IP Address from sk_buff

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

You can use strtol to convert each piece to it's integer form.

Answer 2

Simple. The IP address in "x.x.x.x" format is called dotted-quad for a reason. Each number represents a byte, for a total of 4 bytes in your address.

So, with the 4 byte address, you would simply print the decimal value of each byte.

Quick and dirty example (replace printf with your output function of choice):

unsigned char *addr = (unsigned char*)sk_buff->addr;
printf("%d.%d.%d.%d", addr[0], addr[1], addr[2], addr[3]);

Answer 3

/* Convinience union to __be32 to ip address  */
union ip_address {
    u8 a[4];
    __be32 saddr;
};

IP Address could be obtained a[0].a[1].a[2].a[3]

Answer 4

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.