Help needed linux socket programming in c

esmeco · 10-27-2008, 02:49 PM

Good evening everyone!

I'm doing a small client / server application for sharing files in C, and I am trying to implement the following:

The client of my application sends to the address 255.255.255.255 a message requesting a particular file.In the network there is only one server, however, there may be multiple clients.
The server responds with an ACK if it has the file specified by the client.Until now,everythings' allright.
The question arises now,as I want the client, after receiving the confirmation (ACK) from the server, making sure that the reply message comes from the server or if the message is of uknown origin.As so,I have the following code in Client:

Code:

 ...
msg_tam = recvfrom (socketfd, buffer, BUFFERSIZE, 0, (struct sockaddr *) & serv_addr, & Nbytes);

     if (msg_tam <0)
      Abort ("...");

     buffer [msg_tam] = 0;

    if (SERV_HOST_PORT == ntohs (serv_addr.sin_port) & & strcmp (SERV_HOST_ADDR, (char *) inet_ntoa (serv_addr.sin_addr) == 0)
(
...
)
else
     printf ( "message of unknown origin!");

The SERV_HOST_ADDR is set to "255.255.255.255" so the client can send the file request to the network and the port is also set with a value.Well, the problem arises when the call to the function recvfrom is made, where the ip address of who sent the message is filled in the structure.That address is ,thus, the address where the server is.
As so, when making the comparison between the SERV_HOST_ADDR in the if statement with the address serv_addr.sin_addr filled in the recvfrom call, they will never be the same (even if the reply has come from the server) ,as a it will always enter the else statement.

I would appreciate if someone could help me solve this problem, so that it would be a way the client could check you check whether the reply comes from the server where the client sent the message or from another place.

Thanks in advance for your help!

Mara · 10-27-2008, 04:54 PM

Well, in a normal situation (and I assume this is your case), 255.255.255.255 is a broadcast address and it cannot be assigned to any host. It means that the server has a different IP and that's the one which is in serv_addr. And it will never be that address. You need to find out the server's address somehow.

Also, when it comes to "making sure", everyone can send a message with any source address (if raw sockets are available, but still...). So if it's important, you need to use a different method to check if it was really from the server (in fact, some cryptographic stuff fits here very well...). If you want to know the details, just let me know.

esmeco · 10-27-2008, 05:57 PM

Thanks for the reply!
I really much would like to know how it is done,as I tried the code below
but it didn't work!

Code:

if (host->h_addr_list == NULL)
        printf("No h_addr_list present.\n");
    else
    {
        printf("h_addr_list:\n");
        for (i = 0; he->h_addr_list[i] != 0; i++) 
        {
            memcpy(&addr, he->h_addr_list[i], sizeof(struct in_addr));
            printf("  Addr #%d: %s\n",i,inet_ntoa(addr));
        }
    }

What I wanted to achieve with this code was to obtain all the adresses available,but only two appear in this list which are 127.0.0.1 and 127.0.1.1,but no sign of the 192.xxx.xxx.xxx that is my ethernet address,and also my serv_addr adress!

Mara · 10-28-2008, 06:08 PM

But which function do you use to get the address? It's not a piece from the same fragment as before... So, do you want to obtain the addres on the server or get the right one from the message?