TCP/IP connection on a specific interface

by

Use the bind() function to bind the socket to either 192.168.1.3 or 192.168.1.2 before calling connect(), ConnectEx(), or WSAConnect(). That tells the socket which specific interface to use for the outgoing connection. For example:

SOCKET s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

sockaddr_in localaddr = {0};
localaddr.sin_family = AF_INET;
localaddr.sin_addr.s_addr = inet_addr("192.168.1.3");
bind(s, (sockaddr*)&localaddr, sizeof(localaddr));

sockaddr_in remoteaddr = {0};
remoteaddr.sin_family = AF_INET;
remoteaddr.sin_addr.s_addr = inet_addr("192.168.1.4");
remoteaddr.sin_port = 12345; // whatever the server is listening on
connect(s, (sockaddr*)&remoteaddr, sizeof(remoteaddr));

Alternatively:

addrinfo localhints = {0};
localhints.ai_flags = AI_NUMERICHOST | AI_NUMERICSERV;
localhints.ai_family = AF_INET;
localhints.ai_socktype = SOCK_STREAM;
localhints.ai_protocol = IPPROTO_TCP;

addrinfo *localaddr = NULL;
getaddrinfo("192.168.1.3", "0", &localhints, &localaddr);
bind(s, localaddr->ai_addr, localaddr->ai_addrlen);
freeaddrinfo(localaddr);

addrinfo remotehints = {0};
remotehints.ai_flags = AI_NUMERICHOST | AI_NUMERICSERV;
remotehints.ai_family = AF_INET;
remotehints.ai_socktype = SOCK_STREAM;
remotehints.ai_protocol = IPPROTO_TCP;

addrinfo *remoteaddr = NULL;
getaddrinfo("192.168.1.4", "12345", &remotehints, &remoteaddr);
connect(s, remoteaddr->ai_addr, remoteaddr->ai_addrlen);
freeaddrinfo(remoteaddr);

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