Thread: Choose a particular source port when connecting.

  1. #1
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    Jul 2010
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    21

    Choose a particular source port when connecting.

    Hello!

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

    Code:
    #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:

    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.

  2. #2
    and the hat of int overfl Salem's Avatar
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    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Quote Originally Posted by Salem View Post
    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:

    Code:
    #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.
    Last edited by assiduus; 12-19-2018 at 03:30 AM.

  4. #4
    and the hat of int overfl Salem's Avatar
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    Well for such an ungrateful git, it's a good job I didn't spend any more effort on it.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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