07-08-2019
Just came across this. One thing to be aware of is that all clients will object when you next try to connect them. They will alert on there being a possible man-in-the-middle attack or a DNS attack that is trying to send you to a different host (as determined by the keys) so you would need to get each client to forget the server keys for the machine(s) you are replacing the keys on and re-validate them all, or manually replace the old key with the new on all the clients.
You need to consider all the names that the clients could refer to the server as, be that IP, local hosts, DNS short name, fully qualified DNS name, DNS alias etc. and look for those in ~/.ssh/known_hosts
You will need to do this for every account on every client, so it is not a thing to be done lightly, especially if there are multiple automated jobs that connect with SSH, SCP, SFTP etc. that you need to ensure are not disrupted.
Sorry if I've made you panic, but better that than a massive failure.
I hope that this helps,
Robin
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LEARN ABOUT MOJAVE
ssh-keyscan
SSH-KEYSCAN(1) BSD General Commands Manual SSH-KEYSCAN(1)
NAME
ssh-keyscan -- gather SSH public keys
SYNOPSIS
ssh-keyscan [-46cDHv] [-f file] [-p port] [-T timeout] [-t type] [host | addrlist namelist]
DESCRIPTION
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.
FILES
/etc/ssh/ssh_known_hosts
EXAMPLES
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 -
SEE ALSO
ssh(1), sshd(8)
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, 2006.
AUTHORS
David Mazieres <dm@lcs.mit.edu> wrote the initial version, and Wayne Davison <wayned@users.sourceforge.net> added support for protocol ver-
sion 2.
BSD
March 5, 2018 BSD