ssh-keyscan —
gather SSH public keys from servers
ssh-keyscan |
[-46cDHv]
[-f
file]
[-p
port]
[-T
timeout]
[-t
type]
[host |
addrlist namelist] |
ssh-keyscan is a utility for gathering the public
SSH host keys of a number of hosts. It was designed to aid in building and
verifying
ssh_known_hosts files, the format of
which is documented in
sshd(8).
ssh-keyscan provides a minimal interface suitable
for use by shell and perl scripts.
ssh-keyscan uses non-blocking socket I/O to contact
as many hosts as possible in parallel, so it is very efficient. The keys from
a domain of 1,000 hosts can be collected in tens of seconds, even when some of
those hosts are down or do not run
sshd(8). For
scanning, one does not need login access to the machines that are being
scanned, nor does the scanning process involve any encryption.
Hosts to be scanned may be specified by hostname, address or by CIDR network
range (e.g. 192.168.16/28). If a network range is specified, then all
addresses in that range will be scanned.
The options are as follows:
- -4
- Force ssh-keyscan to use IPv4
addresses only.
- -6
- Force ssh-keyscan to use IPv6
addresses only.
- -c
- Request certificates from target hosts instead of plain
keys.
- -D
- Print keys found as SSHFP DNS records. The default is to
print keys in a format usable as a ssh(1)
known_hosts file.
-
-f
file
- Read hosts or “addrlist namelist” pairs from
file, one per line. If ‘-’
is supplied instead of a filename,
ssh-keyscan will read from the standard
input. Names read from a file must start with an address, hostname or CIDR
network range to be scanned. Addresses and hostnames may optionally be
followed by comma-separated name or address aliases that will be copied to
the output. For example:
- -H
- Hash all hostnames and addresses in the output. Hashed
names may be used normally by ssh(1) and
sshd(8), but they do not reveal identifying
information should the file's contents be disclosed.
-
-p
port
- Connect to port on the
remote host.
-
-T
timeout
- Set the timeout for connection attempts. If
timeout seconds have elapsed since a
connection was initiated to a host or since the last time anything was
read from that host, the connection is closed and the host in question
considered unavailable. The default is 5 seconds.
-
-t
type
- Specify the type of the key to fetch from the scanned
hosts. The possible values are “dsa”, “ecdsa”,
“ed25519”, “ecdsa-sk”,
“ed25519-sk”, or “rsa”. Multiple values may be
specified by separating them with commas. The default is to fetch
“rsa”, “ecdsa”, “ed25519”,
“ecdsa-sk”, and “ed25519-sk” keys.
- -v
- Verbose mode: print debugging messages about progress.
If an ssh_known_hosts file is constructed using
ssh-keyscan without verifying the keys, users
will be vulnerable to
man in the middle attacks.
On the other hand, if the security model allows such a risk,
ssh-keyscan can help in the detection of tampered
keyfiles or man in the middle attacks which have begun after the
ssh_known_hosts file was created.
/etc/ssh/ssh_known_hosts
Print the RSA host key for machine
hostname:
$ ssh-keyscan -t rsa hostname
Search a network range, printing all supported key types:
$ ssh-keyscan 192.168.0.64/25
Find all hosts from the file
ssh_hosts which have
new or different keys from those in the sorted file
ssh_known_hosts:
$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
sort -u - ssh_known_hosts | diff ssh_known_hosts -
ssh(1),
sshd(8)
Using DNS to Securely Publish
Secure Shell (SSH) Key Fingerprints, RFC 4255,
2006.
David Mazieres
<
[email protected]>
wrote the initial version, and
Wayne
Davison
<
[email protected]>
added support for protocol version 2.