Could you provide the piece(s) of code where kernel does this type of operation ?
Of course just suggest me some file where search, I am really curios and I would like understand better this question but at the same time I am unable to find the files where look for.

I had a written a Realtek ethernet card driver long time back (its
been almost 6yrs since) for my own curiosity. So my memory right now
is sketchy! :)

You can start off by gong through the n/w device driver for some card
(eg: drivers/net/8139too.c) and work your way up the stack (next you
can look at net/ipv4/ip_input.c).

The basic idea is that when a packet is rx'ed by the n/w device, it
creates a sk_buff (socket buffer) and passes this up the stack where
it's processed by the various protocol layers (IP, TCP etc). Though
I'm not certain if the n/w device driver populates the various skb
fields in the n/w or host byte order. Only inspecting the code will
tell.

HTH,
-mandeep

Many thanks,
Pietro.


-----Original Message-----
From: Mandeep Sandhu [mailto:mandeepsandhu.chd at gmail.com]
Sent: luned? 15 ottobre 2012 10:33
To: Pietro Paolini
Cc: kernelnewbies at kernelnewbies.org
Subject: Re: Network Byte order not reached reading from a sock RAW

On Mon, Oct 15, 2012 at 1:16 PM, Pietro Paolini [off-list ref] wrote:

quoted

Hello,
Thanks for your answer, my question is why when I read from the buffer data is already in host byte order and not in network byte order.

The 'raw' payload in your packet might still be in the n/w byte order and you will have to convert it using the ntoh*() functions (I'm not 100% sure about this though).

CMIIW, as I haven't touched sockets in a long time! :)

HTH,
-mandeep

quoted

Thanks
Pietro Paolini.


From: Mandeep Sandhu [mailto:mandeepsandhu.chd at gmail.com]
Sent: sabato 13 ottobre 2012 05:30
To: Pietro Paolini
Cc: kernelnewbies at kernelnewbies.org
Subject: Re: Network Byte order not reached reading from a sock RAW


On Oct 12, 2012 9:36 PM, "Pietro Paolini" [off-list ref] wrote:

quoted

Hello,

I am struggling with the byte order question on a x86_32 arch, I am doing some modifications on a program which actually works fine on a MIPS arch.

I do a reading from a RAW socket in this way:

        /* Configure socket */
        if ((fd = socket(AF_INET, SOCK_RAW, IPPROTO_IGMP)) < 0) {
                perror("Error on socket creation, exit");
                exit(1);
        }
          ....
        if (setsockopt(fd, IPPROTO_IP, IP_PKTINFO, (void *)&va, sizeof(va))) {
                perror("Error on setsockopt, exit");
                exit(1);
        }
        va = 0;
        ....
        ....
        struct msghdr mhdr;
        struct __in_pktinfo *pktinfo = NULL;
        ...
        ...
        nrd = recvmsg(env->mrouter_fd, &mhdr, 0);
        ...
        ip = (struct iphdr *)iov.iov_base;

When I print the saddr (or daddr) of the received ip packet it is printed as host byte order instead of what I am expecting, the network byte order. I can just use the htonl() family functions for solve the problem but I would like understand if it is the normal behavior or if there is an issues on my code, or if the device driver of my NIC can influence the question.

I think you should use ntoh*() functions when accessing data rx'ed from the n/w. Network byte order is big endian and your host is little endian, so you'll have convert it to the right order before accessing. You should use hton*() functions when tx'ing data. CMIIW.
HTH,
-mandeep

quoted

Many thanks,
Pietro.


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