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 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:

       -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  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 “dsa”,
               “ecdsa”, “ed25519”, or “rsa”.  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 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

       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 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

       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 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.

Debian                                             May 2, 2017                                    SSH-KEYSCAN(1)