Get IP address and Port number

No matter netcat works in client or server mode, using TCP or UDP, getting the IP address and port number is always the first step (UNIX-domain socket doesn't need it definitely). By default, netcat will work in TCP mode, except you specify -u option. There is a variable which denotes using UDP or not:

int    uflag;                    /* UDP - Default to TCP */
......
while ((ch = getopt(argc, argv, ...))) {
        ......
        switch (ch) {
        ......
        case 'u':
            uflag = 1;
            break;
        ......
        }
}

getaddrinfo() fulfills the duty to get IP address and port number:

 int
 getaddrinfo(const char *hostname, const char *servname,
     const struct addrinfo *hints, struct addrinfo **res);

The third parameter's type is struct addrinfo, which gives a hint of what kind of socket the application wants to use:

struct addrinfo hints;
......
/* Initialize addrinfo structure. */
if (family != AF_UNIX) {
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = family;
    hints.ai_socktype = uflag ? SOCK_DGRAM : SOCK_STREAM;
    hints.ai_protocol = uflag ? IPPROTO_UDP : IPPROTO_TCP;
    if (nflag)
        hints.ai_flags |= AI_NUMERICHOST;
}

The only thing that needs to pay attention is a nflag, which will be set through -n option. It means don't bother to do DNS query or service look-up, the IP address and port number are already provided.

Because server's socket will be used in future bind() operation, it has another extra step than client:

/* Allow nodename to be null. */
hints.ai_flags |= AI_PASSIVE;

This means INADDR_ANY (For IPv4) or IN6ADDR_ANY_INIT (For IPv4) will be used if hostname parameter for getaddrinfo() is NULL.

Now that all is ready, it is time to get IP address, port number and create socket accordingly:

if ((error = getaddrinfo(host, port, &hints, &res0)))
    errx(1, "getaddrinfo: %s", gai_strerror(error));

for (res = res0; res; res = res->ai_next) {
    if ((s = socket(res->ai_family, res->ai_socktype,
        res->ai_protocol)) < 0)
        continue;
    ......
}

Last but not least, don't forget to release the memory of querying result:

freeaddrinfo(res0);

Now you can understand how localhost and www is mapped to IP address and port number in the following commands:

# nc -l localhost www
# nc localhost www