NAME | SYNOPSIS | DESCRIPTION | SECURITY | FILES | EXAMPLES | SEE ALSO | AUTHORS | BUGS | COLOPHON

SSH-KEYSCAN(1)           BSD General Commands Manual          SSH-KEYSCAN(1)

NAME         top

     ssh-keyscan — gather ssh public keys

SYNOPSIS         top

     ssh-keyscan [-46cHv] [-f file] [-p port] [-T timeout] [-t type]
                 [host | addrlist namelist] ...

DESCRIPTION         top

     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 dis‐
             closed.

     -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
             “dsa”, “ecdsa”, “ed25519”, or “rsa” for protocol version 2.
             Multiple values may be specified by separating them with com‐
             mas.  The default is to fetch “rsa”, “ecdsa”, and “ed25519”
             keys.

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

SECURITY         top

     If an ssh_known_hosts file is constructed using ssh-keyscan without
     verifying the keys, users will be vulnerable to 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.

FILES         top

     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         top

     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         top

     ssh(1), sshd(8)

AUTHORS         top

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

BUGS         top

     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.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.  Informa‐
     tion about the project can be found at
     http://www.openssh.com/portable.html.  If you have a bug report for
     this manual page, see http://www.openssh.com/report.html.  This page
     was obtained from the tarball openssh-7.5p1.tar.gz fetched from
     http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2017-09-15.
     If you discover any rendering problems in this HTML version of the
     page, or you believe there is a better or more up-to-date source for
     the page, or you have corrections or improvements to the information in
     this COLOPHON (which is not part of the original manual page), send a
     mail to man-pages@man7.org

BSD                           November 8, 2015                           BSD