ssh-keyscan.1

   1 .\"     $OpenBSD: ssh-keyscan.1,v 1.49 2023/02/10 06:41:53 jmc 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 $Mdocdate: February 10 2023 $
  10 .Dt SSH-KEYSCAN 1
  11 .Os
  12 .Sh NAME
  13 .Nm ssh-keyscan
  14 .Nd gather SSH public keys from servers
  15 .Sh SYNOPSIS
  16 .Nm ssh-keyscan
  17 .Op Fl 46cDHv
  18 .Op Fl f Ar file
  19 .Op Fl O Ar option
  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 .Sh DESCRIPTION
  25 .Nm
  26 is a utility for gathering the public SSH host keys of a number of
  27 hosts.
  28 It was designed to aid in building and verifying
  29 .Pa ssh_known_hosts
  30 files,
  31 the format of which is documented in
  32 .Xr sshd 8 .
  33 .Nm
  34 provides a minimal interface suitable for use by shell and perl
  35 scripts.
  36 .Pp
  37 .Nm
  38 uses non-blocking socket I/O to contact as many hosts as possible in
  39 parallel, so it is very efficient.
  40 The keys from a domain of 1,000
  41 hosts can be collected in tens of seconds, even when some of those
  42 hosts are down or do not run
  43 .Xr sshd 8 .
  44 For scanning, one does not need
  45 login access to the machines that are being scanned, nor does the
  46 scanning process involve any encryption.
  47 .Pp
  48 Hosts to be scanned may be specified by hostname, address or by CIDR
  49 network range (e.g. 192.168.16/28).
  50 If a network range is specified, then all addresses in that range will
  51 be scanned.
  52 .Pp
  53 The options are as follows:
  54 .Bl -tag -width Ds
  55 .It Fl 4
  56 Force
  57 .Nm
  58 to use IPv4 addresses only.
  59 .It Fl 6
  60 Force
  61 .Nm
  62 to use IPv6 addresses only.
  63 .It Fl c
  64 Request certificates from target hosts instead of plain keys.
  65 .It Fl D
  66 Print keys found as SSHFP DNS records.
  67 The default is to print keys in a format usable as a
  68 .Xr ssh 1
  69 .Pa known_hosts
  70 file.
  71 .It Fl f Ar file
  72 Read hosts or
  73 .Dq addrlist namelist
  74 pairs from
  75 .Ar file ,
  76 one per line.
  77 If
  78 .Sq -
  79 is supplied instead of a filename,
  80 .Nm
  81 will read from the standard input.
  82 Names read from a file must start with an address, hostname or CIDR network
  83 range to be scanned.
  84 Addresses and hostnames may optionally be followed by comma-separated name
  85 or address aliases that will be copied to the output.
  86 For example:
  87 .Bd -literal
  88 192.168.11.0/24
  89 10.20.1.1
  90 happy.example.org
  91 10.0.0.1,sad.example.org
  92 .Ed
  93 .It Fl H
  94 Hash all hostnames and addresses in the output.
  95 Hashed names may be used normally by
  96 .Xr ssh 1
  97 and
  98 .Xr sshd 8 ,
  99 but they do not reveal identifying information should the file's contents
 100 be disclosed.
 101 .It Fl O Ar option
 102 Specify a key/value option.
 103 At present, only a single option is supported:
 104 .Bl -tag -width Ds
 105 .It Cm hashalg Ns = Ns Ar algorithm
 106 Selects a hash algorithm to use when printing SSHFP records using the
 107 .Fl D
 108 flag.
 109 Valid algorithms are
 110 .Dq sha1
 111 and
 112 .Dq sha256 .
 113 The default is to print both.
 114 .El
 115 .It Fl p Ar port
 116 Connect to
 117 .Ar port
 118 on the remote host.
 119 .It Fl T Ar timeout
 120 Set the timeout for connection attempts.
 121 If
 122 .Ar timeout
 123 seconds have elapsed since a connection was initiated to a host or since the
 124 last time anything was read from that host, the connection is
 125 closed and the host in question considered unavailable.
 126 The default is 5 seconds.
 127 .It Fl t Ar type
 128 Specify the type of the key to fetch from the scanned hosts.
 129 The possible values are
 130 .Dq dsa ,
 131 .Dq ecdsa ,
 132 .Dq ed25519 ,
 133 .Dq ecdsa-sk ,
 134 .Dq ed25519-sk ,
 135 or
 136 .Dq rsa .
 137 Multiple values may be specified by separating them with commas.
 138 The default is to fetch
 139 .Dq rsa ,
 140 .Dq ecdsa ,
 141 .Dq ed25519 ,
 142 .Dq ecdsa-sk ,
 143 and
 144 .Dq ed25519-sk
 145 keys.
 146 .It Fl v
 147 Verbose mode:
 148 print debugging messages about progress.
 149 .El
 150 .Pp
 151 If an ssh_known_hosts file is constructed using
 152 .Nm
 153 without verifying the keys, users will be vulnerable to
 154 .Em man in the middle
 155 attacks.
 156 On the other hand, if the security model allows such a risk,
 157 .Nm
 158 can help in the detection of tampered keyfiles or man in the middle
 159 attacks which have begun after the ssh_known_hosts file was created.
 160 .Sh FILES
 161 .Pa /etc/ssh/ssh_known_hosts
 162 .Sh EXAMPLES
 163 Print the RSA host key for machine
 164 .Ar hostname :
 165 .Pp
 166 .Dl $ ssh-keyscan -t rsa hostname
 167 .Pp
 168 Search a network range, printing all supported key types:
 169 .Pp
 170 .Dl $ ssh-keyscan 192.168.0.64/25
 171 .Pp
 172 Find all hosts from the file
 173 .Pa ssh_hosts
 174 which have new or different keys from those in the sorted file
 175 .Pa ssh_known_hosts :
 176 .Bd -literal -offset indent
 177 $ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e
 178         sort -u - ssh_known_hosts | diff ssh_known_hosts -
 179 .Ed
 180 .Sh SEE ALSO
 181 .Xr ssh 1 ,
 182 .Xr sshd 8
 183 .Rs
 184 .%D 2006
 185 .%R RFC 4255
 186 .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
 187 .Re
 188 .Sh AUTHORS
 189 .An -nosplit
 190 .An David Mazieres Aq Mt dm@lcs.mit.edu
 191 wrote the initial version, and
 192 .An Wayne Davison Aq Mt wayned@users.sourceforge.net
 193 added support for protocol version 2.