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10 \hypersetup{pdftitle={GNUnet C Tutorial},
12 pdfauthor={Christian Grothoff <christian@grothoff.org>},
13 pdfkeywords={p2p,search,gnunet,tutorial}
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23 literate={*}{{\char42}}1
27 \newcommand{\exercise}[1]{\noindent\begin{boxedminipage}{\textwidth}{\bf Exercise:} #1 \end{boxedminipage}}
32 \large {A Tutorial for GNUnet 0.9.x (C version)}
34 Christian Grothoff $\qquad$ Bart Polot $\qquad$ Matthias Wachs
38 This tutorials explains how to install GNUnet on a GNU/Linux system ond gives an introduction how
39 GNUnet can be used to develop a Peer-to-Peer application. Detailed installation instructions for
40 various operating systems and a detailed list of all dependencies can found on our website at
41 \url{https://gnunet.org/installation}.
43 \textbf{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:
44 \url{https://gnunet.org/contact_information}}
47 \section{Installing GNUnet}
48 First of all you have to install a current version of GNUnet. You can download a
49 tarball of a stable version from GNU FTP mirrors or obtain the latest development
50 version from our Subversion repository.
52 Most of the time you should prefer to download the stable version since with the
53 latest development version things can be broken, functionality can be changed or tests
54 can fail. You should only use the development version if you know that you require a
55 certain feature or a certain issue has been fixed since the last release.
57 \subsection{Obtaining a stable version}
58 You can download the latest stable version of GNUnet from GNU FTP mirrors:
60 \url{ftp://ftp.gnu.org/gnu/gnunet/gnunet-0.9.5a.tar.gz}
62 You should also download the signature file and verify the integrity of the tarball.
64 \url{ftp://ftp.gnu.org/gnu/gnunet/gnunet-0.9.5a.tar.gz.sig}
66 To verify the signature you should first import the GPG key used to sign the tarball
68 $ gpg --keyserver keys.gnupg.net --recv-keys 48426C7E
70 And use this key to verify the tarball's signature
72 $ gpg --verify gnunet-0.9.5a.tar.gz.sig gnunet-0.9.5a.tar.gz
74 After successfully verifying the integrity you can extract the tarball using
76 $ tar xvzf gnunet-0.9.5a.tar.gz
77 $ mv gnunet-0.9.5a gnunet # we will use the directory "gnunet" in the reminder of this document
81 \subsection{Installing Build Tool Chain and Dependencies}
82 To successfully compile GNUnet you need the tools to build GNUnet and the required dependencies.
83 Please have a look at \url{https://gnunet.org/dependencies} for a list of required dependencies
84 and \url{https://gnunet.org/generic_installation} for specific instructions for your operating system.
86 Please check the notes at the end of the configure process about required dependencies.
88 For GNUNet bootstrapping support and the http(s) plugin you should install \texttt{libcurl}.
89 For the filesharing service you should install at least one of the datastore backends \texttt{mysql},
90 \texttt{sqlite} or \texttt{postgresql}.
92 \subsection{Obtaining the latest version from Subversion}
93 The latest development version can obtained from our Subversion (\textit{svn}) repository. To obtain
94 the code you need Subversion installed and checkout the repository using:
95 \lstset{language=bash}
97 $ svn checkout https://gnunet.org/svn/gnunet
99 After cloning the repository you have to execute
100 \lstset{language=bash}
106 The remainder of this tutorial assumes that you have SVN HEAD checked out.
108 \subsection{Compiling and Installing GNUnet}
110 First, you need to install the latest {\tt
111 libgnupgerror}\footnote{\url{ftp://ftp.gnupg.org/gcrypt/libgpg-error/libgpg-error-1.11.tar.bz2}}
112 and {\tt libgcrypt} version from Git. The current GNUnet code uses
113 ECC functions not available in any released version of libgcrypt.
115 \lstset{language=bash}
117 $ git clone git://git.gnupg.org/libgcrypt.git
120 $ ./configure ; $ make install
124 Assuming all dependencies are installed, the following commands will compile and install GNUnet in your
125 home directory. You can specify the directory where GNUnet will be installed by changing the \lstinline|--prefix| value when calling \lstinline|./configure|. If you do not specifiy a prefix, GNUnet is installed in the directory \lstinline|/usr/local|. When developing new applications you may want to enable
126 verbose logging by adding \lstinline|--enable-logging=verbose|:
128 \lstset{language=bash}
130 $ ./configure --prefix=$HOME --enable-logging
135 After installing GNUnet you have to set the \lstinline|GNUNET_PREFIX| environmental variable used by GNUnet to detect it's installation directory and add your GNUnet installation to your path environmental variable.
136 This configuration is only valid for the current shell session, so you should add \lstinline|export GNUNET_PREFIX=$HOME| to your \lstinline|.bash_rc| or \lstinline|.profile| to be sure the environment variable is always set. In addition you have to create the \lstinline|.gnunet| directory in your home directory where GNUnet stores it's data and an empty GNUnet configuration file:
138 \lstset{language=bash}
140 $ export GNUNET_PREFIX=$HOME
141 $ export PATH=$PATH:$GNUNET_PREFIX/bin
142 $ echo export GNUNET_PREFIX=$HOME >> ~/.bashrc
143 $ echo export PATH=$GNUNET_PREFIX/bin:$PATH >> ~/.bashrc
145 $ touch ~/.gnunet/gnunet.conf
149 \subsection{Common Issues - Check your GNUnet installation}
150 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.
154 should return \lstinline|$GNUNET_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:
159 check your {\tt PATH} variable to ensure GNUnet's {\tt bin} directory is included.
161 GNUnet provides tests for all of it's subcomponents. Run
165 to execute tests for all components. {\tt make check} traverses all subdirectories in {\tt src}.
166 For every subdirectory you should get a message like this:
169 make[2]: Entering directory `/home/mwachs/gnunet/contrib'
170 PASS: test_gnunet_prefix
176 If you see a message like this:
179 Mar 12 16:57:56-642482 resolver-api-19449 ERROR Must specify `HOSTNAME' for `resolver' in configuration!
180 Mar 12 16:57:56-642573 test_program-19449 ERROR Assertion failed at resolver_api.c:204.
181 /bin/bash: line 5: 19449 Aborted (core dumped) ${dir}$tst
184 double check your {\tt GNUNET\_PREFIX} environmental variable and double check the steps performed in ~\ref{sub:install}
186 \section{Background: GNUnet Architecture}
187 GNUnet is organized in layers and services. Each service is composed of a
188 main service implementation and a client library for other programs to use
189 the service's functionality, described by an API. This approach is shown in
190 figure~\ref{fig:service}. Some services provide an additional command line
191 tool to enable the user to interact with the service.
193 Very often it is other GNUnet services that will use these APIs to build the
194 higher layers of GNUnet on top of the lower ones. Each layer expands or extends
195 the functionality of the service below (for instance, to build a mesh on top of
196 a DHT). See figure ~\ref{fig:interaction} for an illustration of this approach.
201 \begin{subfigure}[b]{0.3\textwidth}
203 \includegraphics[width=\textwidth]{figs/Service.pdf}
204 \caption{Service with API and network protocol}
208 \begin{subfigure}[b]{0.3\textwidth}
210 \includegraphics[width=\textwidth]{figs/System.pdf}
211 \caption{Service interaction}
212 \label{fig:interaction}
215 \caption{GNUnet's layered system architecture}
218 The main service implementation runs as a standalone process in the operating
219 system and the client code runs as part of the client program, so crashes of a
220 client do not affect the service process or other clients. The service and the
221 clients communicate via a message protocol to be defined and implemented by
224 \section{First Steps with GNUnet}
226 \subsection{Configure your peer}
227 First of all we need to configure your peer. Each peer is started with a configuration containing settings for GNUnet itself and it's services. This configuration is based on the default configuration shipped with GNUnet and can be modified. The default configuration is located in the {\tt \$GNUNET\_PREFIX/share/gnunet/config.d} directory. When starting a peer, you can specify a customized configuration using the the {\tt$-c$} 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.
229 Since we want to start additional peers later, we need
230 some modifications from the default configuration. We need to create a separate service home and a file containing our modifications for this peer:
236 Now add the following lines to peer1.conf to use this directory. For simplified usage we want to prevent
237 the peer to connect to the GNUnet network since this could lead to confusing output. This modifications will replace the default settings:
240 $ SERVICEHOME = ~/gnunet1/ # Use this directory to store GNUnet data
242 $ SERVERS = # prevent bootstrapping
245 \subsection{Start a peer}
246 Each GNUnet instance (called peer) has an identity (\textit{peer ID}) based on a
247 cryptographic public private key pair. The peer ID is the printable hash of the
248 public key. So before starting the peer, you may want to just generate the peer's private
249 key using the command
250 \lstset{language=bash}
252 $ gnunet-peerinfo -c ~/peer1.conf -s
254 You should see an output containing the peer ID similar to:
255 \lstset{language=bash}
257 I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.
260 GNUnet services are controlled by a master service the so called \textit{Automatic Restart Manager} (ARM).
261 ARM starts, stops and even restarts services automatically or on demand when a client connects.
262 You interact with the ARM service using the \lstinline|gnunet-arm| tool.
263 GNUnet can then be started with \lstinline|gnunet-arm -s| and stopped with
264 \lstinline|gnunet-arm -e|. An additional service not automatically started
265 can be started using \lstinline|gnunet-arm -i <service name>| and stopped
266 using \lstinline|gnunet-arm -k <servicename>|.
268 \subsection{Monitor a peer}
269 In this section, we will monitor the behaviour of our peer's DHT service with respect to a
270 specific key. First we will start GNUnet and then start the DHT service and use the DHT monitor tool
271 to monitor the PUT and GET commands we issue ussing the \lstinline|gnunet-dht-put| and
272 \lstinline|gnunet-dht-get| command. Using the ``monitor'' line given below, you can observe the behavior of
273 your own peer's DHT with respect to the specified KEY:
275 \lstset{language=bash}
277 $ gnunet-arm -c ~/peer1.conf -s # start gnunet with all default services
278 $ gnunet-arm -c ~/peer1.conf -i dht # start DHT service
279 $ cd ~/gnunet/src/dht;
280 $ ./gnunet-dht-monitor -c ~/peer1.conf -k KEY
282 Now open a separate terminal and change again to the \lstinline|gnunet/src/dht| directory:
284 $ cd ~/gnunet/src/dht
285 $ ./gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE # put VALUE under KEY in the DHT
286 $ ./gnunet/src/dht/gnunet-dht-get ~/peer1.conf -k KEY # get key KEY from the DHT
287 $ gnunet-statistics -c ~/peer1.conf # print statistics about current GNUnet state
288 $ gnunet-statistics -c ~/peer1.conf -s dht # print statistics about DHT service
291 \subsection{Starting Two Peers by Hand}
292 \subsubsection{Setup a second peer}
293 We will now start a second peer on your machine.
294 For the second peer, you will need to manually create a modified
295 configuration file to avoid conflicts with ports and directories.
296 A peers configuration file is by default located in {\tt ~/.gnunet/gnunet.conf}.
297 This file is typically very short or even empty as only the differences to the
298 defaults need to be specified. The defaults are located in
299 many files in the {\tt \$GNUNET\_PREFIX/share/gnunet/config.d} directory.
301 To configure the second peer, use the files {\tt
302 \$GNUNET\_PREFIX/share/gnunet/config.d} as a template for your main
305 \lstset{language=bash}
307 $ cat $GNUNET_PREFIX/share/gnunet/config.d/*.conf > peer2.conf
309 Now you have to edit {\tt peer2.conf} and change:
312 \item{\texttt{SERVICEHOME} under \texttt{PATHS}}
313 \item{Every (uncommented) value for ``\texttt{PORT}'' (add 10000) in any
314 section (the option may be commented out if \texttt{PORT} is
315 prefixed by "\#", in this case, UNIX domain sockets are used
316 and the PORT option does not need to be touched) }
317 \item{Every value for ``\texttt{UNIXPATH}'' in any section (e.g. by adding a "-p2" suffix)}
319 to a fresh, unique value. Make sure that the \texttt{PORT} numbers stay
320 below 65536. From now on, whenever you interact with the second
321 peer, you need to specify {\tt -c peer2.conf} as an additional
322 command line argument.
324 Now, generate the 2nd peer's private key:
326 \lstset{language=bash}
328 $ gnunet-peerinfo -s -c peer2.conf
332 This may take a while, generate entropy using your keyboard or mouse
333 as needed. Also, make sure the output is different from the {\tt
334 gnunet-peerinfo} output for the first peer (otherwise you made an
335 error in the configuration).
337 \subsubsection{Start the second peer and connect the peers}
338 Then, you can start a second peer using:
339 \lstset{language=bash}
341 $ gnunet-arm -c peer2.conf -s
342 $ gnunet-arm -c peer2.conf -i dht
343 $ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
344 $ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY
346 If you want the two peers to connect, you have multiple options:
349 \item UDP neighbour discovery (automatic)
350 \item Setup a bootstrap server
351 \item Connect manually
353 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 \texttt{peer1.conf} to enable bootstrapping server:
359 Then change {\tt peer2.conf} and replace the ``\texttt{SERVERS}'' line in the ``\texttt{[hostlist]}'' section with
360 ``\texttt{http://localhost:8080/}''. Restart both peers using:
362 $ gnunet-arm -c peer1.conf -e # stop first peer
363 $ gnunet-arm -c peer1.conf -s # start first peer
364 $ gnunet-arm -c peer2.conf -s # start second peer
367 Note that if you start your peers without changing these settings, they
368 will use the ``global'' hostlist servers of the GNUnet P2P network and
369 likely connect to those peers. At that point, debugging might become
370 tricky as you're going to be connected to many more peers and would
371 likely observe traffic and behaviors that are not explicitly controlled
374 \subsubsection{How to connect manually}
375 If you want to use the \texttt{peerinfo} tool to connect your peers, you should:
378 \item{Remove {\tt hostlist} from {\tt DEFAULTSERVICES} (to not connect to the global GNUnet)}
379 \item{Start both peers running {\tt gnunet-arm -c peer1.conf -s} and {\tt gnunet-arm -c peer2.conf -s}}
380 \item{Get \texttt{HELLO} message of the first peer running {\tt gnunet-peerinfo -c peer1.conf -g}}
381 \item{Give the output to the second peer by running {\tt gnunet-peerinfo -c peer2.conf -p '<output>'}}
384 Check that they are connected using {\tt gnunet-core -c peer1.conf}, which should give you the other peer's
387 $ gnunet-core -c peer1.conf
388 Peer `9TVUCS8P5A7ILLBGO6JSTSSN2B44H3D2MUIFJMLKAITC0I22UVFBFP1H8NRK2IA35VKAK16LLO0MFS7TAQ9M1KNBJ4NGCHP3JPVULDG'
391 \subsection{Starting Peers Using the Testbed Service}
393 GNUnet's testbed service is used for testing scenarios where a number of peers
394 are to be started. The testbed can manage peers on a single host or on multiple
395 hosts in a distributed fashion. On a single affordable computer, it should be
396 possible to run around tens of peers without drastically increasing the load on the
399 The testbed service can be access through its API
400 \texttt{include/gnunet\_testbed\_service.h}. The API provides many routines for
401 managing a group of peers. It also provides a helper function
402 \texttt{GNUNET\_TESTBED\_test\_run()} to quickly setup a minimalistic testing
403 environment on a single host.
405 This function takes a configuration file which will be used as a template
406 configuration for the peers. The testbed takes care of modifying relevant
407 options in the peers' configuration such as SERVICEHOME, PORT, UNIXPATH to
408 unique values so that peers run without running into conflicts. It also checks
409 and assigns the ports in configurations only if they are free.
411 Additionally, the testbed service also reads its options from the same
412 configuration file. Various available options and details about them can be
413 found in the testbed default configuration file \texttt{src/testbed/testbed.conf}.
415 With the testbed API, a sample test case can be structured as follows:
416 \lstinputlisting[language=C]{testbed_test.c}
417 The source code for the above listing can be found at
418 \url{https://gnunet.org/svn/gnunet/doc/testbed_test.c}. After installing GNUnet, the above source code can be compiled as:
419 \lstset{language=bash}
421 $ export CPPFLAGS="-I/path/to/gnunet/headers"
422 $ export LDFLAGS="-L/path/to/gnunet/libraries"
423 $ gcc -o testbed-test testbed_test.c -lgnunettestbed -lgnunetdht -lgnunetutil
425 The \texttt{CPPFLAGS} and \texttt{LDFLAGS} are necessary if GNUnet is installed
426 into a different directory other than \texttt{/usr/local}.
428 All of testbed API's peer management functions treat management actions as
429 operations and return operation handles. It is expected that the operations
430 begin immediately, but they may get delayed (to balance out load on the system).
431 The program using the API then has to take care of marking the operation as
432 ``done'' so that its associated resources can be freed immediately and other
433 waiting operations can be executed. Operations will be canceled if they are
434 marked as ``done'' before their completion.
436 An operation is treated as completed when it succeeds or fails. Completion of
437 an operation is either conveyed as events through \textit{controller event
438 callback} or through respective operation completion callbacks. In functions
439 which support completion notification through both controller event callback and
440 operation completion callback, first the controller event callback will be
441 called. If the operation is not marked as done in that callback or if the
442 callback is given as NULL when creating the operation, the operation completion
443 callback will be called. The API documentation shows which event are to be
444 expected in the controller event notifications. It also documents any
445 exceptional behaviour.
447 Once the peers are started, test cases often need to connect some of the peers'
448 services. Normally, opening a connect to a peer's service requires the peer's
449 configuration. While using testbed, the testbed automatically generates
450 per-peer configuration. Accessing those configurations directly through file
451 system is discouraged as their locations are dynamically created and will be
452 different among various runs of testbed. To make access to these configurations
453 easy, testbed API provides the function
454 \texttt{GNUNET\_TESTBED\_service\_connect()}. This function fetches the
455 configuration of a given peer and calls the \textit{Connect Adapter}.
456 In the example code, it is the \texttt{dht\_ca}. A connect adapter is expected
457 to open the connection to the needed service by using the provided configuration
458 and return the created service connection handle. Successful connection to the
459 needed service is signaled through \texttt{service\_connect\_comp\_cb}.
461 A dual to connect adapter is the \textit{Disconnect Adapter}. This callback is
462 called after the connect adapter has been called when the operation from
463 \texttt{GNUNET\_TESTBED\_service\_connect()} is marked as ``done''. It has to
464 disconnect from the service with the provided service handle (\texttt{op\_result}).
466 \exercise{Find out how many peers you can run on your system.}
468 \exercise{Find out how to create a 2D torus topology by changing the
469 options in the configuration file.\footnote{See \url{https://gnunet.org/content/supported-topologies}}
470 Then use the DHT API to store and retrieve values in the
473 \section{Developing Applications}
474 \subsection{gnunet-ext}
475 To develop a new peer-to-peer application or to extend GNUnet we provide
476 a template build system for writing GNUnet extensions in C. It can be
479 \lstset{language=bash}
481 $ svn checkout https://gnunet.org/svn/gnunet-ext/
484 $ ./configure --prefix=$HOME --with-gnunet=$GNUNET_PREFIX
491 The GNUnet ext template includes examples and a working buildsystem for a new GNUnet service.
492 A common GNUnet service consists of the following parts which will be discussed in detail in the
493 remainder of this document. The functionality of a GNUnet service is implemented in:
497 \item the GNUnet service (\lstinline|gnunet-ext/src/ext/gnunet-service-ext.c|)
498 \item the client API (\lstinline|gnunet-ext/src/ext/ext_api.c|)
499 \item the client application using the service API (\lstinline|gnunet-ext/src/ext/gnunet-ext.c|)
504 The interfaces for these entities are defined in:
507 \item client API interface (\lstinline|gnunet-ext/src/ext/ext.h|)
508 \item the service interface (\lstinline|gnunet-ext/src/include/gnunet_service_SERVICE.h|)
509 \item the P2P protocol (\lstinline|gnunet-ext/src/include/gnunet_protocols_ext.h|)
513 In addition the \texttt{ext} systems provides:
516 \item a test testing the API (\lstinline|gnunet-ext/src/ext/test_ext_api.c|)
517 \item a configuration template for the service (\lstinline|gnunet-ext/src/ext/ext.conf.in|)
521 \subsection{Adapting the Template}
523 The first step for writing any extension with a new service is to
524 ensure that the {\tt ext.conf.in} file contains entries for the
525 \texttt{UNIXPATH}, \texttt{PORT} and \texttt{BINARY} for the service in a section named after
528 If you want to adapt the template rename the {\tt ext.conf.in} to match your
529 services name, you have to modify the \texttt{AC\_OUTPUT} section in {\tt configure.ac}
530 in the \texttt{gnunet-ext} root.
532 \section{Writing a Client Application}
534 When writing any client application (for example, a command-line
535 tool), the basic structure is to start with the {\tt
536 GNUNET\_PROGRAM\_run} function. This function will parse
537 command-line options, setup the scheduler and then invoke the {\tt
538 run} function (with the remaining non-option arguments) and a handle
539 to the parsed configuration (and the configuration file name that was
540 used, which is typically not needed):
544 #include <gnunet/platform.h>
545 #include <gnunet/gnunet_util_lib.h>
553 const struct GNUNET_CONFIGURATION_Handle *cfg)
560 main (int argc, char *const *argv)
562 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
563 GNUNET_GETOPT_OPTION_END
566 GNUNET_PROGRAM_run (argc,
569 gettext_noop ("binary description text"),
570 options, &run, NULL)) ? ret : 1;
574 \subsection{Handling command-line options}
576 Options can then be added easily by adding global variables and
577 expanding the {\tt options} array. For example, the following would
578 add a string-option and a binary flag (defaulting to {\tt NULL} and
579 {\tt GNUNET\_NO} respectively):
582 static char *string_option;
586 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
587 {'s', "name", "SOMESTRING",
588 gettext_noop ("text describing the string_option NAME"), 1,
589 &GNUNET_GETOPT_set_string, &string_option},
591 gettext_noop ("text describing the flag option"), 0,
592 &GNUNET_GETOPT_set_one, &a_flag},
593 GNUNET_GETOPT_OPTION_END
598 Issues such as displaying some helpful text describing options using
599 the {\tt --help} argument and error handling are taken care of when
600 using this approach. Other {\tt GNUNET\_GETOPT\_}-functions can be used
601 to obtain integer value options, increment counters, etc. You can
602 even write custom option parsers for special circumstances not covered
603 by the available handlers.
605 Inside the {\tt run} method, the program would perform the
606 application-specific logic, which typically involves initializing and
607 using some client library to interact with the service. The client
608 library is supposed to implement the IPC whereas the service provides
609 more persistent P2P functions.
611 \exercise{Add a few command-line options and print them inside
612 of {\tt run}. What happens if the user gives invalid arguments?}
614 \subsection{Writing a Client Library}
616 The first and most important step in writing a client library is to
617 decide on an API for the library. Typical API calls include
618 connecting to the service, performing application-specific requests
619 and cleaning up. Many examples for such service APIs can be found
620 in the {\tt gnunet/src/include/gnunet\_*\_service.h} files.
622 Then, a client-service protocol needs to be designed. This typically
623 involves defining various message formats in a header that will be
624 included by both the service and the client library (but is otherwise
625 not shared and hence located within the service's directory and not
626 installed by {\tt make install}). Each message must start with a {\tt
627 struct GNUNET\_MessageHeader} and must be shorter than 64k. By
628 convention, all fields in IPC (and P2P) messages must be in big-endian
629 format (and thus should be read using {\tt ntohl} and similar
630 functions and written using {\tt htonl} and similar functions).
631 Unique message types must be defined for each message struct in the
632 {\tt gnunet\_protocols.h} header (or an extension-specific include
635 \subsubsection{Connecting to the Service}
637 Before a client library can implement the application-specific protocol
638 with the service, a connection must be created:
642 struct GNUNET_CLIENT_Connection *client;
643 client = GNUNET_CLIENT_connect ("service-name", cfg);
646 As a result a {\tt GNUNET\_CLIENT\_Connection} handle is returned
647 which has to used in later API calls related to this service.
648 The complete client API can be found in {\tt gnunet\_client\_lib.h}
650 \subsubsection{GNUnet Messages}
652 In GNUnet, messages are always sent beginning with a {\tt struct GNUNET\_MessageHeader}
653 in big endian format. This header defines the size and the type of the
654 message, the payload follows after this header.
658 struct GNUNET_MessageHeader
662 * The length of the struct (in bytes, including the length field itself),
663 * in big-endian format.
665 uint16_t size GNUNET_PACKED;
668 * The type of the message (GNUNET_MESSAGE_TYPE_XXXX), in big-endian format.
670 uint16_t type GNUNET_PACKED;
675 Existing message types are defined in {\tt gnunet\_protocols.h}\\
676 A common way to create a message is:
680 struct GNUNET_MessageHeader *msg =
681 GNUNET_malloc(payload_size + sizeof(struct GNUNET_MessageHeader));
682 msg->size = htons(payload_size + sizeof(struct GNUNET_MessageHeader));
683 msg->type = htons(GNUNET_MY_MESSAGE_TYPE);
684 memcpy(&msg[1], &payload, payload_size);
688 \exercise{Define a message struct that includes a 32-bit
689 unsigned integer in addition to the standard GNUnet MessageHeader.
690 Add a C struct and define a fresh protocol number for your message.}
693 \subsubsection{Sending Requests to the Service}
695 Any client-service protocol must start with the client sending the
696 first message to the service, since services are only notified about
697 (new) clients upon receiving a the first message.
699 Clients can transmit messages to the service using the
700 {\tt GNUNET\_CLIENT\_notify\_transmit\_ready} API:
704 transmit_cb (void *cls, size_t size, void *buf)
707 if (NULL == buf) { handle_error(); return 0; }
708 GNUNET_assert (size >= msg_size);
709 memcpy (buf, my_msg, msg_size);
715 th = GNUNET_CLIENT_notify_transmit_ready (client,
723 The client-service protocoll calls {\tt GNUNET\_CLIENT\_notify\_transmit\_ready}
724 to be notified when the client is ready to send data to the service.
725 Besides other arguments, you have to pass the client returned
726 from the {\tt connect} call, the message size and the callback function to
727 call when the client is ready to send.
729 Only a single transmission request can be queued per client at the
730 same time using this API. The handle {\tt th} can be used to cancel
731 the request if necessary (for example, during shutdown).
733 When {\tt transmit\_cb} is called the message is copied in the buffer provided and
734 the number of bytes copied into the buffer is returned. {\tt transmit\_cb}
735 could also return 0 if for some reason no message
736 could be constructed; this is not an error and the connection to the
737 service will persist in this case.
739 \exercise{Define a helper function to transmit a 32-bit
740 unsigned integer (as payload) to a service using some given client
744 \subsubsection{Receiving Replies from the Service}
746 Clients can receive messages from the service using the
747 {\tt GNUNET\_CLIENT\_receive} API:
752 * Function called with messages from stats service.
755 * @param msg message received, NULL on timeout or fatal error
758 receive_message (void *cls, const struct GNUNET_MessageHeader *msg)
760 struct MyArg *arg = cls;
766 GNUNET_CLIENT_receive (client,
773 It should be noted that this receive call only receives a single
774 message. To receive additional messages, {\tt
775 GNUNET\_CLIENT\_receive} must be called again.
777 \exercise{Expand your helper function to receive a
778 response message (for example, containing just the GNUnet MessageHeader
779 without any payload). Upon receiving the service's response, you should
780 call a callback provided to your helper function's API. You'll need to
781 define a new 'struct' to hold your local context (``closure'').}
784 \subsection{Writing a user interface}
786 Given a client library, all it takes to access a service now is to
787 combine calls to the client library with parsing command-line
790 \exercise{Call your client API from your {\tt run} method
791 in your client application to send a request to the service.
792 For example, send a 32-bit integer value based on a number given
793 at the command-line to the service.}
797 \section{Writing a Service}
799 Before you can test the client you've written so far, you'll need to also
800 implement the corresponding service.
803 \subsection{Code Placement}
805 New services are placed in their own subdirectory under {\tt gnunet/src}.
806 This subdirectory should contain the API implementation file {\tt SERVICE\_api.c},
807 the description of the client-service protocol {\tt SERVICE.h} and P2P protocol
808 {\tt SERVICE\_protocol.h}, the implementation of the service itself
809 {\tt gnunet-service-SERVICE.h} and several files for tests, including test code
810 and configuration files.
812 \subsection{Starting a Service}
814 The key API definitions for starting services are:
817 typedef void (*GNUNET_SERVICE_Main) (void *cls,
818 struct GNUNET_SERVER_Handle *server,
819 const struct GNUNET_CONFIGURATION_Handle *cfg);
820 int GNUNET_SERVICE_run (int argc,
822 const char *serviceName,
823 enum GNUNET_SERVICE_Options opt,
824 GNUNET_SERVICE_Main task,
828 Here is a starting point for your main function for your service:
832 static void my_main (void *cls,
833 struct GNUNET_SERVER_Handle *server,
834 const struct GNUNET_CONFIGURATION_Handle *cfg)
839 int main (int argc, char *const*argv)
842 GNUNET_SERVICE_run (argc, argv, "my",
843 GNUNET_SERVICE_OPTION_NONE,
850 \exercise{Write a stub service that processes no messages at all
851 in your code. Create a default configuration for it, integrate it
852 with the build system and start the service from {\tt
853 gnunet-service-arm} using {\tt gnunet-arm -i NAME}.}
856 \subsection{Receiving Requests from Clients}
858 Inside of the {\tt my\_main} method, a service typically registers for
859 the various message types from clients that it supports by providing
860 a handler function, the message type itself and possibly a fixed
861 message size (or 0 for variable-size messages):
866 handle_set (void *cls,
867 struct GNUNET_SERVER_Client *client,
868 const struct GNUNET_MessageHeader *message)
870 GNUNET_SERVER_receive_done (client, GNUNET_OK);
873 handle_get (void *cls,
874 struct GNUNET_SERVER_Client *client,
875 const struct GNUNET_MessageHeader *message)
877 GNUNET_SERVER_receive_done (client, GNUNET_OK);
880 static void my_main (void *cls,
881 struct GNUNET_SERVER_Handle *server,
882 const struct GNUNET_CONFIGURATION_Handle *cfg)
884 static const struct GNUNET_SERVER_MessageHandler handlers[] = {
885 {&handle_set, NULL, GNUNET_MESSAGE_TYPE_MYNAME_SET, 0},
886 {&handle_get, NULL, GNUNET_MESSAGE_TYPE_MYNAME_GET, 0},
889 GNUNET_SERVER_add_handlers (server, handlers);
890 /* do more setup work */
894 Each handler function {\bf must} eventually (possibly in some
895 asynchronous continuation) call {\tt GNUNET\_SERVER\_receive\_done}.
896 Only after this call additional messages from the same client may
897 be processed. This way, the service can throttle processing messages
898 from the same client. By passing {\tt GNUNET\_SYSERR}, the service
899 can close the connection to the client, indicating an error.
901 Services must check that client requests are well-formed and must not
902 crash on protocol violations by the clients. Similarly, client
903 libraries must check replies from servers and should gracefully report
904 errors via their API.
907 \exercise{Change the service to ``handle'' the message from your
908 client (for now, by printing a message). What happens if you
909 forget to call {\tt GNUNET\_SERVER\_receive\_done}?}
912 \subsection{Responding to Clients}
914 Servers can send messages to clients using the
915 {\tt GNUNET\_SERVER\_notify\_transmit\_ready} API:
920 transmit_cb (void *cls, size_t size, void *buf)
923 if (NULL == buf) { handle_error(); return 0; }
924 GNUNET_assert (size >= msg_size);
925 memcpy (buf, my_msg, msg_size);
931 struct GNUNET_SERVER_TransmitHandle *th;
932 th = GNUNET_SERVER_notify_transmit_ready (client,
939 Only a single transmission request can be queued per client
940 at the same time using this API.
941 Additional APIs for sending messages to clients can be found
942 in the {\tt gnunet\_server\_lib.h} header.
945 \exercise{Change the service respond to the request from your
946 client. Make sure you handle malformed messages in both directions.}
949 \section{Interacting directly with other Peers using the CORE Service}
951 One of the most important services in GNUnet is the \texttt{CORE} service
952 managing connections between peers and handling encryption between peers.
954 One of the first things any service that extends the P2P protocol typically does
955 is connect to the \texttt{CORE} service using:
959 #include <gnunet/gnunet_core_service.h>
961 struct GNUNET_CORE_Handle *
962 GNUNET_CORE_connect (const struct GNUNET_CONFIGURATION_Handle *cfg,
964 GNUNET_CORE_StartupCallback init,
965 GNUNET_CORE_ConnectEventHandler connects,
966 GNUNET_CORE_DisconnectEventHandler disconnects,
967 GNUNET_CORE_MessageCallback inbound_notify,
968 int inbound_hdr_only,
969 GNUNET_CORE_MessageCallback outbound_notify,
970 int outbound_hdr_only,
971 const struct GNUNET_CORE_MessageHandler *handlers);
974 \subsection{New P2P connections}
976 Before any traffic with a different peer can be exchanged, the peer must be
977 known to the service. This is notified by the \texttt{CORE} {\tt connects} callback,
978 which communicates the identity of the new peer to the service:
984 const struct GNUNET_PeerIdentity * peer)
986 /* Save identity for later use */
987 /* Optional: start sending messages to peer */
991 \exercise{Create a service that connects to the \texttt{CORE}. Then
992 start (and connect) two peers and print a message once your connect
993 callback is invoked.}
995 \subsection{Receiving P2P Messages}
997 To receive messages from \texttt{CORE}, services register a set of handlers
998 (parameter {\tt *handlers} in the \lstinline|GNUNET_CORE_connect| call that are called by \texttt{CORE}
999 when a suitable message arrives.
1004 callback_function_for_type_one(void *cls,
1005 const struct GNUNET_PeerIdentity *peer,
1006 const struct GNUNET_MessageHeader *message)
1009 return GNUNET_OK; /* or GNUNET_SYSERR to close the connection */
1013 * Functions to handle messages from core
1015 static struct GNUNET_CORE_MessageHandler core_handlers[] = {
1016 {&callback_function_for_type_one, GNUNET_MESSAGE_TYPE_MYSERVICE_TYPE_ONE, 0},
1022 \exercise{Start one peer with a new service that has a message
1023 handler and start a second peer that only has your ``old'' service
1024 without message handlers. Which ``connect'' handlers are invoked when
1025 the two peers are connected? Why?}
1028 \subsection{Sending P2P Messages}
1030 In response to events (connect, disconnect, inbound messages,
1031 timing, etc.) services can then use this API to transmit messages:
1036 (*GNUNET_CONNECTION_TransmitReadyNotify) (void *cls,
1040 /* Fill "*buf" with up to "size" bytes, must start with GNUNET_MessageHeader */
1041 return n; /* Total size of the message put in "*buf" */
1044 struct GNUNET_CORE_TransmitHandle *
1045 GNUNET_CORE_notify_transmit_ready (struct GNUNET_CORE_Handle *handle,
1046 int cork, uint32_t priority,
1047 struct GNUNET_TIME_Relative maxdelay,
1048 const struct GNUNET_PeerIdentity *target,
1050 GNUNET_CONNECTION_TransmitReadyNotify notify,
1054 \exercise{Write a service that upon connect sends messages as
1055 fast as possible to the other peer (the other peer should run a
1056 service that ``processes'' those messages). How fast is the
1057 transmission? Count using the STATISTICS service on both ends. Are
1058 messages lost? How can you transmit messages faster? What happens if
1059 you stop the peer that is receiving your messages?}
1062 \subsection{End of P2P connections}
1064 If a message handler returns {\tt GNUNET\_SYSERR}, the remote peer shuts down or
1065 there is an unrecoverable network disconnection, CORE notifies the service that
1066 the peer disconnected. After this notification no more messages will be received
1067 from the peer and the service is no longer allowed to send messages to the peer.
1068 The disconnect callback looks like the following:
1073 disconnects (void *cls,
1074 const struct GNUNET_PeerIdentity * peer)
1076 /* Remove peer's identity from known peers */
1077 /* Make sure no messages are sent to peer from now on */
1081 \exercise{Fix your service to handle peer disconnects.}
1083 \section{Using the DHT}
1084 The DHT allows to store data so other peers in the P2P network can
1085 access it and retrieve data stored by any peers in the network.
1086 This section will explain how to use the DHT. Of course, the first
1087 thing to do is to connect to the DHT service:
1090 dht_handle = GNUNET_DHT_connect (cfg, parallel_requests);
1092 The second parameter indicates how many requests in parallel to expect.
1093 It is not a hard limit, but a good approximation will make the DHT more
1096 \subsection{Storing data in the DHT}
1097 Since the DHT is a dynamic environment (peers join a leave frequently)
1098 the data that we put in the DHT does not stay there indefinitely. It is
1099 important to ``refresh'' the data periodically by simply storing it again,
1100 in order to make sure other peers can access it.
1102 The put API call offers a callback to signal that the PUT request has been
1103 sent. This does not guarantee that the data is accessible to others peers,
1104 or even that is has been stored, only that the service has requested to
1105 a neighboring peer the retransmission of the PUT request towards its final
1106 destination. Currently there is no feedback about whether or not the data
1107 has been sucessfully stored or where it has been stored. In order to improve
1108 the availablilty of the data and to compensate for possible errors, peers leaving
1109 and other unfavorable events, just make several PUT requests!
1114 message_sent_cont (void *cls, const struct GNUNET_SCHEDULER_TaskContext *tc)
1116 /* Request has left local node */
1119 struct GNUNET_DHT_PutHandle *
1120 GNUNET_DHT_put (struct GNUNET_DHT_Handle *handle,
1121 const struct GNUNET_HashCode * key,
1122 uint32_t desired_replication_level,
1123 enum GNUNET_DHT_RouteOption options, /* Route options, see next call */
1124 enum GNUNET_BLOCK_Type type, size_t size, const void *data,
1125 struct GNUNET_TIME_Absolute exp, /* When does the data expire? */
1126 struct GNUNET_TIME_Relative timeout, /* How long to try to send the request */
1127 GNUNET_DHT_PutContinuation cont,
1131 \exercise{Store a value in the DHT periodically to make sure it is available
1132 over time. You might consider using the function GNUNET\_SCHEDULER\_add\_delayed and
1133 call GNUNET\_DHT\_put from inside a helper function.}
1136 \subsection{Obtaining data from the DHT}
1137 As we saw in the previous example, the DHT works in an asynchronous mode.
1138 Each request to the DHT is executed ``in the background'' and the API
1139 calls return immediately. In order to receive results from the DHT, the
1140 API provides a callback. Once started, the request runs in the service,
1141 the service will try to get as many results as possible (filtering out
1142 duplicates) until the timeout expires or we explicitly stop the request.
1143 It is possible to give a ``forever'' timeout with
1144 {\tt GNUNET\_TIME\_UNIT\_FOREVER\_REL}.
1146 If we give a route option {\tt GNUNET\_DHT\_RO\_RECORD\_ROUTE} the callback
1147 will get a list of all the peers the data has travelled, both on the PUT
1148 path and on the GET path.
1152 get_result_iterator (void *cls, struct GNUNET_TIME_Absolute expiration,
1153 const struct GNUNET_HashCode * key,
1154 const struct GNUNET_PeerIdentity *get_path,
1155 unsigned int get_path_length,
1156 const struct GNUNET_PeerIdentity *put_path,
1157 unsigned int put_path_length,
1158 enum GNUNET_BLOCK_Type type, size_t size, const void *data)
1160 /* Do stuff with the data and/or route */
1162 GNUNET_DHT_get_stop (get_handle);
1166 GNUNET_DHT_get_start (dht_handle,
1170 GNUNET_DHT_RO_NONE, /* Route options */
1171 NULL, /* xquery: not used here */
1172 0, /* xquery size */
1173 &get_result_iterator,
1177 \exercise{Store a value in the DHT and after a while retrieve it. Show the IDs of all
1178 the peers the requests have gone through. In order to convert a peer ID to a string, use
1179 the function GNUNET\_i2s. Pay attention to the route option parameters in both calls!}
1181 \subsection{Implementing a block plugin}
1183 In order to store data in the DHT, it is necessary to provide a block
1184 plugin. The DHT uses the block plugin to ensure that only well-formed
1185 requests and replies are transmitted over the network.
1187 The block plugin should be put in a file {\tt
1188 plugin\_block\_SERVICE.c} in the service's respective directory. The
1189 mandatory functions that need to be implemented for a block plugin are
1190 described in the following sections.
1192 \subsubsection{Validating requests and replies}
1194 The evaluate function should validate a reply or a request. It returns
1195 a {\tt GNUNET\_BLOCK\_EvaluationResult}, which is an enumeration. All
1196 possible answers are in {\tt gnunet\_block\_lib.h}. The function will
1197 be called with a {\tt reply\_block} argument of {\tt NULL} for
1198 requests. Note that depending on how {\tt evaluate} is called, only
1199 some of the possible return values are valid. The specific meaning of
1200 the {\tt xquery} argument is application-specific. Applications that
1201 do not use an extended query should check that the {\tt xquery\_size}
1202 is zero. The Bloom filter is typically used to filter duplicate
1207 static enum GNUNET_BLOCK_EvaluationResult
1208 block_plugin_SERVICE_evaluate (void *cls,
1209 enum GNUNET_BLOCK_Type type,
1210 const GNUNET_HashCode * query,
1211 struct GNUNET_CONTAINER_BloomFilter **bf,
1215 const void *reply_block,
1216 size_t reply_block_size)
1218 /* Verify type, block and bloomfilter */
1222 Note that it is mandatory to detect duplicate replies in this
1223 function and return the respective status code. Duplicate
1224 detection should be done by setting the respective bits in
1225 the Bloom filter {\tt bf}. Failure to do so may cause replies
1226 to circle in the network.
1228 \subsubsection{Deriving a key from a reply}
1230 The DHT can operate more efficiently if it is possible to derive a key
1231 from the value of the corresponding block. The {\tt get\_key}
1232 function is used to obtain the key of a block --- for example, by
1233 means of hashing. If deriving the key is not possible, the function
1234 should simply return {\tt GNUNET\_SYSERR} (the DHT will still work
1235 just fine with such blocks).
1240 block_plugin_SERVICE_get_key (void *cls, enum GNUNET_BLOCK_Type type,
1241 const void *block, size_t block_size,
1242 GNUNET_HashCode * key)
1244 /* Store the key in the key argument, return GNUNET_OK on success. */
1248 \subsubsection{Initialization of the plugin}
1250 The plugin is realized as a shared C library. The library must export
1251 an initialization function which should initialize the plugin. The
1252 initialization function specifies what block types the plugin cares
1253 about and returns a struct with the functions that are to be used for
1254 validation and obtaining keys (the ones just defined above).
1259 libgnunet_plugin_block_SERVICE_init (void *cls)
1261 static enum GNUNET_BLOCK_Type types[] =
1263 GNUNET_BLOCK_TYPE_SERVICE_BLOCKYPE, /* list of blocks we care about, from gnunet_block_lib.h */
1264 GNUNET_BLOCK_TYPE_ANY /* end of list */
1266 struct GNUNET_BLOCK_PluginFunctions *api;
1268 api = GNUNET_malloc (sizeof (struct GNUNET_BLOCK_PluginFunctions));
1269 api->evaluate = &block_plugin_SERICE_evaluate;
1270 api->get_key = &block_plugin_SERVICE_get_key;
1276 \subsubsection{Shutdown of the plugin}
1278 Following GNUnet's general plugin API concept, the plugin must
1279 export a second function for cleaning up. It usually does very
1285 libgnunet_plugin_block_SERVICE_done (void *cls)
1287 struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
1295 \subsubsection{Integration of the plugin with the build system}
1297 In order to compile the plugin, the {\tt Makefile.am} file for the
1298 service should contain a rule similar to this:
1300 \lstset{language=make}
1302 plugin_LTLIBRARIES = \
1303 libgnunet_plugin_block_SERVICE.la
1304 libgnunet_plugin_block_SERVICE_la_SOURCES = \
1305 plugin_block_SERVICE.c
1306 libgnunet_plugin_block_SERVICE_la_LIBADD = \
1307 $(top_builddir)/src/hello/libgnunethello.la \
1308 $(top_builddir)/src/block/libgnunetblock.la \
1309 $(top_builddir)/src/util/libgnunetutil.la
1310 libgnunet_plugin_block_SERVICE_la_LDFLAGS = \
1311 $(GN_PLUGIN_LDFLAGS)
1312 libgnunet_plugin_block_SERVICE_la_DEPENDENCIES = \
1313 $(top_builddir)/src/block/libgnunetblock.la
1318 \exercise{Write a block plugin that accepts all queries
1319 and all replies but prints information about queries and replies
1320 when the respective validation hooks are called.}
1324 \subsection{Monitoring the DHT}
1325 It is possible to monitor the functioning of the local DHT service. When monitoring
1326 the DHT, the service will alert the monitoring program of any events,
1327 both started locally or received for routing from another peer. The are three different
1328 types of events possible: a GET request, a PUT request or a response (a reply to
1331 Since the different events have different associated data, the API gets 3
1332 different callbacks (one for each message type) and optional type and key parameters,
1333 to allow for filtering of messages. When an event happens, the appropiate callback
1334 is called with all the information about the event.
1338 get_callback (void *cls,
1339 enum GNUNET_DHT_RouteOption options,
1340 enum GNUNET_BLOCK_Type type,
1342 uint32_t desired_replication_level,
1343 unsigned int path_length,
1344 const struct GNUNET_PeerIdentity *path,
1345 const struct GNUNET_HashCode * key)
1350 get_resp_callback (void *cls,
1351 enum GNUNET_BLOCK_Type type,
1352 const struct GNUNET_PeerIdentity *get_path,
1353 unsigned int get_path_length,
1354 const struct GNUNET_PeerIdentity *put_path,
1355 unsigned int put_path_length,
1356 struct GNUNET_TIME_Absolute exp,
1357 const struct GNUNET_HashCode * key,
1364 put_callback (void *cls,
1365 enum GNUNET_DHT_RouteOption options,
1366 enum GNUNET_BLOCK_Type type,
1368 uint32_t desired_replication_level,
1369 unsigned int path_length,
1370 const struct GNUNET_PeerIdentity *path,
1371 struct GNUNET_TIME_Absolute exp,
1372 const struct GNUNET_HashCode * key,
1378 monitor_handle = GNUNET_DHT_monitor_start (dht_handle,
1379 block_type, /* GNUNET_BLOCK_TYPE_ANY for all */
1380 key, /* NULL for all */
1388 \section{Debugging with {\tt gnunet-arm}}
1390 Even if services are managed by {\tt gnunet-arm}, you can start them with
1391 {\tt gdb} or {\tt valgrind}. For example, you could add the following lines
1392 to your configuration file to start the DHT service in a {\tt gdb} session in a
1397 PREFIX=xterm -e gdb --args
1400 Alternatively, you can stop a service that was started via ARM and run it manually:
1402 \lstset{language=bash}
1405 $ gdb --args gnunet-service-dht -L DEBUG
1406 $ valgrind gnunet-service-dht -L DEBUG
1410 Assuming other services are well-written, they will automatically re-integrate the
1411 restarted service with the peer.
1413 GNUnet provides a powerful logging mechanism providing log levels \texttt{ERROR},
1414 \texttt{WARNING}, \texttt{INFO} and \texttt{DEBUG}. The current log level is
1415 configured using the \lstinline|$GNUNET_FORCE_LOG| environmental variable.
1416 The \texttt{DEBUG} level is only available if \lstinline|--enable-logging=verbose| was used when
1417 running \texttt{configure}. More details about logging can be found under
1418 \url{https://gnunet.org/logging}.
1420 You should also probably enable the creation of core files, by setting
1421 {\tt ulimit}, and echo'ing 1 into {\tt /proc/sys/kernel/core\_uses\_pid}.
1422 Then you can investigate the core dumps with {\tt gdb}, which is often
1423 the fastest method to find simple errors.
1425 \exercise{Add a memory leak to your service and obtain a trace
1426 pointing to the leak using {\tt valgrind} while running the service
1427 from {\tt gnunet-service-arm}.}