Use socket connections for TCP tracing

[fujiapple852]

Current approach

The current approach for tcp probing is to construct a TcpPacket and then send_to it over channel constructed with:

TransportChannelType::Layer4(TransportProtocol::Ipv4(IpNextHeaderProtocols::Tcp))

Which is effectively:

socket(AF_INET, SOCK_RAW, 6)

Problem

With this setup, response packets (syn / rst) are never delivered to the socket as, presumably, we have never called bind or connect. We're essentially sending a tcp packet. as a datagram.

Whilst this works to a point (we can receive the icmp TimeExceeded responses to get the bulk of the trace), we don't receive syn / rst / ack and so we don't ever get the response from the target.

Solution

The solution is to create a normal (not raw socket) TCP connection to the target.

The tool mtr takes a similar approach, see construct_ip4_packet() and open_stream_socket() in construct_unix.c where it goes through a normal socket -> bind -> connect setup.

See also receive_replies_from_probe_socket() in probe_unix.c for where the the per-probe TCP socket is checked using select and a successful connection or ECONNREFUSED is used to indicate that the target has responded.