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	1	.\" $OpenBSD: ssh-keyscan.1,v 1.9 2001/08/02 18:37:35 mpech Exp $
	2	.\"
	3	.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>.
	4	.\"
	5	.\" Modification and redistribution in source and binary forms is
	6	.\" permitted provided that due credit is given to the author and the
	7	.\" OpenBSD project by leaving this copyright notice intact.
	8	.\"
	9	.Dd January 1, 1996
	10	.Dt SSH-KEYSCAN 1
	11	.Os
	12	.Sh NAME
	13	.Nm ssh-keyscan
	14	.Nd gather ssh public keys
	15	.Sh SYNOPSIS
	16	.Nm ssh-keyscan
	17	.Op Fl t Ar timeout
	18	.Op Ar -- \| host \| addrlist namelist
	19	.Op Fl f Ar files ...
	20	.Sh DESCRIPTION
	21	.Nm
	22	is a utility for gathering the public ssh host keys of a number of
	23	hosts. It was designed to aid in building and verifying
	24	.Pa ssh_known_hosts
	25	files.
	26	.Nm
	27	provides a minimal interface suitable for use by shell and perl
	28	scripts.
	29	.Pp
	30	.Nm
	31	uses non-blocking socket I/O to contact as many hosts as possible in
	32	parallel, so it is very efficient. The keys from a domain of 1,000
	33	hosts can be collected in tens of seconds, even when some of those
	34	hosts are down or do not run ssh. You do not need login access to the
	35	machines you are scanning, nor does the scanning process involve
	36	any encryption.
	37	.Pp
	38	The options are as follows:
	39	.Bl -tag -width Ds
	40	.It Fl t
	41	Set the timeout for connection attempts. If
	42	.Pa timeout
	43	seconds have elapsed since a connection was initiated to a host or since the
	44	last time anything was read from that host, then the connection is
	45	closed and the host in question considered unavailable. Default is 5
	46	seconds.
	47	.It Fl f
	48	Read hosts or
	49	.Pa addrlist namelist
	50	pairs from this file, one per line.
	51	If
	52	.Pa -
	53	is supplied instead of a filename,
	54	.Nm
	55	will read hosts or
	56	.Pa addrlist namelist
	57	pairs from the standard input.
	58	.El
	59	.Sh SECURITY
	60	If you make an ssh_known_hosts file using
	61	.Nm
	62	without verifying the keys, you will be vulnerable to
	63	.I man in the middle
	64	attacks.
	65	On the other hand, if your security model allows such a risk,
	66	.Nm
	67	can help you detect tampered keyfiles or man in the middle attacks which
	68	have begun after you created your ssh_known_hosts file.
	69	.Sh EXAMPLES
	70	Print the host key for machine
	71	.Pa hostname :
	72	.Bd -literal
	73	ssh-keyscan hostname
	74	.Ed
	75	.Pp
	76	Find all hosts from the file
	77	.Pa ssh_hosts
	78	which have new or different keys from those in the sorted file
	79	.Pa ssh_known_hosts :
	80	.Bd -literal
	81	$ ssh-keyscan -f ssh_hosts \| sort -u - ssh_known_hosts \| \e\
	82	diff ssh_known_hosts -
	83	.Ed
	84	.Sh FILES
	85	.Pa Input format:
	86	1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
	87	.Pp
	88	.Pa Output format:
	89	host-or-namelist bits exponent modulus
	90	.Pp
	91	.Pa /etc/ssh_known_hosts
	92	.Sh BUGS
	93	It generates "Connection closed by remote host" messages on the consoles
	94	of all the machines it scans.
	95	This is because it opens a connection to the ssh port, reads the public
	96	key, and drops the connection as soon as it gets the key.
	97	.Sh SEE ALSO
	98	.Xr ssh 1 ,
	99	.Xr sshd 8
	100	.Sh AUTHORS
	101	David Mazieres <dm@lcs.mit.edu>