[openssh.git] / ssh-keyscan.1

.\"	$OpenBSD: ssh-keyscan.1,v 1.8 2001/06/23 17:48:18 itojun Exp $
.\"
.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>.
.\"
.\" Modification and redistribution in source and binary forms is
.\" permitted provided that due credit is given to the author and the
.\" OpenBSD project by leaving this copyright notice intact.
.\"
.Dd January 1, 1996
.Dt SSH-KEYSCAN 1
.Os
.Sh NAME
.Nm ssh-keyscan
.Nd gather ssh public keys
.Sh SYNOPSIS
.Nm ssh-keyscan
.Op Fl t Ar timeout
.Op Ar -- | host | addrlist namelist
.Op Fl f Ar files ...
.Sh DESCRIPTION
.Nm
is a utility for gathering the public ssh host keys of a number of
hosts.  It was designed to aid in building and verifying
.Pa ssh_known_hosts
files.
.Nm
provides a minimal interface suitable for use by shell and perl
scripts.
.Pp
.Nm
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 ssh.  You do not need login access to the
machines you are scanning, nor does the scanning process involve
any encryption.
.Sh SECURITY
If you make an ssh_known_hosts file using
.Nm
without verifying the keys, you will be vulnerable to
.I man in the middle
attacks.
On the other hand, if your security model allows such a risk,
.Nm
can help you detect tampered keyfiles or man in the middle attacks which
have begun after you created your ssh_known_hosts file.
.Sh OPTIONS
.Bl -tag -width Ds
.It Fl t
Set the timeout for connection attempts.  If
.Pa 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.  Default is 5
seconds.
.It Fl f
Read hosts or
.Pa addrlist namelist
pairs from this file, one per line.
If
.Pa -
is supplied instead of a filename,
.Nm
will read hosts or
.Pa addrlist namelist
pairs from the standard input.
.El
.Sh EXAMPLES
Print the host key for machine
.Pa hostname :
.Bd -literal
ssh-keyscan hostname
.Ed
.Pp
Find all hosts from the file
.Pa ssh_hosts
which have new or different keys from those in the sorted file
.Pa ssh_known_hosts :
.Bd -literal
$ ssh-keyscan -f ssh_hosts | sort -u - ssh_known_hosts | \e\ 
	diff ssh_known_hosts -
.Ed
.Sh FILES
.Pa Input 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
.Pp
.Pa Output format:
host-or-namelist bits exponent modulus
.Pp
.Pa /etc/ssh_known_hosts
.Sh BUGS
It generates "Connection closed by remote host" messages on the consoles
of all the machines it scans.
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.
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr sshd 8
.Sh AUTHORS
David Mazieres <dm@lcs.mit.edu>
Commit	Line	Data
3730bb22	1	.\" $OpenBSD: ssh-keyscan.1,v 1.8 2001/06/23 17:48:18 itojun Exp $
4371658c	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
aa144206	7	.\" OpenBSD project by leaving this copyright notice intact.
23c2a7a5	8	.\"
f6fdbddf	9	.Dd January 1, 1996
4371658c	10	.Dt SSH-KEYSCAN 1
f6fdbddf	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
05cc0c99	35	machines you are scanning, nor does the scanning process involve
f6fdbddf	36	any encryption.
	37	.Sh SECURITY
	38	If you make an ssh_known_hosts file using
	39	.Nm
	40	without verifying the keys, you will be vulnerable to
	41	.I man in the middle
	42	attacks.
	43	On the other hand, if your security model allows such a risk,
	44	.Nm
	45	can help you detect tampered keyfiles or man in the middle attacks which
	46	have begun after you created your ssh_known_hosts file.
	47	.Sh OPTIONS
	48	.Bl -tag -width Ds
	49	.It Fl t
3730bb22	50	Set the timeout for connection attempts. If
f6fdbddf	51	.Pa timeout
	52	seconds have elapsed since a connection was initiated to a host or since the
	53	last time anything was read from that host, then the connection is
	54	closed and the host in question considered unavailable. Default is 5
	55	seconds.
	56	.It Fl f
3730bb22	57	Read hosts or
f6fdbddf	58	.Pa addrlist namelist
	59	pairs from this file, one per line.
	60	If
	61	.Pa -
	62	is supplied instead of a filename,
	63	.Nm
3730bb22	64	will read hosts or
f6fdbddf	65	.Pa addrlist namelist
f6fdbddf	66	pairs from the standard input.
9616313f	67	.El
f6fdbddf	68	.Sh EXAMPLES
f6fdbddf	69	Print the host key for machine
	70	.Pa hostname :
	71	.Bd -literal
	72	ssh-keyscan hostname
	73	.Ed
	74	.Pp
	75	Find all hosts from the file
	76	.Pa ssh_hosts
	77	which have new or different keys from those in the sorted file
	78	.Pa ssh_known_hosts :
	79	.Bd -literal
4371658c	80	$ ssh-keyscan -f ssh_hosts \| sort -u - ssh_known_hosts \| \e\
f6fdbddf	81	diff ssh_known_hosts -
f6fdbddf	82	.Ed
f6fdbddf	83	.Sh FILES
f6fdbddf	84	.Pa Input format:
	85	1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
	86	.Pp
	87	.Pa Output format:
	88	host-or-namelist bits exponent modulus
	89	.Pp
	90	.Pa /etc/ssh_known_hosts
	91	.Sh BUGS
	92	It generates "Connection closed by remote host" messages on the consoles
	93	of all the machines it scans.
	94	This is because it opens a connection to the ssh port, reads the public
	95	key, and drops the connection as soon as it gets the key.
	96	.Sh SEE ALSO
4371658c	97	.Xr ssh 1 ,
f6fdbddf	98	.Xr sshd 8
a5a2da3b	99	.Sh AUTHORS
f6fdbddf	100	David Mazieres <dm@lcs.mit.edu>