Manual Pages  — SSH-KEYSCAN

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.

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. Input is expected in the 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
-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.

$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
        sort -u - ssh_known_hosts | diff ssh_known_hosts -

ssh(1), sshd(8)

RFC 4255, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, 2006.