1922990 - IPv6 preference breaks with VPNs

Status: RESOLVED INVALID

(Core :: Networking: DNS, defect)

Description

[lucas]

Attachment: Screenshot_20241007-005020_1.png

User Agent: Mozilla/5.0 (Android 14; Mobile; rv:131.0) Gecko/131.0 Firefox/131.0

Steps to reproduce:

1. Install RethinkDNS
2. On it, Settings > Network > Choose IP version > Automatic "experimental"
3. Go to test-ipv6.com

Actual results:

IPv6 preference breaks! Chromium (Cromite tested) still prefers IPv6.

Expected results:

Firefox should still prefer IPv6, similarly to Chromium and such.

Comment 1

[lucas]

This also happened with pretty much any other VPN app I used, in this case I'm not even using a VPN, just a blocker proxy kind of thing. e.g. Mullvad

Comment 2

[edgul]

Are you sure that your VPN provider supports ipv6 and that you've configured it correctly?
IIUC: most VPNs just disable ipv6 to prevent ipv6 leaks.

I tried with mullvad VPN with enabled ipv6 and firefox (10/10) actually does better than chromium (9/10) on https://test-ipv6.com/.
Details: Chromium fails the Test if your ISP's DNS server uses IPv6 test.

Comment 3

[edgul]

Just re-read your report. What do you mean "preference"? I'm guessing here that because of happy eyeballs we can expect ipv4 to race ipv6.

Comment 4

[lucas]

Very sorry! I thought I answered this before when I read it on e-mail, I guess I got trolled by my own conscience.
In this case, I don't have Mullvad or anything at the moment, but certain VPN apps do end up breaking preference which is happy eyeballs yes.
I suspect this has something to do with ULA address use, it seems actually not just Firefox-based browsers do this, but also many other apps, with the exception of Chromium-based browsers.

This might not be in Firefox's scope. You can close this if you believe so.

(It is kind of weird that Chromium or Firefox would behave differently in test-ipv6.com test for ISP's DNS, that is way more less to do with the client, you might be using a different DNS in the browser itself)

My previous experience with Mullvad seems to have something to do with the server connected, Brazil causing issues with preference for some reason.

Comment 5

[Valentin Gosu [:valentin] (he/him)]

Hi Lucas, does setting the network.dns.preferIPv6 pref to true fix this for you?

Comment 6

[lucas]

Nope

Comment 7

[Kershaw Chang [:kershaw]]

Hi Lucas,
Are you able to reproduce this with Firefox desktop?
If yes, could you try to capture a http log?

Thanks.

Comment 8

[lucas]

Nope, I don't really have a VPN to test in the desktop.
I'll leave this to the VPN guys to fix as it seems like a few other VPNs do not have this issue.