CODEWITHSUNDEEP
c++linuxsocketsudp
Napoleone1981
Napoleone1981

Reputation: 117

How to set source address when sending using and UDP socket

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 UDP protocol. The software bind the listen socket on "0.0.0.0" and use a new socket every time it needs to send some data to the other party. With wireshark I saw that when it sends data the source IP is the phisycal one... How can I set the source address of the sending socket to the Virtual one??

NOTE: Whit ifconfig I see only eth0 with the physical address...

Upvotes: 7

Views: 10806

Answers (2)

cnicutar
cnicutar

Reputation: 182794

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" in chapter "IP Routing" of "Guide to IP Layer Network Administration with Linux".

Upvotes: 9

Liv
Liv

Reputation: 6124

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.

Upvotes: 0

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