ssh-keyscan(1)

NAME

       ssh-keyscan - gather ssh public keys

SYNOPSIS

       ssh-keyscan  [-46Hv] [-f file] [-p port] [-T timeout] [-t type] [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:

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

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

       -f file
              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.

       -H     Hash all hostnames and addresses in the  output.   Hashed  names
              may  be  used  normally  by ssh and sshd, but they do not reveal
              identifying information should the file's contents be disclosed.

       -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''.

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

SECURITY

       If a ssh_known_hosts file is constructed using ssh-keyscan without ver-
       ifying  the  keys,  users will be vulnerable to maninthemiddle 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 cre-
       ated.

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-dss''.

       /etc/ssh/ssh_known_hosts

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 -

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.

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.

                                January 1, 1996                 SSH-KEYSCAN(1)