gather ssh public keys
[host | addrlist namelist] ...
is a utility for gathering the public
ssh host keys of a number of hosts. It was designed to aid in building and
provides a minimal interface suitable
for use by shell and perl scripts.
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:
- Forces ssh-keyscan to use IPv4
- Forces ssh-keyscan to use IPv6
- Request certificates from target hosts instead of plain
- 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.
- 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.
- 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 “dsa”, “ecdsa”,
“ed25519”, or “rsa”. Multiple values may be
specified by separating them with commas. The default is to fetch
“rsa”, “ecdsa”, and “ed25519”
- Verbose mode. Causes
ssh-keyscan to print debugging messages about
If an ssh_known_hosts file is constructed using
without verifying the keys, users
will be vulnerable to man in the middle
On the other hand, if the security model allows such a risk,
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 RSA, DSA, ECDSA, and Ed25519 keys:
host-or-namelist keytype base64-encoded-key
“ssh-dss” or “ssh-rsa”.
Print the rsa host key for machine hostname
Find all hosts from the file ssh_hosts
new or different keys from those in the sorted file
$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
sort -u - ssh_known_hosts | diff ssh_known_hosts -
wrote the initial version, and Wayne
added support for protocol version 2.
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.