Question

My website can be accessed using any of 2 different static IPs (IPv4).

Is it possible to open a TCP connection to another server, specifying which of the 2 IPs to use as a return address ?

x.x.x.x (my server) => z.z.z.z (destination server)
y.y.y.y (my server) => z.z.z.z (destination server)

Error-checking, etc. in this example has been omitted for simplicity sakes :

struct addrinfo hints, *result;
hints.ai_flags = 0;
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
hints.ai_addrlen = 0;
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;

getaddrinfo(domain, "80", &hints, &result);

socket(result->ai_family, result->ai_socktype, result->ai_protocol);

Answer 1

Like @hochl says, you use the bind system call:

struct sockaddr_in sin = { 0 };
int sock;

/* Create a socket address, with a specific port and (local) ipnumber */
sin.sin_family = AF_INET;
sin.sin_port = htons(80);
inet_aton("1.2.3.4", &sin.sin_addr);

/* Create socket */
sock = socket(AF_INET, SOCK_STREAM, 0);

/* Bind socket to the local address */
bind(sock, (struct sockaddr *) &sin, sizeof(sin));

/* Now connect to remote server... */
/* connect(...) */

It should be noted that use of inet_aton is normally discouraged, in favor of inet_pton.