Unable to download mojang.jar on an IPv6-only server

[rudolf-enum]

Expected behavior

I expected the server to spin up normally.

Observed/Actual behavior

When the server has an IPv4 address that doesn't actually route to the internet, it just hangs.

Upon removing the IPv4 address, the server immediately crashes.

$ java -jar /opt/server.jar
Downloading mojang_1.21.11.jar
Failed to download mojang_1.21.11.jar
        at java.base/sun.nio.ch.Net.connect(Net.java:589)
        at java.base/sun.nio.ch.Net.connect(Net.java:578)
        at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:583)
        at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
        at java.base/java.net.Socket.connect(Socket.java:751)
        at java.base/sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:304)
        at java.base/sun.security.ssl.BaseSSLSocketImpl.connect(BaseSSLSocketImpl.java:181)
        at java.base/sun.net.NetworkClient.doConnect(NetworkClient.java:183)
        at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:531)
        at java.base/sun.net.www.http.HttpClient.openServer(HttpClient.java:636)
        at java.base/sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264)
        at java.base/sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:377)
        at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:193)
        at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1257)
        at java.base/sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1143)
        at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:179)
        at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1705)
        at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1629)
        at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:223)
        at java.base/java.net.URL.openStream(URL.java:1325)
        at io.papermc.paperclip.DownloadContext.download(DownloadContext.java:55)
        at io.papermc.paperclip.Paperclip.setupClasspath(Paperclip.java:63)
        at io.papermc.paperclip.Paperclip.main(Paperclip.java:28)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at io.papermc.paperclip.Main.main(Main.java:26)

Steps/models to reproduce

1. Spin up a Linux VM/VPS/whatever with IPv6 connectivity
2. Ensure no IPv4 connectivity (this maybe done by checking ips ip a and deleting any IPv4 addresses, e.g. ip addr del 123.123.123.123/32 dev <DEVICE>)
3. java -jar paper.jar

Plugin and Datapack List

(none yet)

Paper version

I cannot run /version as the server is not coming up.

1.21.11 build #74

Other

The server from the official Minecraft Server Download link (which is currently hosted at piston-data.mojang.com) can be downloaded just fine as it does have an IPv6 address, it's just the .jar itself that doesn't seem to like that.

More and more providers (both internet providers and Cloud/VPS providers) charge non-negligeable amounts of money for an IPv4 address, and most Cloud platforms don't provide free NAT either.

Paper does not even show where it's downloading the server from, and it does not show where it expects the server to be. Ideally, I would like to be able to disallow any outgoing/un-established internet connections for better security.

[electronicboy]

Testing this myself, it appears that unless I set -Djava.net.preferIPv6Addresses=system the thing still tries to use IPv4 even though no real IPv4 addresses exist on the machine, this would generaly be out of our control;

paperclip downloads the jar from piston-data, I'm not really sure what extra data would be expected here, I think that past the first startup it shouldn't do any network stuff as first shot requires a vanilla server jar to patch