Unable to get DNS data in restricted network, no public function to customize Netty's DNS

[SK-Yang]

I'm using Pushy 0.15.2, and I have encounted UnknownHostException: Failed to resolve 'api.push.apple.com' after 2 queries errors in restricted network. After some investigation, it turn out that Netty cannot use sun.net.dns.ResolverConfiguration correctly on Java 17, forced it fallback to use Google DNS instead of local DNS server. Asked Netty's dev and told me that I can specify the dns servers to use when creating the DnsNameResolver via DnsNameServerBuilder. Since it will be deployed onto restricted network which doesn't allow to use public DNS, so this is a MUST choose.

Since it is hidden deep inside the Pushy and I don't think I see there's any public function from Pushy that I can input a DnsNameResolver object. So I want to ask if there's anyway to do this without fiddling Pushy's source code?

[petarov]

There have been a number of issues about this already, but you bring a nice insight about the DnsNameResolver. Looking at this part of the code in Netty 4.1.x (what Pushy currently uses), one should also be able to use the system property io.netty.resolver.dns.defaultNameServerFallback to specify a semicolon-separate CSV list of name servers.

https://github.com/netty/netty/blob/netty-4.1.119.Final/resolver-dns/src/main/java/io/netty/resolver/dns/DefaultDnsServerAddressStreamProvider.java#L102

Assuming you are on Windows, could you confirm that specifying your restricted network's DNS servers via this property would work? It would be an interesting find.

[SK-Yang]

There have been a number of issues about this already, but you bring a nice insight about the DnsNameResolver. Looking at this part of the code in Netty 4.1.x (what Pushy currently uses), one should also be able to use the system property io.netty.resolver.dns.defaultNameServerFallback to specify a semicolon-separate CSV list of name servers.

https://github.com/netty/netty/blob/netty-4.1.119.Final/resolver-dns/src/main/java/io/netty/resolver/dns/DefaultDnsServerAddressStreamProvider.java#L102

Assuming you are on Windows, could you confirm that specifying your restricted network's DNS servers via this property would work? It would be an interesting find.

Yes, it could be solved by using system property before initialize pushy