IPV6 UDP server client program with Poll system call

IPv6 AF_INET6 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,

• IPV6 AF_INET6 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

Enabling IPv6 on the Loopback Interface Before Program Execution

Before executing the program that relies on IPv6, enable the loopback interface for IPv6 using the following command:

sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=0

• This command sets the disable_ipv6 parameter to 0 for the loopback interface (lo), allowing IPv6 functionality.

• Ensure to use this command cautiously and consider the implications, especially on production systems.

• After enabling IPv6, proceed to execute your program that relies on IPv6 functionality.

IPV6 AF_INET6 UDP SOCKET : FAQs

Let us answer few basic questions in this socket

What does socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP) do?

See Answer

This call creates a UDP socket in the IPv6 address family.

When is it appropriate to use SOCK_DGRAM with IPv6?

See Answer

SOCK_DGRAM is used for fast, connectionless communication, making it suitable for scenarios where reliability is less critical, such as real-time applications.

What privileges are required to create an IPv6 UDP socket?

See Answer

No special privileges are typically required to create an IPv6 UDP socket.

Can this socket be used for TCP communication?

See Answer

No, this socket is specifically designed for UDP communication.

How does an IPv6 UDP socket differ from an IPv4 UDP socket?

See Answer

IPv6 UDP sockets handle communication using IPv6 addresses, providing a larger address space compared to IPv4.

Can I use IPv6 UDP sockets in a mixed IPv4/IPv6 environment?

See Answer

Yes, most modern systems support IPv6, allowing communication with both IPv4 and IPv6 peers.

How do I specify a port number for an IPv6 UDP socket?

See Answer

Set the port number in the sin6_port field of the struct sockaddr_in6 structure.

Can I use IPv6 UDP sockets for non-blocking I/O?

See Answer

Yes, you can set IPv6 UDP sockets to non-blocking mode using functions like fcntl or ioctl.

How do I handle connection establishment with IPv6 UDP sockets?

See Answer

UDP is connectionless, so there is no explicit connection establishment. Use the bind function to associate the socket with a local address.

Are there any compatibility issues with older systems when using IPv6 UDP sockets?

See Answer

Compatibility might be an issue on systems lacking IPv6 support. Ensure the target systems support IPv6.

How do I handle socket errors related to network communication?

See Answer

Functions like sendto and recvfrom return -1 on error. Check the return values and use perror or strerror to print detailed error messages.

When handling socket errors, is it important to close the socket?

See Answer

Yes, it’s generally a good practice to close the socket on error to release system resources. Always follow error-handling best practices.

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.

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

See Answer

It uses different structures for each state in the pollfd array.

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

See Answer

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

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

See Answer

Array of pollfd Structures:

Before calling poll, you need to create an array of pollfd structures, where each structure represents a file descriptor and its associated events.

struct pollfd fds[NUM_FDS];

NUM_FDS is the number of file descriptors you want to monitor.

Initialize pollfd Structures:

For each file descriptor you want to monitor, initialize the corresponding pollfd structure with the following information:

fd: The file descriptor to monitor. events: The events of interest (e.g., POLLIN for readability, POLLOUT for writability). revents: Initially set to zero. After the poll call, this field is updated to indicate the events that occurred.

fds[0].fd = fd1;

fds[0].events = POLLIN;

fds[0].revents = 0;

fds[1].fd = fd2;

fds[1].events = POLLIN;

fds[1].revents = 0;

Call poll:

After initializing the pollfd array, call the poll function, providing the array, the number of file descriptors, and a timeout

int ready_fds = poll(fds, NUM_FDS, timeout_ms);

ready_fds will contain the number of file descriptors that are ready.

How does poll Checking Ready File Descriptors?

See Answer

After the poll call, loop through the pollfd array and check the revents field for each file descriptor to determine which events occurred.

for (int i = 0; i < NUM_FDS; ++i) {
        if (fds[i].revents & POLLIN) {
                // File descriptor i is ready for reading
        }

        if (fds[i].revents & POLLOUT) {
                // File descriptor i is ready for writing
        }
        // Check other events if needed (e.g., POLLERR, POLLHUP)
}

What does it mean if poll 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_INET6, SOCK_DGRAM, IPPROTO_UDP);

• 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, 1000);

• 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,

len = 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 <poll.h>

#define BUFFER_SIZE 1024

int server_socket = -1;

static void sigint_handler(int signo)
{
  (void)close(server_socket);
  sleep(2);
  (void)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);
  }
}

void validate_convert_port(
char *port_str,
struct sockaddr_in6 *sock_addr)
{
 int port;

 if (port_str == NULL) {
   perror("Invalid port_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 port = atoi(port_str);

 if (port == 0) {
     perror("Invalid port\n");
     exit(EXIT_FAILURE);
 }

 sock_addr->sin6_port = htons(
 (uint16_t)port);
 printf("Port: %d\n",
 ntohs(sock_addr->sin6_port));
}

int main(int argc, char *argv[])
{
  int ret;
  struct sockaddr_in6
  server_addr,
  client_addr;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];

  socklen_t client_addr_len = sizeof(
  client_addr);
  
  register_signal_handler(SIGINT,
  sigint_handler);

  if (argc != 2) {
   printf("%s <port-number>",
   argv[0]);
   exit(EXIT_FAILURE);
  }

  memset(&server_addr, 0,
  sizeof(server_addr));
  server_addr.sin6_family = AF_INET6;
  server_addr.sin6_addr = in6addr_any;
  validate_convert_port(argv[1],
  &server_addr);

  server_socket = socket(AF_INET6,
                  SOCK_DGRAM,
                  IPPROTO_UDP);

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

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

  if (ret < 0) {
    perror("bind");
    (void)close(server_socket);
    return -2;
  }

  printf("UDP listining\n");

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

  while (1) {

    ret = poll(fds, 1, 1000);

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

    if (fds[0].revents & POLLIN) {
      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 < 0) {
           perror("sendto");
           break;
        }
      } else if (ret < 0) {
         perror("recvfrom");
         break;
      }
      printf("Sentbuffer = %s\n",
      buffer);
    }
  }

  (void)close(server_socket);

  return 0;
}

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server 8080

Port: 8080
UDP listining
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
^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_INET6, SOCK_DGRAM, IPPROTO_UDP);

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

ret = poll(fds, 2, 1000);

• 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,

len = 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 <poll.h>

#define BUFFER_SIZE 1024

int client_socket = -1;

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

void validate_convert_port(
char *port_str,
struct sockaddr_in6 *sock_addr)
{
 int port;

 if (port_str == NULL) {
   perror("Invalid port_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 port = atoi(port_str);

 if (port == 0) {
   perror("Invalid port\n");
   exit(EXIT_FAILURE);
 }

 sock_addr->sin6_port = htons(
 (uint16_t)port);
 printf("Port: %d\n",
 ntohs(sock_addr->sin6_port));
}

void validate_convert_addr(
char *ip_str,
struct sockaddr_in6 *sock_addr)
{
  if (ip_str == NULL) {
   perror("Invalid ip_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 printf("IP Address: %s\n", ip_str);

 if (inet_pton(AF_INET6, ip_str,
 &(sock_addr->sin6_addr)) <= 0) {
    perror("Invalid address\n");
    exit(EXIT_FAILURE);
  }
}

void recv_data(char *buffer)
{
  int ret, len;

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

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

  } else if (len == 0) {
     printf("Connection closed\n");
     exit(EXIT_FAILURE);
   }
}

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(int argc, char *argv[])
{
  int ret, len;
  struct sockaddr_in6
  server_addr;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];
  char *str = "HI";

  register_signal_handler(SIGINT,
  sigint_handler);

  if (argc != 3) {
    printf("%s<port-number><ip-addr>\n",
    argv[0]);
    exit(EXIT_FAILURE);
  }

  memset(&server_addr, 0,
  sizeof(server_addr));
  server_addr.sin6_family = AF_INET6;
  validate_convert_port(argv[1],
  &server_addr);
  validate_convert_addr(argv[2],
  &server_addr);

  client_socket = socket(AF_INET6,
                  SOCK_DGRAM,
                  IPPROTO_UDP);

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

  while (1) {
    ret = sendto(client_socket, str,
    strlen(str), 0,
    (struct sockaddr*)&server_addr,
    sizeof(server_addr));
    printf("sendbuffer = %s\n", str);

    if (ret < 0) {
       perror("send error\n");
       (void)close(client_socket);
       break;
    }

    fds[0].fd = client_socket;
    fds[0].events = POLLIN;

    ret = poll(fds, 2, 1000);

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

    if (fds[0].revents & POLLIN) {
       recv_data(buffer);
    }
  }

  (void)close(client_socket);

  return 0;
}

Step 6: Compile and Execute Client.c

$ gcc -o client client.c

$ sudo ./client 8080 ::1

Port: 8080
IP Address: ::1
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
Received: HELLO
sendbuffer = HI
^CCaught sigINT!

$ sudo ./server 8080 ::1

$ sudo ./client 8080 ::1

program to run with elevated privileges, listen on port 8080, and bind to the loopback address ::1.

<port_number> <ip_address> decided by the user based on the connection.

Enhanced Socket Flexibility with AF_INET6 and PF_INET6 Domains

Default Domain:

By default, the socket is configured to work in the AF_INET6 domain, handling all types of network data.

Additional Domain Support:

We expand the socket’s capabilities to also function in the PF_INET6 domain, allowing it to operate similarly to AF_INET6.

Socket Creation:

We set up a network connection point known as a socket using socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP).

Working Scenario:

Despite the change in domain to PF_INET6, the socket continues to operate the same way, handling general network data.

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

  • IPV6 UDP server client program with Select system call

• current topic

  • IPV6 UDP server client program with Poll system call

• Next topic

  • IPV6 UDP server client program with Epoll system call

• Other sockets

  • //TODO ipv6_raw_af_inet6_icmp

  • //TODO ipv6_raw_af_inet6_tcp

  • //TODO ipv6_raw_af_inet6_udp

  • // TODO ipv6_raw_af_inet6_raw

  • raw_af_packet_raw_htons_ETH_P_ALL

  • raw_af_packet_tcp_htons_ETH_P_ALL

  • raw_af_packet_udp_htons_ETH_P_ALL

• Other IPCs

  • Message_queues

  • NamedPipes

  • Netlink

  • Shared_Memory

  • Shared_Memory_2_FDS

  • SocketPair

  • Timerfd