Session

The minecraft client and server communicate with minecraft.net to validate player sessions. This page describes some of the operations.

Login

To log the player in, the official launcher sends an HTTPS POST (GET appears to suffice as well) request to:

https://login.minecraft.net

with the postdata:

 ?user=<username>&password=<password>&version=<launcher version>

and a "application/x-www-form-urlencoded" Content-Type header.

The current launcher version is "13", sending a value lower "12" than this will cause the server to return "Old version", however you can send any large number and it will return as expected. If the login succeeded, it will return 5 ':' delimited values.

 1343825972000:deprecated:SirCmpwn:7ae9007b9909de05ea58e94199a33b30c310c69c:dba0c48e1c584963b9e93a038a66bb98

1. current version of the game files (not the launcher itself). This is a unix timestamp which the launcher compares to the ~/.minecraft/bin/version file.
2. Previously contained a download ticket for requesting new versions of minecraft.jar from the server. Now contains only "deprecated".
3. case-correct username.
4. sessionId - a unique ID for your current session.
5. UID - currently unused, introduced near August 8th, 2012. Grum says this is the unique ID for the user, potentially for changing Minecraft names in future.

If the request is missing a parameter, the server will return "Bad response". If the login information is incorrect, the server will return "Bad login". If the login information is correct but the account isn't premium, the server will return "User not premium".

Updating

To update, the launcher downloads files from

http://s3.amazonaws.com/MinecraftDownload/

It takes the unix timestamp from the login request and stores it in ~/.minecraft/bin/version
The following files are downloaded from /MinecraftDownload/ in this exact order:

• lwjgl.jar
• jinput.jar
• lwjgl_util.jar
• minecraft.jar

And then the natives.jar appropriate for the following operating systems: windows, linux, macosx, and solaris

• <os-from-list-above>_natives.jar.lzma

When downloading minecraft.jar, GET variables are set as follows: ?user=foo&ticket=deprecated, although they seem unnecessary to successfully downloading the file.
Furthermore, the downloaded files are all verified via MD5 which is sent via ETag by the server on the page for each file.

Further resources are stored in /MinecraftResources. Loading /MinecraftResources provides an XML formatted list of resources, some of which are downloaded by Minecraft and not the launcher.

Keep-alive

Every 6000 ticks, the client sends an HTTPS request to

https://login.minecraft.net/session?name=<username>&session=<session id>

The client discards the server's response.

Snoop

Every 15 minutes, minecraft sends a HTTP POST request giving stats. This can be disabled in server.properties

• client: http://snoop.minecraft.net/client?version=1
• server: http://snoop.minecraft.net/server?version=1

Field	Client	Server	Example	Notes
applet	Yes	No	true
avg_rec_packet_count	No	Yes	0
avg_rec_packet_size	No	Yes	0
avg_sent_packet_count	No	Yes	6
avg_sent_packet_size	No	Yes	44
avg_tick_ms	No	Yes	3
client_brand	Yes	No	vanilla	via ClientBrandRetriever.getClientModName()
cpu_cores	Yes	Yes	2	via Runtime.getRuntime().availableProcessors()
dedicated	No	Yes	true
display_frequency	Yes	No	60	via org.lwjgl.opengl.Display.getDisplayMode().getFrequency()
display_type	Yes	No	windowed	windowed or fullscreen
fps	Yes	No	60
gl_caps[key]	Yes	No	varies by key	via org.lwjgl.opengl.GLContext.getCapabilities().GL_ARB_key
gl_max_texture_size	Yes	No	8192
gui_state	No	Yes	disabled	enabled or disabled
gui_supported	No	Yes	headless	via java.awt.GraphicsEnvironment.isHeadless() (headless or supported)
java_version	Yes	Yes	1.6.0_24	via System.getProperty("java.version")
jvm_arg[n]	Yes	Yes	-Xms2800M	via java.lang.management.ManagementFactory.getRuntimeMXBean().getInputArguments() (-X args only)
jvm_args	Yes	Yes	2	see above
memory_free	Yes	Yes	2429082024	via Runtime.getRuntime().freeMemory()
memory_max	Yes	Yes	2813722624	via Runtime.getRuntime().maxMemory()
memory_total	Yes	Yes	2813722624	via Runtime.getRuntime().totalMemory()
opengl_vendor	Yes	No	NVIDIA Corporation	via GL11.glGetString(GL_VENDOR)
opengl_version	Yes	No	3.3.0 NVIDIA 302.17	via GL11.glGetString(GL_VERSION)
os_architecture	Yes	Yes	amd64	via System.getProperty("os.arch")
os_name	Yes	Yes	Linux	via System.getProperty("os.name")
os_version	Yes	Yes	3.2.0-3-amd64	via System.getProperty("os.version")
players_current	No	Yes	0
players_max	No	Yes	20
players_seen	No	Yes	4	counts files in world/players
server_brand	No	Yes	vanilla	via getServerModName() (hardcoded to "vanilla", overwritten by bukkit et al)
singleplayer	No	Yes	false	whether or not the server is running to support a singleplayer session
snooper_count	Yes	Yes	0	how many times the snooper has reported so far
snooper_token	Yes	Yes	dfe921ac-0293-4e42-8d99-1316cdd626d8	via java.util.UUID.randomUUID().toString() (unique per program launch)
texpack_name	Yes	No	Default
texpack_resolution	Yes	No	16
uses_auth	No	Yes	true	corresponds to online-mode in server.properties
version	Yes	Yes	1.4.2
vsync_enabled	Yes	No	true
whitelist_count	No	Yes	0
whitelist_enabled	No	Yes	false
world[n][chunks_loaded]	No	Yes	978
world[n][difficulty]	No	Yes	1
world[n][dimension]	No	Yes	0
world[n][generator_name]	No	Yes	default
world[n][generator_version]	No	Yes	1
world[n][hardcore]	No	Yes	false
world[n][height]	No	Yes	256
world[n][mode]	No	Yes	SURVIVAL
worlds	No	Yes	3

Joining a Server

See Protocol Encryption for details on encrypting connections.

Client operation

1. Client connects to server
2. Client sends a 0x02 handshake containing the current player name
3. Client receives an 0xFD encryption request with the server's public key and four verification bytes.
4. Client generates a symmetric key.
5. Client sends a HTTP request to

   http://session.minecraft.net/game/joinserver.jsp?user=<username>&sessionId=<session id>&serverId=<server hash>

   If the response is OK then continue, otherwise stop

6. Client sends 0xFC encryption response - client encrypts shared secret and verification token with the server's public key
7. Server checks validity and sends 0xFC encryption response with two empty arrays
8. Both sides enable AES/CFB2 encryption using the shared secret generated by the client
9. Client sends 0xCD client status with a payload of 0 (ready to spawn)
10. Server sends 0x01 login
11. ... receive map chunks, etc...

Server operation

1. Server generates a 1024-bit RSA keypair.
2. ...
3. Server answers TCP connection request and receives a 0x02 handshake from the client.
4. Server sends 0xFD encryption request with its public key and a verification token.
5. Server receives 0xFC encryption response from client and decrypts the shared secret.
6. Server verifies the validity of the verification token. If this isn't verified, it kicks the client.
7. Server sends a HTTP request to

   http://session.minecraft.net/game/checkserver.jsp?user=<username>&serverId=<server hash>

   If it returns YES then the client is authenticated and allowed to join. Otherwise the client will/should be kicked with “Failed to verify username!”

8. Server sends a 0xFC encryption response packet with two empty arrays.
9. Both sides enable AES/CFB2 encryption.
10. Server receives 0xCD client status with a payload of , indicating "ready to spawn"
11. Server sends 0x01 login
12. ... send map chunks, etc...