IP address in the kernel (was: Accessing a user space variable froma kernel module)

[Trevor Hamm <trevor@sask.trlabs.ca>]

On Thu, 2002-08-01 at 17:00, Nicholas Kidd wrote:
> This is probably not what you want but I think you can get the info you are
> looking for by jumping through these hoops:
> 
> strace ifconfig
>     - shows ifconfig reads the info from /proc/net
>     - which means that the ip address is in the kernel
> 
> look at ifconfig source to see how it figures out the ipaddress from /proc
> 
> then look at the procfs code in the kernel and find out how it gets the
> information it displays. 
> 
I spent a couple of days this week researching this problem for my own
project. Using the ifconfig source as my guide, I eventually traced the
IP retrieval through an ioctl call (using cmd=SIOCGIFADDR) all the way
to devinet_ioctl() in $LINUX_SOURCE/net/ipv4/devinet.c. These are the
steps devinet_ioctl() uses to fetch the IP address:

1. Using the interface name (like "eth0"), fetches the "struct
net_device" for the interface using dev_get_by_name().

2. Fetches a "struct in_device" structure from net_device using
in_dev_get().

3. Searches through a linked list of "struct in_ifaddr"'s contained in
in_device for one with a label matching the interface name.

4. The IP address is stored (network-byte order) in the ifa_local field
of struct in_ifaddr.

So far, this is the easiest and most direct method that I've found for
retrieving the IP address from within the kernel.

-- 
Trevor Hamm

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