On 23/03/2022 09:41, Konstantin Meskhidze wrote:

3/22/2022 4:24 PM, Mickaël Salaün пишет:

quoted

[...]

quoted

The remaining question is: should we need to accept 0 as a valid TCP 
port? Can it be used? How does the kernel handle it?

  I agree that must be a check for port 0 in add_rule_net_service(), 
cause unlike most port numbers, port 0 is a reserved port in TCP/IP 
networking, meaning that it should not be used in TCP or UDP messages.
Also network traffic sent across the internet to hosts listening on port 
0 might be generated from network attackers or accidentally by 
applications programmed incorrectly.
Source: https://www.lifewire.com/port-0-in-tcp-and-udp-818145

OK, so denying this port by default without a way to allow it should not 
be an issue. I guess an -EINVAL error would make sense when trying to 
allow this port. This should be documented in a comment (with a link to 
the RFC/section) and a dedicated test should check that behavior.

What is the behavior of firewalls (e.g. Netfiler) when trying to filter 
port 0?

This doesn't seem to be settle though: 
https://www.austingroupbugs.net/view.php?id=1068

Interesting article: 
https://z3r0trust.medium.com/socket-programming-the-bizarre-tcp-ip-port-0-saga-fcfbc0e0a276