[PATCH pve-container] fix #7571: emit mtu in ifupdown-based LXC guests

[PATCH pve-container] fix #7571: emit mtu in ifupdown-based LXC guests

[Ilan Karasik]

From: iamkarasik <ilankarasik@gmail.com>

Signed-off-by: iamkarasik <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});
                 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

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

[Fabian Grünbichler]

Hi!

please see https://pve.proxmox.com/wiki/Developer_Documentation#Software_License_and_Copyright
for our CLA requirements, and send a signed copy as instructed there.

On May 7, 2026 1:00 am, Ilan Karasik wrote:
> From: iamkarasik <ilankarasik@gmail.com>

please add a short description of the commit/patch here, see our
developer documentation for details:

> Do not put in a literal description of the code diff, especially not
> for simpler changes. The commit message should not focus on the
> detailed what's done/changed but rather answer:
>
>    Why this was done otherwise previously (or why this was missing
>    until now).
>
>    Why the new approach is better or necessary (e.g., a
>    bug was found, requirements changed, a prerequisite was merged that
>    enables this).
>
>    What the resulting effect is for users or developers
>    (if not already obvious from the subject line). If there should be
>    no visible/semantic change then explicitly stating so can also be
>    good for non-trivial changes.

Thanks!

> 
> Signed-off-by: iamkarasik <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});
>                  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
> 
> 
> 
> 
> 

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

[Ilan Karasik]

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});
                 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