Updating sshd_config with new list of ciphers

mramirez81 · 05-06-2021, 06:44 PM

Running Centos 7.9.2009 with kernel 5.12.1-1.el7.elrepo.x86_64. I'm trying to update ssh to not use weak ciphers. Running ssh -Q cipher, I get this:

3des-cbc
blowfish-cbc
cast128-cbc
arcfour
arcfour128
arcfour256
aes128-cbc
aes192-cbc
aes256-cbc
rijndael-cbc@lysator.liu.se
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.com

So I added this section to sshd_config. Then systemctl restart sshd. Also tried a reboot. But anytime I rerun ssh -Q cipher, I still get the list above. Not sure how to fix this and need to get this working on about 25 servers. Any help is appreciated.

# Ciphers and keying
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
#RekeyLimit default none

# HostKeyAlgorithms
HostKeyAlgorithms ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa,ssh-dss

# KexAlgorithms
KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha256

I also ran ssh -Q kex and got this below. So as if none of my changes are taking effect:

diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
curve25519-sha256
curve25519-sha256@libssh.org
gss-gex-sha1-
gss-group1-sha1-
gss-group14-sha1-

# MACs
MACs hmac-sha2-256,hmac-sha2-512,hmac-sha1

Turbocapitalist · 05-07-2021, 11:49 AM

That polling method only queries the client and reports what is available in principle to that client, not what the server has actually been configured with. The way to see what is going on with the server would be with the -T option and sshd with or without the -C option.

Code:

/usr/sbin/sshd -T | sort | less

/usr/sbin/sshd -T | grep ciphers

scottieH · 05-11-2021, 06:05 PM

The ciphers should be listed in strongest to weakest order. You have:

Code:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr

Since the ssh daemon will search the ciphers from left-to right and stop on the first one both machines understand, you'll want to reverse your order:

Code:

Ciphers aes256-ctr,aes192-ctr,aes128-ctr

This way, you are guaranteed to have the strongest available to both sides of the connection.