Background:

Minecraft is using JNDI to resolve SRV entries from DNS when connecting to server (code below).

They received a report about this functionality not working (https://bugs.mojang.com/browse/MC-232009) and it seems that following steps are required to reproduce:
· User is on Windows
· IPv6 stack enabled, but no connectivity
· At least one DNSv6 server is configured

The original report mentions this happening only with non-default ports, while it’s just due to that specific server configuration (redirect pointing to the same host, but different port)

Simplified code:

Hashtable<String, String> env = new Hashtable<>();
env.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
env.put("java.naming.provider.url", "dns:");
env.put("com.sun.jndi.dns.timeout.retries", "1");
DirContext context = new InitialDirContext(env);
Attributes attributes = context.getAttributes("_minecraft._tcp." + host, new String[]{"SRV"});
Attribute srvAttribute = attributes.get("srv");

Problem:

SRV resolver is always throwing java.io.UncheckedIOException: java.net.SocketException: Network is unreachable: connect, even if there are also valid IPv4 DNS servers configured.

Analysis:

· JDK internals changed in 15 (JDK-7006496) to now also return IPv6 DNS entries (and usually they are listed before v4 entries)
· Datagram socket started throwing UncheckedIOException on connection failure (JDK-8232817 + JDK-8235783) - https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/nio/ch/DatagramSocketAdaptor.java#L120
· During DNS query (https://github.com/openjdk/jdk/blob/master/src/jdk.naming.dns/share/classes/com/sun/jndi/dns/DnsClient.java#L171-L319) method is supposed to check all servers and ignore failed ones (see doNotRetry variable), but only handles few exception types and not the one thrown from `doUdpQuery` call on line 214.