af_packet udp server client program with Poll system call

AF_PACKET UDP

• In this program, you are going to learn

• How to create a Socket ?

• How to bind a socket ?

• How to send a data ?

• How to recv a data ?

• How to use socket APIs ?

  • socket

  • bind

  • sendto

  • recvfrom

Topics in this section,

• AF_PACKET UDP SOCKET FAQs

• Step 1: Sequence Diagram for SERVER.c

• Step 2: Program for Server.c

• Step 3: Compile and Execute Server.c

• Step 4: Sequence Diagram for CLIENT.c

• Step 5: Program for Client.c

• Step 6: Compile and Execute Client.c

• Summary

AF_PACKET UDP SOCKET : FAQs

Let us answer few basic questions in this socket

What does socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL)) do?

See Answer

This call creates a packet socket that allows receiving all Ethernet frames, regardless of destination address, using the recvfrom function.

How can I handle errors when creating a packet socket for receiving all data?

See Answer

Check the return value of the socket function. If it returns -1, use perror to print a descriptive error message.

What kind of errors can occur when using a packet socket for receiving data?

See Answer

Common errors include permission-related errors (EACCES), socket creation failures (ENOMEM), or invalid arguments (EINVAL).

How do I handle errors when binding a packet socket for receiving data?

See Answer

Check the return value of the bind function. If it returns -1, handle the error by printing a message or taking appropriate corrective action based on the error code.

What is the significance of error code EPERM when dealing with packet sockets?

See Answer

EPERM (Operation not permitted) typically indicates insufficient privileges. Ensure the program has the necessary permissions to create packet sockets.

How do I handle errors when using recvfrom to receive data with a packet socket?

See Answer

Check the return value of recvfrom. If it returns -1, handle the error by printing a message or taking appropriate corrective action.

Is it important to close the packet socket on error?

See Answer

Yes, closing the socket is crucial to release system resources. Always follow error-handling best practices and close sockets on error.

Is it possible to receive packets only from a specific network interface with a packet socket?

See Answer

Yes, use the bind function to associate the packet socket with a specific network interface.

How do I handle errors related to socket file descriptor management?

See Answer

When closing sockets, check for errors using close. Handle errors by printing messages or taking corrective actions.

What is the purpose of the poll system call?

See Answer

To block and wait for activity on one or more file descriptors.

How does poll differ from poll in terms of usability?

See Answer

poll is more efficient than poll for monitoring multiple file descriptors.

What types of file descriptors can be monitored using poll?

See Answer

sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory.

What is the significance of the timeout parameter in the select function?

See Answer

It specifies the maximum duration to wait for any file descriptor to become ready.

How do you handle errors when using the select system call?

See Answer

Check the return value for -1 to detect errors, Use perror to print error messages.

How does select handle a set of file descriptors with different states (e.g., reading, writing, exception)?

See Answer

Preparing File Descriptor Sets:

select(readfds, writefds, exceptfds);

Setting Up Readiness Conditions:

If you are interested in monitoring file descriptors for readability, you add them to the readfds set.

FD_ZERO(&readfds);

FD_SET(fd1, &readfds);

Setting Up Writability Conditions:

If you are interested in monitoring file descriptors for writability, you add them to the writefds set.

FD_ZERO(&writefds);

FD_SET(fd2, &writefds);

Setting Up Exceptional Conditions:

If you are interested in monitoring file descriptors for exceptional conditions, you add them to the exceptfds set.

FD_ZERO(&exceptfds);

FD_SET(fd3, &exceptfds);

How does select Checking Ready File Descriptors?

See Answer

After select returns, you can check the sets to determine which file descriptors are ready for the specified conditions.

if (FD_ISSET(fd1, &readfds)) {
        // fd1 is ready for reading
}
if (FD_ISSET(fd3, &writefds)) {
        // fd2 is ready for writing
}
if (FD_ISSET(fd4, &exceptfds)) {
        // fd3 has an exceptional condition
}

What does it mean if select returns 0?

See Answer

No file descriptors are ready within the specified timeout.

Step 1: Sequence Diagram for SERVER.c

Step 2: Program for Server.c

• There are many functions used in socket. We can classify those functions based on functionalities.

  • Create Socket

  • Bind Socket

  • Poll

  • Recvfrom data_packet

  • Sendto data_packet

  • Close socket

• socket() is used to create a new socket. For example,

server_socket = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL));

• bind() is used to associate the socket with a specific address and port. For example,

ret = bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));

• poll() is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

ret = poll(fds, 1, -1);

• recvfrom is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender’s IP address and port number. For example,

ret = recvfrom(server_socket, buffer, BUFFER_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_len);

• sendto is used to send the encoded message to the specified server address and port using a UDP socket. For example,

ret = sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len);

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(server_socket);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>
#include <netpacket/packet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <poll.h>

#define BUFFER_SIZE 1024

int server_socket;

static void sigint_handler(int signo)
{
  (void)close(server_socket);
  sleep(2);
  printf("Caught sigINT!\n");
  exit(EXIT_SUCCESS);
}

void register_signal_handler(
int signum,
void (*handler)(int))
{
  if (signal(signum, handler) ==
  SIG_ERR) {
     printf("Cannot handle signal\n");
     exit(EXIT_FAILURE);
  }
}

int main(void) 
{
  int ret, i;
  struct sockaddr_ll 
  server_addr, 
  client_addr;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];

  register_signal_handler(SIGINT,
  sigint_handler);

  server_socket = socket(AF_PACKET, 
                  SOCK_DGRAM, 
                  htons(ETH_P_ALL));

  if (server_socket < 0) {
    perror("socket");
    return -1;
  }

  memset(&server_addr, 0, 
  sizeof(server_addr));
  server_addr.sll_family = AF_PACKET;
  server_addr.sll_protocol = 
  htons(ETH_P_ALL);
  server_addr.sll_ifindex = 
  if_nametoindex("lo");

  ret = bind(server_socket, 
  (struct sockaddr*)&server_addr, 
  sizeof(server_addr));
  
  if (ret < 0) {
    perror("bind");
    (void)close(server_socket);
    return -2;
  }

  printf("UDP is listening\n");

  socklen_t client_addr_len = 
  sizeof(client_addr);

  memset(fds, 0, sizeof(fds));
  fds[0].fd = server_socket;
  fds[0].events = POLLIN;

  while (1) {
    ret = poll(fds, 1, -1);

    if (ret < 0) {
      perror("poll");
      break;
    }
 
    if (fds[0].revents & POLLIN) {
      memset(buffer, 0, 
      sizeof(buffer));
      ret = recvfrom(server_socket, 
      buffer, BUFFER_SIZE, 0, 
      (struct sockaddr*)&client_addr, 
      &client_addr_len);

     if (ret > 0) {
        buffer[ret] = '\0';
        printf("Received: %s\n", 
        buffer);

	memset(buffer, 0,
        sizeof(buffer));
        strncpy(buffer, "HELLO",
        strlen("HELLO") + 1);
        buffer[strlen(buffer)
        + 1] = '\0';
	
	ret = sendto(server_socket,
        buffer, strlen(buffer), 0,
        (struct sockaddr *)&client_addr,
        client_addr_len);

        if (ret == -1) {
           perror("sendto error");
           break;
	}
     } else if (ret < 0) {
       perror("recvfrom error");
       break;
     } 
   }
  }

  (void)close(server_socket);

  return 0;
}

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server

UDP is listening
Received: hello server!
sending message = HELLO
Received: hello server!
sending message = HELLO
Received: HELLO
sending message = HELLO
Received: hello server!
sending message = HELLO
Received: hello server!
sending message = HELLO
Received: HELLO
sending message = HELLO
Received: hello server!
^CCaught sigINT!

Step 4: Sequence Diagram for CLIENT.c

Step 5: Program for Client.c

• There are many functions used in socket. We can classify those functions based on functionalities.

  • Create Socket

  • Poll

  • Sendto data_packet

  • Recvfrom data_packet

  • close socket

• socket is used to create a new socket. For example,

client_socket = socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL));

• poll() is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

ret = poll(fds, 1, -1);

• sendto is used to send the encoded message to the specified server address and port using a UDP socket. For example,

ret = sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));

• recvfrom is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender’s IP address and port number. For example,

ret = recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL);

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(client_socket);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>
#include <netpacket/packet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <poll.h>

#define BUFFER_SIZE 1024

int client_socket;

static void sigint_handler(int signo)
{
  (void)close(client_socket);
  sleep(2);
  printf("Caught sigINT!\n");
  exit(EXIT_SUCCESS);
}

void register_signal_handler(
int signum,
void (*handler)(int))
{
  if (signal(signum, handler) ==
  SIG_ERR) {
     printf("Cannot handle signal\n");
     exit(EXIT_FAILURE);
  }
}

int main(void)
{
  int ret, i;
  struct sockaddr_ll 
  server_addr,
  client_addr;
  struct pollfd fds[1];
  char buffer[BUFFER_SIZE];

  register_signal_handler(SIGINT,
  sigint_handler);

  client_socket = socket(AF_PACKET, 
                  SOCK_DGRAM, 
                  htons(ETH_P_ALL));

  if (client_socket < 0) {
    perror("socket");
    return -1;
  }

  memset(&server_addr, 0, 
  sizeof(server_addr));
  server_addr.sll_family = AF_PACKET;
  server_addr.sll_protocol = 
  htons(ETH_P_ALL);
  server_addr.sll_ifindex = 
  if_nametoindex("lo");

  while (1) {
    sprintf(buffer, "%s", "hello server!");
    ret = sendto(client_socket, buffer, 
    strlen(buffer), 0, 
    (struct sockaddr*)&server_addr, 
    sizeof(server_addr));
  
    if (ret < 0) {
      perror("sendto");
      break;
    }
  
    printf("Sentbuffer : %s\n",
    buffer);

    memset(fds, 0, sizeof(fds));
    fds[0].fd = client_socket;
    fds[0].events = POLLIN;

    ret = poll(fds, 1, -1);

    if (ret < 0) {
	perror("poll");
	break;
    }

    if (fds[0].revents & POLLIN) {
       ret = recvfrom(client_socket, buffer, 
       BUFFER_SIZE, 0, NULL, NULL);
	    
       if (ret < 0) {
	  perror("recvfrom");
          break;
       } else if (ret > 0) {
          buffer[ret] = '\0';
          printf("Received : %s\n", 
	  buffer);
       }
    }
  }

  (void)close(client_socket);

  return 0;
}

Step 6: Compile and Execute Client.c

$ gcc -o client client.c

$ sudo ./client

Sentbuffer : hello server!
Received : hello server!
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
Sentbuffer : hello server!
Received : HELLO
^CCaught sigINT!

Enhanced Socket Flexibility with AF_PACKET and PF_PACKET Domains

Default Domain:

By default, the socket is configured to work in the AF_PACKET domain, handling all types of network data.

Additional Domain Support:

We expand the socket’s capabilities to also function in the PF_PACKET domain, allowing it to operate similarly to AF_PACKET.

Socket Creation:

We set up a network connection point known as a socket using socket(PF_PACKET, SOCK_DGRAM, IPPROTO_UDP).

Working Scenario:

Despite the change in domain to PF_PACKET, the socket continues to operate the same way, handling general network data.

Enhanced Protocol Flexibility in Socket Configuration

Default Protocol Support:

By default, the socket is configured to support the capture of all Ethernet frames (ETH_P_ALL protocol).

Additional Protocol:

The socket is designed to seamlessly support an additional protocol, namely ETH_P_PAE.

Socket Creation:

A socket is created using the socket(AF_PACKET, SOCK_DGRAM, htons(ETH_P_PAE)) call.

Working Scenario:

Despite the change in protocol to ETH_P_PAE, the overall working scenario of the socket remains consistent.

Summary

Socket API	Learning
socket	Create a new socket
bind	Associate the socket with a specific address and port
poll	Monitor multiple file descriptors (usually sockets) for read, write, or error conditions.
recvfrom	It provides information about the source (sender) of the data, including the sender’s IP address and port number.
sendto	Send the encoded message to the specified server address and port using a UDP socket.

See Also

• Previous topic

  • af_packet udp server client program with Select system call

• Current topic

  • af_packet udp server client program with Poll system call

• Next topic

  • af_packet udp server client program with Epoll system call

• Other sockets

  • raw_af_packet_tcp_htons_ETH_P_ALL

• Other IPCs

  • Message_queues

  • NamedPipes

  • Netlink

  • Shared_Memory

  • Shared_Memory_2_FDS

  • SocketPair

  • Timerfd