NAME
systemd.socket - Socket unit configurationSYNOPSIS
socket.socketDESCRIPTION
A unit configuration file whose name ends in ".socket" encodes information about an IPC or network socket or a file system FIFO controlled and supervised by systemd, for socket-based activation. This man page lists the configuration options specific to this unit type. See systemd.unit(5) for the common options of all unit configuration files. The common configuration items are configured in the generic [Unit] and [Install] sections. The socket specific configuration options are configured in the [Socket] section. Additional options are listed in systemd.exec(5), which define the execution environment the ExecStartPre=, ExecStartPost=, ExecStopPre= and ExecStopPost= commands are executed in, and in systemd.kill(5), which define the way the processes are terminated, and in systemd.resource-control(5), which configure resource control settings for the processes of the socket. For each socket unit, a matching service unit must exist, describing the service to start on incoming traffic on the socket (see systemd.service(5) for more information about .service units). The name of the .service unit is by default the same as the name of the .socket unit, but can be altered with the Service= option described below. Depending on the setting of the Accept= option described below, this .service unit must either be named like the .socket unit, but with the suffix replaced, unless overridden with Service=; or it must be a template unit named the same way. Example: a socket file foo.socket needs a matching service foo.service if Accept=no is set. If Accept=yes is set, a service template [email protected] must exist from which services are instantiated for each incoming connection. No implicit WantedBy= or RequiredBy= dependency from the socket to the service is added. This means that the service may be started without the socket, in which case it must be able to open sockets by itself. To prevent this, an explicit Requires= dependency may be added. Socket units may be used to implement on-demand starting of services, as well as parallelized starting of services. See the blog stories linked at the end for an introduction. Note that the daemon software configured for socket activation with socket units needs to be able to accept sockets from systemd, either via systemd's native socket passing interface (see sd_listen_fds(3) for details about the precise protocol used and the order in which the file descriptors are passed) or via traditional inetd(8)-style socket passing (i.e. sockets passed in via standard input and output, using StandardInput=socket in the service file). All network sockets allocated through .socket units are allocated in the host's network namespace (see network_namespaces(7)). This does not mean however that the service activated by a configured socket unit has to be part of the host's network namespace as well. It is supported and even good practice to run services in their own network namespace (for example through PrivateNetwork=, see systemd.exec(5)), receiving only the sockets configured through socket-activation from the host's namespace. In such a set-up communication within the host's network namespace is only permitted through the activation sockets passed in while all sockets allocated from the service code itself will be associated with the service's own namespace, and thus possibly subject to a restrictive configuration.AUTOMATIC DEPENDENCIES
Implicit Dependencies
The following dependencies are implicitly added:•Socket units automatically gain a
Before= dependency on the service units they activate.
•Socket units referring to file system
paths (such as AF_UNIX sockets or FIFOs) implicitly gain
Requires= and After= dependencies on all mount units necessary
to access those paths.
•Socket units using the
BindToDevice= setting automatically gain a BindsTo= and
After= dependency on the device unit encapsulating the specified
network interface.
Additional implicit dependencies may be added as result of execution and
resource control parameters as documented in systemd.exec(5) and
systemd.resource-control(5).
Default Dependencies
The following dependencies are added unless DefaultDependencies=no is set:•Socket units automatically gain a
Before= dependency on sockets.target.
•Socket units automatically gain a pair
of After= and Requires= dependency on sysinit.target, and a pair
of Before= and Conflicts= dependencies on shutdown.target. These
dependencies ensure that the socket unit is started before normal services at
boot, and is stopped on shutdown. Only sockets involved with early boot or
late system shutdown should disable DefaultDependencies= option.
OPTIONS
Socket unit files may include [Unit] and [Install] sections, which are described in systemd.unit(5). Socket unit files must include a [Socket] section, which carries information about the socket or FIFO it supervises. A number of options that may be used in this section are shared with other unit types. These options are documented in systemd.exec(5) and systemd.kill(5). The options specific to the [Socket] section of socket units are the following: ListenStream=, ListenDatagram=, ListenSequentialPacket=Specifies an address to listen on for a stream
( SOCK_STREAM), datagram (SOCK_DGRAM), or sequential packet
(SOCK_SEQPACKET) socket, respectively. The address can be written in
various formats:
If the address starts with a slash ("/"), it is read as file system
socket in the AF_UNIX socket family.
If the address starts with an at symbol ("@"), it is read as abstract
namespace socket in the AF_UNIX family. The "@" is replaced
with a NUL character before binding. For details, see unix(7).
If the address string is a single number, it is read as port number to listen on
via IPv6. Depending on the value of BindIPv6Only= (see below) this
might result in the service being available via both IPv6 and IPv4 (default)
or just via IPv6.
If the address string is a string in the format "
v.w.x.y:z", it is interpreted as IPv4 address
v.w.x.y and port z.
If the address string is a string in the format "[
x]:y", it is interpreted as IPv6 address x and port
y. An optional interface scope (interface name or number) may be
specified after a "%" symbol: "[
x]:y%dev". Interface scopes are only useful with
link-local addresses, because the kernel ignores them in other cases. Note
that if an address is specified as IPv6, it might still make the service
available via IPv4 too, depending on the BindIPv6Only= setting (see
below).
If the address string is a string in the format "vsock:
x:y", it is read as CID x on a port y address
in the AF_VSOCK family. The CID is a unique 32-bit integer identifier
in AF_VSOCK analogous to an IP address. Specifying the CID is optional,
and may be set to the empty string.
Note that SOCK_SEQPACKET (i.e. ListenSequentialPacket=) is only
available for AF_UNIX sockets. SOCK_STREAM (i.e.
ListenStream=) when used for IP sockets refers to TCP sockets,
SOCK_DGRAM (i.e. ListenDatagram=) to UDP.
These options may be specified more than once, in which case incoming traffic on
any of the sockets will trigger service activation, and all listed sockets
will be passed to the service, regardless of whether there is incoming traffic
on them or not. If the empty string is assigned to any of these options, the
list of addresses to listen on is reset, all prior uses of any of these
options will have no effect.
It is also possible to have more than one socket unit for the same service when
using Service=, and the service will receive all the sockets configured
in all the socket units. Sockets configured in one unit are passed in the
order of configuration, but no ordering between socket units is specified.
If an IP address is used here, it is often desirable to listen on it before the
interface it is configured on is up and running, and even regardless of
whether it will be up and running at any point. To deal with this, it is
recommended to set the FreeBind= option described below.
ListenFIFO=
Specifies a file system FIFO (see
fifo(7) for details) to listen on. This expects an absolute file system
path as argument. Behavior otherwise is very similar to the
ListenDatagram= directive above.
ListenSpecial=
Specifies a special file in the file system to
listen on. This expects an absolute file system path as argument. Behavior
otherwise is very similar to the ListenFIFO= directive above. Use this
to open character device nodes as well as special files in /proc/ and
/sys/.
ListenNetlink=
Specifies a Netlink family to create a socket
for to listen on. This expects a short string referring to the
AF_NETLINK family name (such as audit or kobject-uevent)
as argument, optionally suffixed by a whitespace followed by a multicast group
integer. Behavior otherwise is very similar to the ListenDatagram=
directive above.
ListenMessageQueue=
Specifies a POSIX message queue name to listen
on (see mq_overview(7) for details). This expects a valid message queue
name (i.e. beginning with "/"). Behavior otherwise is very similar
to the ListenFIFO= directive above. On Linux message queue descriptors
are actually file descriptors and can be inherited between processes.
ListenUSBFunction=
Specifies a USB FunctionFS[1] endpoints
location to listen on, for implementation of USB gadget functions. This
expects an absolute file system path of a FunctionFS mount point as the
argument. Behavior otherwise is very similar to the ListenFIFO=
directive above. Use this to open the FunctionFS endpoint ep0. When using this
option, the activated service has to have the USBFunctionDescriptors=
and USBFunctionStrings= options set.
SocketProtocol=
Takes one of udplite or sctp.
The socket will use the UDP-Lite ( IPPROTO_UDPLITE) or SCTP
(IPPROTO_SCTP) protocol, respectively.
BindIPv6Only=
Takes one of default, both or
ipv6-only. Controls the IPV6_V6ONLY socket option (see ipv6(7)
for details). If both, IPv6 sockets bound will be accessible via both
IPv4 and IPv6. If ipv6-only, they will be accessible via IPv6 only. If
default (which is the default, surprise!), the system wide default
setting is used, as controlled by /proc/sys/net/ipv6/bindv6only, which in turn
defaults to the equivalent of both.
Backlog=
Takes an unsigned 32bit integer argument.
Specifies the number of connections to queue that have not been accepted yet.
This setting matters only for stream and sequential packet sockets. See
listen(2) for details. Note that this value is silently capped by the
"net.core.somaxconn" sysctl, which typically defaults to 4096. By
default this is set to 4294967295, so that the sysctl takes full effect.
BindToDevice=
Specifies a network interface name to bind
this socket to. If set, traffic will only be accepted from the specified
network interfaces. This controls the SO_BINDTODEVICE socket option
(see socket(7) for details). If this option is used, an implicit
dependency from this socket unit on the network interface device unit is
created (see systemd.device(5)). Note that setting this parameter might
result in additional dependencies to be added to the unit (see above).
SocketUser=, SocketGroup=
Takes a UNIX user/group name. When specified,
all AF_UNIX sockets and FIFO nodes in the file system are owned by the
specified user and group. If unset (the default), the nodes are owned by the
root user/group (if run in system context) or the invoking user/group (if run
in user context). If only a user is specified but no group, then the group is
derived from the user's default group.
SocketMode=
If listening on a file system socket or FIFO,
this option specifies the file system access mode used when creating the file
node. Takes an access mode in octal notation. Defaults to 0666.
DirectoryMode=
If listening on a file system socket or FIFO,
the parent directories are automatically created if needed. This option
specifies the file system access mode used when creating these directories.
Takes an access mode in octal notation. Defaults to 0755.
Accept=
Takes a boolean argument. If yes, a service
instance is spawned for each incoming connection and only the connection
socket is passed to it. If no, all listening sockets themselves are passed to
the started service unit, and only one service unit is spawned for all
connections (also see above). This value is ignored for datagram sockets and
FIFOs where a single service unit unconditionally handles all incoming
traffic. Defaults to no. For performance reasons, it is recommended to
write new daemons only in a way that is suitable for Accept=no. A
daemon listening on an AF_UNIX socket may, but does not need to, call
close(2) on the received socket before exiting. However, it must not
unlink the socket from a file system. It should not invoke shutdown(2)
on sockets it got with Accept=no, but it may do so for sockets it got
with Accept=yes set. Setting Accept=yes is mostly useful to
allow daemons designed for usage with inetd(8) to work unmodified with
systemd socket activation.
Note that depending on this setting the services activated by units of this type
are either regular services (in case of Accept=no) or instances
of templated services (in case of Accept=yes). See the
Description section above for a more detailed discussion of the naming rules
of triggered services.
For IPv4 and IPv6 connections, the REMOTE_ADDR environment variable will
contain the remote IP address, and REMOTE_PORT will contain the remote
port. This is the same as the format used by CGI. For SOCK_RAW, the
port is the IP protocol.
It is recommended to set CollectMode=inactive-or-failed for service
instances activated via Accept=yes, to ensure that failed connection
services are cleaned up and released from memory, and do not accumulate.
Writable=
Takes a boolean argument. May only be used in
conjunction with ListenSpecial=. If true, the specified special file is
opened in read-write mode, if false, in read-only mode. Defaults to
false.
FlushPending=
Takes a boolean argument. May only be used
when Accept=no. If yes, the socket's buffers are cleared after the
triggered service exited. This causes any pending data to be flushed and any
pending incoming connections to be rejected. If no, the socket's buffers won't
be cleared, permitting the service to handle any pending connections after
restart, which is the usually expected behaviour. Defaults to no.
MaxConnections=
The maximum number of connections to
simultaneously run services instances for, when Accept=yes is set. If
more concurrent connections are coming in, they will be refused until at least
one existing connection is terminated. This setting has no effect on sockets
configured with Accept=no or datagram sockets. Defaults to 64.
MaxConnectionsPerSource=
The maximum number of connections for a
service per source IP address. This is very similar to the
MaxConnections= directive above. Disabled by default.
KeepAlive=
Takes a boolean argument. If true, the TCP/IP
stack will send a keep alive message after 2h (depending on the configuration
of /proc/sys/net/ipv4/tcp_keepalive_time) for all TCP streams accepted on this
socket. This controls the SO_KEEPALIVE socket option (see
socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults
to false.
KeepAliveTimeSec=
Takes time (in seconds) as argument. The
connection needs to remain idle before TCP starts sending keepalive probes.
This controls the TCP_KEEPIDLE socket option (see socket(7) and the
TCP Keepalive HOWTO[2] for details.) Defaults value is 7200 seconds (2
hours).
KeepAliveIntervalSec=
Takes time (in seconds) as argument between
individual keepalive probes, if the socket option SO_KEEPALIVE has been
set on this socket. This controls the TCP_KEEPINTVL socket option (see
socket(7) and the TCP Keepalive HOWTO[2] for details.) Defaults
value is 75 seconds.
KeepAliveProbes=
Takes an integer as argument. It is the number
of unacknowledged probes to send before considering the connection dead and
notifying the application layer. This controls the TCP_KEEPCNT socket option
(see socket(7) and the TCP Keepalive HOWTO[2] for details.)
Defaults value is 9.
NoDelay=
Takes a boolean argument. TCP Nagle's
algorithm works by combining a number of small outgoing messages, and sending
them all at once. This controls the TCP_NODELAY socket option (see
tcp(7)). Defaults to false.
Priority=
Takes an integer argument controlling the
priority for all traffic sent from this socket. This controls the
SO_PRIORITY socket option (see socket(7) for details.).
DeferAcceptSec=
Takes time (in seconds) as argument. If set,
the listening process will be awakened only when data arrives on the socket,
and not immediately when connection is established. When this option is set,
the TCP_DEFER_ACCEPT socket option will be used (see tcp(7)),
and the kernel will ignore initial ACK packets without any data. The argument
specifies the approximate amount of time the kernel should wait for incoming
data before falling back to the normal behavior of honoring empty ACK packets.
This option is beneficial for protocols where the client sends the data first
(e.g. HTTP, in contrast to SMTP), because the server process will not be woken
up unnecessarily before it can take any action.
If the client also uses the TCP_DEFER_ACCEPT option, the latency of the
initial connection may be reduced, because the kernel will send data in the
final packet establishing the connection (the third packet in the
"three-way handshake").
Disabled by default.
ReceiveBuffer=, SendBuffer=
Takes an integer argument controlling the
receive or send buffer sizes of this socket, respectively. This controls the
SO_RCVBUF and SO_SNDBUF socket options (see socket(7) for
details.). The usual suffixes K, M, G are supported and are understood to the
base of 1024.
IPTOS=
Takes an integer argument controlling the IP
Type-Of-Service field for packets generated from this socket. This controls
the IP_TOS socket option (see ip(7) for details.). Either a
numeric string or one of low-delay, throughput,
reliability or low-cost may be specified.
IPTTL=
Takes an integer argument controlling the IPv4
Time-To-Live/IPv6 Hop-Count field for packets generated from this socket. This
sets the IP_TTL/IPV6_UNICAST_HOPS socket options (see
ip(7) and ipv6(7) for details.)
Mark=
Takes an integer value. Controls the firewall
mark of packets generated by this socket. This can be used in the firewall
logic to filter packets from this socket. This sets the SO_MARK socket
option. See iptables(8) for details.
ReusePort=
Takes a boolean value. If true, allows
multiple bind(2)s to this TCP or UDP port. This controls the
SO_REUSEPORT socket option. See socket(7) for details.
SmackLabel=, SmackLabelIPIn=, SmackLabelIPOut=
Takes a string value. Controls the extended
attributes "security.SMACK64", "security.SMACK64IPIN" and
"security.SMACK64IPOUT", respectively, i.e. the security label of
the FIFO, or the security label for the incoming or outgoing connections of
the socket, respectively. See Smack[3] for details.
SELinuxContextFromNet=
Takes a boolean argument. When true, systemd
will attempt to figure out the SELinux label used for the instantiated service
from the information handed by the peer over the network. Note that only the
security level is used from the information provided by the peer. Other parts
of the resulting SELinux context originate from either the target binary that
is effectively triggered by socket unit or from the value of the
SELinuxContext= option. This configuration option applies only when
activated service is passed in single socket file descriptor, i.e. service
instances that have standard input connected to a socket or services triggered
by exactly one socket unit. Also note that this option is useful only when
MLS/MCS SELinux policy is deployed. Defaults to "false".
PipeSize=
Takes a size in bytes. Controls the pipe
buffer size of FIFOs configured in this socket unit. See fcntl(2) for
details. The usual suffixes K, M, G are supported and are understood to the
base of 1024.
MessageQueueMaxMessages=, MessageQueueMessageSize=
These two settings take integer values and
control the mq_maxmsg field or the mq_msgsize field, respectively, when
creating the message queue. Note that either none or both of these variables
need to be set. See mq_setattr(3) for details.
FreeBind=
Takes a boolean value. Controls whether the
socket can be bound to non-local IP addresses. This is useful to configure
sockets listening on specific IP addresses before those IP addresses are
successfully configured on a network interface. This sets the
IP_FREEBIND/ IPV6_FREEBIND socket option. For robustness reasons
it is recommended to use this option whenever you bind a socket to a specific
IP address. Defaults to false.
Transparent=
Takes a boolean value. Controls the
IP_TRANSPARENT/ IPV6_TRANSPARENT socket option. Defaults to
false.
Broadcast=
Takes a boolean value. This controls the
SO_BROADCAST socket option, which allows broadcast datagrams to be sent
from this socket. Defaults to false.
PassCredentials=
Takes a boolean value. This controls the
SO_PASSCRED socket option, which allows AF_UNIX sockets to
receive the credentials of the sending process in an ancillary message.
Defaults to false.
PassSecurity=
Takes a boolean value. This controls the
SO_PASSSEC socket option, which allows AF_UNIX sockets to
receive the security context of the sending process in an ancillary message.
Defaults to false.
PassPacketInfo=
Takes a boolean value. This controls the
IP_PKTINFO, IPV6_RECVPKTINFO, NETLINK_PKTINFO or
PACKET_AUXDATA socket options, which enable reception of additional
per-packet metadata as ancillary message, on AF_INET, AF_INET6,
AF_UNIX and AF_PACKET sockets. Defaults to false.
Timestamping=
Takes one of "off", "us"
(alias: "usec", "µs") or "ns" (alias:
"nsec"). This controls the SO_TIMESTAMP or
SO_TIMESTAMPNS socket options, and enables whether ingress network
traffic shall carry timestamping metadata. Defaults to off.
TCPCongestion=
Takes a string value. Controls the TCP
congestion algorithm used by this socket. Should be one of
"westwood", "reno", "cubic", "lp" or
any other available algorithm supported by the IP stack. This setting applies
only to stream sockets.
ExecStartPre=, ExecStartPost=
Takes one or more command lines, which are
executed before or after the listening sockets/FIFOs are created and bound,
respectively. The first token of the command line must be an absolute
filename, then followed by arguments for the process. Multiple command lines
may be specified following the same scheme as used for ExecStartPre= of
service unit files.
ExecStopPre=, ExecStopPost=
Additional commands that are executed before
or after the listening sockets/FIFOs are closed and removed, respectively.
Multiple command lines may be specified following the same scheme as used for
ExecStartPre= of service unit files.
TimeoutSec=
Configures the time to wait for the commands
specified in ExecStartPre=, ExecStartPost=, ExecStopPre=
and ExecStopPost= to finish. If a command does not exit within the
configured time, the socket will be considered failed and be shut down again.
All commands still running will be terminated forcibly via SIGTERM, and
after another delay of this time with SIGKILL. (See KillMode= in
systemd.kill(5).) Takes a unit-less value in seconds, or a time span
value such as "5min 20s". Pass "0" to disable the timeout
logic. Defaults to DefaultTimeoutStartSec= from the manager
configuration file (see systemd-system.conf(5)).
Service=
Specifies the service unit name to activate on
incoming traffic. This setting is only allowed for sockets with
Accept=no. It defaults to the service that bears the same name as the
socket (with the suffix replaced). In most cases, it should not be necessary
to use this option. Note that setting this parameter might result in
additional dependencies to be added to the unit (see above).
RemoveOnStop=
Takes a boolean argument. If enabled, any file
nodes created by this socket unit are removed when it is stopped. This applies
to AF_UNIX sockets in the file system, POSIX message queues, FIFOs, as
well as any symlinks to them configured with Symlinks=. Normally, it
should not be necessary to use this option, and is not recommended as services
might continue to run after the socket unit has been terminated and it should
still be possible to communicate with them via their file system node.
Defaults to off.
Symlinks=
Takes a list of file system paths. The
specified paths will be created as symlinks to the AF_UNIX socket path
or FIFO path of this socket unit. If this setting is used, only one
AF_UNIX socket in the file system or one FIFO may be configured for the
socket unit. Use this option to manage one or more symlinked alias names for a
socket, binding their lifecycle together. Note that if creation of a symlink
fails this is not considered fatal for the socket unit, and the socket unit
may still start. If an empty string is assigned, the list of paths is reset.
Defaults to an empty list.
FileDescriptorName=
Assigns a name to all file descriptors this
socket unit encapsulates. This is useful to help activated services identify
specific file descriptors, if multiple fds are passed. Services may use the
sd_listen_fds_with_names(3) call to acquire the names configured for
the received file descriptors. Names may contain any ASCII character, but must
exclude control characters and ":", and must be at most 255
characters in length. If this setting is not used, the file descriptor name
defaults to the name of the socket unit, including its .socket suffix.
TriggerLimitIntervalSec=, TriggerLimitBurst=
Configures a limit on how often this socket
unit may be activated within a specific time interval. The
TriggerLimitIntervalSec= may be used to configure the length of the
time interval in the usual time units "us", "ms",
"s", "min", "h", ... and defaults to 2s (See
systemd.time(7) for details on the various time units understood). The
TriggerLimitBurst= setting takes a positive integer value and specifies
the number of permitted activations per time interval, and defaults to 200 for
Accept=yes sockets (thus by default permitting 200 activations per 2s),
and 20 otherwise (20 activations per 2s). Set either to 0 to disable any form
of trigger rate limiting. If the limit is hit, the socket unit is placed into
a failure mode, and will not be connectible anymore until restarted. Note that
this limit is enforced before the service activation is enqueued.
Check systemd.unit(5), systemd.exec(5), and systemd.kill(5)
for more settings.
SEE ALSO
systemd(1), systemctl(1), systemd-system.conf(5), systemd.unit(5), systemd.exec(5), systemd.kill(5), systemd.resource-control(5), systemd.service(5), systemd.directives(7), sd_listen_fds(3), sd_listen_fds_with_names(3) For more extensive descriptions see the "systemd for Developers" series: Socket Activation[4], Socket Activation, part II[5], Converting inetd Services[6], Socket Activated Internet Services and OS Containers[7].NOTES
- 1.
- USB FunctionFS
- 2.
- TCP Keepalive HOWTO
- 3.
- Smack
- 4.
- Socket Activation
- 5.
- Socket Activation, part II
- 6.
- Converting inetd Services
- 7.
- Socket Activated Internet Services and OS Containers
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