Network or UNIX-domain socket

OpenBSD's netcat can support both network(based on TCP/IP or UDP/IP) and UNIX-domain sockets. netcat will use network sockets by default except you specify -U option. E.g, create and listen on a UNIX-domain socket:

nc -lU /var/tmp/unix-socket

-l option lets netcat work in server mode. There is a lflag which identifies netcat launched as server or client:

int    lflag;                    /* Bind to local port */

Regarding to network socket, netcat has 2 options to specify using IPv4 only or IPv6 only:

 -4      Use IPv4 addresses only.
 -6      Use IPv6 addresses only.

Netcat program has a family variable which denotes using IPv4, IPv6 or UNIX-domain socket:

int family = AF_UNSPEC;

During parsing options, lflag and family will be assigned different values accordingly:

while ((ch = getopt(argc, argv, ...))) {
        ......
        switch (ch) {
        case '4':
            family = AF_INET;
            break;
        case '6':
            family = AF_INET6;
            break;
        case 'U':
            family = AF_UNIX;
            break;
        ......
        case 'l':
            lflag = 1;
            break;
        ......
        }
}

Netcat will accept one or two arguments:

argc -= optind;
argv += optind;
......
/* Cruft to make sure options are clean, and used properly. */
if (argv[0] && !argv[1] && family == AF_UNIX) {
    host = argv[0];
    uport = NULL;
} else if (argv[0] && !argv[1]) {
    if (!lflag)
        usage(1);
    uport = argv[0];
    host = NULL;
} else if (argv[0] && argv[1]) {
    host = argv[0];
    uport = argv[1];
} else
    usage(1);

If it is UNIX-domain, netcat only needs one argument which is the UNIX-domain socket file. If it is network socket, client must have two arguments: server IP address and port; server can omit IP address and only has port.

What IP protocol version will be used by TCP/IP server if neither -4 nor -6 is provided in option? The answer is IPv4. In local_listen function:

/*
 * In the case of binding to a wildcard address
 * default to binding to an ipv4 address.
 */
if (host == NULL && hints.ai_family == AF_UNSPEC)
    hints.ai_family = AF_INET;

"host == NULL" means you don't designate an IP address for server, like following example:

# nc -l 3003