This document explains how to operate a WireGuard server behind a firewall (e.g. pfSense) when the ISP assigns a dynamic IPv6 prefix (for example /56).

The solution publishes the current global IPv6 address (GUA) of the WireGuard server via the OVH DNS API as an AAAA record. Clients connect via FQDN, so prefix changes are handled automatically.

Audience: beginners + admins Goal: stable endpoint vpn.example.org WireGuard port: 51820/udp (standard)

Replace these with real values:

• RFC1918 IPv4:
  • 192.168.0.0/16
  • 192.168.1.0/24
• IPv6 ULA:
  • fdXX:YYYY:ZZZZ::/48
  • fdXX:YYYY:ZZZZ:40::/64
• Public hostname:
  • vpn.example.org
• WireGuard server:
  • IPv4: 192.168.1.10
  • IPv6: fdXX:YYYY:ZZZZ:5::10
• WireGuard interface:
  • wg0
• WAN-facing NIC on WG host:
  • ens18

Your firewall (OPNSense/PfSense or whatever) performs PPPoE and receives:

• a dynamic IPv4 address
• a dynamic IPv6 via DHCPv6
• a dynamic IPv6 delegated prefix (for example /56)

The WireGuard server is placed in a DMZ or internal VLAN and has:

• RFC1918 IPv4
• IPv6 ULA
• a public IPv6 GUA (derived from the delegated prefix)

Because the prefix changes, we:

1. detect the current GUA on the WireGuard server
2. update an AAAA record via OVH API
3. clients always connect via DNS name

No NAT66, no hacks, no manual updates.

Single AAAA record:

vpn.example.org.  AAAA  <current-GUA>

Recommended TTL: 300 seconds

This allows fast updates and stable WireGuard endpoints.

Create a dedicated OVH API token.

Minimal permissions for zone example.org:

• GET /domain/zone/example.org/record
• GET /domain/zone/example.org/record/*
• POST /domain/zone/example.org/record
• PUT /domain/zone/example.org/record/*
• POST /domain/zone/example.org/refresh

Nothing else is required.

apt update
apt install -y wireguard ufw python3 python3-venv ca-certificates

Default policy:

ufw default deny incoming
ufw default allow outgoing

Allow WireGuard (public):

ufw allow 51820/udp comment 'WireGuard Server'

Allow SSH only from internal networks:

ufw allow from 192.168.0.0/16 to any port 22 proto tcp comment 'SSH IPv4 internal'
ufw allow from fdXX:YYYY:ZZZZ::/48 to any port 22 proto tcp comment 'SSH IPv6 internal'
ufw allow route on wg0 from 192.168.0.0/16 to 192.168.0.0/16 comment 'Wireguard v4 forward'
ufw allow route on wg0 from fdXX:YYYY:ZZZZ::/48 to fdXX:YYYY:ZZZZ::/48 comment 'Wireguard v6 forward'

Note: Adjust other rules to your environment

Enable firewall:

ufw enable
ufw status numbered

File: /usr/local/sbin/fetch-gua.sh

#!/usr/bin/env bash
set -euo pipefail
 
IFACE="${IFACE:-ens18}"
 
ip -6 addr show dev "$IFACE" \
 | awk '/inet6/ && /scope global/ && !/temporary/ {print $2}' \
 | cut -d/ -f1 \
 | grep -Ev '^(fc|fd)' \
 | head -n1

Permissions:

chmod 0755 /usr/local/sbin/fetch-gua.sh

Test:

IFACE=ens18 /usr/local/sbin/fetch-gua.sh

/opt/ovh-dyndns-v6/
 ├── venv/
 └── update_aaaa.py
/etc/ovh-dyndns-v6.conf
/usr/local/sbin/ovh-dyndns-v6.sh

python3 -m venv /opt/ovh-dyndns-v6/venv
/opt/ovh-dyndns-v6/venv/bin/pip install --upgrade pip
/opt/ovh-dyndns-v6/venv/bin/pip install ovh

Test:

/opt/ovh-dyndns-v6/venv/bin/python -c 'import ovh; print("ovh ok")'

File: /etc/ovh-dyndns-v6.conf

OVH_ENDPOINT="ovh-eu"
OVH_APP_KEY="REPLACE_ME"
OVH_APP_SECRET="REPLACE_ME"
OVH_CONSUMER_KEY="REPLACE_ME"

OVH_ZONE="example.org"
RECORD_FQDN="vpn.example.org"
TTL="300"

IFACE="ens18"

Permissions:

chmod 600 /etc/ovh-dyndns-v6.conf

File: /opt/ovh-dyndns-v6/update_aaaa.py

#!/usr/bin/env python3
import os, sys, ovh
 
def env(k):
    v = os.getenv(k)
    if not v:
        sys.exit(f"missing env var {k}")
    return v
 
client = ovh.Client(
    endpoint=env("OVH_ENDPOINT"),
    application_key=env("OVH_APP_KEY"),
    application_secret=env("OVH_APP_SECRET"),
    consumer_key=env("OVH_CONSUMER_KEY"),
)
 
zone = env("OVH_ZONE")
fqdn = env("RECORD_FQDN")
gua  = env("GUA")
ttl  = int(env("TTL"))
 
sub = "" if fqdn == zone else fqdn.replace("." + zone, "")
ids = client.get(f"/domain/zone/{zone}/record", fieldType="AAAA", subDomain=sub)
 
if ids:
    rec = client.get(f"/domain/zone/{zone}/record/{ids[0]}")
    if rec["target"] == gua:
        print("no change")
        sys.exit(0)
    client.put(f"/domain/zone/{zone}/record/{ids[0]}", target=gua, ttl=ttl)
    client.post(f"/domain/zone/{zone}/refresh")
    print(f"updated: {gua}")
else:
    client.post(f"/domain/zone/{zone}/record",
                fieldType="AAAA", subDomain=sub, target=gua, ttl=ttl)
    client.post(f"/domain/zone/{zone}/refresh")
    print(f"created: {gua}")

File: /usr/local/sbin/ovh-dyndns-v6.sh

#!/usr/bin/env bash
set -euo pipefail
 
source /etc/ovh-dyndns-v6.conf
 
GUA="$(IFACE="$IFACE" /usr/local/sbin/fetch-gua.sh)"
 
export OVH_ENDPOINT OVH_APP_KEY OVH_APP_SECRET OVH_CONSUMER_KEY
export OVH_ZONE RECORD_FQDN TTL GUA
 
OUT="$(/opt/ovh-dyndns-v6/venv/bin/python /opt/ovh-dyndns-v6/update_aaaa.py)"
 
logger -t ovh-dyndns-v6 "$OUT fqdn=$RECORD_FQDN gua=$GUA ts=$(date -Is)"
echo "$OUT"

Service: /etc/systemd/system/ovh-dyndns-v6.service

[Unit]
Description=OVH DynDNS IPv6 updater
After=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/local/sbin/ovh-dyndns-v6.sh

Timer: /etc/systemd/system/ovh-dyndns-v6.timer

[Unit]
Description=Run OVH DynDNS every 5 minutes

[Timer]
OnBootSec=2min
OnUnitActiveSec=5min
Persistent=true

[Install]
WantedBy=timers.target

Enable:

systemctl daemon-reload
systemctl enable --now ovh-dyndns-v6.timer

Logs:

journalctl -u ovh-dyndns-v6.service
journalctl -t ovh-dyndns-v6

DNS:

dig +short AAAA vpn.example.org

UDP scan (expected open|filtered):

nmap -6 -sU -p51820 vpn.example.org

WireGuard proof:

wg show

Look for recent handshake.

This setup provides a stable IPv6 WireGuard endpoint with a dynamic ISP prefix. The DNS AAAA record is automatically updated via OVH API and systemd timer.

Result: IPv6 works as intended.

want to create a reverse v6/v4 fallback endpoint? The following article might be of interest for you.