SSH-KEYSCAN(1) General Commands Manual SSH-KEYSCAN(1)

ssh-keyscan —
gather ssh public keys

ssh-keyscan [-t timeout] [-- | host | addrlist namelist] [-f files ...]

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. 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 ssh. You do not need login access to the machines you are scanning, nor does the scanning process involve any encryption.

The options are as follows:

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, then the connection is closed and the host in question considered unavailable. Default is 5 seconds.
Read hosts or addrlist namelist pairs from this file, one per line. If - is supplied instead of a filename, ssh-keyscan will read hosts or addrlist namelist pairs from the standard input.

If you make an ssh_known_hosts file using ssh-keyscan without verifying the keys, you will be vulnerable to attacks. On the other hand, if your security model allows such a risk, ssh-keyscan can help you detect tampered keyfiles or man in the middle attacks which have begun after you created your ssh_known_hosts file.

Print the host key for machine hostname: 
ssh-keyscan 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 -f ssh_hosts | sort -u - ssh_known_hosts | \ 
	diff ssh_known_hosts -

Input format: 1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4

Output format: host-or-namelist bits exponent modulus

/etc/ssh_known_hosts

It generates "Connection closed by remote host" messages on the consoles of all the machines it scans. This is because it opens a connection to the ssh port, reads the public key, and drops the connection as soon as it gets the key.

ssh(1), sshd(8)

David Mazieres <dm@lcs.mit.edu>
January 1, 1996 BSD