dnsmasq: failed to create listening socket - Address already in use

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Environment

SUSE Linux Enterprise Server 15 SP6

Situation

A system that runs `named` and also hosts one or more libvirt virtual networks that use `dnsmasq` as a DHCP and DNS server. For example the default network provided by the libvirt-daemon-config-network package.

After upgrading `dnsmasq` to version 2.90 or newer, libvirt virtual networks using `dnsmasq` as a DNS server will fail to start with an "Address already in use" error. For example:

dnsmasq: failed to create listening socket for 192.168.122.1: Address already in use

Resolution

By default, `named` is configured to listen on all local IP addresses. When used in conjunction with libvirt virtual networks, it must be configured to skip addresses used by the virtual networks, allowing `dnsmasq` to listen on those addresses.

The `named` service supports the listen-on directive in /etc/named.conf, which can be used to exclude an address (192.168.122.1 in our example), or the entire network block used by virtual machines (e.g. 192.168.122.0/24 or even 192.168.0.0/16). The following examples illustrate these various levels of exclusion:

listen-on port 53 { !192.168.122.1;    any; };
listen-on port 53 { !192.168.122.0/24; any; };
listen-on port 53 { !192.168.0.0/16;   any; };

Cause

Older versions of `dnsmasq` silently ignored the listen failures, which could lead to confusing or incorrect behavior of name resolution for virtual machines. Version 2.90 now correctly reports an error when it fails to listen on specified addresses. The configuration of the `named` service must be adjusted to exclude addresses used by `dnsmasq`.

Disclaimer

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