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SSH-KEYSCAN(1)		  BSD 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 num-
     ber 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 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 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 identi-
	     fying 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.	 De-
	     fault 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 "dsa",
	     "ecdsa" or	"rsa" for protocol version 2.  Multiple	values may be
	     specified by separating them with commas.	The default is to
	     fetch "rsa" and "ecdsa" keys.

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

     Input format:,,name,,n,,

     Output format for rsa1 keys:

     host-or-namelist bits exponent modulus

     Output format for rsa, dsa	and ecdsa keys:

     host-or-namelist keytype base64-encoded-key

     Where keytype is either "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384",
     "ecdsa-sha2-nistp521", "ssh-dss" or "ssh-rsa".


     Print the rsa 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,ecdsa -f ssh_hosts | \
	     sort -u - ssh_known_hosts | diff ssh_known_hosts -

     ssh(1), sshd(8)

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

     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.

BSD				 July 16, 2013				   BSD


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