dns docs are missing that family = 0 for dns.lookup is allowed

[daprahamian]

• Node.js Version: 12.2
• OS: macOs
• Scope (install, code, runtime, meta, other?): runtime
• Module (and version) (if relevant): dns, net, tls

Quick question: is dns.lookup(url, 0, callback) supported as part of Node's API? The documentation says Must be 4 or 6, and any positive integer other than 4 or 6 will return a TypeError [ERR_INVALID_OPT_VALUE]: The value "2" is invalid for option "family". But passing in 0 does not error. Is this expected? If so, is this something stable that developers can rely on?

// The following are all expected to work
dns.lookup('www.google.com', 4, err => {
 console.log(err); // null
});
dns.lookup('www.google.com', 6, err => {
 console.log(err); // null
});
dns.lookup('www.google.com', err => {
 console.log(err); // null
});

// The following should error
try {
 dns.lookup('www.google.com', 1, () => {});
} catch (e) {
 console.log(e); // TypeError [ERR_INVALID_OPT_VALUE]: The value "1" is invalid for option "family"
}
try {
 dns.lookup('www.google.com', 2, () => {});
} catch (e) {
 console.log(e); // TypeError [ERR_INVALID_OPT_VALUE]: The value "2" is invalid for option "family"
}
try {
 dns.lookup('www.google.com', 3, () => {});
} catch (e) {
 console.log(e); // TypeError [ERR_INVALID_OPT_VALUE]: The value "3" is invalid for option "family"
}
try {
 dns.lookup('www.google.com', 5, () => {});
} catch (e) {
 console.log(e); // TypeError [ERR_INVALID_OPT_VALUE]: The value "5" is invalid for option "family"
}

// Asking about this one:
dns.lookup('www.google.com', 0, err => {
 console.log(err); // null <-- is this correct?
});

[addaleax]

0 stands for AF_UNSPEC, which, quoting the man page, means:

The value AF_UNSPEC indicates that getaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with node and service.

So, the Node.js documentation is clearly wrong, and omitting an important feature here (although it’s the default behaviour). I’m moving this over to the main repo as it’s a documentation bug.

[ZYSzys]

Fixed via #28163.