Daemon Protocol

General

This document describes the protocols used between the various daemons. As Bacula has developed, it has become quite out of date. The general idea still holds true, but the details of the fields for each command, and indeed the commands themselves have changed considerably.

It is intended to be a technical discussion of the general daemon protocols and as such is not targeted at end users but rather at developers and system administrators that want or need to know more of the working details of Bacula.

Low Level Network Protocol

At the lowest level, the network protocol is handled by BSOCK packets which contain a lot of information about the status of the network connection: who is at the other end, etc. Each basic Bacula network read or write actually consists of two low level network read/writes. The first write always sends four bytes of data in machine independent byte order. If data is to follow, the first four bytes are a positive non-zero integer indicating the length of the data that follow in the subsequent write. If the four byte integer is zero or negative, it indicates a special request, a sort of network signaling capability. In this case, no data packet will follow. The low level BSOCK routines expect that only a single thread is accessing the socket at a time. It is advised that multiple threads do not read/write the same socket. If you must do this, you must provide some sort of locking mechanism. It would not be appropriate for efficiency reasons to make every call to the BSOCK routines lock and unlock the packet.

General Daemon Protocol

In general, all the daemons follow the following global rules. There may be exceptions depending on the specific case. Normally, one daemon will be sending commands to another daemon (specifically, the Director to the Storage daemon and the Director to the File daemon).

Commands are always ASCII commands that are upper/lower case dependent as well as space sensitive.
All binary data is converted into ASCII (either with printf statements or using base64 encoding).
All responses to commands sent are always prefixed with a return numeric code where codes in the 1000's are reserved for the Director, the 2000's are reserved for the File daemon, and the 3000's are reserved for the Storage daemon.
Any response that is not prefixed with a numeric code is a command (or subcommand if you like) coming from the other end. For example, while the Director is corresponding with the Storage daemon, the Storage daemon can request Catalog services from the Director. This convention permits each side to send commands to the other daemon while simultaneously responding to commands.
Any response that is of zero length, depending on the context, either terminates the data stream being sent or terminates command mode prior to closing the connection.
Any response that is of negative length is a special sign that normally requires a response. For example, during data transfer from the File daemon to the Storage daemon, normally the File daemon sends continuously without intervening reads. However, periodically, the File daemon will send a packet of length -1 indicating that the current data stream is complete and that the Storage daemon should respond to the packet with an OK, ABORT JOB, PAUSE, etc. This permits the File daemon to efficiently send data while at the same time occasionally “polling” the Storage daemon for his status or any special requests.
Currently, these negative lengths are specific to the daemon, but shortly, the range 0 to -999 will be standard daemon wide signals, while -1000 to -1999 will be for Director user, -2000 to -2999 for the File daemon, and -3000 to -3999 for the Storage daemon.

The Protocol Used Between the Director and the Storage Daemon

Before sending commands to the File daemon, the Director opens a Message channel with the Storage daemon, identifies itself and presents its password. If the password check is OK, the Storage daemon accepts the Director. The Director then passes the Storage daemon, the JobId to be run as well as the File daemon authorization (append, read all, or read for a specific session). The Storage daemon will then pass back to the Director a enabling key for this JobId that must be presented by the File daemon when opening the job. Until this process is complete, the Storage daemon is not available for use by File daemons.

SD: listens
DR: makes connection
DR: Hello <Director-name> calling <password>
SD: 3000 OK Hello
DR: JobId=nnn Allow=(append, read) Session=(*, SessionId)
                    (Session not implemented yet)
SD: 3000 OK Job Authorization=<password>
DR: use device=<device-name> media_type=<media-type>
        pool_name=<pool-name> pool_type=<pool_type>
SD: 3000 OK use device

For the Director to be authorized, the <Director-name> and the <password> must match the values in one of the Storage daemon's Director resources (there may be several Directors that can access a single Storage daemon).

The Protocol Used Between the Director and the File Daemon

A typical conversation might look like the following:

FD: listens
DR: makes connection
DR: Hello <Director-name> calling <password>
FD: 2000 OK Hello
DR: JobId=nnn Authorization=<password>
FD: 2000 OK Job
DR: storage address = <Storage daemon address> port = <port-number>
          name = <DeviceName> mediatype = <MediaType>
FD: 2000 OK storage
DR: include
DR: <directory1>
DR: <directory2>
    ...
DR: Null packet
FD: 2000 OK include
DR: exclude
DR: <directory1>
DR: <directory2>
    ...
DR: Null packet
FD: 2000 OK exclude
DR: full
FD: 2000 OK full
DR: save
FD: 2000 OK save
FD: Attribute record for each file as sent to the
    Storage daemon (described above).
FD: Null packet
FD: <append close responses from Storage daemon>
    e.g.
    3000 OK Volumes = <number of volumes>
    3001 Volume = <volume-id> <start file> <start block>
         <end file> <end block> <volume session-id>
    3002 Volume data = <date/time of last write> <Number bytes written>
         <number errors>
    ... additional Volume / Volume data pairs for volumes 2 .. n
FD: Null packet
FD: close socket

The Save Protocol Between the File Daemon and the Storage Daemon

Once the Director has send a save command to the File daemon, the File daemon will contact the Storage daemon to begin the save.

In what follows: FD: refers to information set via the network from the File daemon to the Storage daemon, and SD: refers to information set from the Storage daemon to the File daemon.

Command and Control Information

Command and control information is exchanged in human readable ASCII commands.

FD: listens
SD: makes connection
FD: append open session = <JobId> [<password>]
SD: 3000 OK ticket = <number>
FD: append data <ticket-number>
SD: 3000 OK data address = <IPaddress> port = <port>

Data Information

The Data information consists of the file attributes and data to the Storage daemon. For the most part, the data information is sent one way: from the File daemon to the Storage daemon. This allows the File daemon to transfer information as fast as possible without a lot of handshaking and network overhead.

However, from time to time, the File daemon needs to do a sort of checkpoint of the situation to ensure that everything is going well with the Storage daemon. To do so, the File daemon sends a packet with a negative length indicating that he wishes the Storage daemon to respond by sending a packet of information to the File daemon. The File daemon then waits to receive a packet from the Storage daemon before continuing.

All data sent are in binary format except for the header packet, which is in ASCII. There are two packet types used data transfer mode: a header packet, the contents of which are known to the Storage daemon, and a data packet, the contents of which are never examined by the Storage daemon.

The first data packet to the Storage daemon will be an ASCII header packet consisting of the following data.

<File-Index> <Stream-Id> <Info> where <File-Index> is a sequential number beginning from one that increments with each file (or directory) sent.

where <Stream-Id> will be 1 for the Attributes record and 2 for uncompressed File data. 3 is reserved for the MD5 signature for the file.

where <Info> transmit information about the Stream to the Storage Daemon. It is a character string field where each character has a meaning. The only character currently defined is 0 (zero), which is simply a place holder (a no op). In the future, there may be codes indicating compressed data, encrypted data, etc.

Immediately following the header packet, the Storage daemon will expect any number of data packets. The series of data packets is terminated by a zero length packet, which indicates to the Storage daemon that the next packet will be another header packet. As previously mentioned, a negative length packet is a request for the Storage daemon to temporarily enter command mode and send a reply to the File daemon. Thus an actual conversation might contain the following exchanges:

FD: <1 1 0> (header packet)
FD: <data packet containing file-attributes>
FD: Null packet
FD: <1 2 0>
FD: <multiple data packets containing the file data>
FD: Packet length = -1
SD: 3000 OK
FD: <2 1 0>
FD: <data packet containing file-attributes>
FD: Null packet
FD: <2 2 0>
FD: <multiple data packets containing the file data>
FD: Null packet
FD: Null packet
FD: append end session <ticket-number>
SD: 3000 OK end
FD: append close session <ticket-number>
SD: 3000 OK Volumes = <number of volumes>
SD: 3001 Volume = <volumeid> <start file> <start block>
     <end file> <end block> <volume session-id>
SD: 3002 Volume data = <date/time of last write> <Number bytes written>
     <number errors>
SD: ... additional Volume / Volume data pairs for
     volumes 2 .. n
FD: close socket

The information returned to the File daemon by the Storage daemon in response to the append close session is transmit in turn to the Director.