sth...@nethelp.no wrote:

Well, "random" was probably misleading, I'm sorry. It should better be called "arbitrary", I think.

It _is_ predictable, it is the first address currently configured on the interface. But doing so is (was) an arbitrary decision.

Of course, if you remove the first address, it will simply use the next one (which will then become the first one).

On the other hand, if you need to guarantee that a certain address is used as source IP for outgoing connections, then you should explicitly bind the socket to that address. Many programs have an option to do that, or -- if they don't -- it's usually not too difficult to insert a bind(2) call into the source yourself. Another way to do it is to run the program inside a jail; you don't even have to set up a chroot if you don't want to: # jail / `hostname` $IP /path/to/program

I would advise against relying on the current behaviour that the kernel always picks the first address as the source address for a subnet for unbound sockets.

Best regards Oliver

"And believe me, as a C++ programmer, I don't hesitate to question the decisions of language designers. After a decent amount of C++ exposure, Python's flaws seem ridiculously small." -- Ville Vainio