CODEWITHSUNDEEP
csocketsnetwork-programmingraw-sockets
Ricky
Ricky

Reputation: 705

Raw sockets receiving messages sent by itself

I was trying to write some codes using raw sockets, while I observed some strange phenomenon. Consider the code:

                int rsfd = socket(AF_INET,SOCK_RAW,253);
                if(rsfd<0)
                {
                    perror("Raw socket not created");
                }
                else
                {
                    struct sockaddr_in addr2;
                    memset(&addr2,0,sizeof(addr2));
                    addr2.sin_family = AF_INET;
                    addr2.sin_addr.s_addr = inet_addr("127.0.0.2");
            /*       if(connect(rsfd,(struct sockaddr*)&addr2,sizeof(addr2))<0)
                    {
                        perror("Could not connect");continue;
                    } */
                }

Now if I remove the commented portion, whatever message I am sending through this rsfd is also being received by itself. On the other end I have already bound a socket with the ip address 127.0.0.2. When I printed the ip address of the sender socket, it is printing 127.0.0.1 but still it is receiving packets which is meant for 127.0.0.2. This problem was solved when I added that connect request which is mentioned in the commented portion. This seems weird because on the other side, no one is accepting or listen on this address and moreover, I am using sendto and recvfrom functions for sending and receiving packets which is used for connection less sockets. My question is, why is this happening? How is this connect request solvong the problem here?

Upvotes: 0

Views: 902

Answers (1)

John Bollinger
John Bollinger

Reputation: 180276

Now if I [don't connect() the socket], whatever message I am sending through this rsfd is also being received by itself.

I note first that raw sockets are an extension to POSIX. Linux offers them, and I think other systems do too, but details of their behavior are not certain to be consistent across implementations.

With that said, the problem seems likely to be that you are not bind()ing your socket to any address. On Linux, for example, the docs for raw sockets note that

A raw socket can be bound to a specific local address using the bind(2) call. If it isn't bound, all packets with the specified IP protocol are received.

(Emphasis added.) On a system where raw sockets have that behavior, if you're sending packets to an IP loopback address via a raw IP socket that is neither bound nor connected then yes, the source socket will receive them, or at least may do.

It's unclear why connecting the socket solves the problem, or why it is even successful at all. The behavior of connect() is unspecified for socket types other than the standard ones, SOCK_DGRAM, SOCK_STREAM, and SOCK_SEQPACKET. However, the behavior you observe is consistent with connect() having an effect on raw sockets like that it has on datagram sockets, which are also connectionless:

If the socket sockfd is of type SOCK_DGRAM, then addr is the address to which datagrams are sent by default, and the only address from which datagrams are received.

Instead of relying on that discovered behavior, however, I suggest following the documented (at least on Linux) procedure of binding the socket to an address (including a port), and communicating with it at that address.

Upvotes: 1

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