SSH-KEYSCAN(1) BSD General Commands Manual SSH-KEYSCAN(1)
ssh-keyscan - gather ssh public keys
ssh-keyscan [-46Hv] [-f file] [-p port] [-T timeout] [-t type]
[host | addrlist namelist] [...]
ssh-keyscan is a utility for gathering the public ssh host keys of a num
ber 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 pos
sible 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 pro
cess 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.
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 identi
fying information should the files contents be disclosed.
Port to connect to on the remote host.
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.
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 speci
fied by separating them with commas. The default is rsa1.
-v Verbose mode. Causes ssh-keyscan to print debugging messages
about its progress.
If a ssh_known_hosts file is constructed using ssh-keyscan without veri
fying 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.
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.
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 -
David Mazieres firstname.lastname@example.org wrote the initial version, and Wayne
Davison email@example.com added support for protocol version
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.
BSD January 1, 1996 BSD