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The Yahoo! Messenger Protocol is an application layer protocol running most of the time over TCP, but in some cases over HTTP as well. Throughout this document, we will speak about the YMSG packets, after stripping out any other protocol data, but will mention this other data if it is of relevance. This document is incomplete. For anything not mentioned here, refer to the source of libyahoo2.

The YMSG protocol was documented by Philip Tellis as part of the libyahoo2 project. The original document is available at http://libyahoo2.sourceforge.net/ymsg-9.txt

Contents 1 The YMSG packet structure 2 Status codes 3 Session states 3.1 Authentication 3.2 Messaging state 4 Requests 4.1 Authentication 4.2 Sending a message 4.3 Send typing start/stop notification 4.4 Set status 4.5 Logoff 4.6 Keep alive - sent every 20 minutes 4.7 Add buddy 4.8 Remove buddy 4.9 Reject buddy add 5 See also 6 External links

The YMSG packet structure is as follows (each byte is represented by 5 spaces in the following diagram, including the | at the end):

    <------- 4B -------><------- 4B -------><---2B--->
   +-------------------+-------------------+---------+
   |   Y   M   S   G   |      version      | pkt_len |
   +---------+---------+---------+---------+---------+
   | service |      status       |    session_id     |
   +---------+-------------------+-------------------+
   |                                                 |
   :                    D A T A                      :
   |                   0 - 65535*                    |
   +-------------------------------------------------+
   

YMSG

The first four bytes of all packets are always YMSG - the protocol name.

version

The next four bytes are for the protocol version number. For version 9, these are 0x09 0x00 0x00 0x00

NOTE: The last three bytes of this may just be padding bytes.

pkt_len

A two byte value, in network byte order, stating how many bytes are in the _data_ section of the packet. In practice, this value does not exceed about 1000.

service

This is an opcode that tells the client/server what kind of service is requested/being responded to. There are 45 known services. See the services section of this document for a full listing.

status

In case of a response from the server, indicates the status of the request (success/failure/etc.). For a request, it is 0 in most cases, except for packets that set the user's status (set status, typing notify, etc.)

session

The session id is used primarily when connecting through a HTTP id proxy. It is set in all cases, but has no effect in a direct connection. When the client sends the first packet, it is 0, the server responds with a session id that is used by the client and the server in all further packets. The server may change the session id, in which case the client must use the new session id henceforth.

DATA

The data section is pkt_len bytes long, having the theoretical limit of 65535 bytes, since the length field is two bytes long. Practically though, the data section does not exceed about 1000 bytes. This section consists of a series of key/value pairs. All keys are numeric strings. The packet contains their numeric values in the ASCII character set. e.g.:

1 == 0x31, 21 == 0x32 0x31

All numeric fields are stored in network byte order. i.e. Most significant byte first.

The maximum number of digits in a key is unknown, although keys of up to three digits have been seen. Every key and value is terminated by a two byte sequence of 0xc0 0x80. Some keys may have empty values. The actual keys sent, and their meanings depend on the service in use. e.g. The packet data to send an instant message looks like this:

0x30 0xc080 yahoo_id 0xc080 0x31 0xc080 active_id 0xc080 0x35 
0xc080 recipient_id 0xc080 0x3134 0xc080 message_text 0xc080

The 0xc080 byte sequence is a separator. The values 0x30, 0x31, 0x35 and 0x3134 are the keys. Convert them to their ASCII equivalents and you get 0, 1, 5, 14 (0x3134 == 0x31 0x34)

♦== Services == There are 45 known services at the moment, although more may exist. All known services are listed below along with the hex values that they correspond to. Any service without a hex value is one more than the previous value. i.e. YAHOO_SERVICE_LOGOFF=0x02 and YAHOO_SERVICE_ISBACK=0x04.

YAHOO_SERVICE_LOGON            = 0x01
YAHOO_SERVICE_LOGOFF
YAHOO_SERVICE_ISAWAY
YAHOO_SERVICE_ISBACK
YAHOO_SERVICE_IDLE             = 0x05
YAHOO_SERVICE_MESSAGE
YAHOO_SERVICE_IDACT
YAHOO_SERVICE_IDDEACT
YAHOO_SERVICE_MAILSTAT
YAHOO_SERVICE_USERSTAT         = 0x0a
YAHOO_SERVICE_NEWMAIL
YAHOO_SERVICE_CHATINVITE
YAHOO_SERVICE_CALENDAR
YAHOO_SERVICE_NEWPERSONALMAIL
YAHOO_SERVICE_NEWCONTACT       = 0x0f
YAHOO_SERVICE_ADDIDENT         = 0x10
YAHOO_SERVICE_ADDIGNORE
YAHOO_SERVICE_PING
YAHOO_SERVICE_GROUPRENAME
YAHOO_SERVICE_SYSMESSAGE       = 0x14
YAHOO_SERVICE_PASSTHROUGH2     = 0x16
YAHOO_SERVICE_CONFINVITE       = 0x18
YAHOO_SERVICE_CONFLOGON
YAHOO_SERVICE_CONFDECLINE      = 0x1a
YAHOO_SERVICE_CONFLOGOFF
YAHOO_SERVICE_CONFADDINVITE
YAHOO_SERVICE_CONFMSG
YAHOO_SERVICE_CHATLOGON
YAHOO_SERVICE_CHATLOGOFF       = 0x1f
YAHOO_SERVICE_CHATMSG          = 0x20
YAHOO_SERVICE_GAMELOGON        = 0x28
YAHOO_SERVICE_GAMELOGOFF
YAHOO_SERVICE_GAMEMSG          = 0x2a
YAHOO_SERVICE_FILETRANSFER     = 0x46
YAHOO_SERVICE_VOICECHAT        = 0x4a
YAHOO_SERVICE_NOTIFY           = 0x4b
YAHOO_SERVICE_P2PFILEXFER      = 0x4d
YAHOO_SERVICE_PEERTOPEER       = 0x4f
YAHOO_SERVICE_AUTHRESP         = 0x54
YAHOO_SERVICE_LIST             = 0x55
YAHOO_SERVICE_AUTH             = 0x57
YAHOO_SERVICE_ADDBUDDY         = 0x83
YAHOO_SERVICE_REMBUDDY
YAHOO_SERVICE_IGNORECONTACT
YAHOO_SERVICE_REJECTCONTACT
YAHOO_SERVICE_PICTURE          = 0xbe

Most of the service codes should be self explanatory. Those that aren't are listed here:

IDACT/IDDEACT - activate/deactivate an identity
NOTIFY        - typing/game notification
FILETRASNFER  - transfer a file using the yahoo filetransfer server as an intermediate
P2PFILEXFER   - transfer a file between two peers, yahoo server not used
PEERTOPEER    - check if peer to peer connections are possible
AUTH          - Send initial login packet (username), response contains challenge string
AUTHRESP      - Send response to challenge string, or, if received from server, contains reason for login failure
LOGON/LOGOFF  - a buddy logged in/out

The status code is a four byte value. Most status codes are two bytes long. The status codes (in decimal except for offline and typing) are:

YAHOO_STATUS_AVAILABLE    = 0
YAHOO_STATUS_BRB
YAHOO_STATUS_BUSY
YAHOO_STATUS_NOTATHOME
YAHOO_STATUS_NOTATDESK
YAHOO_STATUS_NOTINOFFICE  = 5
YAHOO_STATUS_ONPHONE
YAHOO_STATUS_ONVACATION
YAHOO_STATUS_OUTTOLUNCH
YAHOO_STATUS_STEPPEDOUT   = 9
YAHOO_STATUS_INVISIBLE    = 12
YAHOO_STATUS_CUSTOM       = 99
YAHOO_STATUS_IDLE         = 999
YAHOO_STATUS_OFFLINE      = 0x5a55aa56
YAHOO_STATUS_TYPING       = 0x16

You may choose either AVAILABLE or INVISIBLE as your initial login status. TYPING is used only when sending a TYPING notification packet.

A Yahoo! session has two states, Authentication and Messaging.

The session starts in the authentication state. The client sends the username to the server. The server responds with a challenge string. The client responds to this challenge with two response strings. If authentication is successful, the connection goes into the messaging state, else, an error response is sent back.

After successful authentication, the session goes into the messaging state. The server sends the buddy list, ignore list, identity list and a list of cookies to the client. These might all be sent in a single packet. It then sends the list of online buddies along with their status codes. All this is sent without the client requesting anything.

At this time, any offline messages are also delivered to the client.

In the messaging state, a client may send/receive messages, join conferences, send/receive files, change state, etc.

Messaging state is terminated when the user goes offline by sending a LOGOFF packet.

The first packet sent from the client is the authentication request packet. This consists of the user's yahoo id, or any identity corresponding to that yahoo id. The AUTH packet has one key/value pair.

service: YAHOO_SERVICE_AUTH
status:  YAHOO_STATUS_AVAILABLE

1: yahoo_id

The server responds with a Challenge string. The client then hashes the username and password with this challenge string, and sends it back as an AUTH_RESP packet.

service: YAHOO_SERVICE_AUTHRESP
status:  initial login status

0: yahoo_id
6: response_string_1
96: response_string_2
1: active_id

service: YAHOO_SERVICE_MESSAGE
status:  0

0: yahoo_id
1: active_id
5: recipient_id
14: message to send

service: YAHOO_SERVICE_NOTIFY
status:  YAHOO_STATUS_TYPING

4: active_id
5: recipient_id
13: 1 or 0 depending on whether this is a typing start or stop             packet
14: 
49: TYPING /* The literal string */

service: YAHOO_SERVICE_ISBACK or YAHOO_SERVICE_ISAWAY
status:  the status to set to

10: status_code if custom_status
19: custom away message
47: 0 or 1 depending on whether it is away or not

service: YAHOO_SERVICE_LOGOFF
status:  YAHOO_STATUS_AVAILABLE

no key value pairs

service: YAHOO_SERVICE_PING
status:  YAHOO_STATUS_AVAILABLE

no key value pairs

service: YAHOO_SERVICE_ADDBUDDY
status:  YAHOO_STATUS_AVAILABLE

2: yahoo_id
7: buddy_to_add
65: group to add to

service: YAHOO_SERVICE_REMBUDDY
status:  YAHOO_STATUS_AVAILABLE

1: yahoo_id
7: buddy_to_remove
65: group to remove from

service: YAHOO_SERVICE_REJECTCONTACT
status:  YAHOO_STATUS_AVAILABLE

1: yahoo_idnnm,
7: buddy_to_reject
14: reject message

• YMSG
• Comparison of instant messaging protocols

• ymsg-9 protocol docs on the libyahoo2 page
• YMSG Java API - Yahoo! Instant Messenger Support for Java
• Hamsam - Multi-protocol instant messaging API for Java
• libyahoo2 - A C library for Yahoo! Messenger
• Net::YMSG - Perl Interface to the Yahoo! Messenger IM protocol
• YahEh - Open Source VB Yahoo Chat Client
• Yahoo! Coders Cookbook
• Exploiting Yahoo Messenger Protocol - Demonstration of a Yahoo invisible status plugin
• Tracking the Yahoo users in 24 hours - Yahoo protocol web based service
• Yahoo-Owned