ssh-keyscan(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  suit-
       able 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 connec-
              tion  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 ver-
       ifying 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/ssh/ssh_known_hosts

BUGS

       It  generates  "Connection  closed by remote host" messages on the con-
       soles 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 ver-
       sion 2.

                                January 1, 1996                 SSH-KEYSCAN(1)