ssh-keyscan - gather ssh public keys
ssh-keyscan [-v46] [-p port] [-T timeout] [-t type] [-f
[host | addrlist namelist] [...]
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
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
involve any encryption.
The options are as follows:
Port to connect to on the remote host.
Set the timeout for connection attempts. If timeout
elapsed since a connection was initiated to a host
or since the
last time anything was read from that host, then the
is closed and the host in question considered unavailable. Default
is 5 seconds.
Specifies the type of the key to fetch from the
The possible values are ``rsa1'' for protocol version 1 and
``rsa'' or ``dsa'' for protocol version 2. Multiple
be specified by separating them with commas. The
Read hosts or addrlist namelist pairs from this
file, one per
line. If - is supplied instead of a filename,
read hosts or addrlist namelist pairs from the standard input.
-v Verbose mode. Causes ssh-keyscan to print debugging
about its progress.
-4 Forces ssh-keyscan to use IPv4 addresses only.
-6 Forces ssh-keyscan to use IPv6 addresses only.
If a ssh_known_hosts file is constructed using ssh-keyscan
the keys, users will be vulnerable to man in the
On the other hand, if the security model allows such a risk,
can help in the detection of tampered keyfiles or man in the
which have begun after the ssh_known_hosts file was
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
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 <email@example.com> wrote the initial version,
Wayne Davison <firstname.lastname@example.org> added support
It generates "Connection closed by remote host" messages on
of all the machines it scans if the server is older than
This is because it opens a connection to the ssh port, reads
key, and drops the connection as soon as it gets the key.
OpenBSD 3.6 January 1, 1996
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