Add a DNS server which always responds as its own IP addresses.
Additionally add a tsnet TailscaleIPs() function to return the
IP addresses, both IPv4 and IPv6.
Updates https://github.com/tailscale/tailscale/issues/1748
Signed-off-by: Denton Gentry <dgentry@tailscale.com>
No ListenPacket support yet, but Listen with a udp network type fit
easier into netstack's model to start.
Then added an example of using it to cmd/sniproxy with a little udp
:53 handler.
No tests in tsnet yet because we don't have support for dialing over
UDP in tsnet yet. When that's done, a new test can test both sides.
Updates #5871
Updates #1748
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
