ssh-keyscan —
gather SSH public keys
ssh-keyscan |
[-46cDHv]
[-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 number of hosts. It was designed to aid in building and
verifying
ssh_known_hosts files, the format of
which is documented in
sshd(8).
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
sshd(8). 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
- Force ssh-keyscan to use IPv4
addresses only.
- -6
- Force ssh-keyscan to use IPv6
addresses only.
- -c
- Request certificates from target hosts instead of plain
keys.
- -D
- Print keys found as SSHFP DNS records. The default is to
print keys in a format usable as a
ssh(1)
known_hosts file.
- -f
file
- Read hosts or “addrlist namelist” pairs from
file, one per line. If ‘-’
is supplied instead of a filename,
ssh-keyscan will read from the standard
input. Input is expected in the 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
- -H
- Hash all hostnames and addresses in the output. Hashed
names may be used normally by
ssh(1) and
sshd(8), but they do not
reveal identifying information should the file's contents be
disclosed.
- -p
port
- Connect to port 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, the connection is closed and the host in question
considered unavailable. The default is 5 seconds.
- -t
type
- Specify 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: print debugging messages about progress.
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.
/etc/ssh/ssh_known_hosts
Print the RSA host key for machine
hostname:
$ ssh-keyscan -t rsa 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(1),
sshd(8)
Using DNS to Securely Publish
Secure Shell (SSH) Key Fingerprints, RFC 4255,
2006.
David Mazieres
<
dm@lcs.mit.edu>
wrote the initial version, and
Wayne
Davison
<
wayned@users.sourceforge.net>
added support for protocol version 2.