Ubuntu Key Exchange Algo

I am trying to test the connectivity to several network devices, with Ansible installed on Ubuntu 20.04.2 LTS, using ansible ad-hoc.

The problem: SSH is not working as the device's key exchange method is only ssh-RSA, the server doesn't support that. Trying to enforce ssh-RSA but I know it's not available as it wasn't sent as one of the key exchange methods on the cipher negotiation.

Ansible_output:

    (venv) omera@sandbox:~/code/ansible/play_06$ ansible all -m ping
edge_02 | UNREACHABLE! => {
    "changed": false,
    "msg": "Failed to connect to the host via ssh: Unable to negotiate with 192.168.1.201 port 22: no matching key exchange method found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1",
    "unreachable": true
}
edge_01 | UNREACHABLE! => {
    "changed": false,
    "msg": "Failed to connect to the host via ssh: Unable to negotiate with 192.168.1.200 port 22: no matching key exchange method found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1",
    "unreachable": true
}
core_01 | UNREACHABLE! => {
    "changed": false,
    "msg": "Failed to connect to the host via ssh: Unable to negotiate with 192.168.1.202 port 22: no matching key exchange method found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1",
    "unreachable": true

edge_02_debug_output:

Edge_02#

    *Jun  7 07:49:14.738: SSH0: starting SSH control process
    *Jun  7 07:49:14.738: SSH0: sent protocol version id SSH-1.99-Cisco-1.25
    *Jun  7 07:49:14.741: SSH0: protocol version id is - SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.2
    *Jun  7 07:49:14.741: SSH2 0: Server certificate trustpoint not found. Skipping hostkey algo = x509v3-ssh-rsa
    *Jun  7 07:49:14.741: SSH2 0: kexinit sent: hostkey algo = ssh-rsa
    *Jun  7 07:49:14.741: SSH2 0: kexinit sent: encryption algo = aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
    *Jun  7 07:49:14.741: SSH2 0: kexinit sent: mac algo = hmac-sha1,hmac-sha1-96
    *Jun  7 07:49:14.741: SSH2 0: SSH2_MSG_KEXINIT sent
    *Jun  7 07:49:14.741: SSH2 0: SSH2_MSG_KEXINIT received
    *Jun  7 07:49:14.741: SSH2 0: kex: client->server enc:aes128-ctr mac:hmac-sha1 
    *Jun  7 07:49:14.741: SSH2 0: kex: server->client enc:aes128-ctr mac:hmac-sha1 
    *Jun  7 07:49:14.741: %SSH-3-NO_MATCH: No matching kex algorithm found: client curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,ext-info-c server diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1

my concern is that is it possible to set RSA as the key exchange method on ubuntu (ansible is using sshpass)?

ii  sshpass  1.06-1   amd64  Non-interactive ssh password authentication

Solution 1:

On default ansible uses OpenSSH, which is a bit picky with older IOS versions. You have to enable the Diffie–Hellman key exchange and some older ciphers in ~/.ssh/config.

KexAlgorithms +diffie-hellman-group1-sha1
Ciphers +aes256-cbc,aes192-cbc,aes128-cbc,3des-cbc

Alternatively, you might try paramiko for the transport.

[defaults]
inventory = /root/hosts
host_key_checking=False
timeout = 30
transport = paramiko

While these instructions are generally true for IOS, I'm fairly certain that you are facing a similar issue on IOU device too.

Source