ssh-keyscan Man page

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


ssh-keyscan — gather ssh public keys


ssh-keyscan [-46cHv] [-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
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 identi‐
fying 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 “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.

If an 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.

Input format:,,name,,n,,

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”.


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 –


ssh, sshd(8)

David Mazieres wrote the initial version, and Wayne
Davison 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 November 8, 2015 BSD