Choose a particular source port when connecting.

[assiduus]

Hello!

The following code does work (the most minimalistic version chosen for clarity) as I connect using port 2100, not a random one:

#include <arpa/inet.h>

int main(void) {

        int sock;
        struct sockaddr_in myaddr, remaddr;
        socklen_t addrlen;

        sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

        remaddr.sin_family = AF_INET;
        remaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
        remaddr.sin_port = htons(20000);

        myaddr.sin_family = AF_INET;
        myaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
        myaddr.sin_port = htons(21000);

        bind(sock, (struct sockaddr *) &myaddr, sizeof(struct sockaddr_in));

        addrlen = sizeof(myaddr);

        connect(sock, (struct sockaddr *) &remaddr, sizeof(remaddr));

        return 0;
}

BUT it doesn't use getaddrinfo() which I assume is the preferred way nowadays. The following code:

#include <unistd.h>
#include <string.h>
#include <netdb.h>

int main(void) {

        int sfd, addr;
        struct addrinfo hints, *servinfo, *sp;

        memset(&hints, 0, sizeof(struct addrinfo));

        hints.ai_family = AF_INET;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_protocol = IPPROTO_TCP;

        addr = getaddrinfo("127.0.0.1", "20000", &hints, &servinfo);
        for(sp = servinfo; sp != NULL; sp = sp->ai_next) {
                if ((sfd = socket(sp->ai_family, sp->ai_socktype, sp->ai_protocol)) == -1) {
                        continue;
                }
                if ((connect(sfd, sp->ai_addr, sp->ai_addrlen) == -1)) {
                        continue;
                }
                break;
        }
        freeaddrinfo(servinfo);

        return 0;
}

works but it doesn't bind() first to force the code to use the source port of my choosing and not the random one.

Question: Where does bind() fit in in the above implementation? Can it be done at all with getaddrinfo()?

Cheers.

[Salem]

Yes, read Beej's Guide to Network Programming

[assiduus]

Yes, read Beej's Guide to Network Programming

First of all, 1337 answer on your part. There's ZERO information there about changing your source port when connecting to a server. Read it yourself.

Secondly, the answer is:

#include <string.h>
#include <netdb.h>

void binding(int sock);

int main(void) {

        int sfd, addr;
        struct addrinfo hints, *servinfo, *sp;

        memset(&hints, 0, sizeof(struct addrinfo));

        hints.ai_family = AF_INET;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_protocol = IPPROTO_TCP;

        addr = getaddrinfo("127.0.0.1", "20000", &hints, &servinfo);
        for(sp = servinfo; sp != NULL; sp = sp->ai_next) {
                if ((sfd = socket(sp->ai_family, sp->ai_socktype, sp->ai_protocol)) == -1) {
                        continue;
                }
                binding(sfd);
                if ((connect(sfd, sp->ai_addr, sp->ai_addrlen)) == -1) {
                        continue;
                }
                break;
        }
        freeaddrinfo(servinfo);

        return 0;
}

void binding(int sock) {

        int sfd, addr;
        struct addrinfo hints, *servinfo, *sp;

        memset(&hints, 0, sizeof(struct addrinfo));

        hints.ai_family = AF_INET;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_protocol = IPPROTO_TCP;

        addr = getaddrinfo("127.0.0.1", "30000", &hints, &servinfo);
        for(sp = servinfo; sp != NULL; sp = sp->ai_next) {
                if (bind(sock, sp->ai_addr, sp->ai_addrlen) == -1) {
                        continue;
                }
                break;
        }
}

Thanks for nothing.

Cheers.

[Salem]

Well for such an ungrateful git, it's a good job I didn't spend any more effort on it.