ssh-keyscan(1)

NAME

       ssh-keyscan - gather ssh public keys

SYNOPSIS

       ssh-keyscan [-46cHv] [-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.

       -c     Request certificates from target hosts instead of plain keys.

       -f file
              Read hosts or ``addrlist namelist'' pairs  from  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
              ``dsa'', ``ecdsa'', ``ed25519'', or ``rsa'' for protocol version
              2.   Multiple  values  may  be specified by separating them with
              commas.   The  default  is  to  fetch  ``rsa'',  ``ecdsa'',  and
              ``ed25519'' keys.

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

SECURITY

       If an ssh_known_hosts file is  constructed  using  ssh-keyscan  without
       verifying 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, DSA, ECDSA, and Ed25519 keys:

       host-or-namelist keytype base64-encoded-key

       Where    keytype    is    either    ``ecdsa-sha2-nistp256'',   ``ecdsa-
       sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-ed25519'',  ``ssh-dss''
       or ``ssh-rsa''.

       /etc/ssh/ssh_known_hosts

EXAMPLES

       Print the rsa 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,ecdsa,ed25519 -f ssh_hosts | \
            sort -u - ssh_known_hosts | diff ssh_known_hosts -

SEE ALSO

       ssh(1), sshd(8)

AUTHORS

       -nosplit

       David Mazieres <Mt dm@lcs.mit.edu> wrote the initial version, and

       Wayne Davison <Mt wayned@users.sourceforge.net> added support for  pro-
       tocol 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.

                               November 8 2015                  SSH-KEYSCAN(1)