[PATCH v3] fix #7571: emit mtu in ifupdown-based LXC guests

[Ilan Karasik <ilankarasik@gmail.com>]

From: iamkarasik <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. This
configuration applies to static, dhcp, and manual configurations.

Signed-off-by: Ilan Karasik <ilankarasik@gmail.com>
---
 src/PVE/LXC/Setup/Debian.pm | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/PVE/LXC/Setup/Debian.pm b/src/PVE/LXC/Setup/Debian.pm
index 837397b..fd4a7d4 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;
         }
     }
@@ -298,6 +299,7 @@ sub setup_network {
     my $done_auto = {};
     my $done_v4_hash = {};
     my $done_v6_hash = {};
+    my $done_mtu_hash = {};
 
     my ($os, $version) = ($conf->{ostype}, $self->{version});
     my $print_section = sub {
@@ -317,7 +319,12 @@ sub setup_network {
             if (!defined($net->{address})) {
                 # no address => no iface line
             } elsif ($net->{address} =~ /^(dhcp|manual)$/) {
-                $interfaces .= "iface $ifname inet $1\n\n";
+                $interfaces .= "iface $ifname inet $1\n";
+                if (defined($net->{mtu}) && !$done_mtu_hash->{$ifname}) {
+                    $interfaces .= "\tmtu $net->{mtu}\n";
+                    $done_mtu_hash->{$ifname} = 1;
+                }
+                $interfaces .= "\n";
             } else {
                 $interfaces .= "iface $ifname inet static\n";
                 if (
@@ -329,6 +336,10 @@ sub setup_network {
                     $interfaces .= "\taddress $net->{address}\n" if defined($net->{address});
                     $interfaces .= "\tnetmask $net->{netmask}\n" if defined($net->{netmask});
                 }
+                if (defined($net->{mtu}) && !$done_mtu_hash->{$ifname}) {
+                    $interfaces .= "\tmtu $net->{mtu}\n";
+                    $done_mtu_hash->{$ifname} = 1;
+                }
                 remove_gateway_scripts($section->{attr});
                 if (defined(my $gw = $net->{gateway})) {
                     if ($net->{needsroute}) {
@@ -348,7 +359,12 @@ sub setup_network {
             if (!defined($net->{address6})) {
                 # no address => no iface line
             } elsif ($net->{address6} =~ /^(auto|dhcp|manual)$/) {
-                $interfaces .= "iface $ifname inet6 $1\n\n";
+                $interfaces .= "iface $ifname inet6 $1\n";
+                if (defined($net->{mtu}) && !$done_mtu_hash->{$ifname}) {
+                    $interfaces .= "\tmtu $net->{mtu}\n";
+                    $done_mtu_hash->{$ifname} = 1;
+                }
+                $interfaces .= "\n";
             } else {
                 $interfaces .= "iface $ifname inet6 static\n";
                 if (
@@ -360,6 +376,10 @@ sub setup_network {
                     $interfaces .= "\taddress $net->{address6}\n" if defined($net->{address6});
                     $interfaces .= "\tnetmask $net->{netmask6}\n" if defined($net->{netmask6});
                 }
+                if (defined($net->{mtu}) && !$done_mtu_hash->{$ifname}) {
+                    $interfaces .= "\tmtu $net->{mtu}\n";
+                    $done_mtu_hash->{$ifname} = 1;
+                }
                 remove_gateway_scripts($section->{attr});
                 if (defined(my $gw = $net->{gateway6})) {
                     if ($net->{needsroute6}) {
@@ -450,6 +470,7 @@ sub setup_network {
                     || $aname eq 'netmask'
                     || $aname eq 'gateway'
                     || $aname eq 'broadcast'
+                    || $aname eq 'mtu'
                 ) {
                     # skip
                 } else {
-- 
2.51.2