allow OVN networks to go below 1280 MTU if IPv6 not used

[MostafaTarek124eru]

Please confirm

• I have searched existing issues to check if an issue already exists for the bug I encountered.

Distribution

Ubuntu

Distribution version

Noble 24.04

Output of snap list --all lxd core20 core22 core24 snapd

core20  20260211        2769   latest/stable  canonical✓  base
core22  20260225        2411   latest/stable  canonical✓  base
core24  20260317        1587   latest/stable  canonical✓  base
lxd     5.21.4-aee7e08  38767  5.21/stable    canonical✓  in-cohort
snapd   2.74.1          26382  latest/stable  canonical✓  snapd

System info

  network.ovn.northbound_connection: ssl:10.0.0.1:6641,ssl:10.0.0.2:6641,ssl:10.0.0.3:6641
  user.microcloud: 2.1.2
api_status: stable
api_version: "1.0"
  driver: lxc | qemu
  driver_version: 6.0.4 | 8.2.2
  kernel: Linux
  kernel_architecture: x86_64
  kernel_features:
    bpf_token: "false"
    idmapped_mounts: "true"
    netnsid_getifaddrs: "true"
    seccomp_listener: "true"
    seccomp_listener_continue: "true"
    uevent_injection: "true"
    unpriv_binfmt: "true"
    unpriv_fscaps: "true"
  kernel_version: 6.8.0-100-generic
  lxc_features:
    cgroup2: "true"
    core_scheduling: "true"
    devpts_fd: "true"
    idmapped_mounts_v2: "true"
    mount_injection_file: "true"
    network_gateway_device_route: "true"
    network_ipvlan: "true"
    network_l2proxy: "true"
    network_phys_macvlan_mtu: "true"
    network_veth_router: "true"
    pidfd: "true"
    seccomp_allow_deny_syntax: "true"
    seccomp_notify: "true"
    seccomp_proxy_send_notify_fd: "true"
  os_name: Ubuntu
  os_version: "24.04"
  project: default
  server: lxd
  server_clustered: true
  server_event_mode: full-mesh
  server_version: 5.21.4
  server_lts: true
  storage: ceph | cephfs
  storage_version: 17.2.9 | 17.2.9

Instance log

n/a

Expected behavior

If we don't care about IPv6, we should be allowed to set the bridge.mtu to lower value than 1280

Actual behavior

returns an error when I try to go lower than 1280 for reference my physical host mtu is 1300 so I need to set the bridge to 1242
Config parsing error: Invalid value for network "default" option "bridge.mtu": Out of MTU range (1280-16384) "1242
might be related to this validation

lxd/shared/validate/validate.go

Line 464 in d7defba

// Anything below 68 and the kernel doesn't allow IPv4, anything below 1280 and the kernel doesn't allow IPv6.

Steps to reproduce

1. bootstrap microcloud or lxd.
2. lxc network edit UPLINK
3. change bridge.mtu to lower value than the fixed 1442

Information to attach

• Any relevant kernel output (dmesg)
• Instance configuration (lxc config show <instance> --expanded)
• Main daemon log (at /var/log/lxd/lxd.log or /var/snap/lxd/common/lxd/logs/lxd.log)
• If a lxc command fails, output of the command with --debug
• Output of the daemon with --debug (or use lxc monitor while reproducing the issue)

[tomponline]

Which network type are you trying to set bridge.mtu on?

Is this on the OVN network itself or an uplink bridge network?

[MostafaTarek124eru]

sorry, OVN. editing title.

lxc network show default
name: default
description: ""
type: ovn
managed: true
status: Created
config:
  bridge.mtu: "1442"
  ipv4.address: 10.123.45.1/24
  ipv4.nat: "true"
  network: UPLINK
  volatile.network.ipv4.address: 10.0.0.27