Debugging

Minecraft's client and server can log every packet which can be helpful for debugging servers.

   -Dlog4j.configurationFile=fullpathtoconfigfile.xml

The -Dlog4j.configurationFile=fullpathtoconfigfile.xml should be placed:

• In the JVM Arguments section of the launcher
• Before the -jar in the server launch command

where the config file should contain

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN" packages="net.minecraft,com.mojang">
    <Appenders>
        <Console name="SysOut" target="SYSTEM_OUT">
            <PatternLayout pattern="[%d{HH:mm:ss}] [%t/%level]: %msg%n" />
        </Console>
        <Queue name="ServerGuiConsole">
            <PatternLayout pattern="[%d{HH:mm:ss} %level]: %msg%n" />
        </Queue>
        <RollingRandomAccessFile name="File" fileName="logs/latest.log" filePattern="logs/%d{yyyy-MM-dd}-%i.log.gz">
            <PatternLayout pattern="[%d{HH:mm:ss}] [%t/%level]: %msg%n" />
            <Policies>
                <TimeBasedTriggeringPolicy />
                <OnStartupTriggeringPolicy />
            </Policies>
        </RollingRandomAccessFile>
    </Appenders>
    <Loggers>
        <Root level="debug">
            <filters>
                <MarkerFilter marker="NETWORK_PACKETS" onMatch="ACCEPT" onMismatch="NEUTRAL" />
            </filters>
            <AppenderRef ref="SysOut"/>
            <AppenderRef ref="File"/>
            <AppenderRef ref="ServerGuiConsole"/>
        </Root>
    </Loggers> 
</Configuration>

this will display packets in the launcher's console like so:

Client> [22:56:42] [Netty Client IO #5/DEBUG]: OUT: [PLAY:3] iq[]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:21] gw[id=1907, xa=4, ya=0, za=3]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:25] hg[id=1907, rot=-28]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:21] gw[id=1903, xa=11, ya=0, za=0]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:22] gy[id=1580, yRot=-49, xRot=0]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:25] hg[id=1580, rot=-49]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:18] hl[id=1025, x=0.00, y=-0.08, z=-0.01]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:21] gw[id=1025, xa=-5, ya=0, za=-2]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:25] hg[id=1340, rot=85]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:25] hg[id=1809, rot=-66]
Client> [22:56:42] [Netty Client IO #5/DEBUG]:  IN: [PLAY:25] hg[id=1471, rot=-103]