1SSH-KEYSCAN(1)             OpenBSD Reference Manual             SSH-KEYSCAN(1)
2
3NAME
4     ssh-keyscan - gather ssh public keys
5
6SYNOPSIS
7     ssh-keyscan [-46Hv] [-f file] [-p port] [-T timeout] [-t type]
8                 [host | addrlist namelist] ...
9
10DESCRIPTION
11     ssh-keyscan is a utility for gathering the public ssh host keys of a
12     number of hosts.  It was designed to aid in building and verifying
13     ssh_known_hosts files.  ssh-keyscan provides a minimal interface suitable
14     for use by shell and perl scripts.
15
16     ssh-keyscan uses non-blocking socket I/O to contact as many hosts as
17     possible in parallel, so it is very efficient.  The keys from a domain of
18     1,000 hosts can be collected in tens of seconds, even when some of those
19     hosts are down or do not run ssh.  For scanning, one does not need login
20     access to the machines that are being scanned, nor does the scanning
21     process involve any encryption.
22
23     The options are as follows:
24
25     -4      Forces ssh-keyscan to use IPv4 addresses only.
26
27     -6      Forces ssh-keyscan to use IPv6 addresses only.
28
29     -f file
30             Read hosts or addrlist namelist pairs from this file, one per
31             line.  If - is supplied instead of a filename, ssh-keyscan will
32             read hosts or addrlist namelist pairs from the standard input.
33
34     -H      Hash all hostnames and addresses in the output.  Hashed names may
35             be used normally by ssh and sshd, but they do not reveal
36             identifying information should the file's contents be disclosed.
37
38     -p port
39             Port to connect to on the remote host.
40
41     -T timeout
42             Set the timeout for connection attempts.  If timeout seconds have
43             elapsed since a connection was initiated to a host or since the
44             last time anything was read from that host, then the connection
45             is closed and the host in question considered unavailable.
46             Default is 5 seconds.
47
48     -t type
49             Specifies the type of the key to fetch from the scanned hosts.
50             The possible values are ``rsa1'' for protocol version 1 and
51             ``dsa'', ``ecdsa'' or ``rsa'' for protocol version 2.  Multiple
52             values may be specified by separating them with commas.  The
53             default is to fetch ``rsa'' and ``ecdsa'' keys.
54
55     -v      Verbose mode.  Causes ssh-keyscan to print debugging messages
56             about its progress.
57
58SECURITY
59     If an ssh_known_hosts file is constructed using ssh-keyscan without
60     verifying the keys, users will be vulnerable to man in the middle
61     attacks.  On the other hand, if the security model allows such a risk,
62     ssh-keyscan can help in the detection of tampered keyfiles or man in the
63     middle attacks which have begun after the ssh_known_hosts file was
64     created.
65
66FILES
67     Input format:
68
69     1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
70
71     Output format for rsa1 keys:
72
73     host-or-namelist bits exponent modulus
74
75     Output format for rsa, dsa and ecdsa keys:
76
77     host-or-namelist keytype base64-encoded-key
78
79     Where keytype is either ``ecdsa-sha2-nistp256'', ``ecdsa-sha2-nistp384'',
80     ``ecdsa-sha2-nistp521'', ``ssh-dss'' or ``ssh-rsa''.
81
82     /etc/ssh/ssh_known_hosts
83
84EXAMPLES
85     Print the rsa host key for machine hostname:
86
87     $ ssh-keyscan hostname
88
89     Find all hosts from the file ssh_hosts which have new or different keys
90     from those in the sorted file ssh_known_hosts:
91
92     $ ssh-keyscan -t rsa,dsa,ecdsa -f ssh_hosts | \
93             sort -u - ssh_known_hosts | diff ssh_known_hosts -
94
95SEE ALSO
96     ssh(1), sshd(8)
97
98AUTHORS
99     David Mazieres <dm@lcs.mit.edu> wrote the initial version, and Wayne
100     Davison <wayned@users.sourceforge.net> added support for protocol version
101     2.
102
103BUGS
104     It generates "Connection closed by remote host" messages on the consoles
105     of all the machines it scans if the server is older than version 2.9.
106     This is because it opens a connection to the ssh port, reads the public
107     key, and drops the connection as soon as it gets the key.
108
109OpenBSD 5.3                     April 11, 2012                     OpenBSD 5.3
110