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Java Proxy Fix

This is a Java agent that remaps all calls to java.net.URL.openConnection(java.net.Proxy proxy) to java.net.URL.openConnection(), meaning that URL.openConnection will always ignore the proxy supplied to it and use the system proxy instead (e.g. the proxy specified by the http.proxyHost and https.proxyHost system properties.

This is useful for MITMing communication between Minecraft servers and Minecraft authentication servers (such as Drasl). Minecraft often calls URL.openConnection(Proxy.NO_PROXY) when communicating with authentication servers (see usages of YggdrasilAuthenticationService in the Minecraft source). So we can use this Java agent to get the Minecraft server to respect -Dhttps.proxyHost and inspect the traffic using mitmproxy or similar.

I could have sworn I'd seen something like this before, but I couldn't find it, so I wrote another one with some help from the LLM.

Example usage

java -Xmx512M -Xms512M \
	-Dminecraft.api.env=custom \
	-Dminecraft.api.auth.host=https://drasl.unmojang.org/auth \
	-Dminecraft.api.account.host=https://drasl.unmojang.org/account \
	-Dminecraft.api.session.host=https://drasl.unmojang.org/session \
	-Dminecraft.api.services.host=https://drasl.unmojang.org/services \
	-Djavax.net.ssl.trustStore=/home/user/cacerts -Djavax.net.ssl.trustStorePassword=changeit -Dhttp.proxyHost=localhost -Dhttp.proxyPort=8080 -Dhttps.proxyHost=localhost -Dhttps.proxyPort=8080 \
	-javaagent:ProxyFix-1.0-SNAPSHOT-jar-with-dependencies.jar \
	-jar server.jar nogui

See also https://github.com/unmojang/drasl/blob/master/doc/troubleshooting.md.