Re: [PATCH v2] fix #7571: emit mtu in ifupdown-based LXC guests

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

On June 17, 2026 8:32 pm, Ilan Karasik wrote:
> This fixes a bug where custom mtu settings applied to an LXC container
> are silently ignored during network setup for Debian-based guests
> using ifupdown.
> 
> Because the Proxmox network setup was failing to write the mtu value
> to the guest's `/etc/network/interfaces` file, the container would
> always default to an mtu of 1500.
> 
> This patch resolves the issue by writing the `mtu <value>` directive into the
> guest's network configuration file at /etc/network/interfaces.
> 
> Signed-off-by: Ilan Karasik <ilankarasik@gmail.com>
> ---
>  src/PVE/LXC/Setup/Debian.pm | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/src/PVE/LXC/Setup/Debian.pm b/src/PVE/LXC/Setup/Debian.pm
> index 837397b..bbf73d5 100644
> --- a/src/PVE/LXC/Setup/Debian.pm
> +++ b/src/PVE/LXC/Setup/Debian.pm
> @@ -284,6 +284,7 @@ sub setup_network {
>                      $net->{needsroute6} = 1;
>                  }
>              }
> +            $net->{mtu} = $d->{mtu} if defined($d->{mtu});
>              $networks->{ $d->{name} } = $net if keys %$net;
>          }
>      }
> @@ -337,6 +338,7 @@ sub setup_network {
>                          $interfaces .= "\tgateway $gw\n";
>                      }
>                  }
> +                $interfaces .= "\tmtu $net->{mtu}\n" if defined($net->{mtu});

sorry for the long turnaround time!

should we also set this higher up in the dhcp/manual part?

>                  foreach my $attr (@{ $section->{attr} }) {
>                      $interfaces .= "\t$attr\n";
>                  }
> @@ -450,6 +452,7 @@ sub setup_network {
>                      || $aname eq 'netmask'
>                      || $aname eq 'gateway'
>                      || $aname eq 'broadcast'
> +                    || $aname eq 'mtu'
>                  ) {
>                      # skip
>                  } else {
> -- 
> 2.51.2
> 
> 
> 
> 
>