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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
126 Assuming all dependencies are installed, the following commands will compile and install GNUnet in your
127 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
128 verbose logging by adding \lstinline|--enable-logging=verbose|:
130 \lstset{language=bash}
132 $ ./configure --prefix=$HOME --enable-logging
137 After installing GNUnet you have to set the \lstinline|GNUNET_PREFIX| environmental variable used by GNUnet to detect it's installation directory and add your GNUnet installation to your path environmental variable.
138 This configuration is only valid for the current shell session, so you should add \lstinline|export GNUNET_PREFIX=$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:
140 \lstset{language=bash}
142 $ export GNUNET_PREFIX=$HOME
143 $ export PATH=$PATH:$GNUNET_PREFIX/bin
144 $ echo export GNUNET_PREFIX=$HOME >> ~/.bashrc
145 $ echo export PATH=$GNUNET_PREFIX/bin:$PATH >> ~/.bashrc
147 $ touch ~/.gnunet/gnunet.conf
151 \subsection{Common Issues - Check your GNUnet installation}
152 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.
156 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:
161 check your {\tt PATH} variable to ensure GNUnet's {\tt bin} directory is included.
163 GNUnet provides tests for all of it's subcomponents. Run
167 to execute tests for all components. {\tt make check} traverses all subdirectories in {\tt src}.
168 For every subdirectory you should get a message like this:
171 make[2]: Entering directory `/home/mwachs/gnunet/contrib'
172 PASS: test_gnunet_prefix
178 If you see a message like this:
181 Mar 12 16:57:56-642482 resolver-api-19449 ERROR Must specify `HOSTNAME' for `resolver' in configuration!
182 Mar 12 16:57:56-642573 test_program-19449 ERROR Assertion failed at resolver_api.c:204.
183 /bin/bash: line 5: 19449 Aborted (core dumped) ${dir}$tst
186 double check your {\tt GNUNET\_PREFIX} environmental variable and double check the steps performed in ~\ref{sub:install}
188 \section{Background: GNUnet Architecture}
189 GNUnet is organized in layers and services. Each service is composed of a
190 main service implementation and a client library for other programs to use
191 the service's functionality, described by an API. This approach is shown in
192 figure~\ref{fig:service}. Some services provide an additional command line
193 tool to enable the user to interact with the service.
195 Very often it is other GNUnet services that will use these APIs to build the
196 higher layers of GNUnet on top of the lower ones. Each layer expands or extends
197 the functionality of the service below (for instance, to build a mesh on top of
198 a DHT). See figure ~\ref{fig:interaction} for an illustration of this approach.
203 \begin{subfigure}[b]{0.3\textwidth}
205 \includegraphics[width=\textwidth]{figs/Service.pdf}
206 \caption{Service with API and network protocol}
210 \begin{subfigure}[b]{0.3\textwidth}
212 \includegraphics[width=\textwidth]{figs/System.pdf}
213 \caption{Service interaction}
214 \label{fig:interaction}
217 \caption{GNUnet's layered system architecture}
220 The main service implementation runs as a standalone process in the operating
221 system and the client code runs as part of the client program, so crashes of a
222 client do not affect the service process or other clients. The service and the
223 clients communicate via a message protocol to be defined and implemented by
226 \section{First Steps with GNUnet}
228 \subsection{Configure your peer}
229 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.
231 Since we want to start additional peers later, we need
232 some modifications from the default configuration. We need to create a separate service home and a file containing our modifications for this peer:
238 Now add the following lines to peer1.conf to use this directory. For simplified usage we want to prevent
239 the peer to connect to the GNUnet network since this could lead to confusing output. This modifications will replace the default settings:
242 $ SERVICEHOME = ~/gnunet1/ # Use this directory to store GNUnet data
244 $ SERVERS = # prevent bootstrapping
247 \subsection{Start a peer}
248 Each GNUnet instance (called peer) has an identity (\textit{peer ID}) based on a
249 cryptographic public private key pair. The peer ID is the printable hash of the
250 public key. So before starting the peer, you may want to just generate the peer's private
251 key using the command
252 \lstset{language=bash}
254 $ gnunet-peerinfo -c ~/peer1.conf -s
256 You should see an output containing the peer ID similar to:
257 \lstset{language=bash}
259 I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.
262 GNUnet services are controlled by a master service the so called \textit{Automatic Restart Manager} (ARM).
263 ARM starts, stops and even restarts services automatically or on demand when a client connects.
264 You interact with the ARM service using the \lstinline|gnunet-arm| tool.
265 GNUnet can then be started with \lstinline|gnunet-arm -s| and stopped with
266 \lstinline|gnunet-arm -e|. An additional service not automatically started
267 can be started using \lstinline|gnunet-arm -i <service name>| and stopped
268 using \lstinline|gnunet-arm -k <servicename>|.
270 \subsection{Monitor a peer}
271 In this section, we will monitor the behaviour of our peer's DHT service with respect to a
272 specific key. First we will start GNUnet and then start the DHT service and use the DHT monitor tool
273 to monitor the PUT and GET commands we issue ussing the \lstinline|gnunet-dht-put| and
274 \lstinline|gnunet-dht-get| command. Using the ``monitor'' line given below, you can observe the behavior of
275 your own peer's DHT with respect to the specified KEY:
277 \lstset{language=bash}
279 $ gnunet-arm -c ~/peer1.conf -s # start gnunet with all default services
280 $ gnunet-arm -c ~/peer1.conf -i dht # start DHT service
281 $ cd ~/gnunet/src/dht;
282 $ ./gnunet-dht-monitor -c ~/peer1.conf -k KEY
284 Now open a separate terminal and change again to the \lstinline|gnunet/src/dht| directory:
286 $ cd ~/gnunet/src/dht
287 $ ./gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE # put VALUE under KEY in the DHT
288 $ ./gnunet/src/dht/gnunet-dht-get ~/peer1.conf -k KEY # get key KEY from the DHT
289 $ gnunet-statistics -c ~/peer1.conf # print statistics about current GNUnet state
290 $ gnunet-statistics -c ~/peer1.conf -s dht # print statistics about DHT service
293 \subsection{Starting Two Peers by Hand}
294 \subsubsection{Setup a second peer}
295 We will now start a second peer on your machine.
296 For the second peer, you will need to manually create a modified
297 configuration file to avoid conflicts with ports and directories.
298 A peers configuration file is by default located in {\tt ~/.gnunet/gnunet.conf}.
299 This file is typically very short or even empty as only the differences to the
300 defaults need to be specified. The defaults are located in
301 many files in the {\tt \$GNUNET\_PREFIX/share/gnunet/config.d} directory.
303 To configure the second peer, use the files {\tt
304 \$GNUNET\_PREFIX/share/gnunet/config.d} as a template for your main
307 \lstset{language=bash}
309 $ cat $GNUNET_PREFIX/share/gnunet/config.d/*.conf > peer2.conf
311 Now you have to edit {\tt peer2.conf} and change:
314 \item{\texttt{SERVICEHOME} under \texttt{PATHS}}
315 \item{Every (uncommented) value for ``\texttt{PORT}'' (add 10000) in any
316 section (the option may be commented out if \texttt{PORT} is
317 prefixed by "\#", in this case, UNIX domain sockets are used
318 and the PORT option does not need to be touched) }
319 \item{Every value for ``\texttt{UNIXPATH}'' in any section (e.g. by adding a "-p2" suffix)}
321 to a fresh, unique value. Make sure that the \texttt{PORT} numbers stay
322 below 65536. From now on, whenever you interact with the second
323 peer, you need to specify {\tt -c peer2.conf} as an additional
324 command line argument.
326 Now, generate the 2nd peer's private key:
328 \lstset{language=bash}
330 $ gnunet-peerinfo -s -c peer2.conf
334 This may take a while, generate entropy using your keyboard or mouse
335 as needed. Also, make sure the output is different from the {\tt
336 gnunet-peerinfo} output for the first peer (otherwise you made an
337 error in the configuration).
339 \subsubsection{Start the second peer and connect the peers}
340 Then, you can start a second peer using:
341 \lstset{language=bash}
343 $ gnunet-arm -c peer2.conf -s
344 $ gnunet-arm -c peer2.conf -i dht
345 $ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
346 $ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY
348 If you want the two peers to connect, you have multiple options:
351 \item UDP neighbour discovery (automatic)
352 \item Setup a bootstrap server
353 \item Connect manually
355 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:
361 Then change {\tt peer2.conf} and replace the ``\texttt{SERVERS}'' line in the ``\texttt{[hostlist]}'' section with
362 ``\texttt{http://localhost:8080/}''. Restart both peers using:
364 $ gnunet-arm -c peer1.conf -e # stop first peer
365 $ gnunet-arm -c peer1.conf -s # start first peer
366 $ gnunet-arm -c peer2.conf -s # start second peer
369 Note that if you start your peers without changing these settings, they
370 will use the ``global'' hostlist servers of the GNUnet P2P network and
371 likely connect to those peers. At that point, debugging might become
372 tricky as you're going to be connected to many more peers and would
373 likely observe traffic and behaviors that are not explicitly controlled
376 \subsubsection{How to connect manually}
377 If you want to use the \texttt{peerinfo} tool to connect your peers, you should:
380 \item{Remove {\tt hostlist} from {\tt DEFAULTSERVICES} (to not connect to the global GNUnet)}
381 \item{Start both peers running {\tt gnunet-arm -c peer1.conf -s} and {\tt gnunet-arm -c peer2.conf -s}}
382 \item{Get \texttt{HELLO} message of the first peer running {\tt gnunet-peerinfo -c peer1.conf -g}}
383 \item{Give the output to the second peer by running {\tt gnunet-peerinfo -c peer2.conf -p '<output>'}}
386 Check that they are connected using {\tt gnunet-core -c peer1.conf}, which should give you the other peer's
389 $ gnunet-core -c peer1.conf
390 Peer `9TVUCS8P5A7ILLBGO6JSTSSN2B44H3D2MUIFJMLKAITC0I22UVFBFP1H8NRK2IA35VKAK16LLO0MFS7TAQ9M1KNBJ4NGCHP3JPVULDG'
393 \subsection{Starting Peers Using the Testbed Service}
395 GNUnet's testbed service is used for testing scenarios where a number of peers
396 are to be started. The testbed can manage peers on a single host or on multiple
397 hosts in a distributed fashion. On a single affordable computer, it should be
398 possible to run around tens of peers without drastically increasing the load on the
401 The testbed service can be access through its API
402 \texttt{include/gnunet\_testbed\_service.h}. The API provides many routines for
403 managing a group of peers. It also provides a helper function
404 \texttt{GNUNET\_TESTBED\_test\_run()} to quickly setup a minimalistic testing
405 environment on a single host.
407 This function takes a configuration file which will be used as a template
408 configuration for the peers. The testbed takes care of modifying relevant
409 options in the peers' configuration such as SERVICEHOME, PORT, UNIXPATH to
410 unique values so that peers run without running into conflicts. It also checks
411 and assigns the ports in configurations only if they are free.
413 Additionally, the testbed service also reads its options from the same
414 configuration file. Various available options and details about them can be
415 found in the testbed default configuration file \texttt{src/testbed/testbed.conf}.
417 With the testbed API, a sample test case can be structured as follows:
418 \lstinputlisting[language=C]{testbed_test.c}
419 The source code for the above listing can be found at
420 \url{https://gnunet.org/svn/gnunet/doc/testbed_test.c}. After installing GNUnet, the above source code can be compiled as:
421 \lstset{language=bash}
423 $ export CPPFLAGS="-I/path/to/gnunet/headers"
424 $ export LDFLAGS="-L/path/to/gnunet/libraries"
425 $ gcc $CPPFLAGS $LDFLAGS -o testbed-test testbed_test.c -lgnunettestbed -lgnunetdht -lgnunetutil
427 The \texttt{CPPFLAGS} and \texttt{LDFLAGS} are necessary if GNUnet is installed
428 into a different directory other than \texttt{/usr/local}.
430 All of testbed API's peer management functions treat management actions as
431 operations and return operation handles. It is expected that the operations
432 begin immediately, but they may get delayed (to balance out load on the system).
433 The program using the API then has to take care of marking the operation as
434 ``done'' so that its associated resources can be freed immediately and other
435 waiting operations can be executed. Operations will be canceled if they are
436 marked as ``done'' before their completion.
438 An operation is treated as completed when it succeeds or fails. Completion of
439 an operation is either conveyed as events through \textit{controller event
440 callback} or through respective operation completion callbacks. In functions
441 which support completion notification through both controller event callback and
442 operation completion callback, first the controller event callback will be
443 called. If the operation is not marked as done in that callback or if the
444 callback is given as NULL when creating the operation, the operation completion
445 callback will be called. The API documentation shows which event are to be
446 expected in the controller event notifications. It also documents any
447 exceptional behaviour.
449 Once the peers are started, test cases often need to connect some of the peers'
450 services. Normally, opening a connect to a peer's service requires the peer's
451 configuration. While using testbed, the testbed automatically generates
452 per-peer configuration. Accessing those configurations directly through file
453 system is discouraged as their locations are dynamically created and will be
454 different among various runs of testbed. To make access to these configurations
455 easy, testbed API provides the function
456 \texttt{GNUNET\_TESTBED\_service\_connect()}. This function fetches the
457 configuration of a given peer and calls the \textit{Connect Adapter}.
458 In the example code, it is the \texttt{dht\_ca}. A connect adapter is expected
459 to open the connection to the needed service by using the provided configuration
460 and return the created service connection handle. Successful connection to the
461 needed service is signaled through \texttt{service\_connect\_comp\_cb}.
463 A dual to connect adapter is the \textit{Disconnect Adapter}. This callback is
464 called after the connect adapter has been called when the operation from
465 \texttt{GNUNET\_TESTBED\_service\_connect()} is marked as ``done''. It has to
466 disconnect from the service with the provided service handle (\texttt{op\_result}).
468 \exercise{Find out how many peers you can run on your system.}
470 \exercise{Find out how to create a 2D torus topology by changing the
471 options in the configuration file.\footnote{See \url{https://gnunet.org/content/supported-topologies}}
472 Then use the DHT API to store and retrieve values in the
475 \section{Developing Applications}
476 \subsection{gnunet-ext}
477 To develop a new peer-to-peer application or to extend GNUnet we provide
478 a template build system for writing GNUnet extensions in C. It can be
481 \lstset{language=bash}
483 $ svn checkout https://gnunet.org/svn/gnunet-ext/
486 $ ./configure --prefix=$HOME --with-gnunet=$GNUNET_PREFIX
493 The GNUnet ext template includes examples and a working buildsystem for a new GNUnet service.
494 A common GNUnet service consists of the following parts which will be discussed in detail in the
495 remainder of this document. The functionality of a GNUnet service is implemented in:
499 \item the GNUnet service (\lstinline|gnunet-ext/src/ext/gnunet-service-ext.c|)
500 \item the client API (\lstinline|gnunet-ext/src/ext/ext_api.c|)
501 \item the client application using the service API (\lstinline|gnunet-ext/src/ext/gnunet-ext.c|)
506 The interfaces for these entities are defined in:
509 \item client API interface (\lstinline|gnunet-ext/src/ext/ext.h|)
510 \item the service interface (\lstinline|gnunet-ext/src/include/gnunet_service_SERVICE.h|)
511 \item the P2P protocol (\lstinline|gnunet-ext/src/include/gnunet_protocols_ext.h|)
515 In addition the \texttt{ext} systems provides:
518 \item a test testing the API (\lstinline|gnunet-ext/src/ext/test_ext_api.c|)
519 \item a configuration template for the service (\lstinline|gnunet-ext/src/ext/ext.conf.in|)
523 \subsection{Adapting the Template}
525 The first step for writing any extension with a new service is to
526 ensure that the {\tt ext.conf.in} file contains entries for the
527 \texttt{UNIXPATH}, \texttt{PORT} and \texttt{BINARY} for the service in a section named after
530 If you want to adapt the template rename the {\tt ext.conf.in} to match your
531 services name, you have to modify the \texttt{AC\_OUTPUT} section in {\tt configure.ac}
532 in the \texttt{gnunet-ext} root.
534 \section{Writing a Client Application}
536 When writing any client application (for example, a command-line
537 tool), the basic structure is to start with the {\tt
538 GNUNET\_PROGRAM\_run} function. This function will parse
539 command-line options, setup the scheduler and then invoke the {\tt
540 run} function (with the remaining non-option arguments) and a handle
541 to the parsed configuration (and the configuration file name that was
542 used, which is typically not needed):
546 #include <gnunet/platform.h>
547 #include <gnunet/gnunet_util_lib.h>
555 const struct GNUNET_CONFIGURATION_Handle *cfg)
562 main (int argc, char *const *argv)
564 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
565 GNUNET_GETOPT_OPTION_END
568 GNUNET_PROGRAM_run (argc,
571 gettext_noop ("binary description text"),
572 options, &run, NULL)) ? ret : 1;
576 \subsection{Handling command-line options}
578 Options can then be added easily by adding global variables and
579 expanding the {\tt options} array. For example, the following would
580 add a string-option and a binary flag (defaulting to {\tt NULL} and
581 {\tt GNUNET\_NO} respectively):
584 static char *string_option;
588 static const struct GNUNET_GETOPT_CommandLineOption options[] = {
589 {'s', "name", "SOMESTRING",
590 gettext_noop ("text describing the string_option NAME"), 1,
591 &GNUNET_GETOPT_set_string, &string_option},
593 gettext_noop ("text describing the flag option"), 0,
594 &GNUNET_GETOPT_set_one, &a_flag},
595 GNUNET_GETOPT_OPTION_END
600 Issues such as displaying some helpful text describing options using
601 the {\tt --help} argument and error handling are taken care of when
602 using this approach. Other {\tt GNUNET\_GETOPT\_}-functions can be used
603 to obtain integer value options, increment counters, etc. You can
604 even write custom option parsers for special circumstances not covered
605 by the available handlers.
607 Inside the {\tt run} method, the program would perform the
608 application-specific logic, which typically involves initializing and
609 using some client library to interact with the service. The client
610 library is supposed to implement the IPC whereas the service provides
611 more persistent P2P functions.
613 \exercise{Add a few command-line options and print them inside
614 of {\tt run}. What happens if the user gives invalid arguments?}
616 \subsection{Writing a Client Library}
618 The first and most important step in writing a client library is to
619 decide on an API for the library. Typical API calls include
620 connecting to the service, performing application-specific requests
621 and cleaning up. Many examples for such service APIs can be found
622 in the {\tt gnunet/src/include/gnunet\_*\_service.h} files.
624 Then, a client-service protocol needs to be designed. This typically
625 involves defining various message formats in a header that will be
626 included by both the service and the client library (but is otherwise
627 not shared and hence located within the service's directory and not
628 installed by {\tt make install}). Each message must start with a {\tt
629 struct GNUNET\_MessageHeader} and must be shorter than 64k. By
630 convention, all fields in IPC (and P2P) messages must be in big-endian
631 format (and thus should be read using {\tt ntohl} and similar
632 functions and written using {\tt htonl} and similar functions).
633 Unique message types must be defined for each message struct in the
634 {\tt gnunet\_protocols.h} header (or an extension-specific include
637 \subsubsection{Connecting to the Service}
639 Before a client library can implement the application-specific protocol
640 with the service, a connection must be created:
644 struct GNUNET_CLIENT_Connection *client;
645 client = GNUNET_CLIENT_connect ("service-name", cfg);
648 As a result a {\tt GNUNET\_CLIENT\_Connection} handle is returned
649 which has to used in later API calls related to this service.
650 The complete client API can be found in {\tt gnunet\_client\_lib.h}
652 \subsubsection{GNUnet Messages}
654 In GNUnet, messages are always sent beginning with a {\tt struct GNUNET\_MessageHeader}
655 in big endian format. This header defines the size and the type of the
656 message, the payload follows after this header.
660 struct GNUNET_MessageHeader
664 * The length of the struct (in bytes, including the length field itself),
665 * in big-endian format.
667 uint16_t size GNUNET_PACKED;
670 * The type of the message (GNUNET_MESSAGE_TYPE_XXXX), in big-endian format.
672 uint16_t type GNUNET_PACKED;
677 Existing message types are defined in {\tt gnunet\_protocols.h}\\
678 A common way to create a message is:
682 struct GNUNET_MessageHeader *msg =
683 GNUNET_malloc(payload_size + sizeof(struct GNUNET_MessageHeader));
684 msg->size = htons(payload_size + sizeof(struct GNUNET_MessageHeader));
685 msg->type = htons(GNUNET_MY_MESSAGE_TYPE);
686 memcpy(&msg[1], &payload, payload_size);
690 \exercise{Define a message struct that includes a 32-bit
691 unsigned integer in addition to the standard GNUnet MessageHeader.
692 Add a C struct and define a fresh protocol number for your message.}
695 \subsubsection{Sending Requests to the Service}
697 Any client-service protocol must start with the client sending the
698 first message to the service, since services are only notified about
699 (new) clients upon receiving a the first message.
701 Clients can transmit messages to the service using the
702 {\tt GNUNET\_CLIENT\_notify\_transmit\_ready} API:
706 transmit_cb (void *cls, size_t size, void *buf)
709 if (NULL == buf) { handle_error(); return 0; }
710 GNUNET_assert (size >= msg_size);
711 memcpy (buf, my_msg, msg_size);
717 th = GNUNET_CLIENT_notify_transmit_ready (client,
725 The client-service protocoll calls {\tt GNUNET\_CLIENT\_notify\_transmit\_ready}
726 to be notified when the client is ready to send data to the service.
727 Besides other arguments, you have to pass the client returned
728 from the {\tt connect} call, the message size and the callback function to
729 call when the client is ready to send.
731 Only a single transmission request can be queued per client at the
732 same time using this API. The handle {\tt th} can be used to cancel
733 the request if necessary (for example, during shutdown).
735 When {\tt transmit\_cb} is called the message is copied in the buffer provided and
736 the number of bytes copied into the buffer is returned. {\tt transmit\_cb}
737 could also return 0 if for some reason no message
738 could be constructed; this is not an error and the connection to the
739 service will persist in this case.
741 \exercise{Define a helper function to transmit a 32-bit
742 unsigned integer (as payload) to a service using some given client
746 \subsubsection{Receiving Replies from the Service}
748 Clients can receive messages from the service using the
749 {\tt GNUNET\_CLIENT\_receive} API:
754 * Function called with messages from stats service.
757 * @param msg message received, NULL on timeout or fatal error
760 receive_message (void *cls, const struct GNUNET_MessageHeader *msg)
762 struct MyArg *arg = cls;
768 GNUNET_CLIENT_receive (client,
775 It should be noted that this receive call only receives a single
776 message. To receive additional messages, {\tt
777 GNUNET\_CLIENT\_receive} must be called again.
779 \exercise{Expand your helper function to receive a
780 response message (for example, containing just the GNUnet MessageHeader
781 without any payload). Upon receiving the service's response, you should
782 call a callback provided to your helper function's API. You'll need to
783 define a new 'struct' to hold your local context (``closure'').}
786 \subsection{Writing a user interface}
788 Given a client library, all it takes to access a service now is to
789 combine calls to the client library with parsing command-line
792 \exercise{Call your client API from your {\tt run} method
793 in your client application to send a request to the service.
794 For example, send a 32-bit integer value based on a number given
795 at the command-line to the service.}
799 \section{Writing a Service}
801 Before you can test the client you've written so far, you'll need to also
802 implement the corresponding service.
805 \subsection{Code Placement}
807 New services are placed in their own subdirectory under {\tt gnunet/src}.
808 This subdirectory should contain the API implementation file {\tt SERVICE\_api.c},
809 the description of the client-service protocol {\tt SERVICE.h} and P2P protocol
810 {\tt SERVICE\_protocol.h}, the implementation of the service itself
811 {\tt gnunet-service-SERVICE.h} and several files for tests, including test code
812 and configuration files.
814 \subsection{Starting a Service}
816 The key API definitions for starting services are:
819 typedef void (*GNUNET_SERVICE_Main) (void *cls,
820 struct GNUNET_SERVER_Handle *server,
821 const struct GNUNET_CONFIGURATION_Handle *cfg);
822 int GNUNET_SERVICE_run (int argc,
824 const char *serviceName,
825 enum GNUNET_SERVICE_Options opt,
826 GNUNET_SERVICE_Main task,
830 Here is a starting point for your main function for your service:
834 static void my_main (void *cls,
835 struct GNUNET_SERVER_Handle *server,
836 const struct GNUNET_CONFIGURATION_Handle *cfg)
841 int main (int argc, char *const*argv)
844 GNUNET_SERVICE_run (argc, argv, "my",
845 GNUNET_SERVICE_OPTION_NONE,
852 \exercise{Write a stub service that processes no messages at all
853 in your code. Create a default configuration for it, integrate it
854 with the build system and start the service from {\tt
855 gnunet-service-arm} using {\tt gnunet-arm -i NAME}.}
858 \subsection{Receiving Requests from Clients}
860 Inside of the {\tt my\_main} method, a service typically registers for
861 the various message types from clients that it supports by providing
862 a handler function, the message type itself and possibly a fixed
863 message size (or 0 for variable-size messages):
868 handle_set (void *cls,
869 struct GNUNET_SERVER_Client *client,
870 const struct GNUNET_MessageHeader *message)
872 GNUNET_SERVER_receive_done (client, GNUNET_OK);
875 handle_get (void *cls,
876 struct GNUNET_SERVER_Client *client,
877 const struct GNUNET_MessageHeader *message)
879 GNUNET_SERVER_receive_done (client, GNUNET_OK);
882 static void my_main (void *cls,
883 struct GNUNET_SERVER_Handle *server,
884 const struct GNUNET_CONFIGURATION_Handle *cfg)
886 static const struct GNUNET_SERVER_MessageHandler handlers[] = {
887 {&handle_set, NULL, GNUNET_MESSAGE_TYPE_MYNAME_SET, 0},
888 {&handle_get, NULL, GNUNET_MESSAGE_TYPE_MYNAME_GET, 0},
891 GNUNET_SERVER_add_handlers (server, handlers);
892 /* do more setup work */
896 Each handler function {\bf must} eventually (possibly in some
897 asynchronous continuation) call {\tt GNUNET\_SERVER\_receive\_done}.
898 Only after this call additional messages from the same client may
899 be processed. This way, the service can throttle processing messages
900 from the same client. By passing {\tt GNUNET\_SYSERR}, the service
901 can close the connection to the client, indicating an error.
903 Services must check that client requests are well-formed and must not
904 crash on protocol violations by the clients. Similarly, client
905 libraries must check replies from servers and should gracefully report
906 errors via their API.
909 \exercise{Change the service to ``handle'' the message from your
910 client (for now, by printing a message). What happens if you
911 forget to call {\tt GNUNET\_SERVER\_receive\_done}?}
914 \subsection{Responding to Clients}
916 Servers can send messages to clients using the
917 {\tt GNUNET\_SERVER\_notify\_transmit\_ready} API:
922 transmit_cb (void *cls, size_t size, void *buf)
925 if (NULL == buf) { handle_error(); return 0; }
926 GNUNET_assert (size >= msg_size);
927 memcpy (buf, my_msg, msg_size);
933 struct GNUNET_SERVER_TransmitHandle *th;
934 th = GNUNET_SERVER_notify_transmit_ready (client,
941 Only a single transmission request can be queued per client
942 at the same time using this API.
943 Additional APIs for sending messages to clients can be found
944 in the {\tt gnunet\_server\_lib.h} header.
947 \exercise{Change the service respond to the request from your
948 client. Make sure you handle malformed messages in both directions.}
951 \section{Interacting directly with other Peers using the CORE Service}
953 One of the most important services in GNUnet is the \texttt{CORE} service
954 managing connections between peers and handling encryption between peers.
956 One of the first things any service that extends the P2P protocol typically does
957 is connect to the \texttt{CORE} service using:
961 #include <gnunet/gnunet_core_service.h>
963 struct GNUNET_CORE_Handle *
964 GNUNET_CORE_connect (const struct GNUNET_CONFIGURATION_Handle *cfg,
966 GNUNET_CORE_StartupCallback init,
967 GNUNET_CORE_ConnectEventHandler connects,
968 GNUNET_CORE_DisconnectEventHandler disconnects,
969 GNUNET_CORE_MessageCallback inbound_notify,
970 int inbound_hdr_only,
971 GNUNET_CORE_MessageCallback outbound_notify,
972 int outbound_hdr_only,
973 const struct GNUNET_CORE_MessageHandler *handlers);
976 \subsection{New P2P connections}
978 Before any traffic with a different peer can be exchanged, the peer must be
979 known to the service. This is notified by the \texttt{CORE} {\tt connects} callback,
980 which communicates the identity of the new peer to the service:
986 const struct GNUNET_PeerIdentity * peer)
988 /* Save identity for later use */
989 /* Optional: start sending messages to peer */
993 \exercise{Create a service that connects to the \texttt{CORE}. Then
994 start (and connect) two peers and print a message once your connect
995 callback is invoked.}
997 \subsection{Receiving P2P Messages}
999 To receive messages from \texttt{CORE}, services register a set of handlers
1000 (parameter {\tt *handlers} in the \lstinline|GNUNET_CORE_connect| call that are called by \texttt{CORE}
1001 when a suitable message arrives.
1006 callback_function_for_type_one(void *cls,
1007 const struct GNUNET_PeerIdentity *peer,
1008 const struct GNUNET_MessageHeader *message)
1011 return GNUNET_OK; /* or GNUNET_SYSERR to close the connection */
1015 * Functions to handle messages from core
1017 static struct GNUNET_CORE_MessageHandler core_handlers[] = {
1018 {&callback_function_for_type_one, GNUNET_MESSAGE_TYPE_MYSERVICE_TYPE_ONE, 0},
1024 \exercise{Start one peer with a new service that has a message
1025 handler and start a second peer that only has your ``old'' service
1026 without message handlers. Which ``connect'' handlers are invoked when
1027 the two peers are connected? Why?}
1030 \subsection{Sending P2P Messages}
1032 In response to events (connect, disconnect, inbound messages,
1033 timing, etc.) services can then use this API to transmit messages:
1038 (*GNUNET_CONNECTION_TransmitReadyNotify) (void *cls,
1042 /* Fill "*buf" with up to "size" bytes, must start with GNUNET_MessageHeader */
1043 return n; /* Total size of the message put in "*buf" */
1046 struct GNUNET_CORE_TransmitHandle *
1047 GNUNET_CORE_notify_transmit_ready (struct GNUNET_CORE_Handle *handle,
1048 int cork, uint32_t priority,
1049 struct GNUNET_TIME_Relative maxdelay,
1050 const struct GNUNET_PeerIdentity *target,
1052 GNUNET_CONNECTION_TransmitReadyNotify notify,
1056 \exercise{Write a service that upon connect sends messages as
1057 fast as possible to the other peer (the other peer should run a
1058 service that ``processes'' those messages). How fast is the
1059 transmission? Count using the STATISTICS service on both ends. Are
1060 messages lost? How can you transmit messages faster? What happens if
1061 you stop the peer that is receiving your messages?}
1064 \subsection{End of P2P connections}
1066 If a message handler returns {\tt GNUNET\_SYSERR}, the remote peer shuts down or
1067 there is an unrecoverable network disconnection, CORE notifies the service that
1068 the peer disconnected. After this notification no more messages will be received
1069 from the peer and the service is no longer allowed to send messages to the peer.
1070 The disconnect callback looks like the following:
1075 disconnects (void *cls,
1076 const struct GNUNET_PeerIdentity * peer)
1078 /* Remove peer's identity from known peers */
1079 /* Make sure no messages are sent to peer from now on */
1083 \exercise{Fix your service to handle peer disconnects.}
1085 \section{Using the DHT}
1086 The DHT allows to store data so other peers in the P2P network can
1087 access it and retrieve data stored by any peers in the network.
1088 This section will explain how to use the DHT. Of course, the first
1089 thing to do is to connect to the DHT service:
1092 dht_handle = GNUNET_DHT_connect (cfg, parallel_requests);
1094 The second parameter indicates how many requests in parallel to expect.
1095 It is not a hard limit, but a good approximation will make the DHT more
1098 \subsection{Storing data in the DHT}
1099 Since the DHT is a dynamic environment (peers join a leave frequently)
1100 the data that we put in the DHT does not stay there indefinitely. It is
1101 important to ``refresh'' the data periodically by simply storing it again,
1102 in order to make sure other peers can access it.
1104 The put API call offers a callback to signal that the PUT request has been
1105 sent. This does not guarantee that the data is accessible to others peers,
1106 or even that is has been stored, only that the service has requested to
1107 a neighboring peer the retransmission of the PUT request towards its final
1108 destination. Currently there is no feedback about whether or not the data
1109 has been sucessfully stored or where it has been stored. In order to improve
1110 the availablilty of the data and to compensate for possible errors, peers leaving
1111 and other unfavorable events, just make several PUT requests!
1116 message_sent_cont (void *cls, const struct GNUNET_SCHEDULER_TaskContext *tc)
1118 /* Request has left local node */
1121 struct GNUNET_DHT_PutHandle *
1122 GNUNET_DHT_put (struct GNUNET_DHT_Handle *handle,
1123 const struct GNUNET_HashCode * key,
1124 uint32_t desired_replication_level,
1125 enum GNUNET_DHT_RouteOption options, /* Route options, see next call */
1126 enum GNUNET_BLOCK_Type type, size_t size, const void *data,
1127 struct GNUNET_TIME_Absolute exp, /* When does the data expire? */
1128 struct GNUNET_TIME_Relative timeout, /* How long to try to send the request */
1129 GNUNET_DHT_PutContinuation cont,
1133 \exercise{Store a value in the DHT periodically to make sure it is available
1134 over time. You might consider using the function GNUNET\_SCHEDULER\_add\_delayed and
1135 call GNUNET\_DHT\_put from inside a helper function.}
1138 \subsection{Obtaining data from the DHT}
1139 As we saw in the previous example, the DHT works in an asynchronous mode.
1140 Each request to the DHT is executed ``in the background'' and the API
1141 calls return immediately. In order to receive results from the DHT, the
1142 API provides a callback. Once started, the request runs in the service,
1143 the service will try to get as many results as possible (filtering out
1144 duplicates) until the timeout expires or we explicitly stop the request.
1145 It is possible to give a ``forever'' timeout with
1146 {\tt GNUNET\_TIME\_UNIT\_FOREVER\_REL}.
1148 If we give a route option {\tt GNUNET\_DHT\_RO\_RECORD\_ROUTE} the callback
1149 will get a list of all the peers the data has travelled, both on the PUT
1150 path and on the GET path.
1154 get_result_iterator (void *cls, struct GNUNET_TIME_Absolute expiration,
1155 const struct GNUNET_HashCode * key,
1156 const struct GNUNET_PeerIdentity *get_path,
1157 unsigned int get_path_length,
1158 const struct GNUNET_PeerIdentity *put_path,
1159 unsigned int put_path_length,
1160 enum GNUNET_BLOCK_Type type, size_t size, const void *data)
1162 /* Do stuff with the data and/or route */
1164 GNUNET_DHT_get_stop (get_handle);
1168 GNUNET_DHT_get_start (dht_handle,
1172 GNUNET_DHT_RO_NONE, /* Route options */
1173 NULL, /* xquery: not used here */
1174 0, /* xquery size */
1175 &get_result_iterator,
1179 \exercise{Store a value in the DHT and after a while retrieve it. Show the IDs of all
1180 the peers the requests have gone through. In order to convert a peer ID to a string, use
1181 the function GNUNET\_i2s. Pay attention to the route option parameters in both calls!}
1183 \subsection{Implementing a block plugin}
1185 In order to store data in the DHT, it is necessary to provide a block
1186 plugin. The DHT uses the block plugin to ensure that only well-formed
1187 requests and replies are transmitted over the network.
1189 The block plugin should be put in a file {\tt
1190 plugin\_block\_SERVICE.c} in the service's respective directory. The
1191 mandatory functions that need to be implemented for a block plugin are
1192 described in the following sections.
1194 \subsubsection{Validating requests and replies}
1196 The evaluate function should validate a reply or a request. It returns
1197 a {\tt GNUNET\_BLOCK\_EvaluationResult}, which is an enumeration. All
1198 possible answers are in {\tt gnunet\_block\_lib.h}. The function will
1199 be called with a {\tt reply\_block} argument of {\tt NULL} for
1200 requests. Note that depending on how {\tt evaluate} is called, only
1201 some of the possible return values are valid. The specific meaning of
1202 the {\tt xquery} argument is application-specific. Applications that
1203 do not use an extended query should check that the {\tt xquery\_size}
1204 is zero. The Bloom filter is typically used to filter duplicate
1209 static enum GNUNET_BLOCK_EvaluationResult
1210 block_plugin_SERVICE_evaluate (void *cls,
1211 enum GNUNET_BLOCK_Type type,
1212 const GNUNET_HashCode * query,
1213 struct GNUNET_CONTAINER_BloomFilter **bf,
1217 const void *reply_block,
1218 size_t reply_block_size)
1220 /* Verify type, block and bloomfilter */
1224 Note that it is mandatory to detect duplicate replies in this
1225 function and return the respective status code. Duplicate
1226 detection should be done by setting the respective bits in
1227 the Bloom filter {\tt bf}. Failure to do so may cause replies
1228 to circle in the network.
1230 \subsubsection{Deriving a key from a reply}
1232 The DHT can operate more efficiently if it is possible to derive a key
1233 from the value of the corresponding block. The {\tt get\_key}
1234 function is used to obtain the key of a block --- for example, by
1235 means of hashing. If deriving the key is not possible, the function
1236 should simply return {\tt GNUNET\_SYSERR} (the DHT will still work
1237 just fine with such blocks).
1242 block_plugin_SERVICE_get_key (void *cls, enum GNUNET_BLOCK_Type type,
1243 const void *block, size_t block_size,
1244 GNUNET_HashCode * key)
1246 /* Store the key in the key argument, return GNUNET_OK on success. */
1250 \subsubsection{Initialization of the plugin}
1252 The plugin is realized as a shared C library. The library must export
1253 an initialization function which should initialize the plugin. The
1254 initialization function specifies what block types the plugin cares
1255 about and returns a struct with the functions that are to be used for
1256 validation and obtaining keys (the ones just defined above).
1261 libgnunet_plugin_block_SERVICE_init (void *cls)
1263 static enum GNUNET_BLOCK_Type types[] =
1265 GNUNET_BLOCK_TYPE_SERVICE_BLOCKYPE, /* list of blocks we care about, from gnunet_block_lib.h */
1266 GNUNET_BLOCK_TYPE_ANY /* end of list */
1268 struct GNUNET_BLOCK_PluginFunctions *api;
1270 api = GNUNET_malloc (sizeof (struct GNUNET_BLOCK_PluginFunctions));
1271 api->evaluate = &block_plugin_SERICE_evaluate;
1272 api->get_key = &block_plugin_SERVICE_get_key;
1278 \subsubsection{Shutdown of the plugin}
1280 Following GNUnet's general plugin API concept, the plugin must
1281 export a second function for cleaning up. It usually does very
1287 libgnunet_plugin_block_SERVICE_done (void *cls)
1289 struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
1297 \subsubsection{Integration of the plugin with the build system}
1299 In order to compile the plugin, the {\tt Makefile.am} file for the
1300 service should contain a rule similar to this:
1302 \lstset{language=make}
1304 plugin_LTLIBRARIES = \
1305 libgnunet_plugin_block_SERVICE.la
1306 libgnunet_plugin_block_SERVICE_la_SOURCES = \
1307 plugin_block_SERVICE.c
1308 libgnunet_plugin_block_SERVICE_la_LIBADD = \
1309 $(top_builddir)/src/hello/libgnunethello.la \
1310 $(top_builddir)/src/block/libgnunetblock.la \
1311 $(top_builddir)/src/util/libgnunetutil.la
1312 libgnunet_plugin_block_SERVICE_la_LDFLAGS = \
1313 $(GN_PLUGIN_LDFLAGS)
1314 libgnunet_plugin_block_SERVICE_la_DEPENDENCIES = \
1315 $(top_builddir)/src/block/libgnunetblock.la
1320 \exercise{Write a block plugin that accepts all queries
1321 and all replies but prints information about queries and replies
1322 when the respective validation hooks are called.}
1326 \subsection{Monitoring the DHT}
1327 It is possible to monitor the functioning of the local DHT service. When monitoring
1328 the DHT, the service will alert the monitoring program of any events,
1329 both started locally or received for routing from another peer. The are three different
1330 types of events possible: a GET request, a PUT request or a response (a reply to
1333 Since the different events have different associated data, the API gets 3
1334 different callbacks (one for each message type) and optional type and key parameters,
1335 to allow for filtering of messages. When an event happens, the appropiate callback
1336 is called with all the information about the event.
1340 get_callback (void *cls,
1341 enum GNUNET_DHT_RouteOption options,
1342 enum GNUNET_BLOCK_Type type,
1344 uint32_t desired_replication_level,
1345 unsigned int path_length,
1346 const struct GNUNET_PeerIdentity *path,
1347 const struct GNUNET_HashCode * key)
1352 get_resp_callback (void *cls,
1353 enum GNUNET_BLOCK_Type type,
1354 const struct GNUNET_PeerIdentity *get_path,
1355 unsigned int get_path_length,
1356 const struct GNUNET_PeerIdentity *put_path,
1357 unsigned int put_path_length,
1358 struct GNUNET_TIME_Absolute exp,
1359 const struct GNUNET_HashCode * key,
1366 put_callback (void *cls,
1367 enum GNUNET_DHT_RouteOption options,
1368 enum GNUNET_BLOCK_Type type,
1370 uint32_t desired_replication_level,
1371 unsigned int path_length,
1372 const struct GNUNET_PeerIdentity *path,
1373 struct GNUNET_TIME_Absolute exp,
1374 const struct GNUNET_HashCode * key,
1380 monitor_handle = GNUNET_DHT_monitor_start (dht_handle,
1381 block_type, /* GNUNET_BLOCK_TYPE_ANY for all */
1382 key, /* NULL for all */
1390 \section{Debugging with {\tt gnunet-arm}}
1392 Even if services are managed by {\tt gnunet-arm}, you can start them with
1393 {\tt gdb} or {\tt valgrind}. For example, you could add the following lines
1394 to your configuration file to start the DHT service in a {\tt gdb} session in a
1399 PREFIX=xterm -e gdb --args
1402 Alternatively, you can stop a service that was started via ARM and run it manually:
1404 \lstset{language=bash}
1407 $ gdb --args gnunet-service-dht -L DEBUG
1408 $ valgrind gnunet-service-dht -L DEBUG
1412 Assuming other services are well-written, they will automatically re-integrate the
1413 restarted service with the peer.
1415 GNUnet provides a powerful logging mechanism providing log levels \texttt{ERROR},
1416 \texttt{WARNING}, \texttt{INFO} and \texttt{DEBUG}. The current log level is
1417 configured using the \lstinline|$GNUNET_FORCE_LOG| environmental variable.
1418 The \texttt{DEBUG} level is only available if \lstinline|--enable-logging=verbose| was used when
1419 running \texttt{configure}. More details about logging can be found under
1420 \url{https://gnunet.org/logging}.
1422 You should also probably enable the creation of core files, by setting
1423 {\tt ulimit}, and echo'ing 1 into {\tt /proc/sys/kernel/core\_uses\_pid}.
1424 Then you can investigate the core dumps with {\tt gdb}, which is often
1425 the fastest method to find simple errors.
1427 \exercise{Add a memory leak to your service and obtain a trace
1428 pointing to the leak using {\tt valgrind} while running the service
1429 from {\tt gnunet-service-arm}.}