IPV6 UDP server client program with Select 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 select system call in network programming?

See Answer

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

How does select help in handling multiple sockets efficiently?

See Answer

It provides a way to wait for readiness on multiple sockets without blocking the entire program.

What types of file descriptors can be monitored using select?

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

  • Select

  • 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));

• select() is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(server_socket + 1, &read_fds, NULL, NULL, NULL);

• 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>

#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;
  fd_set read_fds;
  char buffer[BUFFER_SIZE];

  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;
  validate_convert_port(argv[1],
  &server_addr);
  server_addr.sin6_addr = in6addr_any;

  server_socket = socket(AF_INET6, 
                  SOCK_DGRAM, 
                  IPPROTO_UDP);

  if (server_socket < 0) {
    perror("socket");
    return -3;
  }

  ret = bind(server_socket, 
  (struct sockaddr*)&server_addr, 
  sizeof(server_addr));
  
  if (ret < 0) {
    perror("bind");
    (void)close(server_socket);
    return -4;
  }

  printf("UDP listining\n");

  while (1) {
    FD_ZERO(&read_fds);
    FD_SET(server_socket, &read_fds);

    ret = select(server_socket + 1, 
    &read_fds, NULL, NULL, NULL);

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

    if (FD_ISSET(server_socket, 
     &read_fds)) {
      ret = recvfrom(server_socket, 
      buffer, BUFFER_SIZE, 0, 
      (struct sockaddr*)&client_addr, 
      &client_addr_len);
      
      if (ret > 0) {
        buffer[ret] = '\0';
        (void)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;
        }
        (void)printf("Sent buffer= %s\n",
        buffer);

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

  (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!
Sent buffer= HELLO
Received: HI!
Sent buffer= HELLO
Received: HI!
Sent buffer= HELLO
Received: HI!
Sent buffer= HELLO
Received: HI!
Sent buffer= HELLO
Received: HI!
Sent buffer= HELLO
Received: HI!
^CCaught sigINT!

Step 4 : Sequence Diagram for CLIENT.c

Step 5: program for client.c

• Create Socket

• Select

• 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);

• select is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(client_socket + 1, &read_fds, NULL, NULL, NULL);

• 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);

• 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));

• 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>

#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;
  fd_set read_fds;
  char buffer[BUFFER_SIZE];
  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;
  }

  FD_ZERO(&read_fds);
  FD_SET(client_socket, &read_fds);
  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;
    }
    

    ret = select(client_socket + 1, 
    &read_fds, NULL, NULL, NULL);

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

    if (FD_ISSET(client_socket, 
    &read_fds)) {
      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
select	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

  • Basic example ipv6 udp server and client

• Current topic

  • IPV6 UDP server client program with Select system call

• Next topic

  • IPV6 UDP server client program with Poll 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