Creating sockets

In C language:

It is a means of communication, originally implemented by the University of Berkeley, between applications.

When using the "socket library" in C language, you should include the header files corresponding to this system library and the constants used.

#include <sys/types.h>
#include <netdb.h>
#include <sys/socket.h>
	

The first step is to create a socket via the system call:

int socket (int domaine, int type, int protocole);
	

• the first parameter is the domain. It is an integer which can take as its value one of the following constants:
• AF_INET: to work with IPv4 as network layer.
• AF_INET6: to work with IPv6 as network layer.
• AF_PACKET: to operate directly above the link layer (layer 2) without using the implementation of a network layer (layer 3) of the kernel.
• AF_UNIX: to work with unix sockets and have communication limited to processes residing on the same machine.
  In some cases, this constant is replaced by the synonym AF_LOCAL, which incidentally belongs to Posix terminology.
• The second parameter is the type. It is an integer which can take as its value one of the following constants:
• SOCK_STREAM: the dialogue takes place in connected mode, with flow control from one end of the communication to the other.
• SOCK_DGRAM: communication takes place without connection, by transmission of data packets (i.e. by exchange of "datagram").
• SOCK_RAW: the socket will be used to communicate raw with the indicated protocol. This is what to use with the AF_PACKET domain value.
• The third parameter is the transport protocol used. It is an integer which can take as its value one of the following constants:
• NULL: if the default protocol is chosen for the type of communication indicated.
• IPPROTO_TCP: indicates the TCP protocol. This is the default if the type is SOCK_STREAM.
• IPPROTO_UDP: indicates the UDP protocol. This is the default if the type is SOCK_DGRAM.
• IPPROTO_ICMP: indicates the ICMP protocol. This is one of two values to choose when the domain is AF_INET (or AF_INET6) and the type is SOCK_RAW.
• IPPROTO_RAW: indicates direct communication with the network layer of the indicated domain. This is one of two values to choose when the domain is AF_INET (or AF_INET6) and the type is SOCK_RAW.
• In the case of the AF_PACKET domain, the value of the protocol field will be indicated in the Ethernet frame. If you want to receive all frame types, you will use htons(ETH_P_ALL).
• The returned value is a positive or null integer corresponding to the number of the descriptor allocated by the socket that has just been created or a negative value to indicate an error:
• EMFILE: constant corresponding to a full file table in the kernel.
• ENFILE: constant corresponding to the fact that the limit of the total number of open files on the system has been reached.
• ENOMEM: constant corresponding to the fact that there is not enough space to allocate the necessary buffers. The socket cannot be created until enough resources are freed.
• EINVAL: constant corresponding to a non-existent domain.
• EPROTONOSUPPORT: constant corresponding to a type inconsistent with the protocol or the domain
• EACCES: constant corresponding to the absence of authorization to create a socket of the requested type (for example AF_INET and SOCK_RAW).

Notes:

• In this course, we will only use TCP or UDP internet sockets which are therefore created via one of the following two instructions:

fd_tcp = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
fd_udp = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	

• We have only reserved the socket management memory in the kernel, no communication has yet been initiated.
• There is a library equivalent to the "socket library" for Windows: The "Winsock library" which basically works on the same principle.
• In languages like PHP or Python, there are libraries interfacing with system functions.

$socket_tcp = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
$socket_udp = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
	

sock_tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock_udp = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)