• An Observation:
  • Regardless of the layer you are working on there are some things they have in common
• Some Fundamental Issues:
  • Byte Order
  • Addresses

• The Issue:
  • In a multi-byte entity, the individual bytes can be stored in any order and different processors use different representations
• One Implication:
  • If these entities aren't moved from one processor to another there is no reason to care however network programs don't have this luxury

• The Canonical Representations:
  • Big-Endian - Most significant byte first (i.e., in the smallest address)
  • Little-Endian - Least significant byte first
• Network Byte Order:
  • The Internet uses big-endian representations

• Host Order to Network Order:
  • htons() for "short" values (i.e., of type uint16_t)
  • htonl() for "long" values (i.e., of type uint32_t)
• Network Order to Host Order:
  • ntohs() for "short" values (i.e., of type uint16_t)
  • ntohl() for "long" values (i.e., of type uint32_t)

#include <arpa/inet.h>

Note: These "functions" are commonly defined as macros.

• Purpose:
  • Every host has (one or more) addresses (associated with a hardware interface) that can be used to identify it
• IPv4 Addresses:
  • Four bytes (32 bits)
  • Dotted Quads
• IPv6 Addresses:
  • Sixteen bytes (128 bits)
  • Colon-Seperated Hexadecimals

  // Generic
  struct sockaddr {
    sa_family_t     sa_family;     // AF_UNIX, AF_INET, AF_INET6
    char            sa_data[14];
  };

  // IPv4
  struct in_addr {
    in_addr_t       s_addr;        // 32-bits
  };

  struct sockaddr_in {
    sa_family_t     sin_family;    // AF_INET
    in_port_t       sin_port;      // Port umber
    struct in_addr  sin_addr;
    unsigned char   __pad[X];      // Pad to size of struct sockaddr
  };

  // IPv6
  struct in6_addr {
    uint8_t         s6_addr[16];   // 128 bits
  };

  struct sockaddr_in6 {
    sa_family_t     sin6_family;   // AF_INET6
    in_port_t       sin6_port;     // Port umber
    uint32_t        sin6_flowinfo; // Flow information
    struct in6_addr sin6_addr;
    uint32_t        sin6_scope_id; // Scope ID
  };
  

#include <arpa/inet.h>

Note: This function just converts from one format to another.

#include <arpa/inet.h>

Note: This function just converts from one format to another.

#include <ifaddrs.h>
#include <sys/types.h>

Note: This function reads the information from the file system.

struct ifaddrs {
  struct ifaddrs    *ifa_next;      // Next item in list 
  char              *ifa_name;      // Name of interface 
  unsigned int       ifa_flags;     // Flags from SIOCGIFFLAGS 
  struct sockaddr   *ifa_addr;      // Address of interface 
  struct sockaddr   *ifa_netmask;   // Netmask of interface 
  union {
    struct sockaddr *ifu_broadaddr; // Broadcast address of interface 
    struct sockaddr *ifu_dstaddr;   // Point-to-point destination address 
  } ifa_ifu;
  #define           ifa_broadaddr ifa_ifu.ifu_broadaddr
  #define           ifa_dstaddr   ifa_ifu.ifu_dstaddr
  void              *ifa_data;      // Address-specific data 
};
  

unixexamples/internet/my_address.c

        #include <arpa/inet.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int
main(void)
{
  struct ifaddrs     *first, *current;
  struct sockaddr_in *address;
    
  getifaddrs(&first); // Gets a linked structure of interface addresses

  current = first;
  while (current)
    {
      if ((current->ifa_addr) && (current->ifa_addr->sa_family == AF_INET))
        {
          address = (struct sockaddr_in *)current->ifa_addr;
          printf("%s: %s\n", current->ifa_name, inet_ntoa(address->sin_addr));
        }

      current = current->ifa_next;
    }

  freeifaddrs(first); // Free the memory allocated by getifaddrs()
  exit(0);
}

        

#define _GNU_SOURCE
#define _POSIX_C_SOURCE
#include <unistd.h>

Note: This function reads the information from the file system.

unixexamples/internet/my_name.c

        #define _GNU_SOURCE
#define  _POSIX_C_SOURCE 20112L
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main(void)
{
  char   name[NI_MAXHOST];

  memset(&name, 0, sizeof(name)); 
  gethostname(name, NI_MAXHOST);
  printf("Host: %s\n", name);
  return 0;
}