[openssh.git] / ssh-keyscan.1

.\"	$OpenBSD: ssh-keyscan.1,v 1.5 2001/04/18 16:21:05 ian 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 (for instance 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
.Pp
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
.Pp
.Sh FILES
.Pp
.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 AUTHOR
David Mazieres <dm@lcs.mit.edu>
Commit	Line	Data
05cc0c99	1	.\" $OpenBSD: ssh-keyscan.1,v 1.5 2001/04/18 16:21:05 ian 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
	7	.\" OpenBSD project (for instance by leaving this copyright notice
	8	.\" intact).
23c2a7a5	9	.\"
f6fdbddf	10	.Dd January 1, 1996
4371658c	11	.Dt SSH-KEYSCAN 1
f6fdbddf	12	.Os
	13	.Sh NAME
	14	.Nm ssh-keyscan
	15	.Nd gather ssh public keys
	16	.Sh SYNOPSIS
	17	.Nm ssh-keyscan
	18	.Op Fl t Ar timeout
	19	.Op Ar -- \| host \| addrlist namelist
	20	.Op Fl f Ar files ...
	21	.Sh DESCRIPTION
	22	.Nm
	23	is a utility for gathering the public ssh host keys of a number of
	24	hosts. It was designed to aid in building and verifying
	25	.Pa ssh_known_hosts
	26	files.
	27	.Nm
	28	provides a minimal interface suitable for use by shell and perl
	29	scripts.
	30	.Pp
	31	.Nm
	32	uses non-blocking socket I/O to contact as many hosts as possible in
	33	parallel, so it is very efficient. The keys from a domain of 1,000
	34	hosts can be collected in tens of seconds, even when some of those
	35	hosts are down or do not run ssh. You do not need login access to the
05cc0c99	36	machines you are scanning, nor does the scanning process involve
f6fdbddf	37	any encryption.
	38	.Sh SECURITY
	39	If you make an ssh_known_hosts file using
	40	.Nm
	41	without verifying the keys, you will be vulnerable to
	42	.I man in the middle
	43	attacks.
	44	On the other hand, if your security model allows such a risk,
	45	.Nm
	46	can help you detect tampered keyfiles or man in the middle attacks which
	47	have begun after you created your ssh_known_hosts file.
	48	.Sh OPTIONS
	49	.Bl -tag -width Ds
	50	.It Fl t
	51	Set the timeout for connection attempts. If
	52	.Pa timeout
	53	seconds have elapsed since a connection was initiated to a host or since the
	54	last time anything was read from that host, then the connection is
	55	closed and the host in question considered unavailable. Default is 5
	56	seconds.
	57	.It Fl f
	58	Read hosts or
	59	.Pa addrlist namelist
	60	pairs from this file, one per line.
	61	If
	62	.Pa -
	63	is supplied instead of a filename,
	64	.Nm
	65	will read hosts or
	66	.Pa addrlist namelist
	67	pairs from the standard input.
9616313f	68	.El
f6fdbddf	69	.Sh EXAMPLES
	70	.Pp
	71	Print the host key for machine
	72	.Pa hostname :
	73	.Bd -literal
	74	ssh-keyscan hostname
	75	.Ed
	76	.Pp
	77	Find all hosts from the file
	78	.Pa ssh_hosts
	79	which have new or different keys from those in the sorted file
	80	.Pa ssh_known_hosts :
	81	.Bd -literal
4371658c	82	$ ssh-keyscan -f ssh_hosts \| sort -u - ssh_known_hosts \| \e\
f6fdbddf	83	diff ssh_known_hosts -
	84	.Ed
	85	.Pp
	86	.Sh FILES
	87	.Pp
	88	.Pa Input format:
	89	1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
	90	.Pp
	91	.Pa Output format:
	92	host-or-namelist bits exponent modulus
	93	.Pp
	94	.Pa /etc/ssh_known_hosts
	95	.Sh BUGS
	96	It generates "Connection closed by remote host" messages on the consoles
	97	of all the machines it scans.
	98	This is because it opens a connection to the ssh port, reads the public
	99	key, and drops the connection as soon as it gets the key.
	100	.Sh SEE ALSO
4371658c	101	.Xr ssh 1 ,
f6fdbddf	102	.Xr sshd 8
	103	.Sh AUTHOR
	104	David Mazieres <dm@lcs.mit.edu>