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

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

[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@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}";
     }
-- 
2.47.3


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Re: [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@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}";
>      }



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