[pve-devel] [PATCH pve-network 1/1] fix #5949: avoid dnsmasq segfault when subnet has no gateway

[Fiona Ebner]

Am 26.11.25 um 4:49 PM schrieb Stefan Hanreich:
> When trying to start a guest with a network device in a VNet in a
> simple zone that has DHCP enabled - but no gateway configured - a
> SIGSEGV is triggered in dnsmasq. This seems to be an error in the
> dnsmasq dbus handler that tries to allocate a lease, which fails
> because there is no dhcp-range configured, and then leads to a
> segfault.
> 
> Avoid the situation completely by always configuring a dhcp-range,
> even if there is no gateway configured. Skip configuring the DHCP
> option that returns the router instead, which is the only place in the
> configure_subnet function that uses the gateway.
> 
> Dnsmasq is configured to listen on an interface, so any DHCP messages
> that are received on this interface are dropped, because dnsmasq
> recognizes that there is no IP configured on this interface:
> 
> Nov 26 16:35:49 ipam-test dnsmasq-dhcp[140272]: DHCP packet received on vnet1 which has no address
> Nov 26 16:35:57 ipam-test dnsmasq-dhcp[140272]: DHCP packet received on vnet1 which has no address
> 
> An initial upstream patch to fix the segfault has been submitted here
> [1].
> 
> [1] https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2025q4/018342.html
> 
> Signed-off-by: Stefan Hanreich <s.hanreich at proxmox.com>
> ---
>  src/PVE/Network/SDN/Dhcp/Dnsmasq.pm | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/src/PVE/Network/SDN/Dhcp/Dnsmasq.pm b/src/PVE/Network/SDN/Dhcp/Dnsmasq.pm
> index db22e12..6a1e3b0 100644
> --- a/src/PVE/Network/SDN/Dhcp/Dnsmasq.pm
> +++ b/src/PVE/Network/SDN/Dhcp/Dnsmasq.pm
> @@ -138,9 +138,6 @@ sub add_ip_mapping {
>  sub configure_subnet {
>      my ($class, $config, $dhcpid, $vnetid, $subnet_config) = @_;
>  
> -    die "No gateway defined for subnet $subnet_config->{id}"
> -        if !$subnet_config->{gateway};
> -
>      my $tag = $subnet_config->{id};
>  
>      my ($zone, $network, $mask) = split(/-/, $tag);

1. How "ugly" do you consider the workaround here? How much better than
the segfault from a user perspective? Would it be nicer to go back to
the old behavior with the clear error in the future once the dnsmasq fix
is in downstream? If yes, we should add a reminder comment to do so.

2. What is the situation for IPv6 when there is no gateway? Would it be
worth to keep the early die with the explicit error in the IPv6 case?

> @@ -155,7 +152,7 @@ sub configure_subnet {
>      my $option_string;
>      if (ip_is_ipv6($subnet_config->{network})) {
>          $option_string = 'option6';
> -    } else {
> +    } elsif ($subnet_config->{gateway}) {
>          $option_string = 'option';
>          push @{$config}, "dhcp-option=tag:$tag,$option_string:router,$subnet_config->{gateway}";
>      }