UNIX Socket FAQ

A forum for questions and answers about network programming on Linux and all other Unix-like systems

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#1 2002-07-26 11:10 PM

HectorLasso
Administrator
From: Colombia
Registered: 2002-06-12
Posts: 353

Re: 4.8 How can I bind() to a port number < 1024?

6
From Andrew Gierth (andrew@erlenstar.demon.co.uk):

The restriction on access to ports < 1024 is part of a (fairly weak) security scheme particular to UNIX. The intention is that servers (for example rlogind, rshd) can check the port number of the client, and if it is < 1024, assume the request has been properly authorised at the client end.

The practical upshot of this, is that binding a port number < 1024 is reserved to processes having an effective UID == root.

This can, occasionally, itself present a security problem, e.g. when a server process needs to bind a well-known port, but does not itself need root access (news servers, for example). This is often solved by creating a small program which simply binds the socket, then restores the real userid and exec()s the real server. This program can then be made setuid root.

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