Any gentoo-sources version newer than 5.4.72, when used as a gateway on a LAN (or a similar setup) will break WebRTC protocols such as Google Meet, Discord, etc. (have not done extensive testing but would gather that most similar protocols are affected). In the case of Meet, no video for any participant is ever shown (other than my own), and nobody can see my video, although audio does work. In the case of Discord, no audio/video for other participants is ever shown. Note that every meeting is initiated or joined from inside the LAN, not on the gateway itself. Using plain iptables, firewalld+iptables or firewalld+nftables makes no difference (it was the first thing I tried). I discovered this a few months ago when updating the kernel, and found that reverting to the previous kernel made this work again. I didn't look further into it until now, when I can no longer stay on that old of a kernel :). Using git-bisect I was able to identify the offending commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=02a1b175b0e92d9e0fa5df3957ade8d733ceb6a0 This patch was backported to linux-stable (gentoo-sources) shortly after 5.4.72 released. It appears to still be there in vanilla upstream. I can confirm that reverting this patch in 5.4.109 fixes the issue and webrtc works again. No other protocol/connection seems to be affected. Reproducible: Always Steps to Reproduce: 1. Install any kernel >5.4.72 on a gateway device. 2. Try to use a conferencing application that uses WebRTC (Meet, Discord, etc). Either start or join a meeting from a device that sits in the LAN. Actual Results: Audio and/or video does not work when a meeting is initiated/joined from within the LAN Expected Results: Both audio and video should work when inside the meeting. My C is quite limited, but it appears that this function, from wherever it gets called, returns a different value after the mentioned commit. It used to return: return min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU); Now it returns: mtu = dst_metric_raw(dst, RTAX_MTU); if (mtu) return mtu;
Can you submit this upstream, please at https://bugzilla.kernel.org/ and CC kernel@gentoo.org ? Great job on the bisect. I'm going to ping the author directly.
Reported upstream: https://bugzilla.kernel.org/show_bug.cgi?id=213581 Added kernel@gentoo.org to the CC list in the upstream bug.
Apparently related to honoring the ISP MTU parameter in DHCP. I was receiving a 576 MTU for that route: root@manya ~ # ip route default via W.X.Y.1 dev eno1 proto dhcp src W.X.Y.Z metric 3 mtu 576 172.16.0.0/24 dev enp3s0 proto kernel scope link src 172.16.0.1 172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 172.20.0.0/24 via 172.20.0.37 dev tun0 172.20.0.37 dev tun0 proto kernel scope link src 172.20.0.38 W.X.Y.0/24 dev eno1 proto dhcp scope link src W.X.Y.Z metric 3 mtu 576 root@manya ~ # Commented out the following line in /etc/dhcpcd.conf: option interface_mtu Restarted the eno1 card and had to restart the browsers to test again, the issue seems to be gone. I don't know if this would warrant a change to the default dhcpcd configuration file for that, as I can't tell if it's just my crappy ISP or it could happen to someone else.
Looks like the kernel is doing what's it's asked to do. Reassigning to dhcpcd maintainer to see if something should be considered there or is this a case of the ISP doing faulty things.
This is ISP doing faulty things - dhcpcd is just doing what it's told. interface_mtu is the IPv4 equivalent of accept_ra_mtu in the Linux kernel which is enabled by default. dhcpcd just mirrors this.
