Route IPv6 through WireGuard VPN via radvd and static gateway

Add radvd to gateway role to advertise Pi as high-preference IPv6
default router using the stable ULA prefix (fd1e:.../64). With
FritzBox also sending RAs, devices end up with ECMP between Pi and
FritzBox. To solve this, add network_ipv6_gateway (Pi's link-local)
as a static route with metric 100 to all managed hosts — beats RA
metric 425, ensuring all IPv6 default traffic goes through Pi.

Fix IPv6 MASQUERADE in gateway-apply-rules:
- Direct mode: add MASQUERADE on end0 (LAN devices use ULA source
  addresses not known to FritzBox, so Pi must NAT them)
- FORWARD rules: restrict to RELATED,ESTABLISHED only — previously
  the broad ACCEPT rule passed un-NAT'd packets alongside masqueraded
  copies, causing duplicate SYNs, conntrack corruption, and RSTs
- MASQUERADE/clear rules: match by interface not by source subnet
  (devices may use any source address, not just the ULA prefix)
- VPN mode return traffic: explicitly restrict to wg+→end0 direction

Add network_ipv6_gateway var (optional) to network role NM templates
(ethernet, wifi, bridge) — injects a static IPv6 default route at
metric 100 when set. Add rpi5 static route to FritzBox link-local so
Pi keeps IPv6 after FritzBox RA is disabled.

Force SSH to IPv4 for *.local hosts (AddressFamily inet) — prevents
Ansible from hanging on mDNS returning multiple IPv6 addresses.

Update gateway and pihole READMEs with two-step IPv6 setup process.

Co-Authored-By: Claude.ai

1 files changed, 14 insertions, 12 deletions

diff --git a/roles/gateway/README.md b/roles/gateway/README.md
index cfbe49d3..6c46cc9b 100644
--- a/roles/gateway/README.md
+++ b/roles/gateway/README.md
@@ -19,20 +19,22 @@ gateway_router_interface: "end0"
 
 ### IPv6 Support (Optional)
 
-IPv6 gateway is enabled by setting both `network_ipv6_address` and `gateway_local_ipv6_subnet`:
+IPv6 gateway is enabled by setting `gateway_local_ipv6_subnet` to the ULA prefix advertised by radvd:
 
 ```yaml
-network_ipv6_address: "2a02:xxxx:xxxx:xxxx::254/64"   # Pi's IPv6 address
-gateway_local_ipv6_subnet: "2a02:xxxx:xxxx:xxxx::/64" # LAN IPv6 subnet
+gateway_local_ipv6_subnet: "fd00:xxxx:xxxx:xxxx::/64"  # ULA prefix (stable, router-independent)
 ```
 
-To disable IPv6 entirely, remove both variables from `host_vars`. The role will:
-- Omit `accept_ra` sysctl (no IPv6 interface key written)
-- Skip IPv6 iptables masquerade rules (`gateway_local_ipv6_subnet` defaults to `""`)
-- Leave `net.ipv6.conf.all.forwarding=0`
+When set, this role:
+- Enables `net.ipv6.conf.all.forwarding=1` and `accept_ra=2` on `gateway_router_interface`
+- Deploys radvd advertising the ULA prefix with `AdvDefaultPreference high` — Pi wins as IPv6 default router
+- Adds ip6tables MASQUERADE/FORWARD rules matching by interface (not subnet) so all source addresses are handled
 
-**Note:** `accept_ra` is set per-interface (`gateway_router_interface`) only when IPv6 is enabled.
-This prevents rogue RA acceptance on WireGuard and LAN interfaces.
+**Prerequisite:** Disable the router's RA (FritzBox: Home Network > IPv6 > Router advertisement > ❌) after
+setting `network_ipv6_gateway` in `host_vars` so Pi keeps its static IPv6 default route to the router.
+See `roles/pihole/README.md` for the full two-step setup.
+
+To disable IPv6, leave `gateway_local_ipv6_subnet: ""` (default). The role skips all ip6tables rules and disables radvd.
 
 ## Network Architecture
 
@@ -63,9 +65,9 @@ Client Devices (192.168.1.0/24)
 
 **Direct Mode Characteristics:**
 
-- All traffic NAT'd through router interface (`end0`)
-- Direct IPv4/IPv6 routing to ISP
-- Standard internet routing
+- IPv4 traffic NAT'd through router interface (`end0`)
+- IPv6 forwarded without NAT (globally routable addresses, native routing)
+- Standard internet routing via ISP
 
 ## Requirements