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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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27 \newcommand{\exercise}[1]{\noindent\begin{boxedminipage}{\textwidth}{\bf Exercise:} #1 \end{boxedminipage}}
32 \large {A Tutorial for GNUnet 0.10.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 and gives an introduction on 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 be found on our website at
41 \url{https://gnunet.org/installation}.
43 \textbf{Please read this tutorial carefully since every single step is
44 important and do not hesitate to contact the GNUnet team if you have
45 any questions or problems! Check here how to contact the GNUnet
46 team: \url{https://gnunet.org/contact_information}}
49 \section{Installing GNUnet}
50 First of all you have to install a current version of GNUnet. You can download a
51 tarball of a stable version from GNU FTP mirrors or obtain the latest development
52 version from our Subversion repository.
54 Most of the time you should prefer to download the stable version since with the
55 latest development version things can be broken, functionality can be changed or tests
56 can fail. You should only use the development version if you know that you require a
57 certain feature or a certain issue has been fixed since the last release.
59 \subsection{Obtaining a stable version}
60 You can download the latest stable version of GNUnet from GNU FTP mirrors:
62 \url{ftp://ftp.gnu.org/gnu/gnunet/gnunet-0.10.x.tar.gz}
64 You should also download the signature file and verify the integrity of the tarball.
66 \url{ftp://ftp.gnu.org/gnu/gnunet/gnunet-0.10.x.tar.gz.sig}
68 To verify the signature you should first import the GPG key used to sign the tarball
70 $ gpg --keyserver keys.gnupg.net --recv-keys 48426C7E
72 And use this key to verify the tarball's signature
74 $ gpg --verify gnunet-0.10.x.tar.gz.sig gnunet-0.10.x.tar.gz
76 After successfully verifying the integrity you can extract the tarball using
78 $ tar xvzf gnunet-0.10.x.tar.gz
79 $ mv gnunet-0.10.x gnunet # we will use the directory "gnunet" in the remainder of this document
83 \subsection{Installing Build Tool Chain and Dependencies}
84 To successfully compile GNUnet you need the tools to build GNUnet and the required dependencies.
85 Please have a look at \url{https://gnunet.org/dependencies} for a list of required dependencies
86 and \url{https://gnunet.org/generic_installation} for specific instructions for your operating system.
88 Please check the notes at the end of the configure process about required dependencies.
90 For GNUNet bootstrapping support and the http(s) plugin you should install \texttt{libcurl}.
91 For the filesharing service you should install at least one of the datastore backends \texttt{mysql},
92 \texttt{sqlite} or \texttt{postgresql}.
94 \subsection{Obtaining the latest version from Subversion}
95 The latest development version can obtained from our Subversion (\textit{svn}) repository. To obtain
96 the code you need Subversion installed and checkout the repository using:
97 \lstset{language=bash}
99 $ svn checkout https://gnunet.org/svn/gnunet
101 After cloning the repository you have to execute
102 \lstset{language=bash}
108 The remainder of this tutorial assumes that you have SVN HEAD checked out.
110 \subsection{Compiling and Installing GNUnet}
112 First, you need to install at least {\tt libgnupgerror} version
113 1.12\footnote{\url{ftp://ftp.gnupg.org/gcrypt/libgpg-error/libgpg-error-1.12.tar.bz2}}
114 and {\tt libgcrypt} version
115 1.6\footnote{\url{ftp://ftp.gnupg.org/gcrypt/libgcrypt/libgcrypt-1.6.0.tar.bz2}}.
117 \lstset{language=bash}
119 $ wget ftp://ftp.gnupg.org/gcrypt/libgpg-error/libgpg-error-1.12.tar.bz2
120 $ tar xf libgpg-error-1.12.tar.bz2
121 $ cd libgpg-error-1.12
127 \lstset{language=bash}
129 $ wget ftp://ftp.gnupg.org/gcrypt/libgcrypt/libgcrypt-1.6.0.tar.bz2
130 $ tar xf libgcrypt-1.6.0.tar.bz2
138 Assuming all dependencies are installed, the following commands will
139 compile and install GNUnet in your home directory. You can specify the
140 directory where GNUnet will be installed by changing the
141 \lstinline|--prefix| value when calling \lstinline|./configure|. If
142 you do not specifiy a prefix, GNUnet is installed in the directory
143 \lstinline|/usr/local|. When developing new applications you may want
144 to enable verbose logging by adding
145 \lstinline|--enable-logging=verbose|:
147 \lstset{language=bash}
149 $ ./configure --prefix=$PREFIX --enable-logging
154 After installing GNUnet you have to add your GNUnet installation to your path
155 environmental variable. In addition you have to create the \lstinline|.gnunet|
156 directory in your home directory where GNUnet stores its data and an empty
157 GNUnet configuration file:
159 \lstset{language=bash}
161 $ export PATH=$PATH:$PREFIX/bin
162 $ echo export PATH=$PREFIX/bin:\\$PATH >> ~/.bashrc
164 $ touch ~/.gnunet/gnunet.conf
168 \subsection{Common Issues - Check your GNUnet installation}
169 You should check your installation to ensure that installing GNUnet
170 was successful up to this point. You should be able to access GNUnet's
171 binaries and run GNUnet's self check.
175 should return \lstinline|$PREFIX/bin/gnunet-arm|. It should be
176 located in your GNUnet installation and the output should not be
177 empty. If you see an output like:
182 check your {\tt PATH} variable to ensure GNUnet's {\tt bin} directory is included.
184 GNUnet provides tests for all of its subcomponents. Run
188 to execute tests for all components. {\tt make check} traverses all subdirectories in {\tt src}.
189 For every subdirectory you should get a message like this:
192 make[2]: Entering directory `/home/mwachs/gnunet/contrib'
193 PASS: test_gnunet_prefix
199 If you see a message like this:
202 Mar 12 16:57:56-642482 resolver-api-19449 ERROR Must specify `HOSTNAME' for `resolver' in configuration!
203 Mar 12 16:57:56-642573 test_program-19449 ERROR Assertion failed at resolver_api.c:204.
204 /bin/bash: line 5: 19449 Aborted (core dumped) ${dir}$tst
207 double check the steps performed in ~\ref{sub:install}
209 \section{Background: GNUnet Architecture}
210 GNUnet is organized in layers and services. Each service is composed of a
211 main service implementation and a client library for other programs to use
212 the service's functionality, described by an API. This approach is shown in
213 figure~\ref{fig:service}. Some services provide an additional command line
214 tool to enable the user to interact with the service.
216 Very often it is other GNUnet services that will use these APIs to build the
217 higher layers of GNUnet on top of the lower ones. Each layer expands or extends
218 the functionality of the service below (for instance, to build a mesh on top of
219 a DHT). See figure ~\ref{fig:interaction} for an illustration of this approach.
224 \begin{subfigure}[b]{0.3\textwidth}
226 \includegraphics[width=\textwidth]{figs/Service.pdf}
227 \caption{Service with API and network protocol}
231 \begin{subfigure}[b]{0.3\textwidth}
233 \includegraphics[width=\textwidth]{figs/System.pdf}
234 \caption{Service interaction}
235 \label{fig:interaction}
238 \caption{GNUnet's layered system architecture}
241 The main service implementation runs as a standalone process in the operating
242 system and the client code runs as part of the client program, so crashes of a
243 client do not affect the service process or other clients. The service and the
244 clients communicate via a message protocol to be defined and implemented by
247 \section{First Steps with GNUnet}
249 \subsection{Configure your peer}
250 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 {\tt \$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.
252 Since we want to start additional peers later, we need
253 some modifications from the default configuration. We need to create a separate service home and a file containing our modifications for this peer:
259 Now add the following lines to peer1.conf to use this directory. For
260 simplified usage we want to prevent the peer to connect to the GNUnet
261 network since this could lead to confusing output. This modifications
262 will replace the default settings:
265 GNUNET_HOME = ~/gnunet1/ # Use this directory to store GNUnet data
267 SERVERS = # prevent bootstrapping
270 \subsection{Start a peer}
271 Each GNUnet instance (called peer) has an identity (\textit{peer ID}) based on a
272 cryptographic public private key pair. The peer ID is the printable hash of the
275 GNUnet services are controlled by a master service the so called \textit{Automatic Restart Manager} (ARM).
276 ARM starts, stops and even restarts services automatically or on demand when a client connects.
277 You interact with the ARM service using the \lstinline|gnunet-arm| tool.
278 GNUnet can then be started with \lstinline|gnunet-arm -s| and stopped with
279 \lstinline|gnunet-arm -e|. An additional service not automatically started
280 can be started using \lstinline|gnunet-arm -i <service name>| and stopped
281 using \lstinline|gnunet-arm -k <servicename>|.
283 Once you have started your peer, you can use many other GNUnet commands
284 to interact with it. For example, you can run:
285 \lstset{language=bash}
289 to obtain the public key of your peer.
290 You should see an output containing the peer ID similar to:
291 \lstset{language=bash}
293 I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.
297 \subsection{Monitor a peer}
298 In this section, we will monitor the behaviour of our peer's DHT service with respect to a
299 specific key. First we will start GNUnet and then start the DHT service and use the DHT monitor tool
300 to monitor the PUT and GET commands we issue ussing the \lstinline|gnunet-dht-put| and
301 \lstinline|gnunet-dht-get| commands. Using the ``monitor'' line given below, you can observe the behavior of
302 your own peer's DHT with respect to the specified KEY:
304 \lstset{language=bash}
306 $ gnunet-arm -c ~/peer1.conf -s # start gnunet with all default services
307 $ gnunet-arm -c ~/peer1.conf -i dht # start DHT service
308 $ cd ~/gnunet/src/dht;
309 $ ./gnunet-dht-monitor -c ~/peer1.conf -k KEY
311 Now open a separate terminal and change again to the \lstinline|gnunet/src/dht| directory:
313 $ cd ~/gnunet/src/dht
314 $ ./gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE # put VALUE under KEY in the DHT
315 $ ./gnunet/src/dht/gnunet-dht-get -c ~/peer1.conf -k KEY # get key KEY from the DHT
316 $ gnunet-statistics -c ~/peer1.conf # print statistics about current GNUnet state
317 $ gnunet-statistics -c ~/peer1.conf -s dht # print statistics about DHT service
320 \subsection{Starting Two Peers by Hand}
321 \subsubsection{Setup a second peer}
322 We will now start a second peer on your machine.
323 For the second peer, you will need to manually create a modified
324 configuration file to avoid conflicts with ports and directories.
325 A peers configuration file is by default located in {\tt ~/.gnunet/gnunet.conf}.
326 This file is typically very short or even empty as only the differences to the
327 defaults need to be specified. The defaults are located in
328 many files in the {\tt \$PREFIX/share/gnunet/config.d} directory.
330 To configure the second peer, use the files {\tt
331 \$PREFIX/share/gnunet/config.d} as a template for your main
334 \lstset{language=bash}
336 $ cat $PREFIX/share/gnunet/config.d/*.conf > peer2.conf
338 Now you have to edit {\tt peer2.conf} and change:
341 \item{\texttt{SERVICEHOME} under \texttt{PATHS}}
342 \item{Every (uncommented) value for ``\texttt{PORT}'' (add 10000) in any
343 section (the option may be commented out if \texttt{PORT} is
344 prefixed by "\#", in this case, UNIX domain sockets are used
345 and the PORT option does not need to be touched) }
346 \item{Every value for ``\texttt{UNIXPATH}'' in any section (e.g. by adding a "-p2" suffix)}
348 to a fresh, unique value. Make sure that the \texttt{PORT} numbers stay
349 below 65536. From now on, whenever you interact with the second
350 peer, you need to specify {\tt -c peer2.conf} as an additional
351 command line argument.
353 Now, generate the 2nd peer's private key:
355 \lstset{language=bash}
357 $ gnunet-peerinfo -s -c peer2.conf
361 This may take a while, generate entropy using your keyboard or mouse
362 as needed. Also, make sure the output is different from the {\tt
363 gnunet-peerinfo} output for the first peer (otherwise you made an
364 error in the configuration).
366 \subsubsection{Start the second peer and connect the peers}
367 Then, you can start a second peer using:
368 \lstset{language=bash}
370 $ gnunet-arm -c peer2.conf -s
371 $ gnunet-arm -c peer2.conf -i dht
372 $ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
373 $ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY
375 If you want the two peers to connect, you have multiple options:
378 \item UDP neighbour discovery (automatic)
379 \item Setup a bootstrap server
380 \item Connect manually
382 To setup peer 1 as bootstrapping server change the configuration of
383 the first one to be a hostlist server by adding the following lines to
384 \texttt{peer1.conf} to enable bootstrapping server:
390 Then change {\tt peer2.conf} and replace the ``\texttt{SERVERS}'' line in the ``\texttt{[hostlist]}'' section with
391 ``\texttt{http://localhost:8080/}''. Restart both peers using:
393 $ gnunet-arm -c peer1.conf -e # stop first peer
394 $ gnunet-arm -c peer1.conf -s # start first peer
395 $ gnunet-arm -c peer2.conf -s # start second peer
398 Note that if you start your peers without changing these settings, they
399 will use the ``global'' hostlist servers of the GNUnet P2P network and
400 likely connect to those peers. At that point, debugging might become
401 tricky as you're going to be connected to many more peers and would
402 likely observe traffic and behaviors that are not explicitly controlled
405 \subsubsection{How to connect manually}
406 If you want to use the \texttt{peerinfo} tool to connect your peers, you should:
409 \item{Set {\tt FORCESTART = NO} in section {\tt hostlist} (to not connect to the global GNUnet)}
410 \item{Start both peers running {\tt gnunet-arm -c peer1.conf -s} and {\tt gnunet-arm -c peer2.conf -s}}
411 \item{Get \texttt{HELLO} message of the first peer running {\tt gnunet-peerinfo -c peer1.conf -g}}
412 \item{Give the output to the second peer by running {\tt gnunet-peerinfo -c peer2.conf -p '<output>'}}
415 Check that they are connected using {\tt gnunet-core -c peer1.conf}, which should give you the other peer's
418 $ gnunet-core -c peer1.conf
419 Peer `9TVUCS8P5A7ILLBGO6 [...shortened...] 1KNBJ4NGCHP3JPVULDG'
422 \subsection{Starting Peers Using the Testbed Service}
424 GNUnet's testbed service is used for testing scenarios where a number of peers
425 are to be started. The testbed can manage peers on a single host or on multiple
426 hosts in a distributed fashion. On a single affordable computer, it should be
427 possible to run around tens of peers without drastically increasing the load on the
430 The testbed service can be access through its API
431 \texttt{include/gnunet\_testbed\_service.h}. The API provides many routines for
432 managing a group of peers. It also provides a helper function
433 \texttt{GNUNET\_TESTBED\_test\_run()} to quickly setup a minimalistic testing
434 environment on a single host.
436 This function takes a configuration file which will be used as a template
437 configuration for the peers. The testbed takes care of modifying relevant
438 options in the peers' configuration such as SERVICEHOME, PORT, UNIXPATH to
439 unique values so that peers run without running into conflicts. It also checks
440 and assigns the ports in configurations only if they are free.
442 Additionally, the testbed service also reads its options from the same
443 configuration file. Various available options and details about them can be
444 found in the testbed default configuration file \texttt{src/testbed/testbed.conf}.
446 With the testbed API, a sample test case can be structured as follows:
447 % <lynX> Is there a way to pick a more readable font for this include?
448 \lstinputlisting[language=C]{testbed_test.c}
449 The source code for the above listing can be found at
450 \url{https://gnunet.org/svn/gnunet/doc/testbed_test.c}
451 or in the doc folder of your repository check-out.
452 After installing GNUnet, the above source code can be compiled as:
453 \lstset{language=bash}
455 $ export CPPFLAGS="-I/path/to/gnunet/headers"
456 $ export LDFLAGS="-L/path/to/gnunet/libraries"
457 $ gcc $CPPFLAGS $LDFLAGS -o testbed-test testbed_test.c -lgnunettestbed -lgnunetdht -lgnunetutil
459 The \texttt{CPPFLAGS} and \texttt{LDFLAGS} are necessary if GNUnet is installed
460 into a different directory other than \texttt{/usr/local}.
462 All of testbed API's peer management functions treat management actions as
463 operations and return operation handles. It is expected that the operations
464 begin immediately, but they may get delayed (to balance out load on the system).
465 The program using the API then has to take care of marking the operation as
466 ``done'' so that its associated resources can be freed immediately and other
467 waiting operations can be executed. Operations will be canceled if they are
468 marked as ``done'' before their completion.
470 An operation is treated as completed when it succeeds or fails. Completion of
471 an operation is either conveyed as events through \textit{controller event
472 callback} or through respective operation completion callbacks. In functions
473 which support completion notification through both controller event callback and
474 operation completion callback, first the controller event callback will be
475 called. If the operation is not marked as done in that callback or if the
476 callback is given as NULL when creating the operation, the operation completion
477 callback will be called. The API documentation shows which event are to be
478 expected in the controller event notifications. It also documents any
479 exceptional behaviour.
481 Once the peers are started, test cases often need to connect some of the peers'
482 services. Normally, opening a connect to a peer's service requires the peer's
483 configuration. While using testbed, the testbed automatically generates
484 per-peer configuration. Accessing those configurations directly through file
485 system is discouraged as their locations are dynamically created and will be
486 different among various runs of testbed. To make access to these configurations
487 easy, testbed API provides the function
488 \texttt{GNUNET\_TESTBED\_service\_connect()}. This function fetches the
489 configuration of a given peer and calls the \textit{Connect Adapter}.
490 In the example code, it is the \texttt{dht\_ca}. A connect adapter is expected
491 to open the connection to the needed service by using the provided configuration
492 and return the created service connection handle. Successful connection to the
493 needed service is signaled through \texttt{service\_connect\_comp\_cb}.
495 A dual to connect adapter is the \textit{Disconnect Adapter}. This callback is
496 called after the connect adapter has been called when the operation from
497 \texttt{GNUNET\_TESTBED\_service\_connect()} is marked as ``done''. It has to
498 disconnect from the service with the provided service handle (\texttt{op\_result}).
500 \exercise{Find out how many peers you can run on your system.}
502 \exercise{Find out how to create a 2D torus topology by changing the
503 options in the configuration file.\footnote{See \url{https://gnunet.org/content/supported-topologies}}
504 Then use the DHT API to store and retrieve values in the
507 \section{Developing Applications}
508 \subsection{gnunet-ext}
509 To develop a new peer-to-peer application or to extend GNUnet we provide
510 a template build system for writing GNUnet extensions in C. It can be
513 \lstset{language=bash}
515 $ svn checkout https://gnunet.org/svn/gnunet-ext/
518 $ ./configure --prefix=$PREFIX --with-gnunet=$PREFIX
525 The GNUnet ext template includes examples and a working buildsystem for a new GNUnet service.
526 A common GNUnet service consists of the following parts which will be discussed in detail in the
527 remainder of this document. The functionality of a GNUnet service is implemented in:
531 \item the GNUnet service (\lstinline|gnunet-ext/src/ext/gnunet-service-ext.c|)
532 \item the client API (\lstinline|gnunet-ext/src/ext/ext_api.c|)
533 \item the client application using the service API (\lstinline|gnunet-ext/src/ext/gnunet-ext.c|)
538 The interfaces for these entities are defined in:
541 \item client API interface (\lstinline|gnunet-ext/src/ext/ext.h|)
542 \item the service interface (\lstinline|gnunet-ext/src/include/gnunet_service_SERVICE.h|)
543 \item the P2P protocol (\lstinline|gnunet-ext/src/include/gnunet_protocols_ext.h|)
547 In addition the \texttt{ext} systems provides:
550 \item a test testing the API (\lstinline|gnunet-ext/src/ext/test_ext_api.c|)
551 \item a configuration template for the service (\lstinline|gnunet-ext/src/ext/ext.conf.in|)
555 \subsection{Adapting the Template}
557 The first step for writing any extension with a new service is to
558 ensure that the {\tt ext.conf.in} file contains entries for the
559 \texttt{UNIXPATH}, \texttt{PORT} and \texttt{BINARY} for the service in a section named after
562 If you want to adapt the template rename the {\tt ext.conf.in} to match your
563 services name, you have to modify the \texttt{AC\_OUTPUT} section in {\tt configure.ac}
564 in the \texttt{gnunet-ext} root.
566 \section{Writing a Client Application}
568 When writing any client application (for example, a command-line
569 tool), the basic structure is to start with the {\tt
570 GNUNET\_PROGRAM\_run} function. This function will parse
571 command-line options, setup the scheduler and then invoke the {\tt
572 run} function (with the remaining non-option arguments) and a handle
573 to the parsed configuration (and the configuration file name that was
574 used, which is typically not needed):
578 #include <gnunet/platform.h>
579 #include <gnunet/gnunet_util_lib.h>
587 const struct GNUNET_CONFIGURATION_Handle *cfg)
594 main (int argc, char *const *argv)
596 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
597 GNUNET_GETOPT_OPTION_END
600 GNUNET_PROGRAM_run (argc,
603 gettext_noop ("binary description text"),
604 options, &run, NULL)) ? ret : 1;
608 \subsection{Handling command-line options}
610 Options can then be added easily by adding global variables and
611 expanding the {\tt options} array. For example, the following would
612 add a string-option and a binary flag (defaulting to {\tt NULL} and
613 {\tt GNUNET\_NO} respectively):
616 static char *string_option;
620 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
621 {'s', "name", "SOMESTRING",
622 gettext_noop ("text describing the string_option NAME"), 1,
623 &GNUNET_GETOPT_set_string, &string_option},
625 gettext_noop ("text describing the flag option"), 0,
626 &GNUNET_GETOPT_set_one, &a_flag},
627 GNUNET_GETOPT_OPTION_END
629 string_option = NULL;
630 a_flag = GNUNET_SYSERR;
634 Issues such as displaying some helpful text describing options using
635 the {\tt --help} argument and error handling are taken care of when
636 using this approach. Other {\tt GNUNET\_GETOPT\_}-functions can be used
637 to obtain integer value options, increment counters, etc. You can
638 even write custom option parsers for special circumstances not covered
639 by the available handlers. To check if an argument was specified by the
640 user you initialize the variable with a specific value (e.g. NULL for
641 a string and GNUNET\_SYSERR for a integer) and check after parsing
642 happened if the values were modified.
644 Inside the {\tt run} method, the program would perform the
645 application-specific logic, which typically involves initializing and
646 using some client library to interact with the service. The client
647 library is supposed to implement the IPC whereas the service provides
648 more persistent P2P functions.
650 \exercise{Add a few command-line options and print them inside
651 of {\tt run}. What happens if the user gives invalid arguments?}
653 \subsection{Writing a Client Library}
655 The first and most important step in writing a client library is to
656 decide on an API for the library. Typical API calls include
657 connecting to the service, performing application-specific requests
658 and cleaning up. Many examples for such service APIs can be found
659 in the {\tt gnunet/src/include/gnunet\_*\_service.h} files.
661 Then, a client-service protocol needs to be designed. This typically
662 involves defining various message formats in a header that will be
663 included by both the service and the client library (but is otherwise
664 not shared and hence located within the service's directory and not
665 installed by {\tt make install}). Each message must start with a {\tt
666 struct GNUNET\_MessageHeader} and must be shorter than 64k. By
667 convention, all fields in IPC (and P2P) messages must be in big-endian
668 format (and thus should be read using {\tt ntohl} and similar
669 functions and written using {\tt htonl} and similar functions).
670 Unique message types must be defined for each message struct in the
671 {\tt gnunet\_protocols.h} header (or an extension-specific include
674 \subsubsection{Connecting to the Service}
676 Before a client library can implement the application-specific protocol
677 with the service, a connection must be created:
681 struct GNUNET_CLIENT_Connection *client;
682 client = GNUNET_CLIENT_connect ("service-name", cfg);
685 As a result a {\tt GNUNET\_CLIENT\_Connection} handle is returned
686 which has to used in later API calls related to this service.
687 The complete client API can be found in {\tt gnunet\_client\_lib.h}
689 \subsubsection{GNUnet Messages}
691 In GNUnet, messages are always sent beginning with a {\tt struct GNUNET\_MessageHeader}
692 in big endian format. This header defines the size and the type of the
693 message, the payload follows after this header.
697 struct GNUNET_MessageHeader
701 * The length of the struct (in bytes, including the length field itself),
702 * in big-endian format.
704 uint16_t size GNUNET_PACKED;
707 * The type of the message (GNUNET_MESSAGE_TYPE_XXXX), in big-endian format.
709 uint16_t type GNUNET_PACKED;
714 Existing message types are defined in {\tt gnunet\_protocols.h}\\
715 A common way to create a message is:
719 struct GNUNET_MessageHeader *msg =
720 GNUNET_malloc(payload_size + sizeof(struct GNUNET_MessageHeader));
721 msg->size = htons(payload_size + sizeof(struct GNUNET_MessageHeader));
722 msg->type = htons(GNUNET_MY_MESSAGE_TYPE);
723 memcpy(&msg[1], &payload, payload_size);
727 \exercise{Define a message struct that includes a 32-bit
728 unsigned integer in addition to the standard GNUnet MessageHeader.
729 Add a C struct and define a fresh protocol number for your message.
730 (Protocol numbers in gnunet-ext are defined in \lstinline|gnunet-ext/src/include/gnunet_protocols_ext.h|)}
733 \subsubsection{Sending Requests to the Service}
735 Any client-service protocol must start with the client sending the
736 first message to the service, since services are only notified about
737 (new) clients upon receiving a the first message.
739 Clients can transmit messages to the service using the
740 {\tt GNUNET\_CLIENT\_notify\_transmit\_ready} API:
744 transmit_cb (void *cls, size_t size, void *buf)
747 if (NULL == buf) { /* handle error here */; return 0; }
748 GNUNET_assert (size >= msg_size);
749 memcpy (buf, my_msg, msg_size);
755 th = GNUNET_CLIENT_notify_transmit_ready (client,
763 The client-service protocoll calls {\tt GNUNET\_CLIENT\_notify\_transmit\_ready}
764 to be notified when the client is ready to send data to the service.
765 Besides other arguments, you have to pass the client returned
766 from the {\tt connect} call, the message size and the callback function to
767 call when the client is ready to send.
769 Only a single transmission request can be queued per client at the
770 same time using this API. The handle {\tt th} can be used to cancel
771 the request if necessary (for example, during shutdown).
773 When {\tt transmit\_cb} is called the message is copied in the buffer provided and
774 the number of bytes copied into the buffer is returned. {\tt transmit\_cb}
775 could also return 0 if for some reason no message
776 could be constructed; this is not an error and the connection to the
777 service will persist in this case.
779 \exercise{Define a helper function to transmit a 32-bit
780 unsigned integer (as payload) to a service using some given client
784 \subsubsection{Receiving Replies from the Service}
786 Clients can receive messages from the service using the
787 {\tt GNUNET\_CLIENT\_receive} API:
792 * Function called with messages from stats service.
795 * @param msg message received, NULL on timeout or fatal error
798 receive_message (void *cls, const struct GNUNET_MessageHeader *msg)
800 struct MyArg *arg = cls;
806 GNUNET_CLIENT_receive (client,
813 It should be noted that this receive call only receives a single
814 message. To receive additional messages, {\tt
815 GNUNET\_CLIENT\_receive} must be called again.
817 \exercise{Expand your helper function to receive a
818 response message (for example, containing just the GNUnet MessageHeader
819 without any payload). Upon receiving the service's response, you should
820 call a callback provided to your helper function's API. You'll need to
821 define a new 'struct' to hold your local context (``closure'').}
824 \subsection{Writing a user interface}
826 Given a client library, all it takes to access a service now is to
827 combine calls to the client library with parsing command-line
830 \exercise{Call your client API from your {\tt run} method
831 in your client application to send a request to the service.
832 For example, send a 32-bit integer value based on a number given
833 at the command-line to the service.}
837 \section{Writing a Service}
839 Before you can test the client you've written so far, you'll need to also
840 implement the corresponding service.
843 \subsection{Code Placement}
845 New services are placed in their own subdirectory under {\tt gnunet/src}.
846 This subdirectory should contain the API implementation file {\tt SERVICE\_api.c},
847 the description of the client-service protocol {\tt SERVICE.h} and P2P protocol
848 {\tt SERVICE\_protocol.h}, the implementation of the service itself
849 {\tt gnunet-service-SERVICE.h} and several files for tests, including test code
850 and configuration files.
852 \subsection{Starting a Service}
854 The key API definitions for starting services are:
857 typedef void (*GNUNET_SERVICE_Main) (void *cls,
858 struct GNUNET_SERVER_Handle *server,
859 const struct GNUNET_CONFIGURATION_Handle *cfg);
860 int GNUNET_SERVICE_run (int argc,
862 const char *serviceName,
863 enum GNUNET_SERVICE_Options opt,
864 GNUNET_SERVICE_Main task,
868 Here is a starting point for your main function for your service:
872 static void my_main (void *cls,
873 struct GNUNET_SERVER_Handle *server,
874 const struct GNUNET_CONFIGURATION_Handle *cfg)
879 int main (int argc, char *const*argv)
882 GNUNET_SERVICE_run (argc, argv, "my",
883 GNUNET_SERVICE_OPTION_NONE,
890 \exercise{Write a stub service that processes no messages at all
891 in your code. Create a default configuration for it, integrate it
892 with the build system and start the service from {\tt
893 gnunet-service-arm} using {\tt gnunet-arm -i NAME}.}
896 \subsection{Receiving Requests from Clients}
898 Inside of the {\tt my\_main} method, a service typically registers for
899 the various message types from clients that it supports by providing
900 a handler function, the message type itself and possibly a fixed
901 message size (or 0 for variable-size messages):
906 handle_set (void *cls,
907 struct GNUNET_SERVER_Client *client,
908 const struct GNUNET_MessageHeader *message)
910 GNUNET_SERVER_receive_done (client, GNUNET_OK);
913 handle_get (void *cls,
914 struct GNUNET_SERVER_Client *client,
915 const struct GNUNET_MessageHeader *message)
917 GNUNET_SERVER_receive_done (client, GNUNET_OK);
920 static void my_main (void *cls,
921 struct GNUNET_SERVER_Handle *server,
922 const struct GNUNET_CONFIGURATION_Handle *cfg)
924 static const struct GNUNET_SERVER_MessageHandler handlers[] = {
925 {&handle_set, NULL, GNUNET_MESSAGE_TYPE_MYNAME_SET, 0},
926 {&handle_get, NULL, GNUNET_MESSAGE_TYPE_MYNAME_GET, 0},
929 GNUNET_SERVER_add_handlers (server, handlers);
930 /* do more setup work */
934 Each handler function {\bf must} eventually (possibly in some
935 asynchronous continuation) call {\tt GNUNET\_SERVER\_receive\_done}.
936 Only after this call additional messages from the same client may
937 be processed. This way, the service can throttle processing messages
938 from the same client. By passing {\tt GNUNET\_SYSERR}, the service
939 can close the connection to the client, indicating an error.
941 Services must check that client requests are well-formed and must not
942 crash on protocol violations by the clients. Similarly, client
943 libraries must check replies from servers and should gracefully report
944 errors via their API.
947 \exercise{Change the service to ``handle'' the message from your
948 client (for now, by printing a message). What happens if you
949 forget to call {\tt GNUNET\_SERVER\_receive\_done}?}
952 \subsection{Responding to Clients}
954 Servers can send messages to clients using the
955 {\tt GNUNET\_SERVER\_notify\_transmit\_ready} API:
960 transmit_cb (void *cls, size_t size, void *buf)
963 if (NULL == buf) { handle_error(); return 0; }
964 GNUNET_assert (size >= msg_size);
965 memcpy (buf, my_msg, msg_size);
971 struct GNUNET_SERVER_TransmitHandle *th;
972 th = GNUNET_SERVER_notify_transmit_ready (client,
979 Only a single transmission request can be queued per client
980 at the same time using this API.
981 Additional APIs for sending messages to clients can be found
982 in the {\tt gnunet\_server\_lib.h} header.
985 \exercise{Change the service respond to the request from your
986 client. Make sure you handle malformed messages in both directions.}
988 \section{Interacting directly with other Peers using the CORE Service}
990 One of the most important services in GNUnet is the \texttt{CORE} service
991 managing connections between peers and handling encryption between peers.
993 One of the first things any service that extends the P2P protocol typically does
994 is connect to the \texttt{CORE} service using:
998 #include <gnunet/gnunet_core_service.h>
1000 struct GNUNET_CORE_Handle *
1001 GNUNET_CORE_connect (const struct GNUNET_CONFIGURATION_Handle *cfg,
1003 GNUNET_CORE_StartupCallback init,
1004 GNUNET_CORE_ConnectEventHandler connects,
1005 GNUNET_CORE_DisconnectEventHandler disconnects,
1006 GNUNET_CORE_MessageCallback inbound_notify,
1007 int inbound_hdr_only,
1008 GNUNET_CORE_MessageCallback outbound_notify,
1009 int outbound_hdr_only,
1010 const struct GNUNET_CORE_MessageHandler *handlers);
1013 \subsection{New P2P connections}
1015 Before any traffic with a different peer can be exchanged, the peer must be
1016 known to the service. This is notified by the \texttt{CORE} {\tt connects} callback,
1017 which communicates the identity of the new peer to the service:
1022 connects (void *cls,
1023 const struct GNUNET_PeerIdentity * peer)
1025 /* Save identity for later use */
1026 /* Optional: start sending messages to peer */
1030 \exercise{Create a service that connects to the \texttt{CORE}. Then
1031 start (and connect) two peers and print a message once your connect
1032 callback is invoked.}
1034 \subsection{Receiving P2P Messages}
1036 To receive messages from \texttt{CORE}, services register a set of handlers
1037 (parameter {\tt *handlers} in the \lstinline|GNUNET_CORE_connect| call that are called by \texttt{CORE}
1038 when a suitable message arrives.
1043 callback_function_for_type_one(void *cls,
1044 const struct GNUNET_PeerIdentity *peer,
1045 const struct GNUNET_MessageHeader *message)
1048 return GNUNET_OK; /* or GNUNET_SYSERR to close the connection */
1052 * Functions to handle messages from core
1054 static struct GNUNET_CORE_MessageHandler core_handlers[] = {
1055 {&callback_function_for_type_one, GNUNET_MESSAGE_TYPE_MYSERVICE_TYPE_ONE, 0},
1061 \exercise{Start one peer with a new service that has a message
1062 handler and start a second peer that only has your ``old'' service
1063 without message handlers. Which ``connect'' handlers are invoked when
1064 the two peers are connected? Why?}
1067 \subsection{Sending P2P Messages}
1069 In response to events (connect, disconnect, inbound messages,
1070 timing, etc.) services can then use this API to transmit messages:
1075 (*GNUNET_CONNECTION_TransmitReadyNotify) (void *cls,
1079 /* Fill "*buf" with up to "size" bytes, must start with GNUNET_MessageHeader */
1080 return n; /* Total size of the message put in "*buf" */
1083 struct GNUNET_CORE_TransmitHandle *
1084 GNUNET_CORE_notify_transmit_ready (struct GNUNET_CORE_Handle *handle,
1085 int cork, uint32_t priority,
1086 struct GNUNET_TIME_Relative maxdelay,
1087 const struct GNUNET_PeerIdentity *target,
1089 GNUNET_CONNECTION_TransmitReadyNotify notify,
1093 \exercise{Write a service that upon connect sends messages as
1094 fast as possible to the other peer (the other peer should run a
1095 service that ``processes'' those messages). How fast is the
1096 transmission? Count using the STATISTICS service on both ends. Are
1097 messages lost? How can you transmit messages faster? What happens if
1098 you stop the peer that is receiving your messages?}
1101 \subsection{End of P2P connections}
1103 If a message handler returns {\tt GNUNET\_SYSERR}, the remote peer shuts down or
1104 there is an unrecoverable network disconnection, CORE notifies the service that
1105 the peer disconnected. After this notification no more messages will be received
1106 from the peer and the service is no longer allowed to send messages to the peer.
1107 The disconnect callback looks like the following:
1112 disconnects (void *cls,
1113 const struct GNUNET_PeerIdentity * peer)
1115 /* Remove peer's identity from known peers */
1116 /* Make sure no messages are sent to peer from now on */
1120 \exercise{Fix your service to handle peer disconnects.}
1122 \section{Storing peer-specific data using the PEERSTORE service}
1124 GNUnet's PEERSTORE service offers a persistorage for arbitrary peer-specific data.
1125 Other GNUnet services can use the PEERSTORE to store, retrieve and monitor data records.
1126 Each data record stored with PEERSTORE contains the following fields:
1130 \item subsystem: Name of the subsystem responsible for the record.
1131 \item peerid: Identity of the peer this record is related to.
1132 \item key: a key string identifying the record.
1133 \item value: binary record value.
1134 \item expiry: record expiry date.
1137 The first step is to start a connection to the PEERSTORE service:
1139 #include "gnunet_peerstore_service.h"
1141 peerstore_handle = GNUNET_PEERSTORE_connect (cfg);
1143 The service handle \lstinline|peerstore_handle| will be needed for all subsequent
1144 PEERSTORE operations.
1146 \subsection{Storing records}
1148 To store a new record, use the following function:
1150 struct GNUNET_PEERSTORE_StoreContext *
1151 GNUNET_PEERSTORE_store (struct GNUNET_PEERSTORE_Handle *h,
1152 const char *sub_system,
1153 const struct GNUNET_PeerIdentity *peer,
1157 struct GNUNET_TIME_Absolute expiry,
1158 enum GNUNET_PEERSTORE_StoreOption options,
1159 GNUNET_PEERSTORE_Continuation cont,
1163 The \lstinline|options| parameter can either be \lstinline|GNUNET_PEERSTORE_STOREOPTION_MULTIPLE|
1164 which means that multiple values can be stored under the same key combination (subsystem, peerid, key),
1165 or \lstinline|GNUNET_PEERSTORE_STOREOPTION_REPLACE| which means that PEERSTORE will replace any
1166 existing values under the given key combination (subsystem, peerid, key) with the new given value.
1168 The continuation function \lstinline|cont| will be called after the store request is successfully
1169 sent to the PEERSTORE service. This does not guarantee that the record is successfully stored, only
1170 that it was received by the service.
1172 The \lstinline|GNUNET_PEERSTORE_store| function returns a handle to the store operation. This handle
1173 can be used to cancel the store operation only before the continuation function is called:
1176 GNUNET_PEERSTORE_store_cancel (struct GNUNET_PEERSTORE_StoreContext *sc);
1179 \subsection{Retrieving records}
1181 To retrieve stored records, use the following function:
1183 struct GNUNET_PEERSTORE_IterateContext *
1184 GNUNET_PEERSTORE_iterate (struct GNUNET_PEERSTORE_Handle *h,
1185 const char *sub_system,
1186 const struct GNUNET_PeerIdentity *peer,
1188 struct GNUNET_TIME_Relative timeout,
1189 GNUNET_PEERSTORE_Processor callback,
1190 void *callback_cls);
1192 The values of \lstinline|peer| and \lstinline|key| can be \lstinline|NULL|. This allows the
1193 iteration over values stored under any of the following key combinations:
1197 \item (subsystem, peerid)
1198 \item (subsystem, key)
1199 \item (subsystem, peerid, key)
1202 The \lstinline|callback| function will be called once with each retrieved record and once
1203 more with a \lstinline|NULL| record to signal the end of results.
1205 The \lstinline|GNUNET_PEERSTORE_iterate| function returns a handle to the iterate operation. This
1206 handle can be used to cancel the iterate operation only before the callback function is called with
1207 a \lstinline|NULL| record.
1209 \subsection{Monitoring records}
1211 PEERSTORE offers the functionality of monitoring for new records stored under a specific key
1212 combination (subsystem, peerid, key). To start the monitoring, use the following function:
1214 struct GNUNET_PEERSTORE_WatchContext *
1215 GNUNET_PEERSTORE_watch (struct GNUNET_PEERSTORE_Handle *h,
1216 const char *sub_system,
1217 const struct GNUNET_PeerIdentity *peer,
1219 GNUNET_PEERSTORE_Processor callback,
1220 void *callback_cls);
1223 Whenever a new record is stored under the given key combination, the \lstinline|callback| function
1224 will be called with this new record. This will continue until the connection to the PEERSTORE service
1225 is broken or the watch operation is canceled:
1228 GNUNET_PEERSTORE_watch_cancel (struct GNUNET_PEERSTORE_WatchContext *wc);
1231 \subsection{Disconnecting from PEERSTORE}
1233 When the connection to the PEERSTORE service is no longer needed, disconnect using the following
1237 GNUNET_PEERSTORE_disconnect (struct GNUNET_PEERSTORE_Handle *h, int sync_first);
1240 If the \lstinline|sync_first| flag is set to \lstinline|GNUNET_YES|, the API will delay the
1241 disconnection until all store requests are received by the PEERSTORE service. Otherwise,
1242 it will disconnect immediately.
1244 \section{Using the DHT}
1245 The DHT allows to store data so other peers in the P2P network can
1246 access it and retrieve data stored by any peers in the network.
1247 This section will explain how to use the DHT. Of course, the first
1248 thing to do is to connect to the DHT service:
1251 dht_handle = GNUNET_DHT_connect (cfg, parallel_requests);
1253 The second parameter indicates how many requests in parallel to expect.
1254 It is not a hard limit, but a good approximation will make the DHT more
1257 \subsection{Storing data in the DHT}
1258 Since the DHT is a dynamic environment (peers join and leave frequently)
1259 the data that we put in the DHT does not stay there indefinitely. It is
1260 important to ``refresh'' the data periodically by simply storing it again,
1261 in order to make sure other peers can access it.
1263 The put API call offers a callback to signal that the PUT request has been
1264 sent. This does not guarantee that the data is accessible to others peers,
1265 or even that is has been stored, only that the service has requested to
1266 a neighboring peer the retransmission of the PUT request towards its final
1267 destination. Currently there is no feedback about whether or not the data
1268 has been sucessfully stored or where it has been stored. In order to improve
1269 the availablilty of the data and to compensate for possible errors, peers leaving
1270 and other unfavorable events, just make several PUT requests!
1275 message_sent_cont (void *cls, const struct GNUNET_SCHEDULER_TaskContext *tc)
1277 /* Request has left local node */
1280 struct GNUNET_DHT_PutHandle *
1281 GNUNET_DHT_put (struct GNUNET_DHT_Handle *handle,
1282 const struct GNUNET_HashCode * key,
1283 uint32_t desired_replication_level,
1284 enum GNUNET_DHT_RouteOption options, /* Route options, see next call */
1285 enum GNUNET_BLOCK_Type type, size_t size, const void *data,
1286 struct GNUNET_TIME_Absolute exp, /* When does the data expire? */
1287 struct GNUNET_TIME_Relative timeout, /* How long to try to send the request */
1288 GNUNET_DHT_PutContinuation cont,
1292 \exercise{Store a value in the DHT periodically to make sure it is available
1293 over time. You might consider using the function GNUNET\_SCHEDULER\_add\_delayed and
1294 call GNUNET\_DHT\_put from inside a helper function.}
1297 \subsection{Obtaining data from the DHT}
1298 As we saw in the previous example, the DHT works in an asynchronous mode.
1299 Each request to the DHT is executed ``in the background'' and the API
1300 calls return immediately. In order to receive results from the DHT, the
1301 API provides a callback. Once started, the request runs in the service,
1302 the service will try to get as many results as possible (filtering out
1303 duplicates) until the timeout expires or we explicitly stop the request.
1304 It is possible to give a ``forever'' timeout with
1305 {\tt GNUNET\_TIME\_UNIT\_FOREVER\_REL}.
1307 If we give a route option {\tt GNUNET\_DHT\_RO\_RECORD\_ROUTE} the callback
1308 will get a list of all the peers the data has travelled, both on the PUT
1309 path and on the GET path.
1313 get_result_iterator (void *cls, struct GNUNET_TIME_Absolute expiration,
1314 const struct GNUNET_HashCode * key,
1315 const struct GNUNET_PeerIdentity *get_path,
1316 unsigned int get_path_length,
1317 const struct GNUNET_PeerIdentity *put_path,
1318 unsigned int put_path_length,
1319 enum GNUNET_BLOCK_Type type, size_t size, const void *data)
1321 /* Do stuff with the data and/or route */
1323 GNUNET_DHT_get_stop (get_handle);
1327 GNUNET_DHT_get_start (dht_handle,
1331 GNUNET_DHT_RO_NONE, /* Route options */
1332 NULL, /* xquery: not used here */
1333 0, /* xquery size */
1334 &get_result_iterator,
1338 \exercise{Store a value in the DHT and after a while retrieve it. Show the IDs of all
1339 the peers the requests have gone through. In order to convert a peer ID to a string, use
1340 the function GNUNET\_i2s. Pay attention to the route option parameters in both calls!}
1342 \subsection{Implementing a block plugin}
1344 In order to store data in the DHT, it is necessary to provide a block
1345 plugin. The DHT uses the block plugin to ensure that only well-formed
1346 requests and replies are transmitted over the network.
1348 The block plugin should be put in a file {\tt
1349 plugin\_block\_SERVICE.c} in the service's respective directory. The
1350 mandatory functions that need to be implemented for a block plugin are
1351 described in the following sections.
1353 \subsubsection{Validating requests and replies}
1355 The evaluate function should validate a reply or a request. It returns
1356 a {\tt GNUNET\_BLOCK\_EvaluationResult}, which is an enumeration. All
1357 possible answers are in {\tt gnunet\_block\_lib.h}. The function will
1358 be called with a {\tt reply\_block} argument of {\tt NULL} for
1359 requests. Note that depending on how {\tt evaluate} is called, only
1360 some of the possible return values are valid. The specific meaning of
1361 the {\tt xquery} argument is application-specific. Applications that
1362 do not use an extended query should check that the {\tt xquery\_size}
1363 is zero. The Bloom filter is typically used to filter duplicate
1368 static enum GNUNET_BLOCK_EvaluationResult
1369 block_plugin_SERVICE_evaluate (void *cls,
1370 enum GNUNET_BLOCK_Type type,
1371 const GNUNET_HashCode * query,
1372 struct GNUNET_CONTAINER_BloomFilter **bf,
1376 const void *reply_block,
1377 size_t reply_block_size)
1379 /* Verify type, block and bloomfilter */
1383 Note that it is mandatory to detect duplicate replies in this
1384 function and return the respective status code. Duplicate
1385 detection should be done by setting the respective bits in
1386 the Bloom filter {\tt bf}. Failure to do so may cause replies
1387 to circle in the network.
1389 \subsubsection{Deriving a key from a reply}
1391 The DHT can operate more efficiently if it is possible to derive a key
1392 from the value of the corresponding block. The {\tt get\_key}
1393 function is used to obtain the key of a block --- for example, by
1394 means of hashing. If deriving the key is not possible, the function
1395 should simply return {\tt GNUNET\_SYSERR} (the DHT will still work
1396 just fine with such blocks).
1401 block_plugin_SERVICE_get_key (void *cls, enum GNUNET_BLOCK_Type type,
1402 const void *block, size_t block_size,
1403 GNUNET_HashCode * key)
1405 /* Store the key in the key argument, return GNUNET_OK on success. */
1409 \subsubsection{Initialization of the plugin}
1411 The plugin is realized as a shared C library. The library must export
1412 an initialization function which should initialize the plugin. The
1413 initialization function specifies what block types the plugin cares
1414 about and returns a struct with the functions that are to be used for
1415 validation and obtaining keys (the ones just defined above).
1420 libgnunet_plugin_block_SERVICE_init (void *cls)
1422 static enum GNUNET_BLOCK_Type types[] =
1424 GNUNET_BLOCK_TYPE_SERVICE_BLOCKYPE, /* list of blocks we care about, from gnunet_block_lib.h */
1425 GNUNET_BLOCK_TYPE_ANY /* end of list */
1427 struct GNUNET_BLOCK_PluginFunctions *api;
1429 api = GNUNET_malloc (sizeof (struct GNUNET_BLOCK_PluginFunctions));
1430 api->evaluate = &block_plugin_SERICE_evaluate;
1431 api->get_key = &block_plugin_SERVICE_get_key;
1437 \subsubsection{Shutdown of the plugin}
1439 Following GNUnet's general plugin API concept, the plugin must
1440 export a second function for cleaning up. It usually does very
1446 libgnunet_plugin_block_SERVICE_done (void *cls)
1448 struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
1456 \subsubsection{Integration of the plugin with the build system}
1458 In order to compile the plugin, the {\tt Makefile.am} file for the
1459 service \texttt{SERVICE} should contain a rule similar to this:
1461 \lstset{language=make}
1463 plugindir = $(libdir)/gnunet
1465 plugin_LTLIBRARIES = \
1466 libgnunet_plugin_block_ext.la
1467 libgnunet_plugin_block_ext_la_SOURCES = \
1469 libgnunet_plugin_block_ext_la_LIBADD = \
1470 $(prefix)/lib/libgnunethello.la \
1471 $(prefix)/lib/libgnunetblock.la \
1472 $(prefix)/lib/libgnunetutil.la
1473 libgnunet_plugin_block_ext_la_LDFLAGS = \
1474 $(GN_PLUGIN_LDFLAGS)
1475 libgnunet_plugin_block_ext_la_DEPENDENCIES = \
1476 $(prefix)/lib/libgnunetblock.la
1481 \exercise{Write a block plugin that accepts all queries
1482 and all replies but prints information about queries and replies
1483 when the respective validation hooks are called.}
1487 \subsection{Monitoring the DHT}
1488 It is possible to monitor the functioning of the local DHT service. When monitoring
1489 the DHT, the service will alert the monitoring program of any events,
1490 both started locally or received for routing from another peer. The are three different
1491 types of events possible: a GET request, a PUT request or a response (a reply to
1494 Since the different events have different associated data, the API gets 3
1495 different callbacks (one for each message type) and optional type and key parameters,
1496 to allow for filtering of messages. When an event happens, the appropiate callback
1497 is called with all the information about the event.
1501 get_callback (void *cls,
1502 enum GNUNET_DHT_RouteOption options,
1503 enum GNUNET_BLOCK_Type type,
1505 uint32_t desired_replication_level,
1506 unsigned int path_length,
1507 const struct GNUNET_PeerIdentity *path,
1508 const struct GNUNET_HashCode * key)
1513 get_resp_callback (void *cls,
1514 enum GNUNET_BLOCK_Type type,
1515 const struct GNUNET_PeerIdentity *get_path,
1516 unsigned int get_path_length,
1517 const struct GNUNET_PeerIdentity *put_path,
1518 unsigned int put_path_length,
1519 struct GNUNET_TIME_Absolute exp,
1520 const struct GNUNET_HashCode * key,
1527 put_callback (void *cls,
1528 enum GNUNET_DHT_RouteOption options,
1529 enum GNUNET_BLOCK_Type type,
1531 uint32_t desired_replication_level,
1532 unsigned int path_length,
1533 const struct GNUNET_PeerIdentity *path,
1534 struct GNUNET_TIME_Absolute exp,
1535 const struct GNUNET_HashCode * key,
1541 monitor_handle = GNUNET_DHT_monitor_start (dht_handle,
1542 block_type, /* GNUNET_BLOCK_TYPE_ANY for all */
1543 key, /* NULL for all */
1551 \section{Debugging with {\tt gnunet-arm}}
1553 Even if services are managed by {\tt gnunet-arm}, you can start them with
1554 {\tt gdb} or {\tt valgrind}. For example, you could add the following lines
1555 to your configuration file to start the DHT service in a {\tt gdb} session in a
1560 PREFIX=xterm -e gdb --args
1563 Alternatively, you can stop a service that was started via ARM and run it manually:
1565 \lstset{language=bash}
1568 $ gdb --args gnunet-service-dht -L DEBUG
1569 $ valgrind gnunet-service-dht -L DEBUG
1573 Assuming other services are well-written, they will automatically re-integrate the
1574 restarted service with the peer.
1576 GNUnet provides a powerful logging mechanism providing log levels \texttt{ERROR},
1577 \texttt{WARNING}, \texttt{INFO} and \texttt{DEBUG}. The current log level is
1578 configured using the \lstinline|$GNUNET_FORCE_LOG| environmental variable.
1579 The \texttt{DEBUG} level is only available if \lstinline|--enable-logging=verbose| was used when
1580 running \texttt{configure}. More details about logging can be found under
1581 \url{https://gnunet.org/logging}.
1583 You should also probably enable the creation of core files, by setting
1584 {\tt ulimit}, and echo'ing 1 into {\tt /proc/sys/kernel/core\_uses\_pid}.
1585 Then you can investigate the core dumps with {\tt gdb}, which is often
1586 the fastest method to find simple errors.
1588 \exercise{Add a memory leak to your service and obtain a trace
1589 pointing to the leak using {\tt valgrind} while running the service
1590 from {\tt gnunet-service-arm}.}