openvpn works with AES-256-CBC and doesnt work with AES-256-GCM

decehakan

my openvpn server runs with rsa and static dh parameters and runs super fine. On the client and on the server I have chosen cipher AES-256-CBC and when I read through the protocols it has chosen the best cipher AES-256-GCM.

Now I change the cipher on the client and server to AES-256-GCM ( server.conf / client.conf : cipher AES-256-GCM ). Now I can no longer dial in to the VPN because it somehow no longer works. This is crazy and confusing.

If I use AES-256-CBC it is upgeraded to AES-256-GCM.

and if I now select the entry cipher AES-256-GCM on both configs (server / client), it cannot dial in to the server.

Is this because GCM can only be used in conjunction with the entry "dh none"? If so, why is this up-loaded to AES-256-GCM in the case of AES-256-CBC?

Many thanks and best regards

decehakan

TinCanTech

decehakan wrote Is this because GCM can only be used in conjunction with the entry "dh none"?

No. You did something incorrectly.

decehakan

after i delete the --opt-verify option on server.conf and set on client/ server the cipher AES-256-GCM , the client can connect now to the server.

The reason why i couldn't connect was the auth alg which compared from --opt-verify flag
Log:

SENT CONTROL [test12]: 'AUTH_FAILED' (status=1)

But in bothside(server.conf / client.conf) i'm using auth SHA256.

Reading the Documentation
If an AEAD cipher mode (e.g. GCM) is chosen, the specified --auth algorithm is ignored for the data channel, and the authentication method of the AEAD cipher is used instead. Note that alg still specifies the digest used for tls-auth

Means that: If im using GCM cipher, than i dont need the auth alg ???

kind regards

TinCanTech

decehakan wrote Means that: If im using GCM cipher, than i dont need the auth alg ???

Not quite...

AEAD ciphers do not require authentication because it is built-in to the cipher.

However, Openvpn options still requires --auth alg to be correctly configured.

The best solution for you: Do not use --auth alg or --cipher cipher any where in your server and client config files.

The reason is: Openvpn will automatically select --auth SHA1 (Which is the correct default) and then ignore it.
and --cipher BF-CBC (Which is fine for testing) and then be negotiated to cipher AES-256-GCM.

You really do not need --auth alg in any of your configs.

And do not worry about the default --cipher BF-CBC because --cipher is deprecated.
And --data-ciphers trumps --cipher. Default --data-ciphers AES-256-GCM:AES-128-GCM

https://build.openvpn.net/man/openvpn-2.5/openvpn.8.html

Try it for yourself..