There are some other fun edge cases - for example: - If you have a port-forward ... | Hacker News

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richm44 on Aug 18, 2016 | parent | context | favorite | on: TCP Puzzlers

There are some other fun edge cases - for example:

- If you have a port-forward to a machine that is switched off then you can get ICMP network unreachable or ICMP host unreachable as the response to the a SYN in the initial handshake.

This can also happen at any point in the connection. Other ICMP messages can also occur like this (eg. admin prohibited).

It's always worth remembering that the TCP connection is sitting on an underlying network stack that can also signal errors outside of the TCP protocol itself.

rdtsc on Aug 19, 2016 [–]

Ooh fun! I just learned something new. I knew about how blocking ICMP could lead to MTU path discovery issues, but this is different.

Do you know at the socket API level how the ICMP unreachagle is manifested. Looked at connect() error and saw ENETUNREACH -- guessing that's the one?

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