ssh-keyscan.1(1)

     NAME
          ssh-keyscan - gather ssh public keys

     SYNOPSIS
          ssh-keyscan [-v46] [-p port] [-T timeout] [-t type] [-f
          file] [host | addrlist namelist] [...]

     DESCRIPTION
          ssh-keyscan is a utility for gathering the public ssh host
          keys of a number of hosts.  It was designed to aid in
          building and verifying ssh_known_hosts files.  ssh-keyscan
          provides a minimal interface suitable for use by shell and
          perl scripts.

          ssh-keyscan 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.  For scanning, one does not need login access to
          the machines that are being scanned, nor does the scanning
          process involve any encryption.

          The options are as follows:

          -p port
               Port to connect to on the remote host.

          -T timeout
               Set the timeout for connection attempts.  If 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.

          -t type
               Specifies the type of the key to fetch from the scanned
               hosts.  The possible values are ``rsa1'' for protocol
               version 1 and ``rsa'' or ``dsa'' for protocol version
               2.  Multiple values may be specified by separating them
               with commas.  The default is ``rsa1''.

          -f filename
               Read hosts or addrlist namelist pairs from this file,
               one per line.  If - is supplied instead of a filename,
               ssh-keyscan will read hosts or addrlist namelist pairs
               from the standard input.

          -v   Verbose mode.  Causes ssh-keyscan to print debugging
               messages about its progress.

          -4   Forces ssh-keyscan to use IPv4 addresses only.
          -6   Forces ssh-keyscan to use IPv6 addresses only.

     SECURITY
          If a ssh_known_hosts file is constructed using ssh-keyscan
          without verifying the keys, users will be vulnerable to I
          man in the middle attacks.  On the other hand, if the
          security model allows such a risk, ssh-keyscan can help in
          the detection of tampered keyfiles or man in the middle
          attacks which have begun after the ssh_known_hosts file was
          created.

     EXAMPLES
          Print the rsa1 host key for machine hostname:

          $ ssh-keyscan hostname

          Find all hosts from the file ssh_hosts which have new or
          different keys from those in the sorted file
          ssh_known_hosts:

          $ ssh-keyscan -t rsa,dsa -f ssh_hosts | \
               sort -u - ssh_known_hosts | diff ssh_known_hosts -

     FILES
          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

          Output format for rsa1 keys:

          host-or-namelist bits exponent modulus

          Output format for rsa and dsa keys:

          host-or-namelist keytype base64-encoded-key

          Where keytype is either ``ssh-rsa'' or ``ssh-dsa''.

          /etc/openssh/ssh_known_hosts

     BUGS
          It generates "Connection closed by remote host" messages on
          the consoles of all the machines it scans if the server is
          older than version 2.9.  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.

     SEE ALSO
          ssh(1), sshd(8)

     AUTHORS
          David Mazieres <dm@lcs.mit.edu> wrote the initial version,
          and Wayne Davison <wayned@users.sourceforge.net> added
          support for protocol version 2.