"wg ping" primitive for implementing prioritized endpoint lists?

[Adam Joseph]

Hi, thanks so much for wireguard, it is awesome.

Is there any way to test if a peer is reachable at a particular ip:port
endpoint, without disturbing an existing, working exchange of packets
with that peer at some other ip:port?  Sort of like a "wireguard ping"
that avoids modifying the kernel's wg_peer.endpoint field the way "wg
set peer endpoint; /bin/ping" would.

The use case here is a mobile client like a laptop that knows how to
reach a server via either a preferred LAN IP (when the laptop is on the
LAN) or via a less-desirable public internet address for the server.
In a perfect world packets from the LAN to the server's public internet
address wouldn't be trombone-routed.  Sadly, in real life they usually
are -- for reasons beyond the control of the endpoints' administrators.

With a primitive "wg ping" it would be possible to write a userspace
tool to opportunistically upgrade a peer to a more-preferred endpoint
or fall back to a less-preferred endpoint when connectivity is lost.
More complex policies are possible too, along with improvements on
the wireguard-tools/contrib/nat-hole-punching tool.

In terms of implementation, one possibility is to allow an ICMP socket
to mark its outbound packets "hey wireguard, if you find yourself
encrypting this packet, after doing so send the resulting encrypted
packet [and MESSAGE_HANDSHAKE_INITIATION it might provoke] to $IP:$PORT
regardless of wg_peer.endpoint."  If the peer does not currently have a
valid handshake (e.g. the timer just expired) then the ping will
provoke a MESSAGE_HANDSHAKE_INITIATION, and the peer's
MESSAGE_HANDSHAKE_RESPONSE will cause
wg_socket_set_peer_endpoint_from_skb() to be called, which will update
wg_peer.endpoint.  It would be nice if there were a way to prevent
this, but I don't see any easy way to accomplish it.

  - a