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

[Ilan Karasik <ilankarasik@gmail.com>]

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