Monday, April 7, 2014

tcp client bind() to a non-random port

A Barclays TCP interview asked

Q: When a tcp client runs connect(), can it specify a client-side port rather than using a random port assigned by the system?
A: use bind() -- http://stackoverflow.com/questions/4118241/what-client-side-situations-need-bind

Motivation?
* I feel the client port number can work like a rudimentary tag for a "special" client thread
* similarly, debugging -- http://stackoverflow.com/questions/347636/determining-the-tcp-port-number-to-which-client-got-bound
* firewall filtering on client port -- http://stackoverflow.com/questions/4118241/what-client-side-situations-need-bind
* some servers expect client to use a low port -- http://stackoverflow.com/questions/4118241/what-client-side-situations-need-bind

Note client bind() can also specify a particular client ip address (multihoming). Client side bind() defines the local port and interface address for the connection. This connect() does an implicit bind("0.0.0.0", 0) if one has not been done previously (with zero being taken as "any"). See http://stackoverflow.com/questions/12763268/why-is-bind-used-in-tcp-why-is-it-used-only-on-server-side-and-not-in-client.

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New York (Time Square), NY, United States
http://www.linkedin.com/in/tanbin