undefined | Better HN

0 pointsnathanappere14y ago0 comments

I believe this is incorrect. The server usually listen on one port, but everytime it does an accept, a different random port is used, and the client start talking to the server on that new port.

0 comments

caf14y ago

This is a surprisingly common misconception. When you accept, you get a new socket. but it is on the same local port. You can readily see this by running 'netstat' on a busy server.

ay14y ago

Another way to see that there are no magic "dynamically selected" potrs is to run on the server "tcpdump src x.x.x.x or dst x.x.x.x" where the x.x.x.x is your client address - and check the outputs - the packets will have ports in them.

Alternatively one can read RFC793 or, better, Stevens' "TCP/IP Illustrated".

nathanappereOP14y ago

Just checked it and you're right. I believed I had encoutered the behaviour I described but I do not remember in which context. Anyway glad I learned something!

dsl14y ago

FTP.

pavelkaroukin14y ago

I believe this comes from how FTP protocol works which do it to some extent and the reason why it do not work trough firewalls well without using passive mode.

tybris14y ago

By that logic, you would have to open every single port on your firewall if you wanted to set up a web server. Fortunately, your OS just looks at the client IP/port to distinguish multiple TCP streams.

j / k navigate · click thread line to collapse