     [file version2.texi not included]

Copyright © 2001-2017 GNUnet e.V.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

A copy of the license is also available from the Free Software Foundation Web site at http://www.gnu.org/licenses/fdl.html.

Alternately, this document is also available under the General Public License, version 3 or later, as published by the Free Software Foundation.  A copy of the license is included in the section entitled ``GNU General Public License''.

A copy of the license is also available from the Free Software Foundation Web site at http://www.gnu.org/licenses/gpl.html.

Introduction
************

This tutorials explains how to install GNUnet on a GNU/Linux system and gives an introduction on how GNUnet can be used to develop a Peer-to-Peer application. Detailed installation instructions for various operating systems and a detailed list of all dependencies can be found on our website at https://gnunet.org/installation and in our Reference Documentation (GNUnet Handbook).

Please read this tutorial carefully since every single step is important and do not hesitate to contact the GNUnet team if you have any questions or problems! Check here how to contact the GNUnet team: https://gnunet.org/contact_information

Installing GNUnet
*****************

First of all you have to install a current version of GNUnet. You can download a tarball of a stable version from GNU FTP mirrors or obtain the latest development version from our Git repository.

Most of the time you should prefer to download the stable version since with the latest development version things can be broken, functionality can be changed or tests can fail. You should only use the development version if you know that you require a certain feature or a certain issue has been fixed since the last release.

Obtaining a stable version
==========================

Download the tarball from https://ftp.gnu.org/gnu/gnunet/gnunet-VERSION.tar.gz.

Make sure to download the associated .sig file and to verify the authenticity of the tarball against it, like this:

     $ wget https://ftp.gnu.org/gnu/gnunet/gnunet-VERSION.tar.gz.sig
     $ gpg --verify-files gnunet-VERSION.tar.gz.sig

If this command fails because you do not have the required public key, then you need to run this command to import it:

     $ gpg --keyserver keys.gnupg.net --recv-keys 48426C7E

and rerun the gpg --verify-files command.

Note: The pub key to sign the 0.10.1 release has been revoked. You will get an error message stating that there is no known public key or that it has been revoked. The next release of GNUnet will have a valid signature again. We are sorry for the inconvenience this causes. Another possible source you could use is our "gnunet" git repository which has mandatory signed commits by every developer.

Now you can extract the tarball and rename the resulting directory to gnunet which we will be using in the remainder of this document.

     $ tar xvzf gnunet-VERSION.tar.gz
     $ mv gnunet-VERSION gnunet
     $ cd gnunet

However, please note that stable versions can be very outdated. As a developer you are strongly encouraged to use the version from git (https://gnunet.org/git/).

Installing Build Tool Chain and Dependencies
============================================

To successfully compile GNUnet, you need the tools to build GNUnet and the required dependencies. Please take a look at the GNUnet Reference Documentation (see The GNUnet Reference Documentation) for a list of required dependencies and (see The GNUnet Reference Documentation) read its Installation chapter for specific instructions for your operating system. Please check the notes at the end of the configure process about required dependencies.

For GNUnet bootstrapping support and the HTTP(S) plugin you should install libgnurl (https://gnunet.org/gnurl). For the filesharing service you should install at least one of the datastore backends. MySQL, SQlite and PostgreSQL are supported.

Obtaining the latest version from Git
=====================================

The latest development version can obtained from our Git repository. To obtain the code you need to have Git installed, which is required for obtaining the repository via:

     $ git clone https://gnunet.org/git/gnunet

After cloning the repository you have to execute the bootstrap script in the new directory:

     $ cd gnunet
     $ ./bootstrap

The remainder of this tutorial will assume that you have the Git branch ``master'' checked out.

Compiling and Installing GNUnet
===============================

Note: This section is a duplication of the more in depth see The GNUnet Reference Documentation.

First, you need to install libgnupgerror ≥ 1.27 and libgcrypt ≥ 1.7.6:

     $ export GNUPGFTP="https://www.gnupg.org/ftp/gcrypt"
     $ wget $GNUPGFTP/libgpg-error/libgpg-error-1.27.tar.bz2
     $ tar xf libgpg-error-1.27.tar.bz2
     $ cd libgpg-error-1.27
     $ ./configure
     $ sudo make install
     $ cd ..

     $ export GNUPGFTP="https://www.gnupg.org/ftp/gcrypt"
     $ wget $GNUPGFTP/libgcrypt/libgcrypt-1.7.6.tar.bz2
     $ tar xf libgcrypt-1.7.6.tar.bz2
     $ cd libgcrypt-1.7.6
     $ ./configure
     $ sudo make install
     $ cd ..

Installation
------------

Assuming all dependencies are installed, the following commands will compile and install GNUnet in your home directory. You can specify the directory where GNUnet will be installed by changing the --prefix value when calling ./configure.  If you do not specifiy a prefix, GNUnet is installed in the directory /usr/local. When developing new applications you may want to enable verbose logging by adding --enable-logging=verbose:

     $ ./configure --prefix=$PREFIX --enable-logging
     $ make
     $ make install

After installing GNUnet you have to add your GNUnet installation to your path environmental variable. In addition you have to create the .config directory in your home directory (unless it already exists) where GNUnet stores its data and an empty GNUnet configuration file:

     $ export PATH=$PATH:$PREFIX/bin
     $ echo export PATH=$PREFIX/bin:\\$PATH >> ~/.bashrc
     $ mkdir ~/.config/
     $ touch ~/.config/gnunet.conf

Common Issues - Check your GNUnet installation
==============================================

You should check your installation to ensure that installing GNUnet was successful up to this point. You should be able to access GNUnet's binaries and run GNUnet's self check.

     $ which gnunet-arm

should return $PREFIX/bin/gnunet-arm. It should be located in your GNUnet installation and the output should not be empty. If you see an output like:

     $ which gnunet-arm

check your PATH variable to ensure GNUnet's bin directory is included.

GNUnet provides tests for all of its subcomponents. Run

     $ make check

to execute tests for all components. make check traverses all subdirectories in src. For every subdirectory you should get a message like this:

     make[2]: Entering directory `/home/$USER/gnunet/contrib'
     PASS: test_gnunet_prefix
     =============
     1 test passed
     =============

Introduction to GNUnet Architecture
***********************************

GNUnet is organized in layers and services. Each service is composed of a main service implementation and a client library for other programs to use the service's functionality, described by an API. Some services provide an additional command line tool to enable the user to interact with the service.

Very often it is other GNUnet services that will use these APIs to build the higher layers of GNUnet on top of the lower ones. Each layer expands or extends the functionality of the service below (for instance, to build a mesh on top of a DHT).

The main service implementation runs as a standalone process in the operating system and the client code runs as part of the client program, so crashes of a client do not affect the service process or other clients. The service and the clients communicate via a message protocol to be defined and implemented by the programmer.

First Steps with GNUnet
***********************

Configure your peer
===================

First of all we need to configure your peer. Each peer is started with a configuration containing settings for GNUnet itself and its services. This configuration is based on the default configuration shipped with GNUnet and can be modified. The default configuration is located in the $PREFIX/share/gnunet/config.d directory. When starting a peer, you can specify a customized configuration using the the -c command line switch when starting the ARM service and all other services. When using a modified configuration the default values are loaded and only values specified in the configuration file will replace the default values.

Since we want to start additional peers later, we need some modifications from the default configuration. We need to create a separate service home and a file containing our modifications for this peer:

     $ mkdir ~/gnunet1/
     $ touch peer1.conf

Now add the following lines to peer1.conf to use this directory. For simplified usage we want to prevent the peer to connect to the GNUnet network since this could lead to confusing output. This modifications will replace the default settings:

     [PATHS]
     # Use this directory to store GNUnet data
     GNUNET_HOME = ~/gnunet1/
     [hostlist]
     # prevent bootstrapping
     SERVERS =

Start a peer
============

Each GNUnet instance (called peer) has an identity (peer ID) based on a cryptographic public private key pair. The peer ID is the printable hash of the public key.

GNUnet services are controlled by a master service, the so called Automatic Restart Manager (ARM). ARM starts, stops and even restarts services automatically or on demand when a client connects. You interact with the ARM service using the gnunet-arm tool. GNUnet can then be started with gnunet-arm -s and stopped with gnunet-arm -e.  An additional service not automatically started can be started using gnunet-arm -i <service name> and stopped using gnunet-arm -k <servicename>.

Once you have started your peer, you can use many other GNUnet commands to interact with it.  For example, you can run:

     $ gnunet-peerinfo -s

to obtain the public key of your peer.

You should see an output containing the peer ID similar to:

     I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.

Monitor a peer
==============

In this section, we will monitor the behaviour of our peer's DHT service with respect to a specific key. First we will start GNUnet and then start the DHT service and use the DHT monitor tool to monitor the PUT and GET commands we issue ussing the gnunet-dht-put and gnunet-dht-get commands. Using the ``monitor'' line given below, you can observe the behavior of your own peer's DHT with respect to the specified KEY:

     # start gnunet with all default services:
     $ gnunet-arm -c ~/peer1.conf -s
     # start DHT service:
     $ gnunet-arm -c ~/peer1.conf -i dht
     $ cd ~/gnunet/src/dht;
     $ ./gnunet-dht-monitor -c ~/peer1.conf -k KEY

Now open a separate terminal and change again to the gnunet/src/dht directory:

     $ cd ~/gnunet/src/dht
     # put VALUE under KEY in the DHT:
     $ ./gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE
     # get key KEY from the DHT:
     $ ./gnunet/src/dht/gnunet-dht-get -c ~/peer1.conf -k KEY
     # print statistics about current GNUnet state:
     $ gnunet-statistics -c ~/peer1.conf
     # print statistics about DHT service:
     $ gnunet-statistics -c ~/peer1.conf -s dht

Starting Two Peers by Hand
==========================

This section describes how to start two peers on the same machine by hand. The process is rather painful, but the description is somewhat instructive. In practice, you might prefer the automated method (see Starting Peers Using the Testbed Service).

Setup a second peer
-------------------

We will now start a second peer on your machine. For the second peer, you will need to manually create a modified configuration file to avoid conflicts with ports and directories. A peers configuration file is by default located in ~/.gnunet/gnunet.conf. This file is typically very short or even empty as only the differences to the defaults need to be specified.  The defaults are located in many files in the $PREFIX/share/gnunet/config.d directory.

To configure the second peer, use the files $PREFIX/share/gnunet/config.d as a template for your main configuration file:

     $ cat $PREFIX/share/gnunet/config.d/*.conf > peer2.conf

Now you have to edit peer2.conf and change:

  * GNUNET\_TEST\_HOME under PATHS

  * Every (uncommented) value for ``PORT'' (add 10000) in any section (the option may be commented out if PORT is prefixed by "\#", in this case, UNIX domain sockets are used and the PORT option does not need to be touched)

  * Every value for ``UNIXPATH'' in any section (e.g. by adding a "-p2" suffix)

to a fresh, unique value.  Make sure that the PORT numbers stay below 65536. From now on, whenever you interact with the second peer, you need to specify -c peer2.conf as an additional command line argument.

Now, generate the 2nd peer's private key:

     $ gnunet-peerinfo -s -c peer2.conf

This may take a while, generate entropy using your keyboard or mouse as needed.  Also, make sure the output is different from the gnunet-peerinfo output for the first peer (otherwise you made an error in the configuration).

Start the second peer and connect the peers
-------------------------------------------

Then, you can start a second peer using:

     $ gnunet-arm -c peer2.conf -s
     $ gnunet-arm -c peer2.conf -i dht
     $ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
     $ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY

If you want the two peers to connect, you have multiple options:

  * UDP neighbour discovery (automatic)

  * Setup a bootstrap server

  * Connect manually

To setup peer 1 as bootstrapping server change the configuration of the first one to be a hostlist server by adding the following lines to peer1.conf to enable bootstrapping server:

     [hostlist]
     OPTIONS = -p

Then change peer2.conf and replace the ``SERVERS'' line in the ``[hostlist]'' section with ``http://localhost:8080/''.  Restart both peers using:

     # stop first peer
     $ gnunet-arm -c peer1.conf -e
     # start first peer
     $ gnunet-arm -c peer1.conf -s
     # start second peer
     $ gnunet-arm -c peer2.conf -s

Note that if you start your peers without changing these settings, they will use the ``global'' hostlist servers of the GNUnet P2P network and likely connect to those peers.  At that point, debugging might become tricky as you're going to be connected to many more peers and would likely observe traffic and behaviors that are not explicitly controlled by you.

How to connect manually
-----------------------

If you want to use the peerinfo tool to connect your peers, you should:

  * Set FORCESTART = NO in section hostlist (to not connect to the global GNUnet)

  * Start both peers running gnunet-arm -c peer1.conf -s and gnunet-arm -c peer2.conf -s

  * Get HELLO message of the first peer running gnunet-peerinfo -c peer1.conf -g

  * Give the output to the second peer by running gnunet-peerinfo -c peer2.conf -p '<output>'

Check that they are connected using gnunet-core -c peer1.conf, which should give you the other peer's peer identity:

     $ gnunet-core -c peer1.conf
     Peer `9TVUCS8P5A7ILLBGO6 [...shortened...] 1KNBJ4NGCHP3JPVULDG'

Starting Peers Using the Testbed Service
========================================

GNUnet's testbed service is used for testing scenarios where a number of peers are to be started.  The testbed can manage peers on a single host or on multiple hosts in a distributed fashion. On a single affordable computer, it should be possible to run around tens of peers without drastically increasing the load on the system.

The testbed service can be access through its API include/gnunet\_testbed\_service.h.  The API provides many routines for managing a group of peers.  It also provides a helper function GNUNET\_TESTBED\_test\_run() to quickly setup a minimalistic testing environment on a single host.

This function takes a configuration file which will be used as a template configuration for the peers.  The testbed takes care of modifying relevant options in the peers' configuration such as SERVICEHOME, PORT, UNIXPATH to unique values so that peers run without running into conflicts.  It also checks and assigns the ports in configurations only if they are free.

Additionally, the testbed service also reads its options from the same configuration file.  Various available options and details about them can be found in the testbed default configuration file src/testbed/testbed.conf.

With the testbed API, a sample test case can be structured as follows:

          [file testbed_test.c not included]

The source code for the above listing can be found at https://gnunet.org/git/gnunet.git/tree/doc/ documentation/testbed_test.c or in the doc/documentation/ folder of your repository check-out. After installing GNUnet, the above source code can be compiled as:

     $ export CPPFLAGS="-I/path/to/gnunet/headers"
     $ export LDFLAGS="-L/path/to/gnunet/libraries"
     $ gcc $CPPFLAGS $LDFLAGS -o testbed-test testbed_test.c \
      -lgnunettestbed -lgnunetdht -lgnunetutil
     # Generate (empty) configuration
     $ touch template.conf
     # run it (press CTRL-C to stop)
     $ ./testbed-test

The CPPFLAGS and LDFLAGS are necessary if GNUnet is installed into a different directory other than /usr/local.

All of testbed API's peer management functions treat management actions as operations and return operation handles.  It is expected that the operations begin immediately, but they may get delayed (to balance out load on the system). The program using the API then has to take care of marking the operation as ``done'' so that its associated resources can be freed immediately and other waiting operations can be executed.  Operations will be canceled if they are marked as ``done'' before their completion.

An operation is treated as completed when it succeeds or fails. Completion of an operation is either conveyed as events through controller event callback or through respective operation completion callbacks. In functions which support completion notification through both controller event callback and operation completion callback, first the controller event callback will be called.  If the operation is not marked as done in that callback or if the callback is given as NULL when creating the operation, the operation completion callback will be called.  The API documentation shows which event are to be expected in the controller event notifications.  It also documents any exceptional behaviour.

Once the peers are started, test cases often need to connect some of the peers' services.  Normally, opening a connect to a peer's service requires the peer's configuration.  While using testbed, the testbed automatically generates per-peer configuration. Accessing those configurations directly through file system is discouraged as their locations are dynamically created and will be different among various runs of testbed.  To make access to these configurations easy, testbed API provides the function GNUNET\_TESTBED\_service\_connect().  This function fetches the configuration of a given peer and calls the Connect Adapter. In the example code, it is the dht\_ca.  A connect adapter is expected to open the connection to the needed service by using the provided configuration and return the created service connection handle. Successful connection to the needed service is signaled through service\_connect\_comp\_cb.

A dual to connect adapter is the Disconnect Adapter.  This callback is called after the connect adapter has been called when the operation from GNUNET\_TESTBED\_service\_connect() is marked as ``done''. It has to disconnect from the service with the provided service handle (op\_result).

Exercise: Find out how many peers you can run on your system.

Exercise: Find out how to create a 2D torus topology by changing the options in the configuration file. See The GNUnet Reference Documentation, then use the DHT API to store and retrieve values in the network.

Developing Applications
***********************

gnunet-ext
==========

To develop a new peer-to-peer application or to extend GNUnet we provide a template build system for writing GNUnet extensions in C. It can be obtained as follows:

     $ git clone https://gnunet.org/git/gnunet-ext
     $ cd gnunet-ext/
     $ ./bootstrap
     $ ./configure --prefix=$PREFIX --with-gnunet=$PREFIX
     $ make
     $ make install
     $ make check

The GNUnet ext template includes examples and a working buildsystem for a new GNUnet service. A common GNUnet service consists of the following parts which will be discussed in detail in the remainder of this document. The functionality of a GNUnet service is implemented in:

  * the GNUnet service (gnunet-ext/src/ext/gnunet-service-ext.c)

  * the client API (gnunet-ext/src/ext/ext_api.c)

  * the client application using the service API (gnunet-ext/src/ext/gnunet-ext.c)

The interfaces for these entities are defined in:

  * client API interface (gnunet-ext/src/ext/ext.h)

  * the service interface (gnunet-ext/src/include/gnunet_service_SERVICE.h)

  * the P2P protocol (gnunet-ext/src/include/gnunet_protocols_ext.h)

In addition the ext systems provides:

  * a test testing the API (gnunet-ext/src/ext/test_ext_api.c)

  * a configuration template for the service (gnunet-ext/src/ext/ext.conf.in)

Adapting the Template
=====================

The first step for writing any extension with a new service is to ensure that the ext.conf.in file contains entries for the UNIXPATH, PORT and BINARY for the service in a section named after the service.

If you want to adapt the template rename the ext.conf.in to match your services name, you have to modify the AC\_OUTPUT section in configure.ac in the gnunet-ext root.

Writing a Client Application
============================

When writing any client application (for example, a command-line tool), the basic structure is to start with the GNUNET\_PROGRAM\_run function.  This function will parse command-line options, setup the scheduler and then invoke the run function (with the remaining non-option arguments) and a handle to the parsed configuration (and the configuration file name that was used, which is typically not needed):

          [file 001.c not included]

Handling command-line options
-----------------------------

Options can then be added easily by adding global variables and expanding the options array.  For example, the following would add a string-option and a binary flag (defaulting to NULL and GNUNET\_NO respectively):

          [file 002.c not included]

Issues such as displaying some helpful text describing options using the --help argument and error handling are taken care of when using this approach.  Other GNUNET\_GETOPT\_-functions can be used to obtain integer value options, increment counters, etc.  You can even write custom option parsers for special circumstances not covered by the available handlers. To check if an argument was specified by the user you initialize the variable with a specific value (e.g. NULL for a string and GNUNET\_SYSERR for a integer) and check after parsing happened if the values were modified.

Inside the run method, the program would perform the application-specific logic, which typically involves initializing and using some client library to interact with the service.  The client library is supposed to implement the IPC whereas the service provides more persistent P2P functions.

Exercise: Add a few command-line options and print them inside of run.  What happens if the user gives invalid arguments?

Writing a Client Library
------------------------

The first and most important step in writing a client library is to decide on an API for the library.  Typical API calls include connecting to the service, performing application-specific requests and cleaning up.  Many examples for such service APIs can be found in the gnunet/src/include/gnunet\_*\_service.h files.

Then, a client-service protocol needs to be designed.  This typically involves defining various message formats in a header that will be included by both the service and the client library (but is otherwise not shared and hence located within the service's directory and not installed by make install).  Each message must start with a struct GNUNET\_MessageHeader and must be shorter than 64k.  By convention, all fields in IPC (and P2P) messages must be in big-endian format (and thus should be read using ntohl and similar functions and written using htonl and similar functions). Unique message types must be defined for each message struct in the gnunet\_protocols.h header (or an extension-specific include file).

Connecting to the Service
.........................

Before a client library can implement the application-specific protocol with the service, a connection must be created:

          [file 003.c not included]

As a result a GNUNET\_MQ\_Handle is returned which can to used henceforth to transmit messages to the service. The complete MQ API can be found in gnunet\_mq\_lib.h. The hanlders array in the example above is incomplete. Here is where you will define which messages you expect to receive from the service, and which functions handle them. The error\_cb is a function that is to be called whenever there are errors communicating with the service.

Sending messages
................

In GNUnet, messages are always sent beginning with a struct GNUNET\_MessageHeader in big endian format. This header defines the size and the type of the message, the payload follows after this header.

          [file 004.c not included]

Existing message types are defined in gnunet\_protocols.h. A common way to create a message is with an envelope:

          [file 005.c not included]

Exercise: Define a message struct that includes a 32-bit unsigned integer in addition to the standard GNUnet MessageHeader. Add a C struct and define a fresh protocol number for your message. Protocol numbers in gnunet-ext are defined in gnunet-ext/src/include/gnunet_protocols_ext.h

Exercise: Find out how you can determine the number of messages in a message queue.

Exercise: Find out how you can determine when a message you have queued was actually transmitted.

Exercise: Define a helper function to transmit a 32-bit unsigned integer (as payload) to a service using some given client handle.

Receiving Replies from the Service
..................................

Clients can receive messages from the service using the handlers specified in the handlers array we specified when connecting to the service.  Entries in the the array are usually created using one of two macros, depending on whether the message is fixed size or variable size.  Variable size messages are managed using two callbacks, one to check that the message is well-formed, the other to actually process the message.  Fixed size messages are fully checked by the MQ-logic, and thus only need to provide the handler to process the message.  Note that the prefixes check\_ and handle\_ are mandatory.

          [file 006.c not included]

Exercise: Expand your helper function to receive a response message (for example, containing just the struct GNUnet MessageHeader without any payload).  Upon receiving the service's response, you should call a callback provided to your helper function's API.

Exercise: Figure out where you can pass values to the closures (cls).

Writing a user interface
------------------------

Given a client library, all it takes to access a service now is to combine calls to the client library with parsing command-line options.

Exercise: Call your client API from your run() method in your client application to send a request to the service.  For example, send a 32-bit integer value based on a number given at the command-line to the service.

Writing a Service
=================

Before you can test the client you've written so far, you'll need to also implement the corresponding service.

Code Placement
--------------

New services are placed in their own subdirectory under gnunet/src. This subdirectory should contain the API implementation file SERVICE\_api.c, the description of the client-service protocol SERVICE.h and P2P protocol SERVICE\_protocol.h, the implementation of the service itself gnunet-service-SERVICE.h and several files for tests, including test code and configuration files.

Starting a Service
------------------

The key API definition for creating a service is the GNUNET\_SERVICE\_MAIN macro:

          [file 007.c not included]

In addition to the service name and flags, the macro takes three functions, typically called run, client\_connect\_cb and client\_disconnect\_cb as well as an array of message handlers that will be called for incoming messages from clients.

A minimal version of the three central service funtions would look like this:

          [file 008.c not included]

Exercise: Write a stub service that processes no messages at all in your code.  Create a default configuration for it, integrate it with the build system and start the service from gnunet-service-arm using gnunet-arm -i NAME.

Exercise: Figure out how to set the closure (cls) for handlers of a service.

Exercise: Figure out how to send messages from the service back to the client.

Each handler function in the service must eventually (possibly in some asynchronous continuation) call GNUNET\_SERVICE\_client\_continue(). Only after this call additional messages from the same client may be processed. This way, the service can throttle processing messages from the same client.

Exercise: Change the service to ``handle'' the message from your client (for now, by printing a message).  What happens if you forget to call GNUNET\_SERVICE\_client\_continue()?

Interacting directly with other Peers using the CORE Service
============================================================

FIXME: This section still needs to be updated to the lastest API!

One of the most important services in GNUnet is the CORE service managing connections between peers and handling encryption between peers.

One of the first things any service that extends the P2P protocol typically does is connect to the CORE service using:

          [file 009.c not included]

New P2P connections
-------------------

Before any traffic with a different peer can be exchanged, the peer must be known to the service. This is notified by the CORE connects callback, which communicates the identity of the new peer to the service:

          [file 010.c not included]

Note that whatever you return from connects is given as the cls argument to the message handlers for messages from the respective peer.

Exercise: Create a service that connects to the CORE.  Then start (and connect) two peers and print a message once your connect callback is invoked.

Receiving P2P Messages
----------------------

To receive messages from CORE, you pass the desired handlers to the GNUNET\_CORE\_connect() function, just as we showed for services.

It is your responsibility to process messages fast enough or to implement flow control. If an application does not process CORE messages fast enough, CORE will randomly drop messages to not keep a very long queue in memory.

Exercise: Start one peer with a new service that has a message handler and start a second peer that only has your ``old'' service without message handlers.  Which ``connect'' handlers are invoked when the two peers are connected?  Why?

Sending P2P Messages
--------------------

You can transmit messages to other peers using the mq you were given during the connect callback.  Note that the mq automatically is released upon disconnect and that you must not use it afterwards.

It is your responsibility to not over-fill the message queue, GNUnet will send the messages roughly in the order given as soon as possible.

Exercise: Write a service that upon connect sends messages as fast as possible to the other peer (the other peer should run a service that ``processes'' those messages).  How fast is the transmission?  Count using the STATISTICS service on both ends.  Are messages lost? How can you transmit messages faster?  What happens if you stop the peer that is receiving your messages?

End of P2P connections
----------------------

If a message handler returns GNUNET\_SYSERR, the remote peer shuts down or there is an unrecoverable network disconnection, CORE notifies the service that the peer disconnected. After this notification no more messages will be received from the peer and the service is no longer allowed to send messages to the peer. The disconnect callback looks like the following:

          [file 011.c not included]

Exercise: Fix your service to handle peer disconnects.

Storing peer-specific data using the PEERSTORE service
======================================================

GNUnet's PEERSTORE service offers a persistorage for arbitrary peer-specific data. Other GNUnet services can use the PEERSTORE to store, retrieve and monitor data records. Each data record stored with PEERSTORE contains the following fields:

  * subsystem: Name of the subsystem responsible for the record.

  * peerid: Identity of the peer this record is related to.

  * key: a key string identifying the record.

  * value: binary record value.

  * expiry: record expiry date.

The first step is to start a connection to the PEERSTORE service:

          [file 012.c not included]

The service handle peerstore_handle will be needed for all subsequent PEERSTORE operations.

Storing records
---------------

To store a new record, use the following function:

          [file 013.c not included]

The options parameter can either be GNUNET_PEERSTORE_STOREOPTION_MULTIPLE which means that multiple values can be stored under the same key combination (subsystem, peerid, key), or GNUNET_PEERSTORE_STOREOPTION_REPLACE which means that PEERSTORE will replace any existing values under the given key combination (subsystem, peerid, key) with the new given value.

The continuation function cont will be called after the store request is successfully sent to the PEERSTORE service. This does not guarantee that the record is successfully stored, only that it was received by the service.

The GNUNET_PEERSTORE_store function returns a handle to the store operation. This handle can be used to cancel the store operation only before the continuation function is called:

          [file 013.1.c not included]

Retrieving records
------------------

To retrieve stored records, use the following function:

          [file 014.c not included]

The values of peer and key can be NULL. This allows the iteration over values stored under any of the following key combinations:

  * (subsystem)

  * (subsystem, peerid)

  * (subsystem, key)

  * (subsystem, peerid, key)

The callback function will be called once with each retrieved record and once more with a NULL record to signal the end of results.

The GNUNET_PEERSTORE_iterate function returns a handle to the iterate operation. This handle can be used to cancel the iterate operation only before the callback function is called with a NULL record.

Monitoring records
------------------

PEERSTORE offers the functionality of monitoring for new records stored under a specific key combination (subsystem, peerid, key). To start the monitoring, use the following function:

          [file 015.c not included]

Whenever a new record is stored under the given key combination, the callback function will be called with this new record. This will continue until the connection to the PEERSTORE service is broken or the watch operation is canceled:

          [file 016.c not included]

Disconnecting from PEERSTORE
----------------------------

When the connection to the PEERSTORE service is no longer needed, disconnect using the following function:

          [file 017.c not included]

If the sync_first flag is set to GNUNET_YES, the API will delay the disconnection until all store requests are received by the PEERSTORE service. Otherwise, it will disconnect immediately.

Using the DHT
=============

The DHT allows to store data so other peers in the P2P network can access it and retrieve data stored by any peers in the network. This section will explain how to use the DHT. Of course, the first thing to do is to connect to the DHT service:

          [file 018.c not included]

The second parameter indicates how many requests in parallel to expect. It is not a hard limit, but a good approximation will make the DHT more efficient.

Storing data in the DHT
-----------------------

Since the DHT is a dynamic environment (peers join and leave frequently) the data that we put in the DHT does not stay there indefinitely. It is important to ``refresh'' the data periodically by simply storing it again, in order to make sure other peers can access it.

The put API call offers a callback to signal that the PUT request has been sent. This does not guarantee that the data is accessible to others peers, or even that is has been stored, only that the service has requested to a neighboring peer the retransmission of the PUT request towards its final destination. Currently there is no feedback about whether or not the data has been sucessfully stored or where it has been stored. In order to improve the availablilty of the data and to compensate for possible errors, peers leaving and other unfavorable events, just make several PUT requests!

          [file 019.c not included]

Exercise: Store a value in the DHT periodically to make sure it is available over time. You might consider using the function GNUNET\_SCHEDULER\_add\_delayed and call GNUNET\_DHT\_put from inside a helper function.

Obtaining data from the DHT
---------------------------

As we saw in the previous example, the DHT works in an asynchronous mode. Each request to the DHT is executed ``in the background'' and the API calls return immediately. In order to receive results from the DHT, the API provides a callback. Once started, the request runs in the service, the service will try to get as many results as possible (filtering out duplicates) until the timeout expires or we explicitly stop the request. It is possible to give a ``forever'' timeout with GNUNET\_TIME\_UNIT\_FOREVER\_REL.

If we give a route option GNUNET\_DHT\_RO\_RECORD\_ROUTE the callback will get a list of all the peers the data has travelled, both on the PUT path and on the GET path.

          [file 020.c not included]

Exercise: Store a value in the DHT and after a while retrieve it. Show the IDs of all the peers the requests have gone through. In order to convert a peer ID to a string, use the function GNUNET\_i2s. Pay attention to the route option parameters in both calls!

Implementing a block plugin
---------------------------

In order to store data in the DHT, it is necessary to provide a block plugin.  The DHT uses the block plugin to ensure that only well-formed requests and replies are transmitted over the network.

The block plugin should be put in a file plugin\_block\_SERVICE.c in the service's respective directory. The mandatory functions that need to be implemented for a block plugin are described in the following sections.

Validating requests and replies
...............................

The evaluate function should validate a reply or a request. It returns a GNUNET\_BLOCK\_EvaluationResult, which is an enumeration. All possible answers are in gnunet\_block\_lib.h.  The function will be called with a reply\_block argument of NULL for requests.  Note that depending on how evaluate is called, only some of the possible return values are valid.  The specific meaning of the xquery argument is application-specific.  Applications that do not use an extended query should check that the xquery\_size is zero.  The block group is typically used to filter duplicate replies.

          [file 021.c not included]

Note that it is mandatory to detect duplicate replies in this function and return the respective status code.  Duplicate detection is typically done using the Bloom filter block group provided by libgnunetblockgroup.so.  Failure to do so may cause replies to circle in the network.

Deriving a key from a reply
...........................

The DHT can operate more efficiently if it is possible to derive a key from the value of the corresponding block.  The get\_key function is used to obtain the key of a block --- for example, by means of hashing.  If deriving the key is not possible, the function should simply return GNUNET\_SYSERR (the DHT will still work just fine with such blocks).

          [file 022.c not included]

Initialization of the plugin
............................

The plugin is realized as a shared C library.  The library must export an initialization function which should initialize the plugin.  The initialization function specifies what block types the plugin cares about and returns a struct with the functions that are to be used for validation and obtaining keys (the ones just defined above).

          [file 023.c not included]

Shutdown of the plugin
......................

Following GNUnet's general plugin API concept, the plugin must export a second function for cleaning up.  It usually does very little.

          [file 024.c not included]

Integration of the plugin with the build system
...............................................

In order to compile the plugin, the Makefile.am file for the service SERVICE should contain a rule similar to this:

          [file 025.c not included]

Exercise: Write a block plugin that accepts all queries and all replies but prints information about queries and replies when the respective validation hooks are called.

Monitoring the DHT
------------------

It is possible to monitor the functioning of the local DHT service. When monitoring the DHT, the service will alert the monitoring program of any events, both started locally or received for routing from another peer. The are three different types of events possible: a GET request, a PUT request or a response (a reply to a GET).

Since the different events have different associated data, the API gets 3 different callbacks (one for each message type) and optional type and key parameters, to allow for filtering of messages. When an event happens, the appropiate callback is called with all the information about the event.

          [file 026.c not included]

Debugging with gnunet-arm
=========================

Even if services are managed by gnunet-arm, you can start them with gdb or valgrind.  For example, you could add the following lines to your configuration file to start the DHT service in a gdb session in a fresh xterm:

     [dht]
     PREFIX=xterm -e gdb --args

Alternatively, you can stop a service that was started via ARM and run it manually:

     $ gnunet-arm -k dht
     $ gdb --args gnunet-service-dht -L DEBUG
     $ valgrind gnunet-service-dht -L DEBUG

Assuming other services are well-written, they will automatically re-integrate the restarted service with the peer.

GNUnet provides a powerful logging mechanism providing log levels ERROR, WARNING, INFO and DEBUG. The current log level is configured using the $GNUNET_FORCE_LOG environmental variable. The DEBUG level is only available if --enable-logging=verbose was used when running configure. More details about logging can be found under https://gnunet.org/logging.

You should also probably enable the creation of core files, by setting ulimit, and echo'ing 1 into /proc/sys/kernel/core\_uses\_pid. Then you can investigate the core dumps with gdb, which is often the fastest method to find simple errors.

Exercise: Add a memory leak to your service and obtain a trace pointing to the leak using valgrind while running the service from gnunet-service-arm.