don't crash on rDNS failure

Node apparently throws even on valid ipv6 input in certain environments,
probably due to the DNS server returning SERVFAIL.
Guard against it and fallback with the plain IP

Fixes: #4768

server/server.ts

@@ -1015,28 +1015,40 @@ function performAuthentication(this: Socket, data) {
 }
 
 function reverseDnsLookup(ip: string, callback: (hostname: string) => void) {
-	dns.reverse(ip, (reverseErr, hostnames) => {
-		if (reverseErr || hostnames.length < 1) {
-			return callback(ip);
-		}
-
-		dns.resolve(hostnames[0], net.isIP(ip) === 6 ? "AAAA" : "A", (resolveErr, resolvedIps) => {
-			// TODO: investigate SoaRecord class
-			if (!Array.isArray(resolvedIps)) {
+	// node can throw, even if we provide valid input based on the DNS server
+	// returning SERVFAIL it seems: https://github.com/thelounge/thelounge/issues/4768
+	// so we manually resolve with the ip as a fallback in case something fails
+	try {
+		dns.reverse(ip, (reverseErr, hostnames) => {
+			if (reverseErr || hostnames.length < 1) {
 				return callback(ip);
 			}
 
-			if (resolveErr || resolvedIps.length < 1) {
-				return callback(ip);
-			}
+			dns.resolve(
+				hostnames[0],
+				net.isIP(ip) === 6 ? "AAAA" : "A",
+				(resolveErr, resolvedIps) => {
+					// TODO: investigate SoaRecord class
+					if (!Array.isArray(resolvedIps)) {
+						return callback(ip);
+					}
 
-			for (const resolvedIp of resolvedIps) {
-				if (ip === resolvedIp) {
-					return callback(hostnames[0]);
-				}
-			}
+					if (resolveErr || resolvedIps.length < 1) {
+						return callback(ip);
+					}
 
-			return callback(ip);
+					for (const resolvedIp of resolvedIps) {
+						if (ip === resolvedIp) {
+							return callback(hostnames[0]);
+						}
+					}
+
+					return callback(ip);
+				}
+			);
 		});
-	});
+	} catch (err) {
+		log.error(`failed to resolve rDNS for ${ip}, using ip instead`, (err as any).toString());
+		setImmediate(callback, ip); // makes sure we always behave asynchronously
+	}
 }