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

     ssh-keyscan -- gather SSH public keys from	servers

     ssh-keyscan [-46cDHv] [-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 num-
     ber 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 pos-
     sible 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 scan-
     ning 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:,,name,,n,,

     -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 de-
	     fault is 5	seconds.

     -t	type
	     Specify the type of the key to fetch from the scanned hosts.  The
	     possible values are "dsa",	"ecdsa", "ed25519", or "rsa".  Multi-
	     ple 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 veri-
     fying 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 at-
     tacks 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 -

     ssh(1), sshd(8)

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

     David Mazieres <> wrote the initial version,	and Wayne
     Davison <> added support for protocol version

FreeBSD	13.0		       November	30, 2019		  FreeBSD 13.0


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