what is RAW socket in socket programming

Question

When I gone through socket programming I could not clearly understand RAW_SOCKET.

My understanding is

If I open a socket with this option AF_INET , RAW_SOCKET mean's I can create my own header before AF_INET headers but finally the data is send in the format of AF_INET protocol. Is my understanding is correct . If wrong can some explain me.

ThankYou

Answer 1

In every layer, a packet has two disjoint sections: Header and Payload.

non-Raw socket means you can just determine Transport Layer Payload. i.e it is the OS' task to create the Transport, Network, and Data Link layer headers.

Raw socket means you can determine every section of a packet, be it header or payload. Please note that raw socket is a general word. I categorize raw socket into: Network Socket and Data-Link Socket (or alternativly L3 Socket and L2 Socket).

In L3 Socket you can set the header and payload of a packet in the network layer. For example: if a network layer protocol is IPv4, you can determine the IPv4 header and payload. Thus you can set the transport layer header/payload, ICMP header/payload, Routing Protocols header/payload, ... .

In L2 Socket you can set the header and payload of a packet in the data link layer, i.e everything in the packet. Thus you do everything done with L3 Socket + determine ARP header/payload, PPP header/payload, PPPOE header/payload, ... .

Now in programming:

• socket(AF_INET,RAW_SOCKET,...) means L3 socket , Network Layer Protocol = IPv4
• socket(AF_IPX,RAW_SOCKET,...) means L3 socket , Network Layer Protocol = IPX
• socket(AF_INET6,RAW_SOCKET,...) means L3 socket , Network Layer Protocol=IPv6
• socket(AF_PACKET,RAW_SOCKET,...) means L2 socket , Data-link Layer Protocol= Ethernet

The third parameter specify the payload protocol.

Answer 2

RAW_SOCKET allow user to implement it's own transport layer protocol above internet (IP) level . You are responsible for creating and parsing transport level headers and logic behind it. A packet would look like:

-------------------------------------------------------------------
| Ethernet (typically) header | IP header | Your header | payload |
-------------------------------------------------------------------

EDIT: there's good description of raw sockets on Linux man page, or here if you are using Windows.

Answer 3

You can also use SOCK_RAW with "Packet Sockets" that will allow you to have full control over the L2 (Ethernet) and L3 (IP) layers.. meaning you can completely custom-render you packet as it comes out of a NIC..

Details here:

http://www.kernel.org/doc/man-pages/online/pages/man7/packet.7.html

http://austinmarton.wordpress.com/2011/09/14/sending-raw-ethernet-packets-from-a-specific-interface-in-c-on-linux/