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Inter-process communication between SSSD processes

This document describes how the different SSSD processes communicate between one another with a special emphasis on the Sbus protocol. In addition, the document describes the interfaces SSSD might use to communicate with external libraries or programs, such as libkrb5 or how are the requests from NSS and PAM modules received.

An overview of the SSSD architecture

The SSSD consists of several processes, each of them has its own function. The SSSD processes can be one of the following:

  1. the monitor - The purpose of the monitor process is to spawn the other processes, periodically ping them if to check if they are still running and respawn them if not. There is only one instance of the monitor process at a given time.
  2. a data provider - The data provider process communicates with the remote server (i.e. queries the remote server for a user) and updates the cache (i.e. writes the user entry. There is one Data Provider process per remote server.
  3. a responder - The system libraries (such as the Name Service Switch module or the PAM module) communicate with the corresponding responder process. When the responder process receives a query, it checks the cache first and attempts to return the requested data from cache. If the data is not cached (or is expired), the responder sends a message to the Data Provider requesting the cache to be updated. When the Data Provider is done updating the cache, the responder process checks the cache again and returns the updated data. It is important to note that the responder process never returns the data directly from the server, the data is always written to the cache by the Data Provider Process and returned to the calling library in the responder process.
  4. a helper process - The SSSD performs some operations that would be blocking, such as kinit in a special helper sub-process. The sub-processes are forked from the Data Provider processes again for each operation, there is no preforked pool of helper processes. The SSSD establishes pipes towards the processes’ standard input and output to communicate with the child using an ad-hoc wire protocol.

DBus and SBUS

The SSSD uses the DBus protocol to pass messages between the SSSD processes. It should be noted that the core SSSD does NOT listen on or use the system bus. The SSSD only uses the DBus protocol to pass messages between the processes. There is a public SSSD DBus interface, starting with the 1.12 upstream release, but that resides in a separate sub-package and is not related to the interprocess communication between the different components.

A quick overview of the DBus concepts

The DBus protocol consists of several primary components:

  1. The D-BUS Server - Rather than accepting connections and listening for requests on that connection, the D-BUS server is instead used only for establishing connections. A DBus server is identified by its address. Server addresses consist of a transport name followed by a colon, and then an optional, comma-separated list of keys and values in the form key=value, for example unix:path=/tmp/dbus-test.
  2. The D-BUS Connection - Once a D-BUS connection has been made to a D-BUS server, it becomes a peer-to-peer connection. Either end of the connection can listen for method calls or signals, and either end can initiate them.
  3. The D-BUS Message - D-BUS messages come in three primary forms. These are D-BUS method calls, D-BUS Signals and D-BUS errors. D-BUS signals and D-BUS errors are one-way messages from one end of a D-BUS connection to the other, intended to carry a brief message (such as a signal to start or stop a service, or a notification that an error has occurred in the connection). D-BUS errors are usually generated by the internal D-BUS API itself, though they can be generated by your own code as well. D-BUS method calls are the bread-and-butter of the D-BUS protocol. The purpose of these calls is to essentially run a method on a remote process and treat it as if it had been run locally. These calls may (or may not) receive replies from the other end of the connection.
  4. The D-BUS System Bus - The system bus is a special implementation of the D-BUS protocol. It was designed by the freedesktop project to handle communication between the many system daemons. We DO NOT use the system bus in the SSSD.

DBus and S-Bus

For performance reasons, the SSSD works in a completely non-blocking way using the tevent event loop library from the Samba project. To integrate the DBus API with the event loop and provide a level of abstraction, the SSSD uses a wrapper around the D-Bus library called the S-Bus. The S-Bus code can be found in the src/sbus subdirectory. In particular, the wrappers and tevent integration can be found in sssd_dbus_common.c and sssd_dbus_connection.c files. When the message is received, the tevent loop invokes the sbus_message_handler function which is located in sssd_dbus_connection.c. The handler selects the SSSD internal interface that receives the message and invokes the appropriate handler.

Describing the SBUS and public DBus interfaces

Starting with upstream version 1.12, when the SSSD implemented its public DBus interface, the SSSD switched from hardcoding interface names, methods etc. in the source files directly to only describing the interfaces in XML files using the introspection format, which are then used to autogenerate message handlers, property getters and similar. While using generated code might sound odd at first, using a code generator removes a large amount of code duplication, packing and unpacking from DBus types to C types or vice versa, or unpacking DBus message properties (if needed).

The code generator and the generated code are currently used for both the DBus public interface (which is outside the scope of this page) and the internal SBUS communication. The internal SBUS code, however, uses the generated code in a ‘raw’ mode mostly and still does packing/unpacking of the parameters on its own. The reason is that the ‘raw’ code in SSSD predates the code generator, is quite stable and tested and converting it to the easier handlers with unpacked parameters might cause functional regressions.

One example of the canonical XML code might be found in the unit tests, along with the corresponding autogenerated code. The XML files for the internal interfaces, such as the Data Provider can also be inspected. Since all the internal interfaces use the raw approach, the autogenerated code is quite terse and the interface handlers do the packing and unpacking on their own.

An SBus server

An S-Bus server is an abstraction of the DBus server. An S-Bus server is always identified with an UNIX socket located in the directory /var/lib/sss/pipes/private. Two processes act as an S-Bus server in the SSSD:

  1. The monitor - Both the responders and the Data providers establish connection to the monitor after startup. The monitor then periodically sends “pings” to the worker processes to check if they are still up and running. The other S-Bus methods the monitor can invoke include “rotateLogs” to force log rotation or “resetOffline” to force that the next operation attempts to contact the remote server regardless of the connection status. The complete list of methods is in src/monitor/monitor_interfaces.h
    • listens on /var/lib/sss/pipes/private/sbus-monitor.
  2. The Data Providers - The Responder processes connect to the Data Provider processes with a cache update request. The Data Provider then communicates with the remote server, updates the cache and sends a message back to the responder, indicating that the cache was updated. Each responder calls a different DBus method depending on the data type the cache should be updated with. For example, the getAccountInfo method is called from the NSS.
    • listens on /var/lib/sss/pipes/private/sbus-dp_$domain_name

Two kinds of messages

The SSSD sends an SBUS message between two of its components under two different circumstances:

  1. When a request is received, completing the request might require communicating with another subprocess. An example of this is when a getpwnam() call triggers an LDAP search - the NSS responder sends an SBUS message to the Data Provider to update the cache.
  2. Control messages sent by the monitor. The monitor process (aka the sssd process) sends periodical ping messages to all subprocesses it controls. If a subprocess doesn’t respond with a pong message in time, it gets killed and restarted.

This means, there is ongoing sbus communication even though the sssd is otherwise idle.

UNIX signals

Apart from the internal SBUS communication, SSSD also uses UNIX signals for certain functionality - either for communication with external utilities or for cases where the SBUS communication might not work, such as an unresponsive worker process. Below is an overview of the supported signals and their use. The signal handlers are typically integrated with the tevent event loop using its tevent_add_signal call.

  • SIGTERM
    If a responder or a provider process fails to send a pong message to the monitor process after receiving the ping message, the monitor terminates the unresponsive process with a SIGTERM. Also used to terminate helper processes (such as the krb5_child process) in case of a timeout.

  • SIGKILL
    In cases where an unresponsive worker process does not terminate after receiving SIGTERM, the monitor forcibly kills it with SIGILL

  • SIGUSR1
    Can be handled a sssd_be process individually or the monitor process (in that case, the monitor re-sends the signal to all sssd_be processes it handles). Upon receiving this signal, the sssd_be process transitions into the ‘offline’ state. This signal is mostly useful for testing.

  • SIGUSR2
    Similar to the SIGUSR1 signal, the SIGUSR2 would cause an sssd_be process to reset the offline status and retry the next request it receives against a remote server.

  • SIGHUP
    Can be delivered to the sssd process. After receiving SIGHUP, the monitor rotates its logfile and sends a reset method to the managed processes. The managed processes also rotate logfiles. In addition, the sssd_be processes re-read resolv.conf and the sssd_nss process clears the fast in-memory cache.

Local sockets

After startup, the SSSD also creates several local (AF_UNIX) sockets to listen on. These sockets are used by the NSS and PAM modules and also the external programs SSSD integrates with, such as sudo, OpenSSH or autofs. All consumers use the sockets in a similar fashion, so they can be commonly called SSS clients.

The clients all employ a request/response protocol. using its own TLV-encoding. Note that the SSS clients only support synchronous I/O, so it may block (e.g. while waiting for a response). On the other hand, the responders supports asynchronous I/O using its tevent main loop, so it will not block (e.g. while waiting to read from a client).

KDCInfo files

The SSSD might discover additional KDC or Kadmin servers that are not defined in krb5.conf. However, it would still be prudent if tools like kinit or kpasswd could talk to the same servers the SSSD talks to. To this end, the SSSD implements a plugin for libkrb5, located in the sssd_krb5_locator_plugin.c file. When a new KDC is discovered, the sssd_be process writes the IP address of this KDC into a file under the /var/lib/sss/pubconf directory. With the help of the locator plugin, libkrb5 is able to read these files in the pubconf directory and use the KDC servers discovered by the SSSD.