Re: [PATCH container] lxc: debian: avoid 'inet6 auto' with debian's ifupdown2

[Hannes Laimer <h.laimer@proxmox.com>]

lgtm!

Reviewed-by: Hannes Laimer <h.laimer@proxmox.com>

On 2026-09-11 11:56, Gabriel Goller wrote:
> Debian's ifupdown2 (3.0.0-1.3 in bookworm and trixie) does not support
> SLAAC and errors on the 'inet6 auto' stanza (Our ifupdown2 version has
> SLAAC support though).
> 
> Fall back to 'manual' if the container has an ifupdown2 version without
> the 'auto' addon. SLAAC still works there, because the kernel manages
> everything on its own with the default accept_ra and autoconf sysctls.
> This is the same workaround we already use for Alpine, whose busybox
> ifupdown has no 'auto' method either.
> 
> Reported-by: https://forum.proxmox.com/threads/debian-13-lxc-template-ipv4-dhcp-ipv6-slaac-launches-dhcpv6-and-breaks-backups.186269/
> Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
> ---
>  src/PVE/LXC/Setup/Debian.pm | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/src/PVE/LXC/Setup/Debian.pm b/src/PVE/LXC/Setup/Debian.pm
> index 837397bb3ecd..eafca59cfefe 100644
> --- a/src/PVE/LXC/Setup/Debian.pm
> +++ b/src/PVE/LXC/Setup/Debian.pm
> @@ -172,6 +172,21 @@ sub snakeoil_fixup {
>      }
>  }
>  
> +=head3 ifupdown2_has_slaac($self)
> +
> +Checks if the ifupdown2 installation has SLAAC support. Debian's version of
> +ifupdown2 currently doesn't, our version does.
> +
> +=cut
> +
> +sub ifupdown2_has_slaac {
> +    my ($self) = @_;
> +
> +    my $addon_dir = '/usr/share/ifupdown2/addons';
> +
> +    return $self->ct_is_directory($addon_dir) && $self->ct_file_exists("$addon_dir/auto.py");
> +}
> +
>  sub remove_gateway_scripts {
>      my ($attr) = @_;
>      my $length = scalar(@$attr);
> @@ -300,6 +315,7 @@ sub setup_network {
>      my $done_v6_hash = {};
>  
>      my ($os, $version) = ($conf->{ostype}, $self->{version});
> +    my $slaac = ifupdown2_has_slaac($self);
>      my $print_section = sub {
>          return if !$section;
>  
> @@ -348,7 +364,13 @@ 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";
> +                my $method = $1;
> +                # the kernel does SLAAC on its own with the default sysctls, so
> +                # 'manual' is the closest we can get without a working 'auto'
> +                if ($method eq 'auto' && !$slaac) {
> +                    $method = 'manual';
> +                }
> +                $interfaces .= "iface $ifname inet6 $method\n\n";
>              } else {
>                  $interfaces .= "iface $ifname inet6 static\n";
>                  if (