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	1	.\" $OpenBSD: ssh-keyscan.1,v 1.27 2009/10/28 16:38:18 reyk 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 $Mdocdate$
	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	.Bk -words
	18	.Op Fl 46Hv
	19	.Op Fl f Ar file
	20	.Op Fl p Ar port
	21	.Op Fl T Ar timeout
	22	.Op Fl t Ar type
	23	.Op Fl V Ar rdomain
	24	.Op Ar host \| addrlist namelist
	25	.Ar ...
	26	.Ek
	27	.Sh DESCRIPTION
	28	.Nm
	29	is a utility for gathering the public ssh host keys of a number of
	30	hosts.
	31	It was designed to aid in building and verifying
	32	.Pa ssh_known_hosts
	33	files.
	34	.Nm
	35	provides a minimal interface suitable for use by shell and perl
	36	scripts.
	37	.Pp
	38	.Nm
	39	uses non-blocking socket I/O to contact as many hosts as possible in
	40	parallel, so it is very efficient.
	41	The keys from a domain of 1,000
	42	hosts can be collected in tens of seconds, even when some of those
	43	hosts are down or do not run ssh.
	44	For scanning, one does not need
	45	login access to the machines that are being scanned, nor does the
	46	scanning process involve any encryption.
	47	.Pp
	48	The options are as follows:
	49	.Bl -tag -width Ds
	50	.It Fl 4
	51	Forces
	52	.Nm
	53	to use IPv4 addresses only.
	54	.It Fl 6
	55	Forces
	56	.Nm
	57	to use IPv6 addresses only.
	58	.It Fl f Ar file
	59	Read hosts or
	60	.Pa addrlist namelist
	61	pairs from this file, one per line.
	62	If
	63	.Pa -
	64	is supplied instead of a filename,
	65	.Nm
	66	will read hosts or
	67	.Pa addrlist namelist
	68	pairs from the standard input.
	69	.It Fl H
	70	Hash all hostnames and addresses in the output.
	71	Hashed names may be used normally by
	72	.Nm ssh
	73	and
	74	.Nm sshd ,
	75	but they do not reveal identifying information should the file's contents
	76	be disclosed.
	77	.It Fl p Ar port
	78	Port to connect to on the remote host.
	79	.It Fl T Ar timeout
	80	Set the timeout for connection attempts.
	81	If
	82	.Pa timeout
	83	seconds have elapsed since a connection was initiated to a host or since the
	84	last time anything was read from that host, then the connection is
	85	closed and the host in question considered unavailable.
	86	Default is 5 seconds.
	87	.It Fl t Ar type
	88	Specifies the type of the key to fetch from the scanned hosts.
	89	The possible values are
	90	.Dq rsa1
	91	for protocol version 1 and
	92	.Dq rsa
	93	or
	94	.Dq dsa
	95	for protocol version 2.
	96	Multiple values may be specified by separating them with commas.
	97	The default is
	98	.Dq rsa .
	99	.It Fl V Ar rdomain
	100	Set the routing domain.
	101	.It Fl v
	102	Verbose mode.
	103	Causes
	104	.Nm
	105	to print debugging messages about its progress.
	106	.El
	107	.Sh SECURITY
	108	If an ssh_known_hosts file is constructed using
	109	.Nm
	110	without verifying the keys, users will be vulnerable to
	111	.Em man in the middle
	112	attacks.
	113	On the other hand, if the security model allows such a risk,
	114	.Nm
	115	can help in the detection of tampered keyfiles or man in the middle
	116	attacks which have begun after the ssh_known_hosts file was created.
	117	.Sh FILES
	118	.Pa Input format:
	119	.Bd -literal
	120	1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
	121	.Ed
	122	.Pp
	123	.Pa Output format for rsa1 keys:
	124	.Bd -literal
	125	host-or-namelist bits exponent modulus
	126	.Ed
	127	.Pp
	128	.Pa Output format for rsa and dsa keys:
	129	.Bd -literal
	130	host-or-namelist keytype base64-encoded-key
	131	.Ed
	132	.Pp
	133	Where
	134	.Pa keytype
	135	is either
	136	.Dq ssh-rsa
	137	or
	138	.Dq ssh-dss .
	139	.Pp
	140	.Pa /etc/ssh/ssh_known_hosts
	141	.Sh EXAMPLES
	142	Print the
	143	.Pa rsa
	144	host key for machine
	145	.Pa hostname :
	146	.Bd -literal
	147	$ ssh-keyscan hostname
	148	.Ed
	149	.Pp
	150	Find all hosts from the file
	151	.Pa ssh_hosts
	152	which have new or different keys from those in the sorted file
	153	.Pa ssh_known_hosts :
	154	.Bd -literal
	155	$ ssh-keyscan -t rsa,dsa -f ssh_hosts \| \e
	156	sort -u - ssh_known_hosts \| diff ssh_known_hosts -
	157	.Ed
	158	.Sh SEE ALSO
	159	.Xr ssh 1 ,
	160	.Xr sshd 8
	161	.Sh AUTHORS
	162	.An -nosplit
	163	.An David Mazieres Aq dm@lcs.mit.edu
	164	wrote the initial version, and
	165	.An Wayne Davison Aq wayned@users.sourceforge.net
	166	added support for protocol version 2.
	167	.Sh BUGS
	168	It generates "Connection closed by remote host" messages on the consoles
	169	of all the machines it scans if the server is older than version 2.9.
	170	This is because it opens a connection to the ssh port, reads the public
	171	key, and drops the connection as soon as it gets the key.