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

ssh-keyscan —
gather ssh public keys

ssh-keyscan [-46Hv] [-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. 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. 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:

Forces ssh-keyscan to use IPv4 addresses only.
Forces ssh-keyscan to use IPv6 addresses only.
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.
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.
port
Port to connect to on the remote host.
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.
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 “rsa”.
Verbose mode. Causes ssh-keyscan to print debugging messages about its 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.

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 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

Print the rsa1 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 -t rsa,dsa -f ssh_hosts | \
	sort -u - ssh_known_hosts | diff ssh_known_hosts -

ssh(1), sshd(8)

David Mazieres ⟨dm@lcs.mit.edu⟩ wrote the initial version, and Wayne Davison ⟨wayned@users.sourceforge.net⟩ added support for protocol version 2.

It generates "Connection closed by remote host" messages on the consoles of all the machines it scans if the server is older than version 2.9. 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.
November 1, 2008 BSD