Convert source IP address from struct iphdr* to string equivalent using Linux netfilter

Question

I want to convert the source & destination IP addresses from a packet captured using netfilter to char *.

In my netfilter hook function, I have:

sock_buff = skb; // argument 2 of hook function

// ip_header is struct iphdr*
ip_header = (struct iphdr *)skb_network_header(sock_buff);

// now how to convert ip_header->saddr & ip_header->daddr to char *
// ip_header->saddr & ip_header->daddr are of type __be32

Thanks.

Answer 1

The kernel's family of printf() functions has a special format specifier for IP-addresses (%pI4 for IPv4-addresses, %pI6 for IPv6).

So with IPv4, you could use something like:

char source[16];
snprintf(source, 16, "%pI4", &ip_header->saddr); // Mind the &!

Or write to dynamically allocated memory.

If you simply want to print debug-output, you can also use printk(). For the many other features of %p, see this document.

Answer 2

Try in4_pton() function in net/core/utils.c (definition: https://elixir.bootlin.com/linux/latest/source/net/core/utils.c#L118)

#include <linux/inet.h>

char source[16];
in4_pton(source, -1, &ip_header->saddr, '\0', NULL);