TCPDump: display MP_CAPABLE C flag

[matttbe]

Description

As described in the RFC, the C flag is an important info to display:

The third bit, labeled "C", is set to 1 to indicate that the sender of this option will not accept additional MPTCP subflows to the source address and port, and therefore the receiver MUST NOT try to open any additional subflows toward this address and port. This improves efficiency in situations where the sender knows a restriction is in place -- for example, if the sender is behind a strict NAT or operating behind a legacy Layer 4 load balancer.

But for the moment, it is not.

Solution

I guess just showing [C] is too vague, maybe deny-join?

Considered alternatives

Note that Wireshark and ptcpdump displays the info correctly.

Additional context

Thanks to @majek for having reported this.

[matttbe]

@majek: I just noticed TCPDump does support the MP_CAPABLE flags since... 2021!

But... this feature is only available in the master branch, not in the last v4.99 stable version! (and it has not been backported to a v4.99 version)!

I might check later to see if it is possible to have that backported.