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,packet has two disjoint sections: Header, Payload

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

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

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

In L2 Socket you can set header and payload of packet in data link layer, ie everything in 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

Third parameter specify 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/

Answer 4

It's also used for protocols like ICMP (ping), you have to know structure of ICPM packet to create it. Also kernel doesn'n modify your packets

Answer 5

            Once the application creates RAW socket is used to send and
    receive packets from source to destination those all packets are
    treated as datagram on an unconnected socket

            when sending IPv4 data, an application has a choice on
    whether to specify the IPv4 header at the front of the outgoing
    datagram for the packet.

            If the IP_HDRINCL socket option is set to true for an IPv4
    socket (address family of AF_INET), the application must supply the
    IPv4 header in the outgoing data for send operations.

            If this socket option is false (the default setting), then
    the IPv4 header should not be in included the outgoing data for
    send operations.

            It is important to understand that some sockets of type
    SOCK_RAW may receive many unexpected datagrams. For example, a PING
    program may create a socket of type SOCK_RAW to send ICMP echo
    requests and receive responses. While the application is expecting
    ICMP echo responses, if several SOCK_RAW sockets are open on a
    computer at the same time, the same datagrams may be delivered to
    all the open sockets. An application must have a mechanism to
    recognize and to ignore all others.

            For a PING program, such a mechanism might include
    inspecting the received IP header for unique identifiers in the
    ICMP header (the application's process ID, for example)

            TCP data cannot be sent by using raw socket
            Referred from below link : 
                   https://msdn.microsoft.com/en-us/library/windows/desktop/ms740548%28v=vs.85%29.aspx