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Modules » m_spanningtree » Connecting a Server

To connect to a server we first proceed with the authentication phase, as detailed in the section below.

The Authentication Phase

During the authentication phase, a three-way handshake is performed where both servers exchange their passwords. Unlike most IRC servers, the notable exception being ircu, this must be done in lock-step fashion, e.g. one server sends a request then waits for the other server to reply before sending another. Note that once the authentication phase is complete, the data flow moves to being full duplex and no such restrictions apply once the BURST command is received.

Step One - Connection

Server A, wishing to connect to the network, connects to a TCP port which server B is listening on. When the connection is established, it immediately sends the CAPAB and SERVER commands to server B which is waiting for inbound traffic. Up until this point, server B will do nothing but accept the connection and reply with its CAPAB.

Transport-Layer Handshakes

If there are any transport modules attached to the connection, such as GnuTLS, then this is the point where they perform their handshakes. The server should wait before sending anything to ensure the handshake has completed. The details of what is sent at the transport layer are dependent upon the module, and outside the scope of this page.

Step Two - Inbound Authentication

Server B, after receiving server A's SERVER command, analyzes the parameters (namely server name and password) and if it is not happy with the credentials, should instantly send a suitable ERROR reply, and close its connection. However, if server B deems the credentials correct for server A, server B then sends its own SERVER command, with its own credentials in to authenticate to server A.

Step Three - Outbound Authentication

Server A, upon receiving server B's credentials, analyzes them (again, namely the server name and password), and again, if it is not happy, it can send an ERROR reply, and close its connection. If the details are correct however, server A now sends a BURST command (see the commands list below) and begins its network burst.

Step Four - Network Burst

Upon receiving server A's network burst, server B also sends its own network burst, preceded again by the BURST command. When both servers have finished bursting, they send the ENDBURST command. BURST and ENDBURST are sent both for local, and for remote bursts. In the case of remote bursts, the BURST and ENDBURST command must be correctly prefixed with the server ID they originate from.

The order which data is sent during the burst is important, and must be:

  1. The names of any servers connected to this one, directly or otherwise, with relative hop counts, 1 being a direct connection (these will be re-calculated by the receiving side). The server names should be introduced using recursion or similar so that each server only appears after its parent server in the list.

  2. The nicknames of any users, sent using UID, their OPERTYPE if they are an oper.

  3. The membership and privileges of all channels using FJOIN (see the commands list) and the modes (sent using FMODE), topics (sent using FTOPIC) and bans (again, sent using FMODE) for each channel in turn (no specific order of channels).

  4. Any module specific data (which may be a combination of FMODE and any other module-specific commands).

Example Authentication Phase

This is an example of a successful authentication using plaintext password authentication:

>> SERVER password 0 666 :Description here
<< SERVER password 0 491 :ChatSpike InspIRCd test server
>> :066 BURST 1176144272
<< :491 BURST 1176144273

Both servers now operate in duplex mode sending data freely

Challenge-Response and HMAC authentication

InspIRCd also supports challenge-response HMAC authentication for server passwords. This ensures that the password itself is never transmitted in any usable form across the server link.

To facilitate this in a backwards compatible manner, the challenge string is placed into the CAPAB CAPABILITIES line of the servers which support it and have it enabled. The CHALLENGE string should be sufficiently random to prevent brute-forcing and mathematical attacks against it. The current InspIRCd implementation collects entropy from /dev/urandom.

A challenge should never be sent to the other server if:

  1. HMAC is disabled
  2. The m_sha256 module, required for the cryptographic hash, is not loaded

Here is an example of a HMAC enabled authentication phase, which is described in greater detail below the example:

-> SERVER HMAC-SHA256:185ea67956256a3703bae0cb9324a655180f5b7089224539e08cf1003b296f9c 0 666 :Test server
<- SERVER HMAC-SHA256:76a7d469c3dcab86067495d1b3407720fce9f7286400835baa2972e75e891a65 0 491 :"Bollocks" Said pooh, as he caught his testicles in the vice.
-> :666 BURST 1176144272
<- :491 BURST 1176144273

When authenticating via HMAC, both sides of the connection should wait for the complete CAPAB information before sending their SERVER line. This is to allow for both sides to send their challenge, which is encapsulated in the CAPAB CAPABILITIES as shown above.

HMAC implementation details

The challenge string is hashed against the password using a simple HMAC algorithm, as shown below, before being sent back to the other side in the password section:

sha256( (p xor 0x5C) + sha256((p xor 0x36) + m) )

Where 'm' is the challenge string given in the other side's CAPAB, and 'p' is the password you wish to send. Each character of 'p' should be exclusive-or'ed against 0x5C or 0x36, and in the above equation, + indicates string concatenation, not integer addition.

Due to limitations inherent in the protocol and CAPAB notation, the equals (=) symbol may not form part of the CHALLENGE value. Any equals symbols should be translated to non-equals, in the current implementation, they are translated to an underscore (_).

IMPORTANT NOTE: In the current implementation, the SHA256 output is ASCII text, e.g. the hexadecimal hash value and not raw binary data.

When the server replies with its calculated HMAC, it should prepend the value "HMAC-SHA256:" to the hash. This is to distinguish it from a plaintext password, and allow servers which do not support HMAC, such as services and past versions of InspIRCd, to link correctly.

When checking the HMAC against what is expected, the checking server should fall back to plaintext authentication if:

  1. The sender has not sent a HMAC password
  2. We never sent or received a CHALLENGE
  3. HMAC is disabled
  4. The m_sha256 module, required for the cryptographic hash, is not loaded
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