Wireguard Neighborhood (IPv6)

[tlhackque]

TLDR; It seems that WireGuard isn't supporting IPv6 NDP, and it should. 
Use case & a work-around.

Full story:

Configuration:

WireGuard server (Linux, details below) behind a site router that
handles IPv4 NAT & an IPv6 tunnel.

Server LAN has other hosts (and multiple subnets/vlans) - mostly dual stack.

The WireGuard server is able to access the WireGuard peers (clients)
over IPv6.  The other hosts (and the router) are not.

The clients can't even ping the other hosts - the echo replies are
generated, but they end up with an icmp6 unreachable.

It turns out that the other hosts (and router) send an icmp6 Neighbor
Solicitation for the clients, which is never answered.

My interim solution was to implement
https://github.com/setaou/ndp-proxy, which will respond with Neighbor
Advertisements for the entire WireGuard subnet.

This is a rather crude solution - since ndp-proxy doesn't know what
clients are connected, and since it requires one proxy process/wg interface.

It seems to me that WireGuard (in this case on the server) should at
least be responding to Neighbor Solicitations for AllowedIPs of its
active peers... Of course in the case of a WireGuard tunnel between two
such sites, this is symmetric.

I did look at net.ipv6.conf.*.proxy_ndp, but that requires adding each
address - and in any case I couldn't get it to work.  Neither did
advertising the server as a "router" with radvd.

Unless I'm missing something, it seems to me that supporting NDP is the
simplest "it just works" approach in any case...

wireguard-tools v1.0.20210424 - https://git.zx2c4.com/wireguard-tools/

Linux hagrid 5.13.16-200.fc34.x86_64 #1 SMP Mon Sep 13 12:39:36 UTC 2021
x86_64 x86_64 x86_64 GNU/Linux


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