BBR3 on NixOS ( Solution )

[randomizedcoder]

G'day,

If here's any interest, I made a Nix module to add bbr3 to the latest kernel on NixOS.

There are some configurePhase changes required to compile against a pretty new kernel 6.18.3

[das@l2:~]$ cat /proc/sys/net/ipv4/tcp_available_congestion_control
reno cubic bbr3

[das@l2:~]$ uname -a
Linux l2 6.18.3 #1-NixOS SMP PREEMPT_DYNAMIC Fri Jan  2 11:57:31 UTC 2026 x86_64 GNU/Linux

[das@l2:~]$ neofetch
          ▗▄▄▄       ▗▄▄▄▄    ▄▄▄▖            das@l2 
          ▜███▙       ▜███▙  ▟███▛            ------ 
           ▜███▙       ▜███▙▟███▛             OS: NixOS 26.05.20260102.fb7944c (Yarara) x86_64 
            ▜███▙       ▜██████▛              Host: LENOVO 1046 
     ▟█████████████████▙ ▜████▛     ▟▙        Kernel: 6.18.3 
    ▟███████████████████▙ ▜███▙    ▟██▙       Uptime: 6 mins 
           ▄▄▄▄▖           ▜███▙  ▟███▛       Packages: 641 (nix-system), 980 (nix-user) 
          ▟███▛             ▜██▛ ▟███▛        Shell: bash 5.3.3 
         ▟███▛               ▜▛ ▟███▛         Resolution: 3840x2160 
▟███████████▛                  ▟██████████▙   Terminal: /dev/pts/0 
▜██████████▛                  ▟███████████▛   CPU: AMD Ryzen Threadripper PRO 3945WX s (24) @ 4.427GHz 
      ▟███▛ ▟▙               ▟███▛            GPU: AMD ATI Radeon PRO WX 2100 
     ▟███▛ ▟██▙             ▟███▛             Memory: 2725MiB / 128660MiB 
    ▟███▛  ▜███▙           ▝▀▀▀▀
    ▜██▛    ▜███▙ ▜██████████████████▛                                
     ▜▛     ▟████▙ ▜████████████████▛                                 
           ▟██████▙       ▜███▙
          ▟███▛▜███▙       ▜███▙
         ▟███▛  ▜███▙       ▜███▙
         ▝▀▀▀    ▀▀▀▀▘       ▀▀▀▘


[das@l2:~]$ date
Sat Jan  3 04:26:02 PM PST 2026

[das@l2:~]$ 

#
# bbr3-module.nix
#
# Build BBRv3 congestion control from L4S kernel source as out-of-tree module
# Source: https://github.com/L4STeam/linux
#
# BBRv3 is an improved version of BBR with better fairness and L4S awareness
#

{ config, lib, pkgs, ... }:

let
  kernel = config.boot.kernelPackages.kernel;

  # L4S repo commit (testing branch)
  l4sRev = "48b3db6b4a7fd57e2d31db3bb46a3bc6af7bf3ad";

  # BBRv3 module built from L4S kernel source
  bbr3Module = pkgs.stdenv.mkDerivation rec {
    pname = "tcp-bbr3";
    version = "3.0-${kernel.version}";

    src = pkgs.fetchFromGitHub {
      owner = "L4STeam";
      repo = "linux";
      rev = l4sRev;
      hash = "sha256-fqv0xKajFFaVpDuN2B13BpeS6dkjObqAPCEVkJXYN6Q=";
    };

    nativeBuildInputs = kernel.moduleBuildDependencies;

    hardeningDisable = [ "pic" "format" ];

    postUnpack = ''
      # Create a clean build directory
      mkdir -p $TMPDIR/build
      cd $TMPDIR/build
      sourceRoot=$TMPDIR/build
    '';

    configurePhase = ''
      # Copy BBRv3 source to build directory
      cp $src/net/ipv4/tcp_bbr.c tcp_bbr3.c

      # Rename the module to avoid conflict with in-kernel tcp_bbr
      sed -i 's/tcp_bbr_cong_ops/tcp_bbr3_cong_ops/g' tcp_bbr3.c
      sed -i 's/bbr_register/bbr3_register/g' tcp_bbr3.c
      sed -i 's/bbr_unregister/bbr3_unregister/g' tcp_bbr3.c
      sed -i 's/MODULE_DESCRIPTION("TCP BBR/MODULE_DESCRIPTION("TCP BBRv3/g' tcp_bbr3.c

      # Change the registered name to "bbr3"
      sed -i 's/\.name[[:space:]]*=[[:space:]]*"bbr"/.name = "bbr3"/g' tcp_bbr3.c

      # === Kernel API compatibility fixes for 6.18+ ===

      # Fix 1: prandom_u32_max was renamed to get_random_u32_below in kernel 6.2+
      sed -i 's/prandom_u32_max/get_random_u32_below/g' tcp_bbr3.c

      # Fix 2: tso_segs was removed/renamed - the function signature changed
      # Just remove the line since it's optional
      sed -i '/\.tso_segs[[:space:]]*=/d' tcp_bbr3.c

      # Fix 3: cong_control signature changed - need to adapt bbr_main
      # Old: void (*cong_control)(struct sock *sk, const struct rate_sample *rs)
      # New: void (*cong_control)(struct sock *sk, u32 ack, int flag, const struct rate_sample *rs)
      # We create a wrapper function
      sed -i '/^static void bbr_main/,/^{/c\
static void bbr_main_inner(struct sock *sk, const struct rate_sample *rs)\
{' tcp_bbr3.c

      # Add wrapper at end before module registration
      sed -i '/static struct tcp_congestion_ops tcp_bbr3_cong_ops/i\
/* Wrapper for new kernel API */\
static void bbr_main(struct sock *sk, u32 ack, int flag, const struct rate_sample *rs)\
{\
	bbr_main_inner(sk, rs);\
}\
' tcp_bbr3.c

      # Create Kbuild file for out-of-tree build
      echo "obj-m := tcp_bbr3.o" > Kbuild
    '';

    buildPhase = ''
      make -C ${kernel.dev}/lib/modules/${kernel.modDirVersion}/build M=$(pwd) modules
    '';

    installPhase = ''
      runHook preInstall
      mkdir -p $out/lib/modules/${kernel.modDirVersion}/extra
      cp tcp_bbr3.ko $out/lib/modules/${kernel.modDirVersion}/extra/
      runHook postInstall
    '';

    meta = with lib; {
      description = "TCP BBRv3 - Bottleneck Bandwidth and RTT v3 from L4S team";
      homepage = "https://github.com/L4STeam/linux";
      license = licenses.gpl2;
      platforms = platforms.linux;
    };
  };

in {
  options.services.bbr3 = {
    enable = lib.mkEnableOption "BBRv3 congestion control module";
  };

  config = lib.mkIf config.services.bbr3.enable {
    boot.extraModulePackages = [ bbr3Module ];
    boot.kernelModules = [ "tcp_bbr3" ];
  };
}

#
# USAGE:
#
# 1. First, get the correct hash:
#    nix-prefetch-github L4STeam linux --rev 48b3db6b4a7fd57e2d31db3bb46a3bc6af7bf3ad
#
# 2. Replace the placeholder hash above with the actual hash
#
# 3. Add to configuration.nix:
#    imports = [ ./bbr3-module.nix ];
#    services.bbr3.enable = true;
#
# 4. Rebuild:
#    sudo nixos-rebuild switch
#
# 5. Verify:
#    lsmod | grep bbr3
#    cat /proc/sys/net/ipv4/tcp_available_congestion_control
#
# 6. To use BBRv3, add to sysctl.nix:
#    "net.ipv4.tcp_congestion_control" = "bbr3";
#

https://discourse.nixos.org/t/bbr3-tcp-for-nix/73954