KEX diffie-hellman-group-exchange-sha1 isn't offered for Cisco

[Kurtas]

Hello,

I have configured almost all KEX's algorithms, see the list

    kex: [
      'ecdh-sha2-nistp256',
      'ecdh-sha2-nistp384',
      'ecdh-sha2-nistp521',
      'diffie-hellman-group-exchange-sha1',
      'diffie-hellman-group-exchange-sha256',
      'diffie-hellman-group1-sha1',
      'diffie-hellman-group14-sha1',
      'diffie-hellman-group14-sha256',
      'diffie-hellman-group15-sha512',
      'diffie-hellman-group16-sha512',
      'diffie-hellman-group17-sha512',
      'diffie-hellman-group18-sha512',
    ],

But when I'm trying to connect to Cisco then the diffie-hellman-group-exchange-sha1 is never offered and I'm not able to connect there, see

2022-11-02T16:41:15.194Z - debug: Custom crypto binding not available
2022-11-02T16:41:15.195Z - debug: Local ident: 'SSH-2.0-ssh2js1.10.0'
2022-11-02T16:41:15.195Z - debug: Client: Trying 149.196.174.121 on port 22 ...
2022-11-02T16:41:15.318Z - debug: Socket connected
2022-11-02T16:41:15.445Z - debug: Remote ident: 'SSH-2.0-Cisco-1.25'
2022-11-02T16:41:15.445Z - debug: Outbound: Sending KEXINIT
2022-11-02T16:41:15.446Z - debug: Inbound: Handshake in progress
2022-11-02T16:41:15.446Z - debug: Handshake: (local) KEX method: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group15-sha512,diffie-hellman-group16-sha512,diffie-hellman-group17-sha512,diffie-hellman-group18-sha512
2022-11-02T16:41:15.446Z - debug: Handshake: (remote) KEX method: diffie-hellman-group-exchange-sha1
2022-11-02T16:41:15.447Z - debug: Handshake: No matching key exchange algorithm
2022-11-02T16:41:15.447Z - debug: Outbound: Sending DISCONNECT (3)
2022-11-02T16:41:15.448Z - debug: Outbound: Sending DISCONNECT (11)
2022-11-02T16:41:15.469Z - info: Awaiting connectionEndedPromise...
2022-11-02T16:41:15.672Z - debug: Socket ended
2022-11-02T16:41:15.672Z - debug: Socket closed
2022-11-02T16:41:15.673Z - info: SocketClosedPromise finished.

But it is offered for SSH-2.0-ROSSSH - Mikrotik device

2022-11-11T08:35:24.580Z - debug: Remote ident: 'SSH-2.0-ROSSSH'
2022-11-11T08:35:24.580Z - debug: Outbound: Sending KEXINIT
2022-11-11T08:35:24.582Z - debug: Inbound: Handshake in progress
2022-11-11T08:35:24.582Z - debug: Handshake: (local) KEX method: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha1,diffie-hellman-group-exchange-sha256,diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group15-sha512,diffie-hellman-group16-sha512,diffie-hellman-group17-sha512,diffie-hellman-group18-sha512
2022-11-11T08:35:24.582Z - debug: Handshake: (remote) KEX method: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
2022-11-11T08:35:24.582Z - debug: Handshake: KEX algorithm: diffie-hellman-group-exchange-sha1

So the question is why all configured algorithms are not always offered to all devices?
The last version ssh2 (1.11.0) is used

Thanks

[mscdex]

Some SSH implementations are known to be buggy in different ways. In the case of Cisco's implementation, it has a problem with exchanging large diffie-hellman parameters, so we currently disable the group exchange in that case.

It appears OpenSSH is simply capping the number of requested bits (to 4096) when connecting to a Cisco SSH server, so I suppose we could do the same instead of removing the algorithm outright if it proves to work reliably. Unfortunately I don't have any Cisco equipment to test against to verify that that will be the case.

[Kurtas]

I can test changes across a lot of vendors not only Cisco in our network, will that help?

[petercrocker]

I'm happy to test this as well on various Cisco implementations: IOS, IOS-XE, NX-OS. This changed has caused it to fail on our Cisco boxes, so happy to do any verification asap so we can get it included again!

[roman-kaspar]

Please see pull request #1243 which is removing the BAD_DHGEX compatibility flag.

The changes were tested on affected Cisco 1.25 box (and also on other 370 boxes with various SSH implementations in our lab).

Thanks!