1.\"	$NetBSD: ssh-keyscan.1,v 1.4 2010/11/21 18:29:49 adam Exp $
2.\"	$OpenBSD: ssh-keyscan.1,v 1.28 2010/01/09 23:04:13 dtucker Exp $
3.\"
4.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>.
5.\"
6.\" Modification and redistribution in source and binary forms is
7.\" permitted provided that due credit is given to the author and the
8.\" OpenBSD project by leaving this copyright notice intact.
9.\"
10.Dd January 9, 2010
11.Dt SSH-KEYSCAN 1
12.Os
13.Sh NAME
14.Nm ssh-keyscan
15.Nd gather ssh public keys
16.Sh SYNOPSIS
17.Nm ssh-keyscan
18.Bk -words
19.Op Fl 46Hv
20.Op Fl f Ar file
21.Op Fl p Ar port
22.Op Fl T Ar timeout
23.Op Fl t Ar type
24.Op Ar host | addrlist namelist
25.Ar ...
26.Ek
27.Sh DESCRIPTION
28.Nm
29is a utility for gathering the public ssh host keys of a number of
30hosts.
31It was designed to aid in building and verifying
32.Pa ssh_known_hosts
33files.
34.Nm
35provides a minimal interface suitable for use by shell and perl
36scripts.
37.Pp
38.Nm
39uses non-blocking socket I/O to contact as many hosts as possible in
40parallel, so it is very efficient.
41The keys from a domain of 1,000
42hosts can be collected in tens of seconds, even when some of those
43hosts are down or do not run ssh.
44For scanning, one does not need
45login access to the machines that are being scanned, nor does the
46scanning process involve any encryption.
47.Pp
48The options are as follows:
49.Bl -tag -width Ds
50.It Fl 4
51Forces
52.Nm
53to use IPv4 addresses only.
54.It Fl 6
55Forces
56.Nm
57to use IPv6 addresses only.
58.It Fl f Ar file
59Read hosts or
60.Pa addrlist namelist
61pairs from this file, one per line.
62If
63.Pa -
64is supplied instead of a filename,
65.Nm
66will read hosts or
67.Pa addrlist namelist
68pairs from the standard input.
69.It Fl H
70Hash all hostnames and addresses in the output.
71Hashed names may be used normally by
72.Nm ssh
73and
74.Nm sshd ,
75but they do not reveal identifying information should the file's contents
76be disclosed.
77.It Fl p Ar port
78Port to connect to on the remote host.
79.It Fl T Ar timeout
80Set the timeout for connection attempts.
81If
82.Pa timeout
83seconds have elapsed since a connection was initiated to a host or since the
84last time anything was read from that host, then the connection is
85closed and the host in question considered unavailable.
86Default is 5 seconds.
87.It Fl t Ar type
88Specifies the type of the key to fetch from the scanned hosts.
89The possible values are
90.Dq rsa1
91for protocol version 1 and
92.Dq rsa
93or
94.Dq dsa
95for protocol version 2.
96Multiple values may be specified by separating them with commas.
97The default is
98.Dq rsa .
99.It Fl v
100Verbose mode.
101Causes
102.Nm
103to print debugging messages about its progress.
104.El
105.Sh SECURITY
106If an ssh_known_hosts file is constructed using
107.Nm
108without verifying the keys, users will be vulnerable to
109.Em man in the middle
110attacks.
111On the other hand, if the security model allows such a risk,
112.Nm
113can help in the detection of tampered keyfiles or man in the middle
114attacks which have begun after the ssh_known_hosts file was created.
115.Sh FILES
116.Pa Input format:
117.Bd -literal
1181.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
119.Ed
120.Pp
121.Pa Output format for rsa1 keys:
122.Bd -literal
123host-or-namelist bits exponent modulus
124.Ed
125.Pp
126.Pa Output format for rsa and dsa keys:
127.Bd -literal
128host-or-namelist keytype base64-encoded-key
129.Ed
130.Pp
131Where
132.Pa keytype
133is either
134.Dq ssh-rsa
135or
136.Dq ssh-dss .
137.Pp
138.Pa /etc/ssh/ssh_known_hosts
139.Sh EXAMPLES
140Print the
141.Pa rsa
142host key for machine
143.Pa hostname :
144.Bd -literal
145$ ssh-keyscan hostname
146.Ed
147.Pp
148Find all hosts from the file
149.Pa ssh_hosts
150which have new or different keys from those in the sorted file
151.Pa ssh_known_hosts :
152.Bd -literal
153$ ssh-keyscan -t rsa,dsa -f ssh_hosts | \e
154	sort -u - ssh_known_hosts | diff ssh_known_hosts -
155.Ed
156.Sh SEE ALSO
157.Xr ssh 1 ,
158.Xr sshd 8
159.Sh AUTHORS
160.An -nosplit
161.An David Mazieres Aq dm@lcs.mit.edu
162wrote the initial version, and
163.An Wayne Davison Aq wayned@users.sourceforge.net
164added support for protocol version 2.
165.Sh BUGS
166It generates "Connection closed by remote host" messages on the consoles
167of all the machines it scans if the server is older than version 2.9.
168This is because it opens a connection to the ssh port, reads the public
169key, and drops the connection as soon as it gets the key.
170