Connecting a socket, using getaddrinfo, times out

Question

I spent hours trying to find out my issue, just found the solution as I was writing my question (it always help when you need to formalize your issue and explain it). I post it, hopefully it helps someone.

Using getaddrinfo, if I try to connect a socket to my server, doing (what I thought was) exactly what is being explained on tons of website aswell as in the man page sample code of getaddrinfo, it FAILS with a "connection timed out" error message: (Simplifying the code to be more concise)

void connect_UsingGetAddrInfo_Wrong (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    x =  getaddrinfo(hostname, NULL, &hints, &addr);
    //shall rather loop on addr linked list, but this is not the topic here.

    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
    x = connect(socketfd, addr->ai_addr, addr->ai_addrlen);
}

However, I was able to connect a socket to the same server, using gethostbyname method.

void connect_UsingGetHostByName_Deprecated (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    struct hostent DNS, *r;
    char buf[1024];
    x = gethostbyname_r(hostname.c_str(), & DNS, buf, sizeof(buf), & r, & err));
    socketfd = socket(domain, socketType, 0);

    //server.
    sockaddr_in server;
    memset(&server, 0x00, sizeof(server));
    server.sin_family=domain;
    server.sin_port=htons(port);
    memcpy(& server.sin_addr.s_addr, DNS.h_addr, (size_t) DNS.h_length);
    x = connect(socketfd, (struct sockaddr *) & server, sizeof(server));
}

Running code shows that both version correctly retrieve the valid IP address of the server. Still the first one won't connect and will time out. Why ?

Where do these variables `x`, `domain`, `socketType`, `hostname`, `fd` come from? — Tormund Giantsbane, Jun 20 '18 at 13:49
"//simplified loops & error handling for concision" i should have added also int and error var declaration. Since i wanted to post 4 similar piece of code, i tried to keep only the minimum to understand the differences and keep it concise. I could have pasted everything with traces, loops and errors it wouldn't have helped here. let me fix a few declarations then — Pellekrino, Jun 20 '18 at 13:59
I finally added to both questions and answer function prototype and type declaration for every var (hope i didn't miss one). — Pellekrino, Jun 20 '18 at 14:28

Pellekrino · Accepted Answer · 2018-06-20T14:24:44.493

The reason why it kept failing was : to retrieve the addrinfo, I had left the field 'service' equals to NULL. It will still return success and provide you an address (which you can map with getnameinfo to the right IP address). Still the address won't be usable to connect your socket !

I had found an hybrid version of both methods here : https://codereview.stackexchange.com/questions/17863/socket-connect-realization-gethostbyname-or-getnameinfo This one is functional, but i don't buy the casting

void connect_UsingGetAddrInfo_HYBRID (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams. 

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    x =  getaddrinfo(host, NULL, &hints, &addr);
    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);

    //here is the hybrid part
    sockaddr_in servAddr;
    memset(&servAddr, 0, sizeof(servAddr));
    servAddr.sin_family = addr->ai_family;
    servAddr.sin_addr.s_addr = *((uint32_t*) & (((sockaddr_in*)addr->ai_addr)->sin_addr));
    servAddr.sin_port        = htons(port);

    x=connect(socketfd, (struct sockaddr*) &servAddr, sizeof(servAddr));
}

In the end, it helped me find the rootcause :

void connect_UsingGetAddrInfo_FIXED (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    //Precise here the port !
    const char* service = std::to_string(port).c_str();

    x =  getaddrinfo(host, service, &hints, &addr);
    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
    x = connect(socketfd, addr->ai_addr, addr->ai_addrlen);
}

Hope this will help someone one day !

Connecting a socket, using getaddrinfo, times out

1 Answers1