Properly send ICMP

[LeeSmet]

This is a bit difficult to do, but in general we should be able to send some decent ICMP packets to the tun interface if something goes wrong. This includes no route to host in case we try to send to some unknown address.

[LeeSmet]

The main thing we want to do here is No route to host and TTL expired. These need special handling, since they are generated by hosts which act as hops usually. Icmp defines that (part of) the original packet is included in the icmp body, but since that packet is encrypted, the intermediate can't send the actual data. Instead it could send the encrypted data, and then the receiver needs to decrypt the data and reassemble the packet.

A basic test shows that TTL based tools such as traceroute/tracepath can work partially by not setting a body and injecting the packet in the regular L3 stream. The problem is that intermediate hops are not properly shown. For this, we will probably need to implement the solution described above.

[LeeSmet]

Currently traceroute works out of the box, though the implementation needs to be cleaned up a lot (right now it's hacked in there as a POC). The main problem is we can't truncate packets which are too large, so in this case we'll have to return just the ICMP with no data. this is not according to spec, but the best we can do as we can't access the raw intermediate data.

[LeeSmet]

Current state: mechanism to support this is done, Time exceeded is implemented. No route to host is still left to do