Question

I am writing a kernel module which registers a netfilter hook. I am trying to get the ip address of the caller by using the sk_buff->saddr member. Is there a way I can get the IP in human readable i.e. x.x.x.x format?

I found the function inet_ntop() but it doesn’t seem to be available in kernel headers. How do I convert \xC0\xA8\x00\x01 to 192.168.0.1 ?

Answer 1

There are two macros defined in include/linux/kernel.h

NIPQUAD for ipv4 addresses and NIP6 for ipv6 addresses.

#define NIPQUAD(addr) \     ((unsigned char *)&addr)[0], \     ((unsigned char *)&addr)[1], \     ((unsigned char *)&addr)[2], \     ((unsigned char *)&addr)[3]  #define NIP6(addr) \     ntohs((addr).s6_addr16[0]), \     ntohs((addr).s6_addr16[1]), \     ntohs((addr).s6_addr16[2]), \     ntohs((addr).s6_addr16[3]), \     ntohs((addr).s6_addr16[4]), \     ntohs((addr).s6_addr16[5]), \     ntohs((addr).s6_addr16[6]), \     ntohs((addr).s6_addr16[7]) 

There are ample examples in the kernel sources that make use of these to print ip addresses in human-readable format. For instance:

printk(KERN_DEBUG 'Received packet from source address: %d.%d.%d.%d!\n',NIPQUAD(iph->saddr)); 

Hope this helps.