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. For scanning, one does not need login access to the machines that are being scanned, nor does the scanning process involve any encryption.

The options are as follows:

-4

Forces ssh-keyscan to use IPv4 addresses only.

-6

Forces ssh-keyscan to use IPv6 addresses only.

-f file

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.

-H

Hash all hostnames and addresses in the output. Hashed names may be used normally by ssh and sshd but they do not reveal identifying information should the file's contents be disclosed.

-p port

Port to connect to 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, then the connection is closed and the host in question considered unavailable. Default is 5 seconds.

-t type

Specifies the type of the key to fetch from the scanned hosts. The possible values are ``rsa1'' for protocol version 1 and ``rsa'' or ``dsa'' for protocol version 2. Multiple values may be specified by separating them with commas. The default is ``rsa1''

-v

Verbose mode. Causes ssh-keyscan to print debugging messages about its progress.

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 for rsa1 keys:

host-or-namelist bits exponent modulus

Output format for rsa and dsa keys:

host-or-namelist keytype base64-encoded-key

Where keytype is either ``ssh-rsa'' or ``ssh-dss''

/etc/ssh/ssh_known_hosts

$ 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 -t rsa,dsa -f ssh_hosts | \
        sort -u - ssh_known_hosts | diff ssh_known_hosts -