I have set up a socks5 proxy on my network and am able to play single-player with it fine.

When I try to use multiplayer though, the connections to the servers are not being sent through the proxy but are being attempted directly to the servers – which is refused by network policy.

Here is a tcpdump showing minecraft trying to make a direct (unproxied) connection:

14:09:06.141895 IP 10.75.22.4.44171 > 69.197.20.153.25565: Flags [S], seq 2204568652, win 14600, options [mss 1460,sackOK,TS val 474199730 ecr 0,nop,wscale 5], length 0

I start minecraft with:

java -Dhttp.proxyHost=proxy -Dhttp.proxyPort=3128 -Dhttps.proxyHost=proxy -Dhttps.proxyPort=3128 -Dhttp.proxyUser=brianm -Dhttp.proxyPassword=brian -Dhttps.proxyUser=brianm -Dhttps.proxyPassword=brian -Xmx800m -jar Minecraft\ \(1\).jar --proxyHost=proxy --proxyPort=1080 --proxyUser=brianm --proxyPass=brian

I realize that the -Dhttp* java options may not be relevant with regards to socks5 proxies but I have tried the above with and without those parameters and it doesn't work either way.