How to Retrieve TOS Value on a TCP Socket

To retrieve the Type of Service (TOS) value on a TCP socket in a C program, you can use the getsockopt function. The getsockopt function allows you to read socket options. For retrieving the TOS value, you’ll use the IP_TOS option with getsockopt.

Here’s an example of how to do this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main() {
    int sockfd;
    int tos;
    socklen_t optlen;
    struct sockaddr_in server_addr;

    // Create a socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // Initialize server address structure
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    // Connect to the server
    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("connect");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // Retrieve the TOS value
    optlen = sizeof(tos);
    if (getsockopt(sockfd, IPPROTO_IP, IP_TOS, &tos, &optlen) < 0) {
        perror("getsockopt");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    printf("TOS value: %d\n", tos);

    // Close the socket
    close(sockfd);
    return 0;
}

Explanation

1. Creating a Socket:

• The socket function creates a TCP socket.

2. Setting Up Server Address:

• The server_addr structure is initialized with the server’s IP address and port.

3. Connecting to the Server:

• The connect function establishes a connection to the server.

4. Retrieving the TOS Value:

• getsockopt is used to retrieve the TOS value.
• The arguments for getsockopt include the socket descriptor (sockfd), the protocol level (IPPROTO_IP), the option name (IP_TOS), a pointer to store the TOS value (&tos), and the size of the option value (&optlen).

5. Printing the TOS Value:

• The retrieved TOS value is printed to the console.

6. Closing the Socket:

• The close function closes the socket to free resources.

Important Notes

• Error Handling: Always check for errors when calling socket functions and handle them appropriately.
• Permissions: Depending on the system configuration, retrieving or setting the TOS value may require elevated permissions.
• Compatibility: Ensure that your environment supports the required socket options and functions.

This example demonstrates how to retrieve the TOS value for a TCP socket in a C program, which can be useful for debugging or monitoring network traffic characteristics.