Network Programming

IPv4 addresses can be mapped to IPv6 addresses by prefixing the four bytes in the IPv4 address with ::ffff. For example, 1.2.3.4 becomes :ffff:1.2.3.4.

Loopback addresses

• IPv4: 127.0.0.1
• IPv6: ::1

Link-local addresses

These addresses can only be used for communicating between hosts on the same network, routers will not forward packets with these addresses as their destination.

• IPv4: 169.254.x.x
• IPv6: Any address starting FE80, FE90, FEA0 or FEB0.

Private use addresses

Three ranges of addresses are reserved in IPv4 for private internal use (often using NAT):

• 192.168.x.x
• 172.16-31.x.x
• 10.x.x.x

Generally speaking, the host with the private address must initiate communications in order to talk to the outside world.

Example addresses

Any IP addresses in the block 192.0.2.x are reserved for documentation (e.g. example IP addresses). See RFCs 1166, 5735 and 5737 for more details.

Multicast addresses

Multicast addresses potentially refer to an arbitrary number of destinations.

• IPv4: 224-239.x.x.x
• IPv6: Any address starting FF.

Multicasting is an advanced subject and tends only to be used in large networks - especially for delivery of multimedia content.

Port numbers

Port numbers are the same for IPv4 and IPv6 and are represented by a 16 bit unsigned integer, giving a range of 1-65,535 (0 being reserved). Binding to a port below 1024 requires superuser/root privileges, although it is possible for a process to bind as root and then release these privileges and continue running as a different user.

The Internet Assigned Number Authority (IANA) oversees the assignment of well-known port numbers to certain applications and services. For example, port 80 is the standard port for running a HTTP server (and port 443 for HTTP over TLS).

The port number registry can be found online:

http://www.iana.org/assignments/port-numbers

Sockets

There are two main types of sockets within TCP/IP:

• Stream sockets: Use TCP as the end-to-end protocol.
• Datagram sockets: Use UDP as the protocol.

Byte order

There are two possible ways of ordering bytes:

• Big endian: Largest bytes stored first, so abcd is ab, cd.
• Little endian: Smallest bytes stored first, so abcd is cd, ab.

The byte order for networks is always big endian, however different architectures have different host byte orders, and some even allow switching between the two. Fortunately, there are four functions for converting between the two:

• htons(): Host to network short.
• htonl(): Host to network long.
• ntohs(): Network to host short.
• ntohl(): Network to host long.

The header file for these functions on a Linux system is usually arpa/inet.h.

Converting IP addresses to structs

Usually an IP address is represented as a string, e.g. 127.0.0.1 or ::1. However, in order to use these we have to convert them to the relevant struct.

struct sockaddr_in address;
struct sockaddr_in6 address6;

inet_pton(AF_INET, "127.0.0.1", &(address.sin_addr));
inet_pton(AF_INET6, "::1", &(address6.sin6_addr));

The inet_ntop function provides similar functionality in the opposite direction.

Error messages

The gai_strerror function takes the result of a call to getaddrinfo and converts it into a human readable string. Example use:

fprintf(stderr, gai_strerror("getaddrinfo: %s\n", gai_strerror(status));

Compiling C

When compiling C code with GCC, do not include the std=X argument (e.g. std=c99), otherwise you will receive warnings similar to this:

warning: implicit declaration of function ‘getaddrinfo’ [-Werror=implicit-function-declaration]

Basic C program

The most basic C program, using the EXIT_SUCCESS constant in stdlib.h (in practice this will be the same as return 0, but that is less readable and portable) is:

#include <stdlib.h>

int main()
{
  return EXIT_SUCCESS;
}

If you want to process command line arguments, change the declaration of main to:

int main(int argc, char *argv[])

However, your compiler may throw a warning if either of these parameters are unused.

Basic TCP client

The four basic steps for a TCP client to communicate are:

1. Create a TCP socket using socket().
2. Connect to the server using connect().
3. Send/receive data using send() and recv().
4. Close the connection using close().

Creating hints struct

The most basic hints struct for a client can be created like so:

struct addrinfo hints;

memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC; // Use IPv4 or IPv6
hints.socktype = SOCK_STREAM; // TCP

Links

• Beej's Guide to Network Programming
• Operational Code Authentication