Allow WireGuard to listen on port 53

config.cfg

@@ -32,6 +32,9 @@ strongswan_network: 10.19.48.0/24
 strongswan_network_ipv6: 'fd9d:bc11:4020::/48'
 
 # Deploy WireGuard
+# WireGuard will listen on 51820/UDP. You might need to change to another port
+# if your network blocks this one. Be aware that 53/UDP (DNS) is blocked on some
+# mobile data networks.
 wireguard_enabled: true
 wireguard_port: 51820
 # If you're behind NAT or a firewall and you want to receive incoming connections long after network traffic has gone silent.

roles/common/templates/rules.v4.j2

@@ -1,5 +1,5 @@
 {% set subnets = ([strongswan_network] if ipsec_enabled else []) + ([wireguard_network_ipv4] if wireguard_enabled else []) %}
-{% set ports = (['500', '4500'] if ipsec_enabled else []) + ([wireguard_port] if wireguard_enabled else []) %}
+{% set ports = (['500', '4500'] if ipsec_enabled else []) + ([wireguard_port] if wireguard_enabled else []) + ([wireguard_port_actual] if wireguard_enabled and wireguard_port|int == wireguard_port_avoid|int else []) %}
 
 #### The mangle table
 # This table allows us to modify packet headers
@@ -29,6 +29,11 @@ COMMIT
 :PREROUTING ACCEPT [0:0]
 :POSTROUTING ACCEPT [0:0]
 
+{% if wireguard_enabled and wireguard_port|int == wireguard_port_avoid|int %}
+# Handle the special case of allowing access to WireGuard over an already used
+# port like 53
+-A PREROUTING --in-interface {{ ansible_default_ipv4['interface'] }} -p udp --dport {{ wireguard_port_avoid }} -j REDIRECT --to-port {{ wireguard_port_actual }}
+{% endif %}
 # Allow traffic from the VPN network to the outside world, and replies
 -A POSTROUTING -s {{ subnets|join(',') }} -m policy --pol none --dir out -j MASQUERADE
 

roles/common/templates/rules.v6.j2

@@ -1,5 +1,5 @@
 {% set subnets = ([strongswan_network_ipv6] if ipsec_enabled else []) + ([wireguard_network_ipv6] if wireguard_enabled else []) %}
-{% set ports = (['500', '4500'] if ipsec_enabled else []) + ([wireguard_port] if wireguard_enabled else []) %}
+{% set ports = (['500', '4500'] if ipsec_enabled else []) + ([wireguard_port] if wireguard_enabled else []) + ([wireguard_port_actual] if wireguard_enabled and wireguard_port|int == wireguard_port_avoid|int else []) %}
 
 #### The mangle table
 # This table allows us to modify packet headers
@@ -28,6 +28,11 @@ COMMIT
 :PREROUTING ACCEPT [0:0]
 :POSTROUTING ACCEPT [0:0]
 
+{% if wireguard_enabled and wireguard_port|int == wireguard_port_avoid|int %}
+# Handle the special case of allowing access to WireGuard over an already used
+# port like 53
+-A PREROUTING --in-interface {{ ansible_default_ipv6['interface'] }} -p udp --dport {{ wireguard_port_avoid }} -j REDIRECT --to-port {{ wireguard_port_actual }}
+{% endif %}
 # Allow traffic from the VPN network to the outside world, and replies
 -A POSTROUTING -s {{ subnets|join(',') }} -m policy --pol none --dir out -j MASQUERADE
 

roles/wireguard/defaults/main.yml

@@ -3,6 +3,8 @@ wireguard_PersistentKeepalive: 0
 wireguard_config_path: "configs/{{ IP_subject_alt_name }}/wireguard/"
 wireguard_pki_path: "{{ wireguard_config_path }}/.pki/"
 wireguard_interface: wg0
+wireguard_port_avoid: 53
+wireguard_port_actual: 51820
 keys_clean_all: false
 wireguard_dns_servers: >-
   {% if algo_dns_adblocking|default(false)|bool or dns_encryption|default(false)|bool %}

roles/wireguard/templates/server.conf.j2

@@ -1,6 +1,6 @@
 [Interface]
 Address = {{ wireguard_server_ip }}
-ListenPort = {{ wireguard_port }}
+ListenPort = {{ wireguard_port_actual if wireguard_port|int == wireguard_port_avoid|int else wireguard_port }}
 PrivateKey = {{ lookup('file', wireguard_pki_path + '/private/' + IP_subject_alt_name) }}
 SaveConfig = false