xref: /freebsd/crypto/openssh/ssh-keyscan.1 (revision 4a5216a6dc0c3ce4cf5f2d3ee8af0c3ff3402c4f)
1.\"	$OpenBSD: ssh-keyscan.1,v 1.24 2008/04/30 10:14:03 djm 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 April 30 2008
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 Ar host | addrlist namelist
24.Op Ar ...
25.Ek
26.Sh DESCRIPTION
27.Nm
28is a utility for gathering the public ssh host keys of a number of
29hosts.
30It was designed to aid in building and verifying
31.Pa ssh_known_hosts
32files.
33.Nm
34provides a minimal interface suitable for use by shell and perl
35scripts.
36.Pp
37.Nm
38uses non-blocking socket I/O to contact as many hosts as possible in
39parallel, so it is very efficient.
40The keys from a domain of 1,000
41hosts can be collected in tens of seconds, even when some of those
42hosts are down or do not run ssh.
43For scanning, one does not need
44login access to the machines that are being scanned, nor does the
45scanning process involve any encryption.
46.Pp
47The options are as follows:
48.Bl -tag -width Ds
49.It Fl 4
50Forces
51.Nm
52to use IPv4 addresses only.
53.It Fl 6
54Forces
55.Nm
56to use IPv6 addresses only.
57.It Fl f Ar file
58Read hosts or
59.Pa addrlist namelist
60pairs from this file, one per line.
61If
62.Pa -
63is supplied instead of a filename,
64.Nm
65will read hosts or
66.Pa addrlist namelist
67pairs from the standard input.
68.It Fl H
69Hash all hostnames and addresses in the output.
70Hashed names may be used normally by
71.Nm ssh
72and
73.Nm sshd ,
74but they do not reveal identifying information should the file's contents
75be disclosed.
76.It Fl p Ar port
77Port to connect to on the remote host.
78.It Fl T Ar timeout
79Set the timeout for connection attempts.
80If
81.Pa timeout
82seconds have elapsed since a connection was initiated to a host or since the
83last time anything was read from that host, then the connection is
84closed and the host in question considered unavailable.
85Default is 5 seconds.
86.It Fl t Ar type
87Specifies the type of the key to fetch from the scanned hosts.
88The possible values are
89.Dq rsa1
90for protocol version 1 and
91.Dq rsa
92or
93.Dq dsa
94for protocol version 2.
95Multiple values may be specified by separating them with commas.
96The default is
97.Dq rsa .
98.It Fl v
99Verbose mode.
100Causes
101.Nm
102to print debugging messages about its progress.
103.El
104.Sh SECURITY
105If an ssh_known_hosts file is constructed using
106.Nm
107without verifying the keys, users will be vulnerable to
108.Em man in the middle
109attacks.
110On the other hand, if the security model allows such a risk,
111.Nm
112can help in the detection of tampered keyfiles or man in the middle
113attacks which have begun after the ssh_known_hosts file was created.
114.Sh FILES
115.Pa Input format:
116.Bd -literal
1171.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
118.Ed
119.Pp
120.Pa Output format for rsa1 keys:
121.Bd -literal
122host-or-namelist bits exponent modulus
123.Ed
124.Pp
125.Pa Output format for rsa and dsa keys:
126.Bd -literal
127host-or-namelist keytype base64-encoded-key
128.Ed
129.Pp
130Where
131.Pa keytype
132is either
133.Dq ssh-rsa
134or
135.Dq ssh-dss .
136.Pp
137.Pa /etc/ssh/ssh_known_hosts
138.Sh EXAMPLES
139Print the
140.Pa rsa1
141host key for machine
142.Pa hostname :
143.Bd -literal
144$ ssh-keyscan hostname
145.Ed
146.Pp
147Find all hosts from the file
148.Pa ssh_hosts
149which have new or different keys from those in the sorted file
150.Pa ssh_known_hosts :
151.Bd -literal
152$ ssh-keyscan -t rsa,dsa -f ssh_hosts | \e
153	sort -u - ssh_known_hosts | diff ssh_known_hosts -
154.Ed
155.Sh SEE ALSO
156.Xr ssh 1 ,
157.Xr sshd 8
158.Sh AUTHORS
159.An -nosplit
160.An David Mazieres Aq dm@lcs.mit.edu
161wrote the initial version, and
162.An Wayne Davison Aq wayned@users.sourceforge.net
163added support for protocol version 2.
164.Sh BUGS
165It generates "Connection closed by remote host" messages on the consoles
166of all the machines it scans if the server is older than version 2.9.
167This is because it opens a connection to the ssh port, reads the public
168key, and drops the connection as soon as it gets the key.
169