Server crashes on startup if ipv6 is disabled (linux)

[raidekoptix]

What happened?

Server fails to startup with error if kernel option ipv6.disable=1:

Could not bind on 0.0.0.0:30120 - is this address valid and not already in use?
[ citizen-server-impl] Error: Could not bind on 0.0.0.0:30120 - is this address valid and not already in use?

Expected result

Server runs and binds successfully to 0.0.0.0

Reproduction steps

1. Set kernel to boot with IPv6 disabled (ipv6.disable=1)
2. Attempt to start server.

Importancy

There's a workaround

Area(s)

FiveM, FXServer

Specific version(s)

Multiple. Tested on 8102 and 8216

Additional information

I'd really like to keep ipv6.disable on for security reasons. It looks like something in Enet barfs when IPv6 is disabled... I'm thinking somewhere around here:

https://github.com/citizenfx/fivem/blob/7a61eafd2ad6e553335c87a3a315f766db6a58b3/code/components/citizen-server-impl/src/GameServerNet.ENet.cpp#L31

Additional discord conversation is here:

https://discord.com/channels/192358910387159041/1213597178908512296

[raidekoptix]

Additionally, this happens no matter what the bind IP is set to in server.cfg ... Could be 0, could be 127.0.0.1, could be real public ip. This is reproducable on Ubuntu 22.04 and Ubuntu 24.04, I have not tested others but I believe the fellow in the Discord thread mentioned he's on Debian 11 (I do not know what his kernel options are)

[FabianTerhorst]

The reason is most likely that we always bind using the ipv6 address type: https://github.com/citizenfx/fivem/blob/7a61eafd2ad6e553335c87a3a315f766db6a58b3/code/components/citizen-server-impl/src/EnetUv.cpp#L186 but this is very common since this way makes it possible to support both address types without having to manually determinate it.

[AvarianKnight]

Out of curiosity, what is the use case for needing to disable ipv6 "for security reasons"?

[raidekoptix]

The reason is most likely that we always bind using the ipv6 address type

...and that makes sense, but if it fails to see ipv6 support, it should handle this more gracefully and default to the ipv4, no?

Out of curiosity, what is the use case for needing to disable ipv6 "for security reasons"?

Smaller attack surface... allows me to be lazy yet confident that no ipv6 traffic is happening. My provider is kinda loose with VLANs for general population servers and I'd just rather not some link-local shenanigans be a part of my life right now. Also a lot of applications try for v6 first as Fabian mentioned, and I'd rather force them to v4... simplest way to do that is disable the whole v6 stack at kernel level. Most applications handle this gracefully. "for security reasons" was just a (over)simplification of why i disable it globally.

[SloPro]

For what it's worth, works fine here (and has worked for years) with

net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6=1