OpenSSH Requires Password AND Public Key!

Question

I've inherited an old CentOS-6 system with OpenSSH 5.3 installed. And OpenSSH is behaving oddly: Currently the daemon requires the use of a public key and a password in order to login. (It's been this way for as long as I can recall.) And now I need to be able to login with only a password or a key - But not both!

I've verified that these settings are configured on the daemon:

[root@cp ~]# grep -v '#' /etc/ssh/sshd_config | grep -v ^$
Protocol 2
SyslogFacility AUTHPRIV
PermitRootLogin yes
RSAAuthentication yes
PubkeyAuthentication yes
PermitEmptyPasswords no
PasswordAuthentication yes
ChallengeResponseAuthentication yes
GSSAPIAuthentication no
GSSAPICleanupCredentials yes
UsePAM yes
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS
X11Forwarding yes
UseDNS no
Subsystem   sftp    /usr/libexec/openssh/sftp-server
AllowUsers eric@*
AllowUsers techsup@*

I've checked /var/log/secure. And found these two lines after a successful login:

Feb  6 22:33:21 cp sshd[28717]: Accepted keyboard-interactive/pam for root from X.X.X.X port 25075 ssh2
Feb  6 22:33:21 cp sshd[28717]: pam_unix(sshd:session): session opened for user root by (uid=0)

I've tried debugging the login process. But nothing stood out:

eric@cp2:~$ ssh -v [email protected]
OpenSSH_8.9p1 Ubuntu-3ubuntu0.6, OpenSSL 3.0.2 15 Mar 2022
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug1: Connecting to cp.EXAMPLE.com [X.X.X.X] port 22.
debug1: Connection established.
debug1: identity file /home/eric/.ssh/id_rsa type -1
debug1: identity file /home/eric/.ssh/id_rsa-cert type -1
debug1: identity file /home/eric/.ssh/id_ecdsa type -1
debug1: identity file /home/eric/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/eric/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/eric/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/eric/.ssh/id_ed25519 type -1
debug1: identity file /home/eric/.ssh/id_ed25519-cert type -1
debug1: identity file /home/eric/.ssh/id_ed25519_sk type -1
debug1: identity file /home/eric/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/eric/.ssh/id_xmss type -1
debug1: identity file /home/eric/.ssh/id_xmss-cert type -1
debug1: identity file /home/eric/.ssh/id_dsa type -1
debug1: identity file /home/eric/.ssh/id_dsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.6
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: compat_banner: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000002
debug1: Authenticating to cp.EXAMPLE.com:22 as 'root'
debug1: load_hostkeys: fopen /home/eric/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: diffie-hellman-group-exchange-sha256
debug1: kex: host key algorithm: (no match)
Unable to negotiate with X.X.X.X port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss

How can I disable this behavior so that I can login with either a password or a key?

TIA,

Eric Pretorious

Reno, Nevada

Answer 1

The debug shows that all identity files returned errors (type -1) and there is no public key authentication performed. Do you have read permissions to any files in /home/eric/.ssh/?

There is also an error no matching host key type found. Their offer: ssh-rsa,ssh-dss. Use ssh -o HostKeyAlgorithms=+ssh-rsa

Answer 2

To be more explicit: you aren't (yet) trying to 'login' at all

That error does NOT mean your ssh client is trying to authenticate, commonly described as login, with a publickey. It says no matching host key type because your client does not accept the keys from the server/host. This happens at least 6 messages before it tries any kind of client/user authentication, either publickey or password. See where it says Their offer: ssh-rsa,ssh-dss -- both ssh-rsa and ssh-dss are now obsolete and insecure, and disabled by default in recent versions of OpenSSH (since 7.0 and 8.8 respectively).

See SSH No Matching Host Key Type Found
https://unix.stackexchange.com/questions/699192/unable-to-negotiate-with-ip-address-port-22-no-matching-host-key-type-found
https://unix.stackexchange.com/questions/707663/old-linux-rejects-my-ssh-id-rsa-key-from-newly-installed-windows
https://unix.stackexchange.com/questions/693370/can-no-longer-ssh-after-local-os-update-reinstall-no-matching-host-key-type-found
https://unix.stackexchange.com/questions/704177/scp-command-getting-failed-unable-to-negotiate-with-10-100-10-10-port-55-no-matching-host-key-type-found

and either specify or configure [-o]HostKeyAlgorithms={ssh-rsa|ssh-dss|ssh-rsa,ssh-dss}. Once the server key is accepted, if you then want to authenticate/login with an RSA key you will also need [-o]PubkeyAcceptedKeyTypes=ssh-rsa or in 8.5 up the newer and preferred name PubkeyAcceptedAlgorithms as stated in several of those dupes.

Answer 3

As @AlexD pointed out: the remote ssh is an old OpenSSH_5.3 and does not have way more secure ciphers being default in OpenSSH_8.9p1.

There is a reason why ciphers were dropped, so unless your Centos 6 box sits behind a firewall the priority is to upgrade the openssh. You will need a physical access to the machine so if something goes wrong you are not locked out.

You are connecting to: ssh -v [email protected] which means that the remote box should look for your say id_rsa.pub with eric@??? in /root/.ssh/.

Answer 4

Im not sure if CentOS 6 supports it, but I would try to set

RequiredAuthentications2 publickey

in the sshd_config.

https://infosec.mozilla.org/guidelines/openssh