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

[Stefan Hanreich]

the mentioned fix has been upstreamed:

https://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commit;h=eb601683820723df89858cfa695aa131012f1a63

On 11/26/25 4:48 PM, Stefan Hanreich wrote:
> 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);
> @@ -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}";
>      }