY
-
SCTP6-CONNECT:<host>:<port>
- Like SCTP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY
-
SCTP-LISTEN:<port>
- Listens on <port> [TCP service] and accepts an SCTP
connection. The IP version is 4 or the one specified with address option
pf, socat option (-4, -6), or environment variable
SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually blocks
until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog,
accept-timeout, sctp-maxseg, sctp-nodelay, su, reuseaddr, retry,
cool-write
See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN, SCTP-CONNECT
-
SCTP4-LISTEN:<port>
- Like SCTP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY
-
SCTP6-LISTEN:<port>
- Like SCTP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY
-
SOCKET-CONNECT:<domain>:<protocol>:<remote-address>
- Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and connects to the
remote-address. The two socket parameters have to be specified by int
numbers. Consult your OS documentation and include files to find the
appropriate values. The remote-address must be the data representation of
a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO
-
SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>
- Creates a datagram socket using the first three given
socket parameters (see man socket(2)) and sends outgoing data to the
remote-address. The three socket parameters have to be specified by int
numbers. Consult your OS documentation and include files to find the
appropriate values. The remote-address must be the data representation of
a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt,
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV,
SOCKET-RECVFROM
-
SOCKET-LISTEN:<domain>:<protocol>:<local-address>
- Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and waits for
incoming connections on local-address. The two socket parameters have to
be specified by int numbers. Consult your OS documentation and include
files to find the appropriate values. The local-address must be the data
representation of a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options: setsockopt, setsockopt-listen,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO,
SOCKET-SENDTO
-
SOCKET-RECV:<domain>:<type>:<protocol>:<local-address>
- Creates a socket using the three given socket parameters
(see man socket(2)) and binds it to <local-address>. Receives
arriving data. The three parameters have to be specified by int numbers.
Consult your OS documentation and include files to find the appropriate
values. The local-address must be the data representation of a sockaddr
structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET,RANGE
Useful options: range, setsockopt, setsockopt-listen
See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM, SOCKET-SENDTO,
SOCKET-RECVFROM
-
SOCKET-RECVFROM:<domain>:<type>:<protocol>:<local-address>
- Creates a socket using the three given socket parameters
(see man socket(2)) and binds it to <local-address>. Receives
arriving data and sends replies back to the sender. The first three
parameters have to be specified as int numbers. Consult your OS
documentation and include files to find the appropriate values. The
local-address must be the data representation of a sockaddr structure
without sa_family and (BSD) sa_len components.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt, setsockopt-listen
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM, SOCKET-DATAGRAM,
SOCKET-SENDTO, SOCKET-RECV
-
SOCKET-SENDTO:<domain>:<type>:<protocol>:<remote-address>
- Creates a socket using the three given socket parameters
(see man socket(2)). Sends outgoing data to the given address and receives
replies. The three parameters have to be specified as int numbers. Consult
your OS documentation and include files to find the appropriate values.
The remote-address must be the data representation of a sockaddr structure
without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET
Useful options: bind, setsockopt, setsockopt-listen
See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO, SOCKET-DATAGRAM, SOCKET-RECV
SOCKET-RECVFROM
-
SOCKS4:<socks-server>:<host>:<port>
- Connects via <socks-server> [IP address] to
<host> [IPv4 address] on <port> [TCP service], using socks
version 4 protocol over IP version 4 or 6 depending on address
specification, name resolution, or option pf (example).
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Useful options: socksuser, socksport, sourceport, pf, retry
See also: SOCKS4A, PROXY, TCP
-
SOCKS4A:<socks-server>:<host>:<port>
- like SOCKS4, but uses socks protocol version 4a, thus
leaving host name resolution to the socks server.
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
-
STDERR
- Uses file descriptor 2.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
-
STDIN
- Uses file descriptor 0.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
-
STDIO
- Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
-
STDOUT
- Uses file descriptor 1.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
-
SYSTEM:<shell-command>
- Forks a sub process that establishes communication with its
parent process and invokes the specified program with system() . Please
note that <shell-command> [string] must not contain
’,’ or "!!", and that shell meta characters may
have to be protected. After successful program start, socat writes
data to stdin of the process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr,
ctty, setsid, pipes, sigint, sigquit
See also: EXEC
-
TCP:<host>:<port>
- Connects to <port> [TCP service] on <host> [IP
address] using TCP/IP version 4 or 6 depending on address specification,
name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY
Useful options: crnl, bind, pf, connect-timeout, tos, mtudiscover, mss,
nodelay, nonblock, sourceport, retry, readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT, UNIX-CONNECT
-
TCP4:<host>:<port>
- Like TCP, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,IP4,TCP,RETRY
-
TCP6:<host>:<port>
- Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY
-
TCP-LISTEN:<port>
- Listens on <port> [TCP service] and accepts a TCP/IP
connection. The IP version is 4 or the one specified with address option
pf, socat option (-4, -6), or environment variable
SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually blocks
until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog,
accept-timeout, mss, su, reuseaddr, retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, SCTP-LISTEN, UNIX-LISTEN,
OPENSSL-LISTEN, TCP-CONNECT
-
TCP4-LISTEN:<port>
- Like TCP-LISTEN, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY
-
TCP6-LISTEN:<port>
- Like TCP-LISTEN, but only supports IPv6 protocol.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY
-
TUN[:<if-addr>/<bits>]
- Creates a Linux TUN/TAP device and optionally assignes it
the address and netmask given by the parameters. The resulting network
interface is almost ready for use by other processes; socat serves its
"wire side". This address requires read and write access to the
tunnel cloning device, usually /dev/net/tun , as well as permission to set
some ioctl()s. Option iff-up is required to immediately activate the
interface!
Note: If you intend to transfer packets between two Socat "wire
sides" you need a protocol that keeps packet boundaries, e.g.UDP; TCP
might work with option nodelay.
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type, iff-no-pi
See also: ip-recv
-
UDP:<host>:<port>
- Connects to <port> [UDP service] on <host> [IP
address] using UDP/IP version 4 or 6 depending on address specification,
name resolution, or option pf.
Please note that, due to UDP protocol properties, no real connection is
established; data has to be sent for `connecting’ to the server,
and no end-of-file condition can be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP
-
UDP4:<host>:<port>
- Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
-
UDP6:<host>:<port>
- Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
-
UDP-DATAGRAM:<address>:<port>
- Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets arriving on the
local socket are checked for the correct remote port only when option
sourceport is used (this is a change with Socat version 1.7.4.0)
and if their source addresses match RANGE or TCPWRAP options. This address
type can for example be used for implementing symmetric or asymmetric
broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop,
ip-multicast-ttl, ip-multicast-if, ip-add-membership,
ip-add-source-membership, ttl, tos, sourceport, pf
See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO, UDP-RECVFROM, UDP-RECV,
UDP-CONNECT, UDP-LISTEN, IP-DATAGRAM
-
UDP4-DATAGRAM:<address>:<port>
- Like UDP-DATAGRAM, but only supports IPv4 protocol
(example1, example2).
Option groups: FD,SOCKET,IP4, RANGE
-
UDP6-DATAGRAM:<address>:<port>
- Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
-
UDP-LISTEN:<port>
- Waits for a UDP/IP packet arriving on <port> [UDP
service] and `connects’ back to sender. The accepted IP version is
4 or the one specified with option pf. Please note that, due to UDP
protocol properties, no real connection is established; data has to arrive
from the peer first, and no end-of-file condition can be transported. Note
that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options: fork, bind, range, pf
See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN
-
UDP4-LISTEN:<port>
- Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4
-
UDP6-LISTEN:<port>
- Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6
-
UDP-SENDTO:<host>:<port>
- Communicates with the specified peer socket, defined by
<port> [UDP service] on <host> [IP address], using UDP/IP
version 4 or 6 depending on address specification, name resolution, or
option pf. It sends packets to and receives packets from that peer socket
only. This address effectively implements a datagram client. It works well
with socat UDP-RECVFROM and UDP-RECV address peers.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT,
UDP-LISTEN, IP-SENDTO
-
UDP4-SENDTO:<host>:<port>
- Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
-
UDP6-SENDTO:<host>:<port>
- Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
-
UDP-RECVFROM:<port>
- Creates a UDP socket on <port> [UDP service] using
UDP/IP version 4 or 6 depending on option pf. It receives one packet from
an unspecified peer and may send one or more answer packets to that peer.
This mode is particularly useful with fork option where each arriving
packet - from arbitrary peers - is handled by its own sub process. This
allows a behaviour similar to typical UDP based servers like ntpd or
named. This address works well with socat UDP-SENDTO address peers.
Note: When the second address fails before entering the transfer loop the
packet is dropped. Use option retry or forever on the second address to
avoid data loss.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: fork, ttl, tos, bind, sourceport, pf
See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO, UDP-RECV, UDP-CONNECT,
UDP-LISTEN, IP-RECVFROM, UNIX-RECVFROM
-
UDP4-RECVFROM:<port>
- Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
-
UDP6-RECVFROM:<port>
- Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
-
UDP-RECV:<port>
- Creates a UDP socket on <port> [UDP service] using
UDP/IP version 4 or 6 depending on option pf. It receives packets from
multiple unspecified peers and merges the data. No replies are possible.
It works well with, e.g., socat UDP-SENDTO address peers; it behaves
similar to a syslog server.
Note: if you need the fork option, use UDP-RECVFROM in unidirectional mode
(with option -u) instead.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, bind, sourceport, ttl, tos
See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM, UDP-CONNECT,
UDP-LISTEN, IP-RECV, UNIX-RECV
-
UDP4-RECV:<port>
- Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE
-
UDP6-RECV:<port>
- Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
-
UNIX-CONNECT:<filename>
- Connects to <filename> assuming it is a UNIX domain
socket. If <filename> does not exist, this is an error; if
<filename> is not a UNIX domain socket, this is an error; if
<filename> is a UNIX domain socket, but no process is listening,
this is an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
) Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP
-
UNIX-LISTEN:<filename>
- Listens on <filename> using a UNIX domain stream
socket and accepts a connection. If <filename> exists and is not a
socket, this is an error. If <filename> exists and is a UNIX domain
socket, binding to the address fails (use option unlink-early!). Note that
opening this address usually blocks until a client connects. Beginning
with socat version 1.4.3, the file system entry is removed when this
address is closed (but see option unlink-close) (example).
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX
Useful options: fork, umask, mode, user, group, unlink-early
See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV, TCP-LISTEN
-
UNIX-SENDTO:<filename>
- Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain datagram socket. It sends
packets to and receives packets from that peer socket only. Please note
that it might be necessary to bind the local socket to an address (e.g.
/tmp/sock1, which must not exist before). This address type works well
with socat UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT, UDP-SENDTO, IP-SENDTO
-
UNIX-RECVFROM:<filename>
- Creates a UNIX domain datagram socket [<filename>].
Receives one packet and may send one or more answer packets to that peer.
This mode is particularly useful with fork option where each arriving
packet - from arbitrary peers - is handled by its own sub process. This
address works well with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
See the note about RECVFROM addresses.
Useful options: fork
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM,
IP-RECVFROM
-
UNIX-RECV:<filename>
- Creates a UNIX domain datagram socket [<filename>].
Receives packets from multiple unspecified peers and merges the data. No
replies are possible. It can be, e.g., addressed by socat UNIX-SENDTO
address peers. It behaves similar to a syslog server.
Option groups: FD,SOCKET,NAMED,UNIX
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV, IP-RECV
-
UNIX-CLIENT:<filename>
- Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain socket. It first tries to
connect and, if that fails, assumes it is a datagram socket, thus
supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN
-
VSOCK-CONNECT:<cid>:<port>
- Establishes a VSOCK stream connection to the specified
<cid> [VSOCK cid] and <port> [VSOCK port].
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, pf, connect-timeout, retry, readbytes
See also: VSOCK-LISTEN,
-
VSOCK-LISTEN:<port>
- Listens on <port> [VSOCK port] and accepts a VSOCK
connection. Note that opening this address usually blocks until a client
connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RETRY
Useful options: fork, bind, pf, max-children, backlog, su, reuseaddr, retry,
cool-write
See also: VSOCK-CONNECT
-
ABSTRACT-CONNECT:<string>
-
ABSTRACT-LISTEN:<string>
-
ABSTRACT-SENDTO:<string>
-
ABSTRACT-RECVFROM:<string>
-
ABSTRACT-RECV:<string>
-
ABSTRACT-CLIENT:<string>
- The ABSTRACT addresses are almost identical to the related
UNIX addresses except that they do not address file system based sockets
but an alternate UNIX domain address space. To achieve this the socket
address strings are prefixed with "\0" internally. This feature
is available (only?) on Linux. Option groups are the same as with the
related UNIX addresses, except that the ABSTRACT addresses are not member
of the NAMED group.
Address options can be applied to address specifications to influence the
process of opening the addresses and the properties of the resulting data
channels.
For technical reasons not every option can be applied to every address type;
e.g., applying a socket option to a regular file will fail. To catch most
useless combinations as early as in the open phase, the concept of
option
groups was introduced. Each option belongs to one or more option groups.
Options can be used only with address types that support at least one of their
option groups (but see option -g).
Address options have data types that their values must conform to. Every address
option consists of just a keyword or a keyword followed by "=value",
where value must conform to the options type. Some address options manipulate
parameters of system calls; e.g., option sync sets the O_SYNC flag with the
open() call. Other options cause a system or library call; e.g., with option
`ttl=value’ the setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int)) call
is applied. Other options set internal
socat variables that are used
during data transfer; e.g., `crnl’ causes explicit character
conversions. A few options have more complex implementations; e.g., su-d
(substuser-delayed) inquires some user and group infos, stores them, and
applies them later after a possible chroot() call.
If multiple options are given to an address, their sequence in the address
specification has (almost) no effect on the sequence of their
execution/application. Instead,
socat has built in an
option
phase model that tries to bring the options in a useful order. Some
options exist in different forms (e.g., unlink, unlink-early, unlink-late) to
control the time of their execution.
If the same option is specified more than once within one address specification,
with equal or different values, the effect depends on the kind of option.
Options resulting in function calls like setsockopt() cause multiple
invocations. With options that set parameters for a required call like open()
or set internal flags, the value of the last option occurrence is effective.
The existence or semantics of many options are system dependent.
Socat
usually does NOT try to emulate missing libc or kernel features, it just
provides an interface to the underlying system. So, if an operating system
lacks a feature, the related option is simply not available on this platform.
The following paragraphs introduce just the more common address options. For a
more comprehensive reference and to find information about canonical option
names, alias names, option phases, and platforms see file
xio.help.
FD option group
This option group contains options that are applied to a UN*X style file
descriptor, no matter how it was generated. Because all current
socat
address types are file descriptor based, these options may be applied to any
address.
Note: Some of these options are also member of another option group, that
provides another, non-fd based mechanism. For these options, it depends on the
actual address type and its option groups which mechanism is used. The second,
non-fd based mechanism is prioritized.
-
cloexec=<bool>
- Sets the FD_CLOEXEC flag with the fcntl() system call to
value <bool>. If set, the file descriptor is closed on exec() family
function calls. Socat internally handles this flag for the fds it
controls, so in most cases there will be no need to apply this
option.
-
setlk
- Tries to set a discretionary write lock to the whole file
using the fcntl(fd, F_SETLK, ...) system call. If the file is already
locked, this call results in an error. On Linux, when the file permissions
for group are "S" (g-x,g+s), and the file system is locally
mounted with the "mand" option, the lock is mandatory, i.e.
prevents other processes from opening the file.
-
setlkw
- Tries to set a discretionary waiting write lock to the
whole file using the fcntl(fd, F_SETLKW, ...) system call. If the file is
already locked, this call blocks. See option setlk for information about
making this lock mandatory.
-
setlk-rd
- Tries to set a discretionary read lock to the whole file
using the fcntl(fd, F_SETLK, ...) system call. If the file is already
write locked, this call results in an error. See option setlk for
information about making this lock mandatory.
-
setlkw-rd
- Tries to set a discretionary waiting read lock to the whole
file using the fcntl(fd, F_SETLKW, ...) system call. If the file is
already write locked, this call blocks. See option setlk for information
about making this lock mandatory.
-
flock-ex
- Tries to set a blocking exclusive advisory lock to the file
using the flock(fd, LOCK_EX) system call. Socat hangs in this call
if the file is locked by another process.
-
flock-ex-nb
- Tries to set a nonblocking exclusive advisory lock to the
file using the flock(fd, LOCK_EX|LOCK_NB) system call. If the file is
already locked, this option results in an error.
-
flock-sh
- Tries to set a blocking shared advisory lock to the file
using the flock(fd, LOCK_SH) system call. Socat hangs in this call
if the file is locked by another process.
-
flock-sh-nb
- Tries to set a nonblocking shared advisory lock to the file
using the flock(fd, LOCK_SH|LOCK_NB) system call. If the file is already
locked, this option results in an error.
-
lock
- Sets a blocking lock on the file. Uses the setlk or flock
mechanism depending on availability on the particular platform. If both
are available, the POSIX variant (setlkw) is used.
-
user=<user>
- Sets the <user> (owner) of the stream. If the address
is member of the NAMED option group, socat uses the chown() system
call after opening the file or binding to the UNIX domain socket (race
condition!). Without filesystem entry, socat sets the user of the
stream using the fchown() system call. These calls might require root
privilege.
-
user-late=<user>
- Sets the owner of the fd to <user> with the fchown()
system call after opening or connecting the channel. This is useful only
on file system entries.
-
group=<group>
- Sets the <group> of the stream. If the address is
member of the NAMED option group, socat uses the chown() system
call after opening the file or binding to the UNIX domain socket (race
condition!). Without filesystem entry, socat sets the group of the
stream with the fchown() system call. These calls might require group
membership or root privilege.
-
group-late=<group>
- Sets the group of the fd to <group> with the fchown()
system call after opening or connecting the channel. This is useful only
on file system entries.
-
mode=<mode>
- Sets the <mode> [mode_t] (permissions) of the stream.
If the address is member of the NAMED option group and uses the open() or
creat() call, the mode is applied with these. If the address is member of
the NAMED option group without using these system calls, socat uses
the chmod() system call after opening the filesystem entry or binding to
the UNIX domain socket (race condition!). Otherwise, socat sets the
mode of the stream using fchmod() . These calls might require ownership or
root privilege.
-
perm-late=<mode>
- Sets the permissions of the fd to value <mode>
[mode_t] using the fchmod() system call after opening or connecting the
channel. This is useful only on file system entries.
-
append=<bool>
- Always writes data to the actual end of file. If the
address is member of the OPEN option group, socat uses the O_APPEND
flag with the open() system call (example). Otherwise, socat
applies the fcntl(fd, F_SETFL, O_APPEND) call.
-
nonblock=<bool>
- Tries to open or use file in nonblocking mode. Its only
effects are that the connect() call of TCP addresses does not block, and
that opening a named pipe for reading does not block. If the address is
member of the OPEN option group, socat uses the O_NONBLOCK flag
with the open() system call. Otherwise, socat applies the fcntl(fd,
F_SETFL, O_NONBLOCK) call.
-
binary
- Opens the file in binary mode to avoid implicit line
terminator conversions (Cygwin).
-
text
- Opens the file in text mode to force implicit line
terminator conversions (Cygwin).
-
noinherit
- Does not keep this file open in a spawned process
(Cygwin).
-
cool-write
- Takes it easy when write fails with EPIPE or ECONNRESET and
logs the message with notice level instead of error. This
prevents the log file from being filled with useless error messages when
socat is used as a high volume server or proxy where clients often abort
the connection.
This option is experimental.
-
end-close
- Changes the (address dependent) method of ending a
connection to just close the file descriptors. This is useful when the
connection is to be reused by or shared with other processes (example).
Normally, socket connections will be ended with shutdown(2) which terminates
the socket even if it is shared by multiple processes. close(2)
"unlinks" the socket from the process but keeps it active as
long as there are still links from other processes.
Similarly, when an address of type EXEC or SYSTEM is ended, socat usually
will explicitly kill the sub process. With this option, it will just close
the file descriptors.
-
shut-none
- Changes the (address dependent) method of shutting down the
write part of a connection to not do anything.
-
shut-down
- Changes the (address dependent) method of shutting down the
write part of a connection to shutdown(fd, SHUT_WR). Is only useful with
sockets.
-
shut-close
- Changes the (address dependent) method of shutting down the
write part of a connection to close(fd).
-
shut-null
- When one address indicates EOF, socat will send a
zero sized packet to the write channel of the other address to transfer
the EOF condition. This is useful with UDP and other datagram protocols.
Has been tested against netcat and socat with option null-eof.
-
null-eof
- Normally socat will ignore empty (zero size payload)
packets arriving on datagram sockets, so it survives port scans. With this
option socat interprets empty datagram packets as EOF indicator
(see shut-null).
-
ioctl-void=<request>
- Calls ioctl() with the request value as second argument and
NULL as third argument. This option allows utilizing ioctls that are not
explicitly implemented in socat.
-
ioctl-int=<request>:<value>
- Calls ioctl() with the request value as second argument and
the integer value as third argument.
-
ioctl-intp=<request>:<value>
- Calls ioctl() with the request value as second argument and
a pointer to the integer value as third argument.
-
ioctl-bin=<request>:<value>
- Calls ioctl() with the request value as second argument and
a pointer to the given data value as third argument. This data must be
specified in <dalan> form.
-
ioctl-string=<request>:<value>
- Calls ioctl() with the request value as second argument and
a pointer to the given string as third argument. <dalan> form.
NAMED option group
These options work on file system entries.
Please note that, with UNIX domain client addresses, this means the bind entry,
not the target/peer entry.
See also options user, group, and mode.
-
user-early=<user>
- Changes the <user> (owner) of the file system entry
before accessing it, using the chown() system call. This call might
require root privilege.
-
group-early=<group>
- Changes the <group> of the file system entry before
accessing it, using the chown() system call. This call might require group
membership or root privilege.
-
perm-early=<mode>
- Changes the <mode> [mode_t] of the file system entry
before accessing it, using the chmod() system call. This call might
require ownership or root privilege.
-
umask=<mode>
- Sets the umask of the process to <mode> [mode_t]
before accessing the file system entry (useful with UNIX domain sockets!).
This call might affect all further operations of the socat
process!
-
unlink-early
- Unlinks (removes) the file before opening it and even
before applying user-early etc.
-
unlink
- Unlinks (removes) the file before accessing it, but after
user-early etc.
-
unlink-late
- Unlinks (removes) the file after opening it to make it
inaccessible for other processes after a short race condition.
-
unlink-close
- Removes the addresses file system entry when closing the
address. For named pipes, UNIX domain sockets, and the symbolic links of
pty addresses, the default is 1; for created files, opened files, and
generic opened files the default is 0.
OPEN option group
The OPEN group options allow setting flags with the open() system call. E.g.,
option `creat’ sets the O_CREAT flag.
See also options append and nonblock.
-
creat=<bool>
- Creates the file if it does not exist (example).
-
dsync=<bool>
- Blocks write() calls until metainfo is physically written
to media.
-
excl=<bool>
- With option creat, if file exists this is an error.
-
largefile=<bool>
- On 32 bit systems, allows a file larger than 2^31
bytes.
-
noatime
- Sets the O_NOATIME options, so reads do not change the
access timestamp.
-
noctty=<bool>
- Does not make this file the controlling terminal.
-
nofollow=<bool>
- Does not follow symbolic links.
-
nshare=<bool>
- Does not allow sharing this file with other processes.
-
rshare=<bool>
- Does not allow other processes to open this file for
writing.
-
rsync=<bool>
- Blocks write() until metainfo is physically written to
media.
-
sync=<bool>
- Blocks write() until data is physically written to
media.
-
rdonly=<bool>
- Opens the file for reading only.
-
wronly=<bool>
- Opens the file for writing only.
-
trunc
- Truncates the file to size 0 during opening it.
REG and BLK option group
These options are usually applied to a UN*X file descriptor, but their semantics
make sense only on a file supporting random access.
-
seek=<offset>
- Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64
) system call, thus positioning the file pointer absolutely to
<offset> [off_t or off64_t]. Please note that a missing value
defaults to 1, not 0.
-
seek-cur=<offset>
- Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64
) system call, thus positioning the file pointer <offset> [off_t or
off64_t] bytes relatively to its current position (which is usually 0).
Please note that a missing value defaults to 1, not 0.
-
seek-end=<offset>
- Applies the lseek(fd, <offset>, SEEK_END) (or lseek64
) system call, thus positioning the file pointer <offset> [off_t or
off64_t] bytes relatively to the files current end. Please note that a
missing value defaults to 1, not 0.
-
ftruncate=<offset>
- Applies the ftruncate(fd, <offset>) (or ftruncate64
if available) system call, thus truncating the file at the position
<offset> [off_t or off64_t]. Please note that a missing value
defaults to 1, not 0.
-
secrm=<bool>
-
unrm=<bool>
-
compr=<bool>
-
fs-sync=<bool>
-
immutable=<bool>
-
fs-append=<bool>
-
nodump=<bool>
-
fs-noatime=<bool>
-
journal-data=<bool>
-
notail=<bool>
-
dirsync=<bool>
- These options change non standard file attributes on
operating systems and file systems that support these features, like Linux
with ext2fs and successors, xfs, or reiserfs. See man 1 chattr for
information on these options. Please note that there might be a race
condition between creating the file and applying these options.
PROCESS option group
Options of this group change the process properties instead of just affecting
one data channel. For EXEC and SYSTEM addresses and for LISTEN and CONNECT
type addresses with option FORK, these options apply to the child processes
instead of the main socat process.
-
chroot=<directory>
- Performs a chroot() operation to <directory> after
processing the address (example). This call might require root
privilege.
-
chroot-early=<directory>
- Performs a chroot() operation to <directory> before
opening the address. This call might require root privilege.
-
setgid=<group>
- Changes the primary <group> of the process after
processing the address. This call might require root privilege. Please
note that this option does not drop other group related privileges.
-
setgid-early=<group>
- Like setgit but is performed before opening the
address.
-
setuid=<user>
- Changes the <user> (owner) of the process after
processing the address. This call might require root privilege. Please
note that this option does not drop group related privileges. Check if
option su better fits your needs.
-
setuid-early=<user>
- Like setuid but is performed before opening the
address.
-
su=<user>
- Changes the <user> (owner) and groups of the process
after processing the address (example). This call might require root
privilege.
-
su-d=<user>
- Short name for substuser-delayed. Changes the <user>
(owner) and groups of the process after processing the address (example).
The user and his groups are retrieved before a possible chroot() .
This call might require root privilege.
-
setpgid=<pid_t>
- Makes the process a member of the specified process group
<pid_t>. If no value is given, or if the value is 0 or 1, the
process becomes leader of a new process group.
-
setsid
- Makes the process the leader of a new session (example).
READLINE option group
Due to licensing restrictions the readline feature is disabled in Debian (see
BUGS).
These options apply to the readline address type.
-
history=<filename>
- Reads and writes history from/to <filename>
(example).
-
noprompt
- Since version 1.4.0, socat per default tries to determine a
prompt - that is then passed to the readline call - by remembering the
last incomplete line of the output. With this option, socat does not pass
a prompt to readline, so it begins line editing in the first column of the
terminal.
-
noecho=<pattern>
- Specifies a regular pattern for a prompt that prevents the
following input line from being displayed on the screen and from being
added to the history. The prompt is defined as the text that was output to
the readline address after the lastest newline character and before an
input character was typed. The pattern is a regular expression, e.g.
"^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See
regex(7) for details. (example)
-
prompt=<string>
- Passes the string as prompt to the readline function.
readline prints this prompt when stepping through the history. If this
string matches a constant prompt issued by an interactive program on the
other socat address, consistent look and feel can be achieved.
APPLICATION option group
This group contains options that work at data level. Note that these options
only apply to the "raw" data transferred by socat, but not to
protocol data used by addresses like PROXY.
-
cr
- Converts the default line termination character NL
(’\n’, 0x0a) to/from CR (’\r’, 0x0d) when
writing/reading on this channel.
-
crnl
- Converts the default line termination character NL
(’\n’, 0x0a) to/from CRNL ("\r\n", 0x0d0a) when
writing/reading on this channel (example). Note: socat simply strips all
CR characters.
-
ignoreeof
- When EOF occurs on this channel, socat ignores it
and tries to read more data (like "tail -f") (example).
-
readbytes=<bytes>
-
socat reads only so many bytes from this address
(the address provides only so many bytes for transfer and pretends to be
at EOF afterwards). Must be greater than 0.
-
lockfile=<filename>
- If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.
-
waitlock=<filename>
- If lockfile exists, waits until it disappears. When
lockfile does not exist, creates it and continues, unlinks lockfile on
exit.
-
escape=<int>
- Specifies the numeric code of a character that triggers EOF
on the input stream. It is useful with a terminal in raw mode (example).
SOCKET option group
These options are intended for all kinds of sockets, e.g. IP or UNIX domain.
Most are applied with a setsockopt() call.
-
bind=<sockname>
- Binds the socket to the given socket address using the
bind() system call. The form of <sockname> is socket domain
dependent: IP4 and IP6 allow the form
[hostname|hostaddress][:(service|port)] (example), UNIX domain sockets
require <filename>, VSOCK allow the form [cid][:(port)].
-
connect-timeout=<seconds>
- Abort the connection attempt after <seconds>
[timeval] with error status.
-
so-bindtodevice=<interface>
- Binds the socket to the given <interface>. This
option might require root privilege.
-
broadcast
- For datagram sockets, allows sending to broadcast addresses
and receiving packets addressed to broadcast addresses.
-
debug
- Enables socket debugging.
-
dontroute
- Only communicates with directly connected peers, does not
use routers.
-
keepalive
- Enables sending keepalives on the socket.
-
linger=<seconds>
- Blocks shutdown() or close() until data transfers have
finished or the given timeout [int] expired.
-
oobinline
- Places out-of-band data in the input data stream.
-
priority=<priority>
- Sets the protocol defined <priority> [<int>]
for outgoing packets.
-
rcvbuf=<bytes>
- Sets the size of the receive buffer after the socket() call
to <bytes> [int]. With TCP sockets, this value corresponds to the
socket’s maximal window size.
-
rcvbuf-late=<bytes>
- Sets the size of the receive buffer when the socket is
already connected to <bytes> [int]. With TCP sockets, this value
corresponds to the socket’s maximal window size.
-
rcvlowat=<bytes>
- Specifies the minimum number of received bytes [int] until
the socket layer will pass the buffered data to socat.
-
reuseaddr
- Allows other sockets to bind to an address even if parts of
it (e.g. the local port) are already in use by socat
(example).
-
sndbuf=<bytes>
- Sets the size of the send buffer after the socket() call to
<bytes> [int].
-
sndbuf-late=<bytes>
- Sets the size of the send buffer when the socket is
connected to <bytes> [int].
-
sndlowat=<bytes>
- Specifies the minimum number of bytes in the send buffer
until the socket layer will send the data to <bytes> [int].
-
pf=<string>
- Forces the use of the specified IP version or protocol.
<string> can be something like "ip4" or "ip6".
The resulting value is used as first argument to the socket() or
socketpair() calls. This option affects address resolution and the
required syntax of bind and range options.
-
socktype=<type>
- Sets the type of the socket, specified as second argument
to the socket() or socketpair() calls, to <type> [int]. Address
resolution is not affected by this option. Under Linux, 1 means stream
oriented socket, 2 means datagram socket, 3 means raw socket, and 5
seqpacket (stream keeping packet boundaries).
-
protocol
- Sets the protocol of the socket, specified as third
argument to the socket() or socketpair() calls, to <protocol> [int].
Address resolution is not affected by this option. 6 means TCP, 17 means
UDP.
-
reuseport
- Set the SO_REUSEPORT socket option.
-
so-timestamp
- Sets the SO_TIMESTAMP socket option. This enables receiving
and logging of timestamp ancillary messages.
-
setsockopt=<level>:<optname>:<optval>
- Invokes setsockopt() for the socket with the given
parameters. level [int] is used as second argument to setsockopt() and
specifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or SOL_SOCKET for
the socket layer (1 on Linux). optname [int] is the third argument to
setsockopt() and tells which socket option is to be set. For the actual
numbers you might have to look up the appropriate include files of your
system. For the 4th and 5th setsockopt() parameters, value [dalan]
specifies an arbitrary sequence of bytes that are passed to the function
per pointer, with the automatically derived length parameter.
-
setsockopt-int=<level>:<optname>:<optval>
- Like setsockopt, but <optval> is a pointer to int
[int]
-
setsockopt-listen=<level>:<optname>:<optval>
- Like setsockopt, but for listen type addresses it is
applied to the listening socket instead of the connected socket.
-
setsockopt-string=<level>:<optname>:<optval>
- Like setsockopt, but <optval> is a string. This
string is passed to the function with trailing null character, and the
length parameter is automatically derived from the data.
UNIX option group
These options apply to UNIX domain based addresses.
-
unix-tightsocklen=[0|1]
- On socket operations, pass a socket address length that
does not include the whole struct sockaddr_un record but (besides other
components) only the relevant part of the filename or abstract string.
Default is 1.
IP4 and IP6 option groups
These options can be used with IPv4 and IPv6 based sockets.
-
tos=<tos>
- Sets the TOS (type of service) field of outgoing packets to
<tos> [byte] (see RFC 791).
-
ttl=<ttl>
- Sets the TTL (time to live) field of outgoing packets to
<ttl> [byte].
-
ip-options=<data>
- Sets IP options like source routing. Must be given in
binary form, recommended format is a leading "x" followed by an
even number of hex digits. This option may be used multiple times, data
are appended. E.g., to connect to host 10.0.0.1 via some gateway using a
loose source route, use the gateway as address parameter and set a loose
source route using the option ip-options=x8307040a000001 .
IP options are defined in RFC 791.
-
mtudiscover=<0|1|2>
- Takes 0, 1, 2 to never, want, or always use path MTU
discover on this socket.
-
ip-pktinfo
- Sets the IP_PKTINFO socket option. This enables receiving
and logging of ancillary messages containing destination address and
interface (Linux) (example).
-
ip-recverr
- Sets the IP_RECVERR socket option. This enables receiving
and logging of ancillary messages containing detailed error
information.
-
ip-recvopts
- Sets the IP_RECVOPTS socket option. This enables receiving
and logging of IP options ancillary messages (Linux, *BSD).
-
ip-recvtos
- Sets the IP_RECVTOS socket option. This enables receiving
and logging of TOS (type of service) ancillary messages (Linux).
-
ip-recvttl
- Sets the IP_RECVTTL socket option. This enables receiving
and logging of TTL (time to live) ancillary messages (Linux, *BSD).
-
ip-recvdstaddr
- Sets the IP_RECVDSTADDR socket option. This enables
receiving and logging of ancillary messages containing destination address
(*BSD) (example).
-
ip-recvif
- Sets the IP_RECVIF socket option. This enables receiving
and logging of interface ancillary messages (*BSD) (example).
-
ip-add-membership=<multicast-address:interface-address>
-
ip-add-membership=<multicast-address:interface-name>
-
ip-add-membership=<multicast-address:interface-index>
-
ip-add-membership=<multicast-address:interface-address:interface-name>
-
ip-add-membership=<multicast-address:interface-address:interface-index>
- Makes the socket member of the specified multicast group.
This is currently only implemented for IPv4. The option takes the IP
address of the multicast group and info about the desired network
interface. The most common syntax is the first one, while the others are
only available on systems that provide struct mreqn (Linux).
The indices of active network interfaces can be shown using the utility
procan.
-
ip-add-source-membership=<multicast-address:interface-address:source-address>
- Makes the socket member of the specified multicast group
for the specified source, i.e. only multicast traffic from this address is
to be delivered. This is currently only implemented for IPv4.
-
ip-multicast-if=<hostname>
- Specifies hostname or address of the network interface to
be used for multicast traffic.
-
ip-multicast-loop=<bool>
- Specifies if outgoing multicast traffic should loop back to
the interface.
-
ip-multicast-ttl=<byte>
- Sets the TTL used for outgoing multicast traffic. Default
is 1.
-
ip-transparent
- Sets the IP_TRANSPARENT socket option. This option might
require root privilege.
-
res-debug
-
res-aaonly
-
res-usevc
-
res-primary
-
res-igntc
-
res-recurse
-
res-defnames
-
res-stayopen
-
res-dnsrch
- These options set the corresponding resolver (name
resolution) option flags. Append "=0" to clear a default option.
See man resolver(5) for more information on these options. Note: these
options are valid only for the address they are applied to.
-
IP6 option group
These options can only be used on IPv6 based sockets. See IP options for options
that can be applied to both IPv4 and IPv6 sockets.
-
ipv6only=<bool>
- Sets the IPV6_V6ONLY socket option. If 0, the TCP stack
will also accept connections using IPv4 protocol on the same port. The
default is system dependent.
-
ipv6-recvdstopts
- Sets the IPV6_RECVDSTOPTS socket option. This enables
receiving and logging of ancillary messages containing the destination
options.
-
ipv6-recvhoplimit
- Sets the IPV6_RECVHOPLIMIT socket option. This enables
receiving and logging of ancillary messages containing the hoplimit.
-
ipv6-recvhopopts
- Sets the IPV6_RECVHOPOPTS socket option. This enables
receiving and logging of ancillary messages containing the hop
options.
-
ipv6-recvpktinfo
- Sets the IPV6_RECVPKTINFO socket option. This enables
receiving and logging of ancillary messages containing destination address
and interface.
-
ipv6-unicast-hops=link(TYPE_INT)(<int>)
- Sets the IPV6_UNICAST_HOPS socket option. This sets the hop
count limit (TTL) for outgoing unicast packets.
-
ipv6-recvrthdr
- Sets the IPV6_RECVRTHDR socket option. This enables
receiving and logging of ancillary messages containing routing
information.
-
ipv6-tclass
- Sets the IPV6_TCLASS socket option. This sets the transfer
class of outgoing packets.
-
ipv6-recvtclass
- Sets the IPV6_RECVTCLASS socket option. This enables
receiving and logging of ancillary messages containing the transfer class.
TCP option group
These options may be applied to TCP sockets. They work by invoking setsockopt()
with the appropriate parameters.
-
cork
- Doesn’t send packets smaller than MSS (maximal
segment size).
-
defer-accept
- While listening, accepts connections only when data from
the peer arrived.
-
keepcnt=<count>
- Sets the number of keepalives before shutting down the
socket to <count> [int].
-
keepidle=<seconds>
- Sets the idle time before sending the first keepalive to
<seconds> [int].
-
keepintvl=<seconds>
- Sets the interval between two keepalives to <seconds>
[int].
-
linger2=<seconds>
- Sets the time to keep the socket in FIN-WAIT-2 state to
<seconds> [int].
-
mss=<bytes>
- Sets the MSS (maximum segment size) after the socket() call
to <bytes> [int]. This value is then proposed to the peer with the
SYN or SYN/ACK packet (example).
-
mss-late=<bytes>
- Sets the MSS of the socket after connection has been
established to <bytes> [int].
-
nodelay
- Turns off the Nagle algorithm for measuring the RTT (round
trip time).
-
rfc1323
- Enables RFC1323 TCP options: TCP window scale, round-trip
time measurement (RTTM), and protect against wrapped sequence numbers
(PAWS) (AIX).
-
stdurg
- Enables RFC1122 compliant urgent pointer handling
(AIX).
-
syncnt=<count>
- Sets the maximal number of SYN retransmits during connect
to <count> [int].
-
md5sig
- Enables generation of MD5 digests on the packets
(FreeBSD).
-
noopt
- Disables use of TCP options (FreeBSD, MacOSX).
-
nopush
- sets the TCP_NOPUSH socket option (FreeBSD, MacOSX).
-
sack-disable
- Disables use the selective acknowledge feature
(OpenBSD).
-
signature-enable
- Enables generation of MD5 digests on the packets
(OpenBSD).
-
abort-threshold=<milliseconds>
- Sets the time to wait for an answer of the peer on an
established connection (HP-UX).
-
conn-abort-threshold=<milliseconds>
- Sets the time to wait for an answer of the server during
the initial connect (HP-UX).
-
keepinit
- Sets the time to wait for an answer of the server during
connect() before giving up. Value in half seconds, default is 150 (75s)
(Tru64).
-
paws
- Enables the "protect against wrapped sequence
numbers" feature (Tru64).
-
sackena
- Enables selective acknowledge (Tru64).
-
tsoptena
- Enables the time stamp option that allows RTT recalculation
on existing connections (Tru64).
UDP option group
This option may be applied to UDP datagram sockets.
-
udp-ignore-peerport>
- Address UDP-DATAGRAM expects incoming responses to come
from the port specified in its second parameter. With this option, it
accepts packets coming from any port.
SCTP option group
These options may be applied to SCTP stream sockets.
-
sctp-nodelay
- Sets the SCTP_NODELAY socket option that disables the Nagle
algorithm.
-
sctp-maxseg=<bytes>
- Sets the SCTP_MAXSEG socket option to <bytes> [int].
This value is then proposed to the peer with the SYN or SYN/ACK packet.
UDP, TCP, and SCTP option group
Here we find options that are related to the network port mechanism and thus can
be used with UDP, TCP, and SCTP client and server addresses.
-
sourceport=<port>
- For outgoing (client) TCP and UDP connections, it sets the
source <port> using an extra bind() call. With TCP or UDP listen
addresses, socat immediately shuts down the connection if the client does
not use this sourceport. UDP-RECV, UDP-RECVFROM, UDP-SENDTO, and
UDP-DATAGRAM addresses ignore the packet when it does not match.
(example).
-
lowport
- Outgoing (client) TCP and UDP connections with this option
use an unused random source port between 640 and 1023 incl. On UNIX class
operating systems, this requires root privilege, and thus indicates that
the client process is authorized by local root. TCP and UDP listen
addresses with this option immediately shut down the connection if the
client does not use a sourceport <= 1023. This mechanism can provide
limited authorization under some circumstances.
SOCKS option group
When using SOCKS type addresses, some socks specific options can be set.
-
socksport=<tcp service>
- Overrides the default "socks" service or port
1080 for the socks server port with <TCP service>.
-
socksuser=<user>
- Sends the <user> [string] in the username field to
the socks server. Default is the actual user name ($LOGNAME or $USER)
(example).
HTTP option group
Options that can be provided with HTTP type addresses. The only HTTP address
currently implemented is proxy-connect.
-
proxyport=<TCP service>
- Overrides the default HTTP proxy port 8080 with <TCP
service>.
-
ignorecr
- The HTTP protocol requires the use of CR+NL as line
terminator. When a proxy server violates this standard, socat might not
understand its answer. This option directs socat to interprete NL as line
terminator and to ignore CR in the answer. Nevertheless, socat sends CR+NL
to the proxy.
-
proxy-authorization=<username>:<password>
- Provide "basic" authentication to the proxy
server. The argument to the option is used with a
"Proxy-Authorization: Basic" header in base64 encoded form.
Note: username and password are visible for every user on the local machine
in the process list; username and password are transferred to the proxy
server unencrypted (base64 encoded) and might be sniffed.
-
proxy-authorization-file=<filename>
- Like option proxy-authorization, but the credentials are
read from the file and therefore not visible in the process list.
-
resolve
- Per default, socat sends to the proxy a CONNECT request
containing the target hostname. With this option, socat resolves the
hostname locally and sends the IP address. Please note that, according to
RFC 2396, only name resolution to IPv4 addresses is implemented.
RANGE option group
These options check if a connecting client should be granted access. They can be
applied to listening and receiving network sockets. tcp-wrappers options fall
into this group.
-
range=<address-range>
- After accepting a connection, tests if the peer is within
range. For IPv4 addresses, address-range takes the form
address/bits, e.g. 10.0.0.0/8, or address:mask, e.g. 10.0.0.0:255.0.0.0
(example); for IPv6, it is [ip6-address]/bits, e.g. [::1]/128. If the
client address does not match, socat refuses the connection
attempt, issues a warning, and keeps listening/receiving.
-
tcpwrap[=<name>]
- Uses Wietse Venema’s libwrap (tcpd) library to
determine if the client is allowed to connect. The configuration files are
/etc/hosts.allow and /etc/hosts.deny per default, see "man 5
hosts_access" for more information. The optional <name> (type
string) is passed to the wrapper functions as daemon process name
(example). If omitted, the basename of socats invocation (argv[0]) is
passed. If both tcpwrap and range options are applied to an address, both
conditions must be fulfilled to allow the connection.
-
allow-table=<filename>
- Takes the specified file instead of /etc/hosts.allow.
-
deny-table=<filename>
- Takes the specified file instead of /etc/hosts.deny.
-
tcpwrap-etc=<directoryname>
- Looks for hosts.allow and hosts.deny in the specified
directory. Is overridden by options hosts-allow and hosts-deny.
LISTEN option group
Options specific to listening sockets.
-
backlog=<count>
- Sets the backlog value passed with the listen() system call
to <count> [int]. Default is 5.
-
accept-timeout=<seconds>
- End waiting for a connection after <seconds>
[timeval] with error status.
-
max-children=<count>
- Limits the number of concurrent child processes [int].
Default is no limit.
CHILD option group
Options for addresses with multiple connections via child processes.
-
fork
- After establishing a connection, handles its channel in a
child process and keeps the parent process attempting to produce more
connections, either by listening or by connecting in a loop (example).
OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they actually fork off the
child: OPENSSL-LISTEN forks before the SSL handshake, while
OPENSSL-CONNECT forks afterwards. retry and forever options are not
inherited by the child process.
On some operating systems (e.g. FreeBSD) this option does not work for
UDP-LISTEN addresses.
EXEC option group
Options for addresses that invoke a program.
-
path=<string>
- Overrides the PATH environment variable for searching the
program with <string>. This $PATH value is effective in the child
process too.
-
login
- Prefixes argv[0] for the execvp() call with
’-’, thus making a shell behave as login shell.
FORK option group
EXEC or SYSTEM addresses invoke a program using a child process and transfer
data between
socat and the program. The interprocess communication
mechanism can be influenced with the following options. Per default, a
socketpair() is created and assigned to stdin and stdout of the child process,
while stderr is inherited from the
socat process, and the child process
uses file descriptors 0 and 1 for communicating with the main socat process.
-
nofork
- Does not fork a subprocess for executing the program,
instead calls execvp() or system() directly from the actual socat
instance. This avoids the overhead of another process between the program
and its peer, but introduces a lot of restrictions:
- o
- this option can only be applied to the second socat
address.
- o
- it cannot be applied to a part of a dual address.
- o
- the first socat address cannot be OPENSSL or READLINE
- o
- socat options -b, -t, -D, -l, -v, -x become useless
- o
- for both addresses, options ignoreeof, cr, and crnl become
useless
- o
- for the second address (the one with option nofork),
options append, cloexec, flock, user, group, mode, nonblock, perm-late,
setlk, and setpgid cannot be applied. Some of these could be used on the
first address though.
-
pipes
- Creates a pair of unnamed pipes for interprocess
communication instead of a socket pair.
-
openpty
- Establishes communication with the sub process using a
pseudo terminal created with openpty() instead of the default (socketpair
or ptmx).
-
ptmx
- Establishes communication with the sub process using a
pseudo terminal created by opening /dev/ptmx or /dev/ptc
instead of the default (socketpair).
-
pty
- Establishes communication with the sub process using a
pseudo terminal instead of a socket pair. Creates the pty with an
available mechanism. If openpty and ptmx are both available, it uses ptmx
because this is POSIX compliant (example).
-
ctty
- Makes the pty the controlling tty of the sub process
(example).
-
stderr
- Directs stderr of the sub process to its output channel by
making stderr a dup() of stdout (example).
-
fdin=<fdnum>
- Assigns the sub processes input channel to its file
descriptor <fdnum> instead of stdin (0). The program started from
the subprocess has to use this fd for reading data from socat
(example).
-
fdout=<fdnum>
- Assigns the sub processes output channel to its file
descriptor <fdnum> instead of stdout (1). The program started from
the subprocess has to use this fd for writing data to socat
(example).
-
sighup, sigint,
sigquit
- Has socat pass signals of this type to the sub
process. If no address has this option, socat terminates on these signals.
TERMIOS option group
For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the
terminal parameters defined in the UN*X termios mechanism are made available
as address option parameters. Please note that changes of the parameters of
your interactive terminal remain effective after
socat’s
termination, so you might have to enter "reset" or "stty
sane" in your shell afterwards. For EXEC and SYSTEM addresses with option
PTY, these options apply to the pty by the child processes.
-
b0
- Disconnects the terminal.
-
b19200
- Sets the serial line speed to 19200 baud. Some other rates
are possible; use something like socat -hh |grep ’ b[1-9]’
to find all speeds supported by your implementation.
Note: On some operating systems, these options may not be available. Use
ispeed or ospeed instead.
-
echo=<bool>
- Enables or disables local echo.
-
icanon=<bool>
- Sets or clears canonical mode, enabling line buffering and
some special characters.
-
raw
- Sets raw mode, thus passing input and output almost
unprocessed. This option is obsolete, use option rawer or cfmakeraw
instead.
-
rawer
- Makes terminal rawer than raw option. This option
implicitly turns off echo. (example).
-
cfmakeraw
- Sets raw mode by invoking cfmakeraw() or by simulating this
call. This option implicitly turns off echo.
-
ignbrk=<bool>
- Ignores or interpretes the BREAK character (e.g., ^C)
-
brkint=<bool>
-
bs0
-
bs1
-
bsdly=<0|1>
-
clocal=<bool>
-
cr0
cr1
cr2
cr3
- Sets the carriage return delay to 0, 1, 2, or 3,
respectively. 0 means no delay, the other values are terminal
dependent.
-
crdly=<0|1|2|3>
-
cread=<bool>
-
crtscts=<bool>
-
cs5
cs6
cs7
cs8
- Sets the character size to 5, 6, 7, or 8 bits,
respectively.
-
csize=<0|1|2|3>
-
cstopb=<bool>
- Sets two stop bits, rather than one.
-
dsusp=<byte>
- Sets the value for the VDSUSP character that suspends the
current foreground process and reactivates the shell (all except
Linux).
-
echoctl=<bool>
- Echos control characters in hat notation (e.g. ^A)
-
echoe=<bool>
-
echok=<bool>
-
echoke=<bool>
-
echonl=<bool>
-
echoprt=<bool>
-
eof=<byte>
-
eol=<byte>
-
eol2=<byte>
-
erase=<byte>
-
discard=<byte>
-
ff0
-
ff1
-
ffdly=<bool>
-
flusho=<bool>
-
hupcl=<bool>
-
icrnl=<bool>
-
iexten=<bool>
-
igncr=<bool>
-
ignpar=<bool>
-
imaxbel=<bool>
-
inlcr=<bool>
-
inpck=<bool>
-
intr=<byte>
-
isig=<bool>
-
ispeed=<unsigned-int>
- Set the baud rate for incoming data on this line.
See also: ospeed, b19200
-
istrip=<bool>
-
iuclc=<bool>
-
ixany=<bool>
-
ixoff=<bool>
-
ixon=<bool>
-
kill=<byte>
-
lnext=<byte>
-
min=<byte>
-
nl0
- Sets the newline delay to 0.
-
nl1
-
nldly=<bool>
-
noflsh=<bool>
-
ocrnl=<bool>
-
ofdel=<bool>
-
ofill=<bool>
-
olcuc=<bool>
-
onlcr=<bool>
-
onlret=<bool>
-
onocr=<bool>
-
opost=<bool>
- Enables or disables output processing; e.g., converts NL to
CR-NL.
-
ospeed=<unsigned-int>
- Set the baud rate for outgoing data on this line.
See also: ispeed, b19200
-
parenb=<bool>
- Enable parity generation on output and parity checking for
input.
-
parmrk=<bool>
-
parodd=<bool>
-
pendin=<bool>
-
quit=<byte>
-
reprint=<byte>
-
sane
- Brings the terminal to something like a useful default
state.
-
start=<byte>
-
stop=<byte>
-
susp=<byte>
-
swtc=<byte>
-
tab0
-
tab1
-
tab2
-
tab3
-
tabdly=<unsigned-int>
-
time=<byte>
-
tostop=<bool>
-
vt0
-
vt1
-
vtdly=<bool>
-
werase=<byte>
-
xcase=<bool>
-
xtabs
-
i-pop-all
- With UNIX System V STREAMS, removes all drivers from the
stack.
-
i-push=<string>
- With UNIX System V STREAMS, pushes the driver (module) with
the given name (string) onto the stack. For example, to make sure that a
character device on Solaris supports termios etc, use the following
options: i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat
PTY option group
These options are intended for use with the pty address type.
-
link=<filename>
- Generates a symbolic link that points to the actual pseudo
terminal (pty). This might help to solve the problem that ptys are
generated with more or less unpredictable names, making it difficult to
directly access the socat generated pty automatically. With this option,
the user can specify a "fix" point in the file hierarchy that
helps him to access the actual pty (example). Beginning with socat
version 1.4.3, the symbolic link is removed when the address is closed
(but see option unlink-close).
-
wait-slave
- Blocks the open phase until a process opens the slave side
of the pty. Usually, socat continues after generating the pty with opening
the next address or with entering the transfer loop. With the wait-slave
option, socat waits until some process opens the slave side of the pty
before continuing. This option only works if the operating system provides
the poll() system call. And it depends on an undocumented behaviour of
pty’s, so it does not work on all operating systems. It has
successfully been tested on Linux, FreeBSD, NetBSD, and on Tru64 with
openpty.
-
pty-interval=<seconds>
- When the wait-slave option is set, socat periodically
checks the HUP condition using poll() to find if the pty’s slave
side has been opened. The default polling interval is 1s. Use the
pty-interval option [timeval] to change this value.
OPENSSL option group
These options apply to the openssl and openssl-listen address types.
-
cipher=<cipherlist>
- Specifies the list of ciphers that may be used for the
connection. See the man page of ciphers , section CIPHER LIST
FORMAT, for detailed information about syntax, values, and default of
<cipherlist>.
Several cipher strings may be given, separated by ’:’. Some
simple cipher strings:
- 3DES
- Uses a cipher suite with triple DES.
- MD5
- Uses a cipher suite with MD5.
- aNULL
- Uses a cipher suite without authentication.
- NULL
- Does not use encryption.
- HIGH
- Uses a cipher suite with "high" encryption. Note
that the peer must support the selected property, or the negotiation will
fail.
-
method=<ssl-method>
- This option is based on deprecated functions and is only
available when socat was build with option --with-openssl-method.
Use option min-proto-version and maybe max-proto-version instead. Sets the
protocol version to be used. Valid strings (not case sensitive) are:
- SSL2
- Select SSL protocol version 2.
- SSL3
- Select SSL protocol version 3.
- SSL23
- Select the best available SSL or TLS protocol.
- TLS1
- Select TLS protocol version 1.
- TLS1.1
- Select TLS protocol version 1.1.
- TLS1.2
- Select TLS protocol version 1.2. When this option is not
provided OpenSSL negotiates the mothod with its peer.
-
min-proto-version
- This option tells OpenSSL to use this or a later SSL/TLS
protocol version and refuses to accept a lower/older protocol. Valid
syntax is:
- SSL2
- Select SSL protocol version 2.
- SSL3
- Select SSL protocol version 3.
- TLS1
- TLS1.0
- Select TLS protocol version 1.
- TLS1.1
- Select TLS protocol version 1.1.
- TLS1.2
- Select TLS protocol version 1.2.
- TLS1.3
- Select TLS protocol version 1.3.
-
openssl-max-proto-version
- This option is similar to min-proto-version, however, it
disallows use of a higher protocol version. Useful for testing the
peer.
-
verify=<bool>
- Controls check of the peer’s certificate. Default is
1 (true). Disabling verify might open your socket for everyone, making the
encryption useless!
-
cert=<filename>
- Specifies the file with the certificate and private key for
authentication. The certificate must be in OpenSSL format (*.pem). With
openssl-listen, use of this option is strongly recommended. Except with
cipher aNULL, "no shared ciphers" error will occur when no
certificate is given.
-
key=<filename>
- Specifies the file with the private key. The private key
may be in this file or in the file given with the cert option. The party
that has to proof that it is the owner of a certificate needs the private
key.
-
dhparams=<filename>
- Specifies the file with the Diffie Hellman parameters.
These parameters may also be in the file given with the cert option in
which case the dhparams option is not needed.
-
cafile=<filename>
- Specifies the file with the trusted (root) authority
certificates. The file must be in PEM format and should contain one or
more certificates. The party that checks the authentication of its peer
trusts only certificates that are in this file.
-
capath=<dirname>
- Specifies the directory with the trusted (root)
certificates. The directory must contain certificates in PEM format and
their hashes (see OpenSSL documentation)
-
egd=<filename>
- On some systems, openssl requires an explicit source of
random data. Specify the socket name where an entropy gathering daemon
like egd provides random data, e.g. /dev/egd-pool.
-
pseudo
- On systems where openssl cannot find an entropy source and
where no entropy gathering daemon can be utilized, this option activates a
mechanism for providing pseudo entropy. This is achieved by taking the
current time in microseconds for feeding the libc pseudo random number
generator with an initial value. openssl is then feeded with output from
random() calls.
NOTE:This mechanism is not sufficient for generation of secure keys!
-
compress
- Enable or disable the use of compression for a connection.
Setting this to "none" disables compression, setting it to
"auto" lets OpenSSL choose the best available algorithm
supported by both parties. The default is to not touch any
compression-related settings. NOTE: Requires OpenSSL 0.9.8 or higher and
disabling compression with OpenSSL 0.9.8 affects all new connections in
the process.
-
commonname=<string>
- Specify the commonname that the peer certificate must
match. With OPENSSL-CONNECT address this overrides the given hostname or
IP target address; with OPENSSL-LISTEN this turns on check of peer
certificates commonname. This option has only meaning when option verify
is not disabled and the chosen cipher provides a peer certificate.
-
no-sni=<bool>
- Do not use the client side Server Name Indication (SNI)
feature that selects the desired server certificate.
Note: SNI is automatically used since socat version 1.7.4.0 and uses
commonname or the given host name.
-
snihost=<string>
- Set the client side Server Name Indication (SNI) host name
different from the addressed server name or common name. This might be
useful when the server certificate has multiple host names or wildcard
names because the SNI host name is passed in cleartext to the server and
might be eavesdropped; with this option a mock name of the desired
certificate may be transferred.
-
fips
- Enables FIPS mode if compiled in. For info about the FIPS
encryption implementation standard see
http://oss-institute.org/fips-faq.html. This mode might require that the
involved certificates are generated with a FIPS enabled version of
openssl. Setting or clearing this option on one socat address affects all
OpenSSL addresses of this process.
RETRY option group
Options that control retry of some system calls, especially connection attempts.
-
retry=<num>
- Number of retries before the connection or listen attempt
is aborted. Default is 0, which means just one attempt.
-
interval=<timespec>
- Time between consecutive attempts (seconds, [timespec]).
Default is 1 second.
-
forever
- Performs an unlimited number of retry attempts.
TUN option group
Options that control Linux TUN/TAP interface device addresses.
-
tun-device=<device-file>
- Instructs socat to take another path for the TUN clone
device. Default is /dev/net/tun.
-
tun-name=<if-name>
- Gives the resulting network interface a specific name
instead of the system generated (tun0, tun1, etc.)
-
tun-type=[tun|tap]
- Sets the type of the TUN device; use this option to
generate a TAP device. See the Linux docu for the difference between these
types. When you try to establish a tunnel between two TUN devices, their
types should be the same.
-
iff-no-pi
- Sets the IFF_NO_PI flag which controls if the device
includes additional packet information in the tunnel. When you try to
establish a tunnel between two TUN devices, these flags should have the
same values.
-
iff-up
- Sets the TUN network interface status UP. Strongly
recommended.
-
iff-broadcast
- Sets the BROADCAST flag of the TUN network interface.
-
iff-debug
- Sets the DEBUG flag of the TUN network interface.
-
iff-loopback
- Sets the LOOPBACK flag of the TUN network interface.
-
iff-pointopoint
- Sets the POINTOPOINT flag of the TUN device.
-
iff-notrailers
- Sets the NOTRAILERS flag of the TUN device.
-
iff-running
- Sets the RUNNING flag of the TUN device.
-
iff-noarp
- Sets the NOARP flag of the TUN device.
-
iff-promisc
- Sets the PROMISC flag of the TUN device.
-
iff-allmulti
- Sets the ALLMULTI flag of the TUN device.
-
iff-master
- Sets the MASTER flag of the TUN device.
-
iff-slave
- Sets the SLAVE flag of the TUN device.
-
iff-multicast
- Sets the MULTICAST flag of the TUN device.
-
iff-portsel
- Sets the PORTSEL flag of the TUN device.
-
iff-automedia
- Sets the AUTOMEDIA flag of the TUN device.
-
iff-dynamic
- Sets the DYNAMIC flag of the TUN device.
This section explains the different data types that address parameters and
address options can take.
- address-range
- Is currently only implemented for IPv4 and IPv6. See
address-option `range’
- bool
- "0" or "1"; if value is omitted,
"1" is taken.
- byte
- An unsigned int number, read with strtoul() , lower or
equal to UCHAR_MAX .
- command-line
- A string specifying a program name and its arguments,
separated by single spaces.
- data
- This is a more general data specification. The given text
string contains information about the target data type and value.
Generally a leading character specifies the type of the following data
item. In its specific context a default data type may exist.
Currently only the following specifications are implemented:
- i
- A signed integer number, stored in host byte order.
Example: i-1000 (Integer number -1000)
- I
- An unsigned integer number, stored in host byte order.
- l
- A signed long integer number, stored in host byte order.
- L
- An unsigned long integer number, stored in host byte order.
- s
- A signed short integer number, stored in host byte order.
- S
- An unsigned short integer number, stored in host byte
order.
- b
- A signed byte (signed char).
- B
- An unsigned byte (unsigned char).
- x
- Following is an even number of hex digits, stored as
sequence of bytes.
Example: x7f000001 (IP address 127.0.0.1)
- "
- Following is a string that is used with the common
conversions \n \r \t \f \b \a \e \0; the string must be closed with
’"’. Please note that the quotes and backslashes need
to be escaped from shell and socat conversion.
Example: "Hello world!\n"
- ’
- A single char, with the usual conversions. Please note that
the quotes and backslashes need to be escaped from shell and socat
conversion.
Example: ’a’ Data items may be separated with white
space without need to repeat the type specifier again.
- directory
- A string with usual UN*X directory name semantics.
- facility
- The name of a syslog facility in lower case
characters.
- fdnum
- An unsigned int type, read with strtoul() , specifying a
UN*X file descriptor.
- filename
- A string with usual UN*X filename semantics.
- group
- If the first character is a decimal digit, the value is
read with strtoul() as unsigned integer specifying a group id. Otherwise,
it must be an existing group name.
- int
- A number following the rules of the strtol() function with
base "0", i.e. decimal number, octal number with leading
"0", or hexadecimal number with leading "0x". The
value must fit into a C int.
- interface
- A string specifying the device name of a network interface
as shown by ifconfig or procan, e.g. "eth0".
- IP address
- An IPv4 address in numbers-and-dots notation, an IPv6
address in hex notation enclosed in brackets, or a hostname that resolves
to an IPv4 or an IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1
- IPv4 address
- An IPv4 address in numbers-and-dots notation or a hostname
that resolves to an IPv4 address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2
- IPv6 address
- An IPv6 address in hexnumbers-and-colons notation enclosed
in brackets, or a hostname that resolves to an IPv6 address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0],
ip6name.domain.org
- long
- A number read with strtol() . The value must fit into a C
long.
- long long
- A number read with strtoll() . The value must fit into a C
long long.
- off_t
- An implementation dependend signed number, usually 32 bits,
read with strtol or strtoll.
- off64_t
- An implementation dependend signed number, usually 64 bits,
read with strtol or strtoll.
- mode_t
- An unsigned integer, read with strtoul() , specifying mode
(permission) bits.
- pid_t
- A number, read with strtol() , specifying a process
id.
- port
- A uint16_t (16 bit unsigned number) specifying a TCP or UDP
port, read with strtoul() .
- protocol
- An unsigned 8 bit number, read with strtoul() .
- size_t
- An unsigned number with size_t limitations, read with
strtoul .
- sockname
- A socket address. See address-option `bind’
- string
- A sequence of characters, not containing ’\0’
and, depending on the position within the command line, ’:’,
’,’, or "!!". Note that you might have to escape
shell meta characters in the command line.
- TCP service
- A service name, not starting with a digit, that is resolved
by getservbyname() , or an unsigned int 16 bit number read with strtoul()
.
- timeval
- A double float specifying seconds; the number is mapped
into a struct timeval, consisting of seconds and microseconds.
- timespec
- A double float specifying seconds; the number is mapped
into a struct timespec, consisting of seconds and nanoseconds.
- UDP service
- A service name, not starting with a digit, that is resolved
by getservbyname() , or an unsigned int 16 bit number read with strtoul()
.
- unsigned int
- A number read with strtoul() . The value must fit into a C
unsigned int.
- user
- If the first character is a decimal digit, the value is
read with strtoul() as unsigned integer specifying a user id. Otherwise,
it must be an existing user name.
- VSOCK cid
- A uint32_t (32 bit unsigned number) specifying a VSOCK
Context Identifier (CID), read with strtoul() . There are several special
addresses: VMADDR_CID_ANY (-1U) means any address for binding;
VMADDR_CID_HOST (2) is the well-known address of the host.
- VSOCK port
- A uint32_t (32 bit unsigned number) specifying a VSOCK
port, read with strtoul() .
-
socat - TCP4:www.domain.org:80
-
- transfers data between STDIO (-) and a TCP4 connection to
port 80 of host www.domain.org. This example results in an interactive
connection similar to telnet or netcat. The stdin terminal parameters are
not changed, so you may close the relay with ^D or abort it with ^C.
socat -d -d READLINE,history=$HOME/.http_history \
TCP4:www.domain.org:www,crnl
- this is similar to the previous example, but you can edit
the current line in a bash like manner (READLINE) and use the history file
.http_history; socat prints messages about progress (-d -d). The
port is specified by service name (www), and correct network line
termination characters (crnl) instead of NL are used.
-
socat TCP4-LISTEN:www
TCP4:www.domain.org:www
-
- installs a simple TCP port forwarder. With TCP4-LISTEN it
listens on local port "www" until a connection comes in, accepts
it, then connects to the remote host (TCP4) and starts data transfer. It
will not accept a second connection.
socat -d -d -lmlocal2 \
TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \
TCP4:www.domain.org:80,bind=myaddr2
- TCP port forwarder, each side bound to another local IP
address (bind). This example handles an almost arbitrary number of
parallel or consecutive connections by fork’ing a new process after
each accept() . It provides a little security by su’ing to user
nobody after forking; it only permits connections from the private 10
network (range); due to reuseaddr, it allows immediate restart after
master process’s termination, even if some child sockets are not
completely shut down. With -lmlocal2, socat logs to stderr until
successfully reaching the accept loop. Further logging is directed to
syslog with facility local2.
socat TCP4-LISTEN:5555,fork,tcpwrap=script \
EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr
- a simple server that accepts connections (TCP4-LISTEN) and
fork’s a new child process for each connection; every child acts as
single relay. The client must match the rules for daemon process name
"script" in /etc/hosts.allow and /etc/hosts.deny, otherwise it
is refused access (see "man 5 hosts_access"). For
EXEC’uting the program, the child process chroot’s to
/home/sandbox, su’s to user sandbox, and then starts the
program /home/sandbox/bin/myscript. Socat and myscript
communicate via a pseudo tty (pty); myscript’s stderr is redirected
to stdout, so its error messages are transferred via socat to the
connected client.
socat EXEC:"mail.sh [email protected]",fdin=3,fdout=4 \
TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512
-
mail.sh is a shell script, distributed with
socat, that implements a simple SMTP client. It is programmed to
"speak" SMTP on its FDs 3 (in) and 4 (out). The fdin and fdout
options tell socat to use these FDs for communication with the
program. Because mail.sh inherits stdin and stdout while socat does
not use them, the script can read a mail body from stdin. Socat
makes alias1 your local source address (bind), cares for correct network
line termination (crnl) and sends at most 512 data bytes per packet
(mss).
-
socat -,escape=0x0f
/dev/ttyS0,rawer,crnl
-
- opens an interactive connection via the serial line, e.g.
for talking with a modem. rawer sets the console’s and
ttyS0’s terminal parameters to practicable values, crnl converts to
correct newline characters. escape allows terminating the socat process
with character control-O.
socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork \
SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20
- with UNIX-LISTEN, socat opens a listening UNIX
domain socket /tmp/.X11-unix/X1. This path corresponds to local
XWindow display :1 on your machine, so XWindow client connections to
DISPLAY=:1 are accepted. Socat then speaks with the SOCKS4 server
host.victim.org that might permit sourceport 20 based connections due to
an FTP related weakness in its static IP filters. Socat pretends to
be invoked by socksuser nobody, and requests to be connected to loopback
port 6000 (only weak sockd configurations will allow this). So we get a
connection to the victims XWindow server and, if it does not require MIT
cookies or Kerberos authentication, we can start work. Please note that
there can only be one connection at a time, because TCP can establish only
one session with a given set of addresses and ports.
-
socat -u /tmp/readdata,seek-end=0,ignoreeof
-
-
- this is an example for unidirectional data transfer (-u).
Socat transfers data from file /tmp/readdata (implicit address
GOPEN), starting at its current end (seek-end=0 lets socat start
reading at current end of file; use seek=0 or no seek option to first read
the existing data) in a "tail -f" like mode (ignoreeof). The
"file" might also be a listening UNIX domain socket (do not use
a seek option then).
(sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) |
socat - EXEC:'ssh -l user server',pty,setsid,ctty
- EXEC’utes an ssh session to server. Uses a pty for
communication between socat and ssh, makes it ssh’s
controlling tty (ctty), and makes this pty the owner of a new process
group (setsid), so ssh accepts the password from socat.
socat -u TCP4-LISTEN:3334,reuseaddr,fork \
OPEN:/tmp/in.log,creat,append
- implements a simple network based message collector. For
each client connecting to port 3334, a new child process is generated
(option fork). All data sent by the clients are append’ed to the
file /tmp/in.log. If the file does not exist, socat creat’s it.
Option reuseaddr allows immediate restart of the server process.
socat PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \
EXEC:"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer"
- generates a pseudo terminal device (PTY) on the client that
can be reached under the symbolic link $HOME/dev/vmodem0. An
application that expects a serial line or modem can be configured to use
$HOME/dev/vmodem0; its traffic will be directed to a modemserver
via ssh where another socat instance links it to /dev/ttyS0.
socat TCP4-LISTEN:2022,reuseaddr,fork \
PROXY:proxy:www.domain.org:22,proxyport=3128,proxyauth=user:pass
- starts a forwarder that accepts connections on port 2022,
and directs them through the proxy daemon listening on port 3128
(proxyport) on host proxy, using the CONNECT method, where they are
authenticated as "user" with "pass" (proxyauth). The
proxy should establish connections to host www.domain.org on port 22
then.
-
socat -
OPENSSL:server:4443,cafile=server.crt,cert=client.pem
-
- is an OpenSSL client that tries to establish a secure
connection to an SSL server. Option cafile specifies a file that contains
trust certificates: we trust the server only when it presents one of these
certificates and proofs that it owns the related private key. Otherwise
the connection is terminated. With cert a file containing the client
certificate and the associated private key is specified. This is required
in case the server wishes a client authentication; many Internet servers
do not.
The first address (’-’) can be replaced by almost any other
socat address.
-
socat
OPENSSL-LISTEN:4443,reuseaddr,pf=ip4,fork,cert=server.pem,cafile=client.crt
PIPE
-
- is an OpenSSL server that accepts TCP connections, presents
the certificate from the file server.pem and forces the client to present
a certificate that is verified against cafile.crt.
The second address (’PIPE’) can be replaced by almost any
other socat address.
For instructions on generating and distributing OpenSSL keys and
certificates see the additional socat docu socat-openssl.txt.
-
echo |socat -u -
file:/tmp/bigfile,create,largefile,seek=100000000000
-
- creates a 100GB sparse file; this requires a file system
type that supports this (ext2, ext3, reiserfs, jfs; not minix, vfat). The
operation of writing 1 byte might take long (reiserfs: some minutes; ext2:
"no" time), and the resulting file can consume some disk space
with just its inodes (reiserfs: 2MB; ext2: 16KB).
-
socat tcp-l:7777,reuseaddr,fork system:’filan
-i 0 -s >&2’,nofork
-
- listens for incoming TCP connections on port 7777. For each
accepted connection, invokes a shell. This shell has its stdin and stdout
directly connected to the TCP socket (nofork). The shell starts filan and
lets it print the socket addresses to stderr (your terminal window).
-
echo -en "\0\14\0\0\c" |socat -u -
file:/usr/bin/squid.exe,seek=0x00074420
-
- functions as primitive binary editor: it writes the 4 bytes
000 014 000 000 to the executable /usr/bin/squid at offset 0x00074420
(this is a real world patch to make the squid executable from Cygwin run
under Windows, actual per May 2004).
-
socat -
tcp:www.blackhat.org:31337,readbytes=1000
-
- connects to an unknown service and prevents being
flooded.
-
socat -U TCP:target:9999,end-close
TCP-L:8888,reuseaddr,fork
-
- merges data arriving from different TCP streams on port
8888 to just one stream to target:9999. The end-close option prevents the
child processes forked off by the second address from terminating the
shared connection to 9999 (close(2) just unlinks the inode which stays
active as long as the parent process lives; shutdown(2) would actively
terminate the connection).
-
socat -
UDP4-DATAGRAM:192.168.1.0:123,sp=123,broadcast,range=192.168.1.0/24
-
- sends a broadcast to the network 192.168.1.0/24 and
receives the replies of the timeservers there. Ignores NTP packets from
hosts outside this network.
-
socat -
SOCKET-DATAGRAM:2:2:17:x007bxc0a80100x0000000000000000,bind=x007bx00000000x0000000000000000,setsockopt-int=1:6:1,range=x0000xc0a80100x0000000000000000:x0000xffffff00x0000000000000000
- is semantically equivalent to the previous example, but all
parameters are specified in generic form. the value 6 of setsockopt-int is
the Linux value for SO_BROADCAST.
-
socat -
IP4-DATAGRAM:255.255.255.255:44,broadcast,range=10.0.0.0/8
-
- sends a broadcast to the local network(s) using protocol
44. Accepts replies from the private address range only.
-
socat -
UDP4-DATAGRAM:224.255.0.1:6666,bind=:6666,ip-add-membership=224.255.0.1:eth0
-
- transfers data from stdin to the specified multicast
address using UDP. Both local and remote ports are 6666. Tells the
interface eth0 to also accept multicast packets of the given group.
Multiple hosts on the local network can run this command, so all data sent
by any of the hosts will be received by all the other ones. Note that
there are many possible reasons for failure, including IP-filters, routing
issues, wrong interface selection by the operating system, bridges, or a
badly configured switch.
-
socat UDP:host2:4443
TUN:192.168.255.1/24,up
-
- establishes one side of a virtual (but not private!)
network with host2 where a similar process might run, with UDP-L and tun
address 192.168.255.2. They can reach each other using the addresses
192.168.255.1 and 192.168.255.2. Note that streaming eg.via TCP or SSL
does not guarantee to retain packet boundaries and might thus cause packet
loss.
-
socat - VSOCK-CONNECT:2:1234
-
- establishes a VSOCK connection with the host (host is
always reachable with the well-know CID=2) on 1234 port.
-
socat - VSOCK-LISTEN:1234
-
- listens for a VSOCK connection on 1234 port.
-
socat - VSOCK-CONNECT:31:4321,bind:5555
-
- establishes a VSOCK connection with the guest that have
CID=31 on 1234 port, binding the local socket to the 5555 port.
-
socat VSOCK-LISTEN:3333,reuseaddr,fork
VSOCK-CONNECT:42,3333
-
- starts a forwarder that accepts VSOCK connections on port
3333, and directs them to the guest with CID=42 on the same port.
-
socat VSOCK-LISTEN:22,reuseaddr,fork
TCP:localhost:22
-
- forwards VSOCK connections from 22 port to the local SSH
server. Running this in a VM allows you to connect via SSH from the host
using VSOCK, as in the example below.
-
socat TCP4-LISTEN:22222,reuseaddr,fork
VSOCK-CONNECT:33:22
-
- forwards TCP connections from 22222 port to the guest with
CID=33 listening on VSOCK port 22. Running this in the host, allows you to
connect via SSH running "ssh -p 22222 user@localhost", if the
guest runs the example above.
-
socat PTY,link=/var/run/ppp,rawer
INTERFACE:hdlc0
-
- circumvents the problem that pppd requires a serial device
and thus might not be able to work on a synchronous line that is
represented by a network device. socat creates a PTY to make pppd happy,
binds to the network interface hdlc0, and can transfer data between both
devices. Use pppd on device /var/run/ppp then.
-
socat -T 1 -d -d TCP-L:10081,reuseaddr,fork,crlf
SYSTEM:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType:
text/plain\\\n\\\ndate:
\$\(date\)\\\nserver:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT\\\nclient:
\$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n\\\"\"; cat; echo -e
\"\\\"\\\n\\\"\""
- creates a simple HTTP echo server: each HTTP client that
connects gets a valid HTTP reply that contains information about the
client address and port as it is seen by the server host, the host address
(which might vary on multihomed servers), and the original client
request.
-
socat -d -d
UDP4-RECVFROM:9999,so-broadcast,so-timestamp,ip-pktinfo,ip-recverr,ip-recvopts,ip-recvtos,ip-recvttl!!-
SYSTEM:’export; sleep 1’ |grep SOCAT
-
- waits for an incoming UDP packet on port 9999 and prints
the environment variables provided by socat. On BSD based systems you have
to replace ip-pktinfo with ip-recvdstaddr,ip-recvif. Especially
interesting is SOCAT_IP_DSTADDR: it contains the target address of the
packet which may be a unicast, multicast, or broadcast address.
-
echo -e "M-SEARCH * HTTP/1.1\nHOST:
239.255.255.250:1900\nMAN: \"ssdp:discover\"\nMX: 4\nST:
\"ssdp:all\"\n" |socat -
UDP-DATAGRAM:239.255.255.250:1900,crlf
-
- sends an SSDP (Simple Service Discovery Protocol) query to
the local network and collects and outputs the answers received.
-
-
Socat uses a logging mechanism that allows filtering messages by
severity. The severities provided are more or less compatible to the
appropriate syslog priority. With one or up to four occurrences of the -d
command line option, the lowest priority of messages that are issued can be
selected. Each message contains a single uppercase character specifying the
messages severity (one of F, E, W, N, I, or D)
- FATAL:
- Conditions that require unconditional and immediate program
termination.
- ERROR:
- Conditions that prevent proper program processing. Usually
the program is terminated (see option -s).
- WARNING:
- Something did not function correctly or is in a state where
correct further processing cannot be guaranteed, but might be
possible.
- NOTICE:
- Interesting actions of the program, e.g. for supervising
socat in some kind of server mode.
- INFO:
- Description of what the program does, and maybe why it
happens. Allows monitoring the lifecycles of file descriptors.
- DEBUG:
- Description of how the program works, all system or library
calls and their results.
Log messages can be written to stderr, to a file, or to syslog.
On exit,
socat gives status 0 if it terminated due to EOF or inactivity
timeout, with a positive value on error, and with a negative value on fatal
error.
/usr/bin/socat
/usr/bin/filan
/usr/bin/procan
Input variables carry information from the environment to socat, output
variables are set by socat for use in executed scripts and programs.
In the output variables beginning with "SOCAT" this prefix is actually
replaced by the upper case name of the executable or the value of option -lp.
-
SOCAT_DEFAULT_LISTEN_IP (input)
- (Values 4 or 6) Sets the IP version to be used for listen,
recv, and recvfrom addresses if no pf (protocol-family) option is given.
Is overridden by socat options -4 or -6.
-
SOCAT_PREFERRED_RESOLVE_IP (input)
- (Values 0, 4, or 6) Sets the IP version to be used when
resolving target host names when version is not specified by address type,
option pf (protocol-family), or address format. If name resolution does
not return a matching entry, the first result (with differing IP version)
is taken. With value 0, socat always selects the first record and its IP
version.
-
SOCAT_FORK_WAIT (input)
- Specifies the time (seconds) to sleep the parent and child
processes after successful fork(). Useful for debugging.
-
SOCAT_VERSION (output)
- Socat sets this variable to its version string, e.g.
"1.7.0.0" for released versions or e.g.
"1.6.0.1+envvar" for temporary versions; can be used in scripts
invoked by socat.
-
SOCAT_PID (output)
- Socat sets this variable to its process id. In case of fork
address option, SOCAT_PID gets the child processes id. Forking for exec
and system does not change SOCAT_PID.
-
SOCAT_PPID (output)
- Socat sets this variable to its process id. In case of
fork, SOCAT_PPID keeps the pid of the master process.
-
SOCAT_PEERADDR (output)
- With passive socket addresses (all LISTEN and RECVFROM
addresses), this variable is set to a string describing the peers socket
address. Port information is not included.
-
SOCAT_PEERPORT (output)
- With appropriate passive socket addresses (TCP, UDP, and
SCTP - LISTEN and RECVFROM), this variable is set to a string containing
the number of the peer port.
-
SOCAT_SOCKADDR (output)
- With all LISTEN addresses, this variable is set to a string
describing the local socket address. Port information is not included
example
-
SOCAT_SOCKPORT (output)
- With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses,
this variable is set to the local port.
-
SOCAT_TIMESTAMP (output)
- With all RECVFROM addresses where address option
so-timestamp is applied, socat sets this variable to the resulting
timestamp.
-
SOCAT_IP_OPTIONS (output)
- With all IPv4 based RECVFROM addresses where address option
ip-recvopts is applied, socat fills this variable with the IP options of
the received packet.
-
SOCAT_IP_DSTADDR (output)
- With all IPv4 based RECVFROM addresses where address option
ip-recvdstaddr (BSD) or ip-pktinfo (other platforms) is applied, socat
sets this variable to the destination address of the received packet. This
is particularly useful to identify broadcast and multicast addressed
packets.
-
SOCAT_IP_IF (output)
- With all IPv4 based RECVFROM addresses where address option
ip-recvif (BSD) or ip-pktinfo (other platforms) is applied, socat sets
this variable to the name of the interface where the packet was
received.
-
SOCAT_IP_LOCADDR (output)
- With all IPv4 based RECVFROM addresses where address option
ip-pktinfo is applied, socat sets this variable to the address of the
interface where the packet was received.
-
SOCAT_IP_TOS (output)
- With all IPv4 based RECVFROM addresses where address option
ip-recvtos is applied, socat sets this variable to the TOS (type of
service) of the received packet.
-
SOCAT_IP_TTL (output)
- With all IPv4 based RECVFROM addresses where address option
ip-recvttl is applied, socat sets this variable to the TTL (time to live)
of the received packet.
-
SOCAT_IPV6_HOPLIMIT (output)
- With all IPv6 based RECVFROM addresses where address option
ipv6-recvhoplimit is applied, socat sets this variable to the hoplimit
value of the received packet.
-
SOCAT_IPV6_DSTADDR (output)
- With all IPv6 based RECVFROM addresses where address option
ipv6-recvpktinfo is applied, socat sets this variable to the destination
address of the received packet.
-
SOCAT_IPV6_TCLASS (output)
- With all IPv6 based RECVFROM addresses where address option
ipv6-recvtclass is applied, socat sets this variable to the
transfer class of the received packet.
-
SOCAT_OPENSSL_X509_ISSUER (output)
- Issuer field from peer certificate
-
SOCAT_OPENSSL_X509_SUBJECT (output)
- Subject field from peer certificate
-
SOCAT_OPENSSL_X509_COMMONNAME (output)
- commonName entries from peer certificates subject. Multiple
values are separated by " // ".
-
SOCAT_OPENSSL_X509_* (output)
- all other entries from peer certificates subject
-
SOCAT_OPENSSL_X509V3_DNS (output)
- DNS entries from peer certificates extensions -
subjectAltName field. Multiple values are separated by " //
".
-
HOSTNAME (input)
- Is used to determine the hostname for logging (see
-lh).
-
LOGNAME (input)
- Is used as name for the socks client user name if no
socksuser is given.
With options su and su-d, LOGNAME is set to the given user name.
-
USER (input)
- Is used as name for the socks client user name if no
socksuser is given and LOGNAME is empty.
With options su and su-d, USER is set to the given user name.
-
SHELL (output)
- With options su and su-d, SHELL is set to the login shell
of the given user.
-
PATH (output)
- Can be set with option path for exec and system
addresses.
-
HOME (output)
- With options su and su-d, HOME is set to the home directory
of the given user.
The work of the following groups and organizations was invaluable for this
project:
The
FSF (GNU,
http://www.fsf.org/) project with their free and portable
development software and lots of other useful tools and libraries.
The
Linux developers community (
http://www.linux.org/) for providing a
free, open source operating system.
The
Open Group (
http://www.unix-systems.org/) for making their standard
specifications available on the Internet for free.
This man page describes version 1.7.4 of
socat.
Addresses cannot be nested, so a single socat process cannot, e.g., drive ssl
over socks.
Address option ftruncate without value uses default 1 instead of 0.
Verbose modes (-x and/or -v) display line termination characters inconsistently
when address options cr or crnl are used: They show the data
after
conversion in either direction.
The licenses of OpenSSL and GNU Readline are incompatible. Therefore readline
support is disabled in Debian.
Send bug reports to <
[email protected]>
nc(1),
rinetd(8),
openssl(1),
stunnel(8),
rlwrap(1),
setsid(1)
Socat home page
http://www.dest-unreach.org/socat/
Gerhard Rieger <
[email protected]> and contributors