How to set source address when sending using and UDP socket

Question

I\'ve two pc using VRRP for redundancy. So every PC (Linux) has a physical and a Virtual IP address.

I\'ve a software (C++) with a client/server architecture with UD

Answer 1

When the kernel needs to send something through a socket it performs these steps

• if the socket is bound, use that source address
• is the socket is not bound, it looks around for interfaces and selects a source address

So you need to bind(2) your socket to your desired address. For more information: Source Address Selection.

Answer 2

i'm not sure i understand entirely your question, but in terms of writing C/C++ code on linux at low level, you can import the ip.h header from the linux kernel headers which gives you access to the low level IP packet structure. (UDP works on top of IP)

#include <linux/ip.h>

and then look at struct iphdr which is the header for every IP packet sent and that contains a saddr member which you can set programmatically to be the source address.