[Robustness] Crashes if QTYPE is unknown

[bortzmeyer]

There are several robustness issues in zig-dns. If a DNS server using zig-dns is exposed to the public Internet, it can crash (or worse) if the incoming packet is badly formed. Such invalid packets are common on the Internet, either by mistake or as deliberate attack.

First example, when the QTYPE is unknown, instead of returning an error (or, better, the QTYPE as just a number), zig-dns crashes:

...
 Questions {
    Question {
      Name: 
thread 192525 panic: invalid enum value
???:?:?: 0x2c6515 in format__anon_10730 (incorrect)
???:?:?: 0x2bbb9a in formatType__anon_10378 (incorrect)
???:?:?: 0x2bbb23 in format__anon_10367 (incorrect)
???:?:?: 0x2ad010 in print__anon_10104 (incorrect)
???:?:?: 0x29e807 in format__anon_9733 (incorrect)
???:?:?: 0x28c00f in formatType__anon_9306 (incorrect)
???:?:?: 0x28bf36 in format__anon_9294 (incorrect)
???:?:?: 0x276c60 in print__anon_8061 (incorrect)
???:?:?: 0x24338c in print__anon_5074 (incorrect)
???:?:?: 0x22c7db in main (incorrect)
???:?:?: 0x22bf25 in posixCallMainAndExit (incorrect)
???:?:?: 0x22ba11 in _start (incorrect)
???:?:?: 0x0 in ??? (???)
zsh: IOT instruction  ./incorrect

Here is the code to reproduce the issue:

const std = @import("std");
const io = std.io;
const dns = @import("zig-dns/src/dns.zig");

pub fn main() anyerror!void {
    var myallo = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = myallo.allocator();
    const size = 17;
    const data: [size]u8 = .{
        0x0, 0x0, // ID
        0x80, 0x0, // QR=reply, opcode, flags, rcode
        0x0, 0x1, // QDCOUNT
        0x0, 0x0, // ANCOUNT
        0x0, 0x0, // NSCOUNT
        0x0, 0x0, // ARCOUNT
        0x0,  // root domain
        0xfe, 0x1, // RR type NOT REGISTERED UNKNOWN
        0x0, 0x1, // RR class
    };
    const response = try dns.Message.from_bytes(allocator, data[0..size]);
    defer response.deinit();
    std.debug.print("Response:\n{any}\n", .{response});
}

Solving the problem is not easy in Zig. Ma be using non-exhaustive enums? https://ziggit.dev/t/catching-invalid-enum-value-errors/