Debugging Minecraft packets

log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Appenders>
        <Console name="SysOut" target="SYSTEM_OUT">
            <PatternLayout pattern="[%d{HH:mm:ss}] [%t/%level]: %msg%n" />
            <Filters>
               <RegexFilter regex=".*(keep_alive|client_tick_end|set_time|move_player_rot|move_player_pos|move_entity_pos_rot|move_entity_pos|set_entity_motion|rotate_head).*" onMatch="DENY" onMismatch="ACCEPT" />
            </Filters>
        </Console>
        <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>
            <DefaultRolloverStrategy max="1000"/>
        </RollingRandomAccessFile>
    </Appenders>
    <Loggers>
        <!-- Logger specifico per NETWORK_PACKETS -->
        <Logger name="com.mojang.network" level="debug" additivity="false">
            <filters>
                <MarkerFilter marker="NETWORK_PACKETS" onMatch="ACCEPT" onMismatch="DENY" />
            </filters>
            <AppenderRef ref="SysOut"/>
            <AppenderRef ref="File"/>
        </Logger>

        <!-- Root logger per tutti gli altri log -->
        <Root level="debug">
            <AppenderRef ref="SysOut"/>
            <AppenderRef ref="File"/>
        </Root>
    </Loggers> 
</Configuration>

RegexFilter is used to ignore some packets, to avoid log spam. You can enable some if you need them.

Put this file in any folder of your PC and set this java argument in your client:
-Dlog4j.configurationFile="file:///C:/your_path_here/log4j2.xml"