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
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
-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.
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. Input is
expected in the format:
-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.
Connect to port on the remote host.
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.
Specify the type of the key to fetch from the scanned
hosts. The possible values are “dsa”, “ecdsa”, “ed25519”,
or “rsa”. Multiple values may be specified by separating
them with commas. The default is to fetch “rsa”, “ecdsa”,
and “ed25519” 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.
Print the RSA host key for machine hostname:
$ ssh-keyscan -t rsa hostname
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 -
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BSD November 30, 2019 BSD