C Tutorial
This tutorials explains how to install GNUnet on a GNU/Linux system and gives an introduction on how GNUnet can be used to develop a Peer-to-Peer application. Detailed installation instructions for various operating systems and a detailed list of all dependencies can be found on our website at https://docs.gnunet.org/#Installation and in our Reference Documentation (GNUnet Handbook).
Please read this tutorial carefully since every single step is important, and do not hesitate to contact the GNUnet team if you have any questions or problems! Visit this link in your webbrowser to learn how to contact the GNUnet team: https://gnunet.org/en/contact.html
Introduction to GNUnet Architecture
GNUnet is organized in layers and services. Each service is composed of a main service implementation and a client library for other programs to use the service’s functionality, described by an API. Some services provide an additional command line tool to enable the user to interact with the service.
Very often it is other GNUnet services that will use these APIs to build the higher layers of GNUnet on top of the lower ones. Each layer expands or extends the functionality of the service below (for instance, to build a mesh on top of a DHT).
The main service implementation runs as a standalone process in the Operating System and the client code runs as part of the client program, so crashes of a client do not affect the service process or other clients. The service and the clients communicate via a message protocol to be defined and implemented by the programmer.
First Steps with GNUnet
Configure your peer
First of all we need to configure your peer. Each peer is started with a
configuration containing settings for GNUnet itself and its services.
This configuration is based on the default configuration shipped with
GNUnet and can be modified. The default configuration is located in the
$PREFIX/share/gnunet/config.d directory. When starting a peer, you
can specify a customized configuration using the the -c command line
switch when starting the ARM service and all other services. When using
a modified configuration the default values are loaded and only values
specified in the configuration file will replace the default values.
Since we want to start additional peers later, we need some modifications from the default configuration. We need to create a separate service home and a file containing our modifications for this peer:
$ mkdir ~/gnunet1/
$ touch peer1.conf
Now add the following lines to peer1.conf to use this directory. For
simplified usage we want to prevent the peer to connect to the GNUnet
network since this could lead to confusing output. This modifications
will replace the default settings:
[PATHS]
# Use this directory to store GNUnet data
GNUNET_HOME = ~/gnunet1/
[hostlist]
# prevent bootstrapping
SERVERS =
Start a peer
Each GNUnet instance (called peer) has an identity (peer ID) based on a cryptographic public private key pair. The peer ID is the printable hash of the public key.
GNUnet services are controlled by a master service, the so called
Automatic Restart Manager (ARM). ARM starts, stops and even restarts
services automatically or on demand when a client connects. You interact
with the ARM service using the gnunet-arm tool. GNUnet can then be
started with gnunet-arm -s and stopped with gnunet-arm -e. An
additional service not automatically started can be started using
gnunet-arm -i <service name> and stopped using
gnunet-arm -k <servicename>.
Once you have started your peer, you can use many other GNUnet commands to interact with it. For example, you can run:
$ gnunet-core -i
to obtain the public key of your peer.
You should see an output containing the peer ID similar to:
I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.
Monitor a peer
In this section, we will monitor the behaviour of our peer’s DHT service
with respect to a specific key. First we will start GNUnet and then
start the DHT service and use the DHT monitor tool to monitor the PUT
and GET commands we issue ussing the gnunet-dht-put and
gnunet-dht-get commands. Using the “monitor” line given below, you
can observe the behavior of your own peer’s DHT with respect to the
specified KEY:
# start gnunet with all default services:
$ gnunet-arm -c ~/peer1.conf -s
# start DHT service:
$ gnunet-arm -c ~/peer1.conf -i dht
$ gnunet-dht-monitor -c ~/peer1.conf -k KEY
Now open a separate terminal and:
# put VALUE under KEY in the DHT:
$ gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE
# get key KEY from the DHT:
$ gnunet-dht-get -c ~/peer1.conf -k KEY
# print statistics about current GNUnet state:
$ gnunet-statistics -c ~/peer1.conf
# print statistics about DHT service:
$ gnunet-statistics -c ~/peer1.conf -s dht
Starting Two Peers by Hand
This section describes how to start two peers on the same machine by hand. The process is rather painful, but the description is somewhat instructive. In practice, you might prefer the automated method (see Starting Peers Using the Testbed Service).
Setup a second peer
We will now start a second peer on your machine. For the second peer,
you will need to manually create a modified configuration file to avoid
conflicts with ports and directories. A peers configuration file is by
default located in ~/.config/gnunet.conf. This file is typically
very short or even empty as only the differences to the defaults need to
be specified. The defaults are located in many files in the
$PREFIX/share/gnunet/config.d directory.
To configure the second peer, use the files
$PREFIX/share/gnunet/config.d as a template for your main
configuration file:
$ cat $PREFIX/share/gnunet/config.d/*.conf > peer2.conf
Now you have to edit peer2.conf and change:
GNUNET\_TEST\_HOMEunderPATHSEvery (uncommented) value for “
PORT“ (add 10000) in any section (the option may be commented out ifPORTis prefixed by "#", in this case, UNIX domain sockets are used and the PORT option does not need to be touched)Every value for “
UNIXPATH“ in any section (e.g. by adding a "-p2" suffix)
to a fresh, unique value. Make sure that the PORT numbers stay below
65536. From now on, whenever you interact with the second peer, you need
to specify -c peer2.conf as an additional command line argument.
Now, generate the 2nd peer’s private key:
$ gnunet-core -i -c peer2.conf
This may take a while, generate entropy using your keyboard or mouse as needed. Also, make sure the output is different from the gnunet-core output for the first peer (otherwise you made an error in the configuration).
Start the second peer and connect the peers
Then, you can start a second peer using:
$ gnunet-arm -c peer2.conf -s
$ gnunet-arm -c peer2.conf -i dht
$ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
$ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY
If you want the two peers to connect, you have multiple options:
UDP neighbour discovery (automatic)
Setup a bootstrap server
Connect manually
To setup peer 1 as bootstrapping server change the configuration of the
first one to be a hostlist server by adding the following lines to
peer1.conf to enable bootstrapping server:
[hostlist]
OPTIONS = -p
Then change peer2.conf and replace the “SERVERS“ line in the
“[hostlist]“ section with “http://localhost:8080/“.
Restart both peers using:
# stop first peer
$ gnunet-arm -c peer1.conf -e
# start first peer
$ gnunet-arm -c peer1.conf -s
# start second peer
$ gnunet-arm -c peer2.conf -s
Note that if you start your peers without changing these settings, they will use the “global” hostlist servers of the GNUnet P2P network and likely connect to those peers. At that point, debugging might become tricky as you’re going to be connected to many more peers and would likely observe traffic and behaviors that are not explicitly controlled by you.
How to connect manually
If you want to use the peerinfo tool to connect your peers, you
should:
Set
IMMEDIATE_START = NOin sectionhostlist(to not connect to the global GNUnet)Start both peers running
gnunet-arm -c peer1.conf -sandgnunet-arm -c peer2.conf -sGet
HELLOmessage of the first peer runninggnunet-hello -c peer1.conf --export-helloGive the output to the second peer by running
echo "gnunet://hello/..." | gnunet-hello -c peer2.conf --import-hello
Check that they are connected using gnunet-core -s -c peer1.conf, which
should give you the other peer’s peer identity:
$ gnunet-core -s -c peer1.conf
Peer `9TVUCS8P5A7ILLBGO6 [...shortened...] 1KNBJ4NGCHP3JPVULDG'
Starting Peers Using the Testbed Service
GNUnet’s testbed service is used for testing scenarios where a number of peers are to be started. The testbed can manage peers on a single host or on multiple hosts in a distributed fashion. On a single affordable computer, it should be possible to run around tens of peers without drastically increasing the load on the system.
The testbed service can be access through its API
include/gnunet\_testbed\_service.h. The API provides many routines
for managing a group of peers. It also provides a helper function
GNUNET\_TESTBED\_test\_run() to quickly setup a minimalistic testing
environment on a single host.
This function takes a configuration file which will be used as a
template configuration for the peers. The testbed takes care of
modifying relevant options in the peers’ configuration such as
SERVICEHOME, PORT, UNIXPATH to unique values so that peers
run without running into conflicts. It also checks and assigns the ports
in configurations only if they are free.
Additionally, the testbed service also reads its options from the same
configuration file. Various available options and details about them can
be found in the testbed default configuration file
src/testbed/testbed.conf.
With the testbed API, a sample test case can be structured as follows:
#include <unistd.h>
#include <gnunet/platform.h>
#include <gnunet/gnunet_util_lib.h>
#include <gnunet/gnunet_testbed_service.h>
#include <gnunet/gnunet_dht_service.h>
#define NUM_PEERS 20
static struct GNUNET_TESTBED_Operation *dht_op;
static struct GNUNET_DHT_Handle *dht_handle;
struct MyContext
{
int ht_len;
} ctxt;
static int result;
static void
shutdown_task (void *cls)
{
if (NULL != dht_op)
{
GNUNET_TESTBED_operation_done (dht_op);
dht_op = NULL;
dht_handle = NULL;
}
result = GNUNET_OK;
}
static void
service_connect_comp (void *cls,
struct GNUNET_TESTBED_Operation *op,
void *ca_result,
const char *emsg)
{
GNUNET_assert (op == dht_op);
dht_handle = ca_result;
// Do work here...
GNUNET_SCHEDULER_shutdown ();
}
static void *
dht_ca (void *cls, const struct GNUNET_CONFIGURATION_Handle *cfg)
{
struct MyContext *ctxt = cls;
dht_handle = GNUNET_DHT_connect (cfg, ctxt->ht_len);
return dht_handle;
}
static void
dht_da (void *cls, void *op_result)
{
struct MyContext *ctxt = cls;
GNUNET_DHT_disconnect ((struct GNUNET_DHT_Handle *) op_result);
dht_handle = NULL;
}
static void
test_master (void *cls,
struct GNUNET_TESTBED_RunHandle *h,
unsigned int num_peers,
struct GNUNET_TESTBED_Peer **peers,
unsigned int links_succeeded,
unsigned int links_failed)
{
ctxt.ht_len = 10;
dht_op = GNUNET_TESTBED_service_connect
(NULL, peers[0], "dht",
&service_connect_comp, NULL,
&dht_ca, &dht_da, &ctxt);
GNUNET_SCHEDULER_add_shutdown (&shutdown_task, NULL);
}
int
main (int argc, char **argv)
{
int ret;
result = GNUNET_SYSERR;
ret = GNUNET_TESTBED_test_run
("awesome-test", "template.conf",
NUM_PEERS, 0LL,
NULL, NULL, &test_master, NULL);
if ( (GNUNET_OK != ret) || (GNUNET_OK != result) )
return 1;
return 0;
}
The source code for the above listing can be found at
https://git.gnunet.org/gnunet.git/tree/doc/documentation/testbed_test.c
or in the doc/ folder of your repository check-out. After installing
GNUnet, the above source code can be compiled as:
$ export CPPFLAGS="-I/path/to/gnunet/headers"
$ export LDFLAGS="-L/path/to/gnunet/libraries"
$ gcc $CPPFLAGS $LDFLAGS -o testbed-test testbed_test.c \
-lgnunettestbed -lgnunetdht -lgnunetutil
# Generate (empty) configuration
$ touch template.conf
# run it (press CTRL-C to stop)
$ ./testbed-test
The CPPFLAGS and LDFLAGS are necessary if GNUnet is installed
into a different directory other than /usr/local.
All of testbed API’s peer management functions treat management actions as operations and return operation handles. It is expected that the operations begin immediately, but they may get delayed (to balance out load on the system). The program using the API then has to take care of marking the operation as “done” so that its associated resources can be freed immediately and other waiting operations can be executed. Operations will be canceled if they are marked as “done” before their completion.
An operation is treated as completed when it succeeds or fails. Completion of an operation is either conveyed as events through controller event callback or through respective operation completion callbacks. In functions which support completion notification through both controller event callback and operation completion callback, first the controller event callback will be called. If the operation is not marked as done in that callback or if the callback is given as NULL when creating the operation, the operation completion callback will be called. The API documentation shows which event are to be expected in the controller event notifications. It also documents any exceptional behaviour.
Once the peers are started, test cases often need to connect some of the
peers’ services. Normally, opening a connect to a peer’s service
requires the peer’s configuration. While using testbed, the testbed
automatically generates per-peer configuration. Accessing those
configurations directly through file system is discouraged as their
locations are dynamically created and will be different among various
runs of testbed. To make access to these configurations easy, testbed
API provides the function GNUNET\_TESTBED\_service\_connect(). This
function fetches the configuration of a given peer and calls the Connect
Adapter. In the example code, it is the dht\_ca. A connect adapter
is expected to open the connection to the needed service by using the
provided configuration and return the created service connection handle.
Successful connection to the needed service is signaled through
service\_connect\_comp\_cb.
A dual to connect adapter is the Disconnect Adapter. This callback is
called after the connect adapter has been called when the operation from
GNUNET\_TESTBED\_service\_connect() is marked as “done”. It has to
disconnect from the service with the provided service handle
(op\_result).
Exercise: Find out how many peers you can run on your system.
Exercise: Find out how to create a 2D torus topology by changing the options in the configuration file. See section “The GNUnet Reference Documentation” in The GNUnet Reference Documentation, then use the DHT API to store and retrieve values in the network.
Developing Applications
gnunet-ext
To develop a new peer-to-peer application or to extend GNUnet we provide a template build system for writing GNUnet extensions in C. It can be obtained as follows:
$ git clone https://git.gnunet.org/gnunet-ext.git
$ cd gnunet-ext/
$ ./bootstrap
$ ./configure --prefix=$PREFIX --with-gnunet=$PREFIX
$ make
$ make install
$ make check
The GNUnet ext template includes examples and a working buildsystem for a new GNUnet service. A common GNUnet service consists of the following parts which will be discussed in detail in the remainder of this document. The functionality of a GNUnet service is implemented in:
the GNUnet service (gnunet-ext/src/ext/gnunet-service-ext.c)
the client API (gnunet-ext/src/ext/ext_api.c)
the client application using the service API (gnunet-ext/src/ext/gnunet-ext.c)
The interfaces for these entities are defined in:
client API interface (gnunet-ext/src/ext/ext.h)
the service interface (gnunet-ext/src/include/gnunet_service_SERVICE.h)
the P2P protocol (gnunet-ext/src/include/gnunet_protocols_ext.h)
In addition the ext systems provides:
a test testing the API (gnunet-ext/src/ext/test_ext_api.c)
a configuration template for the service (gnunet-ext/src/ext/ext.conf.in)
Adapting the Template
The first step for writing any extension with a new service is to ensure
that the ext.conf.in file contains entries for the UNIXPATH,
PORT and BINARY for the service in a section named after the
service.
If you want to adapt the template rename the ext.conf.in to match
your services name, you have to modify the AC\_OUTPUT section in
configure.ac in the gnunet-ext root.
Writing a Client Application
When writing any client application (for example, a command-line tool),
the basic structure is to start with the GNUNET\_PROGRAM\_run
function. This function will parse command-line options, setup the
scheduler and then invoke the run function (with the remaining
non-option arguments) and a handle to the parsed configuration (and the
configuration file name that was used, which is typically not needed):
#include <gnunet/platform.h>
#include <gnunet/gnunet_util_lib.h>
static int ret;
static void
run (void *cls,
char *const *args,
const char *cfgfile,
const struct GNUNET_CONFIGURATION_Handle *cfg)
{
// main code here
ret = 0;
}
int
main (int argc, char *const *argv)
{
struct GNUNET_GETOPT_CommandLineOption options[] = {
GNUNET_GETOPT_OPTION_END
};
return (GNUNET_OK ==
GNUNET_PROGRAM_run (argc,
argv,
"binary-name",
gettext_noop ("binary description text"),
options, &run, NULL)) ? ret : 1;
}
Handling command-line options
Options can then be added easily by adding global variables and
expanding the options array. For example, the following would add a
string-option and a binary flag (defaulting to NULL and
GNUNET\_NO respectively):
static char *string_option;
static int a_flag;
// ...
struct GNUNET_GETOPT_CommandLineOption options[] = {
GNUNET_GETOPT_option_string ('s', "name", "SOMESTRING",
gettext_noop ("text describing the string_option NAME"),
&string_option},
GNUNET_GETOPT_option_flag ('f', "flag",
gettext_noop ("text describing the flag option"),
&a_flag),
GNUNET_GETOPT_OPTION_END
};
string_option = NULL;
a_flag = GNUNET_SYSERR;
// ...
Issues such as displaying some helpful text describing options using the
--help argument and error handling are taken care of when using this
approach. Other GNUNET\_GETOPT\_-functions can be used to obtain
integer value options, increment counters, etc. You can even write
custom option parsers for special circumstances not covered by the
available handlers. To check if an argument was specified by the user
you initialize the variable with a specific value (e.g. NULL for a
string and GNUNET_SYSERR for a integer) and check after parsing
happened if the values were modified.
Inside the run method, the program would perform the
application-specific logic, which typically involves initializing and
using some client library to interact with the service. The client
library is supposed to implement the IPC whereas the service provides
more persistent P2P functions.
Exercise: Add a few command-line options and print them inside of
run. What happens if the user gives invalid arguments?
Writing a Client Library
The first and most important step in writing a client library is to
decide on an API for the library. Typical API calls include connecting
to the service, performing application-specific requests and cleaning
up. Many examples for such service APIs can be found in the
gnunet/src/include/gnunet\_*\_service.h files.
Then, a client-service protocol needs to be designed. This typically
involves defining various message formats in a header that will be
included by both the service and the client library (but is otherwise
not shared and hence located within the service’s directory and not
installed by make install). Each message must start with a
struct GNUNET\_MessageHeader and must be shorter than 64k. By
convention, all fields in IPC (and P2P) messages must be in big-endian
format (and thus should be read using ntohl and similar functions
and written using htonl and similar functions). Unique message types
must be defined for each message struct in the gnunet\_protocols.h
header (or an extension-specific include file).
Connecting to the Service
Before a client library can implement the application-specific protocol with the service, a connection must be created:
struct GNUNET_MQ_MessageHandlers handlers[] = {
// ...
GNUNET_MQ_handler_end ()
};
struct GNUNET_MQ_Handle *mq;
mq = GNUNET_CLIENT_connect (cfg,
"service-name",
handlers,
&error_cb,
NULL);
As a result a GNUNET\_MQ\_Handle is returned which can to used
henceforth to transmit messages to the service. The complete MQ API can
be found in gnunet\_mq\_lib.h. The handlers array in the example
above is incomplete. Here is where you will define which messages you
expect to receive from the service, and which functions handle them. The
error\_cb is a function that is to be called whenever there are
errors communicating with the service.
Sending messages
In GNUnet, messages are always sent beginning with a
struct GNUNET\_MessageHeader in big endian format. This header
defines the size and the type of the message, the payload follows after
this header.
struct GNUNET_MessageHeader
{
uint16_t size GNUNET_PACKED;
uint16_t type GNUNET_PACKED;
};
Existing message types are defined in gnunet\_protocols.h. A common
way to create a message is with an envelope:
struct GNUNET_MQ_Envelope *env;
struct GNUNET_MessageHeader *msg;
env = GNUNET_MQ_msg_extra (msg, payload_size, GNUNET_MY_MESSAGE_TYPE);
GNUNET_memcpy (&msg[1],
&payload,
payload_size);
// Send message via message queue 'mq'
GNUNET_mq_send (mq, env);
Exercise: Define a message struct that includes a 32-bit unsigned
integer in addition to the standard GNUnet MessageHeader. Add a C struct
and define a fresh protocol number for your message. Protocol numbers in
gnunet-ext are defined in
gnunet-ext/src/include/gnunet_protocols_ext.h
Exercise: Find out how you can determine the number of messages in a message queue.
Exercise: Find out how you can determine when a message you have queued was actually transmitted.
Exercise: Define a helper function to transmit a 32-bit unsigned integer (as payload) to a service using some given client handle.
Receiving Replies from the Service
Clients can receive messages from the service using the handlers
specified in the handlers array we specified when connecting to the
service. Entries in the the array are usually created using one of two
macros, depending on whether the message is fixed size or variable size.
Variable size messages are managed using two callbacks, one to check
that the message is well-formed, the other to actually process the
message. Fixed size messages are fully checked by the MQ-logic, and thus
only need to provide the handler to process the message. Note that the
prefixes check\_ and handle\_ are mandatory.
static void
handle_fix (void *cls, const struct MyMessage *msg)
{
// process 'msg'
}
static int
check_var (void *cls, const struct MyVarMessage *msg)
{
// check 'msg' is well-formed
return GNUNET_OK;
}
static void
handle_var (void *cls, const struct MyVarMessage *msg)
{
// process 'msg'
}
struct GNUNET_MQ_MessageHandler handlers[] = {
GNUNET_MQ_hd_fixed_size (fix,
GNUNET_MESSAGE_TYPE_MY_FIX,
struct MyMessage,
NULL),
GNUNET_MQ_hd_fixed_size (var,
GNUNET_MESSAGE_TYPE_MY_VAR,
struct MyVarMessage,
NULL),
GNUNET_MQ_handler_end ()
};
Exercise: Expand your helper function to receive a response message (for
example, containing just the struct GNUnet MessageHeader without any
payload). Upon receiving the service’s response, you should call a
callback provided to your helper function’s API.
Exercise: Figure out where you can pass values to the closures
(cls).
Writing a user interface
Given a client library, all it takes to access a service now is to combine calls to the client library with parsing command-line options.
Exercise: Call your client API from your run() method in your client
application to send a request to the service. For example, send a 32-bit
integer value based on a number given at the command-line to the
service.
Writing a Service
Before you can test the client you’ve written so far, you’ll need to also implement the corresponding service.
Code Placement
New services are placed in their own subdirectory under gnunet/src.
This subdirectory should contain the API implementation file
SERVICE\_api.c, the description of the client-service protocol
SERVICE.h and P2P protocol SERVICE\_protocol.h, the
implementation of the service itself gnunet-service-SERVICE.h and
several files for tests, including test code and configuration files.
Starting a Service
The key API definition for creating a service is the
GNUNET\_SERVICE\_MAIN macro:
GNUNET_SERVICE_MAIN
("service-name",
GNUNET_SERVICE_OPTION_NONE,
&run,
&client_connect_cb,
&client_disconnect_cb,
NULL,
GNUNET_MQ_hd_fixed_size (...),
GNUNET_MQ_hd_var_size (...),
GNUNET_MQ_handler_end ());
In addition to the service name and flags, the macro takes three
functions, typically called run, client\_connect\_cb and
client\_disconnect\_cb as well as an array of message handlers that
will be called for incoming messages from clients.
A minimal version of the three central service functions would look like this:
static void
run (void *cls,
const struct GNUNET_CONFIGURATION_Handle *c,
struct GNUNET_SERVICE_Handle *service)
{
}
static void *
client_connect_cb (void *cls,
struct GNUNET_SERVICE_Client *c,
struct GNUNET_MQ_Handle *mq)
{
return c;
}
static void
client_disconnect_cb (void *cls,
struct GNUNET_SERVICE_Client *c,
void *internal_cls)
{
GNUNET_assert (c == internal_cls);
}
Exercise: Write a stub service that processes no messages at all in your
code. Create a default configuration for it, integrate it with the build
system and start the service from gnunet-service-arm using
gnunet-arm -i NAME.
Exercise: Figure out how to set the closure (cls) for handlers of a
service.
Exercise: Figure out how to send messages from the service back to the client.
Each handler function in the service must eventually (possibly in
some asynchronous continuation) call
GNUNET\_SERVICE\_client\_continue(). Only after this call additional
messages from the same client may be processed. This way, the service
can throttle processing messages from the same client.
Exercise: Change the service to “handle” the message from your client
(for now, by printing a message). What happens if you forget to call
GNUNET\_SERVICE\_client\_continue()?
Interacting directly with other Peers using the CORE Service
FIXME: This section still needs to be updated to the latest API!
One of the most important services in GNUnet is the CORE service
managing connections between peers and handling encryption between
peers.
One of the first things any service that extends the P2P protocol
typically does is connect to the CORE service using:
#include <gnunet/gnunet_core_service.h>
struct GNUNET_CORE_Handle *
GNUNET_CORE_connect (const struct GNUNET_CONFIGURATION_Handle *cfg,
void *cls,
GNUNET_CORE_StartupCallback init,
GNUNET_CORE_ConnectEventHandler connects,
GNUNET_CORE_DisconnectEventHandler disconnects,
const struct GNUNET_MQ_MessageHandler *handlers);
New P2P connections
Before any traffic with a different peer can be exchanged, the peer must
be known to the service. This is notified by the CORE connects
callback, which communicates the identity of the new peer to the
service:
void *
connects (void *cls,
const struct GNUNET_PeerIdentity *peer,
struct GNUNET_MQ_Handle *mq)
{
return mq;
}
Note that whatever you return from connects is given as the cls
argument to the message handlers for messages from the respective peer.
Exercise: Create a service that connects to the CORE. Then start
(and connect) two peers and print a message once your connect callback
is invoked.
Receiving P2P Messages
To receive messages from CORE, you pass the desired handlers to
the GNUNET\_CORE\_connect() function, just as we showed for
services.
It is your responsibility to process messages fast enough or to implement flow control. If an application does not process CORE messages fast enough, CORE will randomly drop messages to not keep a very long queue in memory.
Exercise: Start one peer with a new service that has a message handler and start a second peer that only has your “old” service without message handlers. Which “connect” handlers are invoked when the two peers are connected? Why?
Sending P2P Messages
You can transmit messages to other peers using the mq you were given
during the connect callback. Note that the mq automatically is
released upon disconnect and that you must not use it afterwards.
It is your responsibility to not over-fill the message queue, GNUnet will send the messages roughly in the order given as soon as possible.
Exercise: Write a service that upon connect sends messages as fast as possible to the other peer (the other peer should run a service that “processes” those messages). How fast is the transmission? Count using the STATISTICS service on both ends. Are messages lost? How can you transmit messages faster? What happens if you stop the peer that is receiving your messages?
End of P2P connections
If a message handler returns GNUNET\_SYSERR, the remote peer shuts
down or there is an unrecoverable network disconnection, CORE notifies
the service that the peer disconnected. After this notification no more
messages will be received from the peer and the service is no longer
allowed to send messages to the peer. The disconnect callback looks like
the following:
void
disconnects (void *cls,
const struct GNUNET_PeerIdentity * peer)
{
/* Remove peer's identity from known peers */
/* Make sure no messages are sent to peer from now on */
}
Exercise: Fix your service to handle peer disconnects.
Storing peer-specific data using the PEERSTORE service
GNUnet’s PEERSTORE service offers a persistorage for arbitrary peer-specific data. Other GNUnet services can use the PEERSTORE to store, retrieve and monitor data records. Each data record stored with PEERSTORE contains the following fields:
subsystem: Name of the subsystem responsible for the record.
peerid: Identity of the peer this record is related to.
key: a key string identifying the record.
value: binary record value.
expiry: record expiry date.
The first step is to start a connection to the PEERSTORE service:
#include "gnunet_peerstore_service.h"
peerstore_handle = GNUNET_PEERSTORE_connect (cfg);
The service handle peerstore_handle will be needed for all
subsequent PEERSTORE operations.
Storing records
To store a new record, use the following function:
struct GNUNET_PEERSTORE_StoreContext *
GNUNET_PEERSTORE_store (struct GNUNET_PEERSTORE_Handle *h,
const char *sub_system,
const struct GNUNET_PeerIdentity *peer,
const char *key,
const void *value,
size_t size,
struct GNUNET_TIME_Absolute expiry,
enum GNUNET_PEERSTORE_StoreOption options,
GNUNET_PEERSTORE_Continuation cont,
void *cont_cls);
The options parameter can either be
GNUNET_PEERSTORE_STOREOPTION_MULTIPLE which means that multiple
values can be stored under the same key combination (subsystem, peerid,
key), or GNUNET_PEERSTORE_STOREOPTION_REPLACE which means that
PEERSTORE will replace any existing values under the given key
combination (subsystem, peerid, key) with the new given value.
The continuation function cont will be called after the store
request is successfully sent to the PEERSTORE service. This does not
guarantee that the record is successfully stored, only that it was
received by the service.
The GNUNET_PEERSTORE_store function returns a handle to the store
operation. This handle can be used to cancel the store operation only
before the continuation function is called:
void
GNUNET_PEERSTORE_store_cancel (struct GNUNET_PEERSTORE_StoreContext
*sc);
Retrieving records
To retrieve stored records, use the following function:
struct GNUNET_PEERSTORE_IterateContext *
GNUNET_PEERSTORE_iterate (struct GNUNET_PEERSTORE_Handle *h,
const char *sub_system,
const struct GNUNET_PeerIdentity *peer,
const char *key,
GNUNET_PEERSTORE_Processor callback,
void *callback_cls);
The values of peer and key can be NULL. This allows the
iteration over values stored under any of the following key
combinations:
(subsystem)
(subsystem, peerid)
(subsystem, key)
(subsystem, peerid, key)
The callback function will be called once with each retrieved record
and once more with a NULL record to signal the end of results.
The GNUNET_PEERSTORE_iterate function returns a handle to the
iterate operation. This handle can be used to cancel the iterate
operation only before the callback function is called with a NULL
record.
Monitoring records
PEERSTORE offers the functionality of monitoring for new records stored under a specific key combination (subsystem, peerid, key). To start the monitoring, use the following function:
struct GNUNET_PEERSTORE_WatchContext *
GNUNET_PEERSTORE_watch (struct GNUNET_PEERSTORE_Handle *h,
const char *sub_system,
const struct GNUNET_PeerIdentity *peer,
const char *key,
GNUNET_PEERSTORE_Processor callback,
void *callback_cls);
Whenever a new record is stored under the given key combination, the
callback function will be called with this new record. This will
continue until the connection to the PEERSTORE service is broken or the
watch operation is canceled:
void
GNUNET_PEERSTORE_watch_cancel (struct GNUNET_PEERSTORE_WatchContext
*wc);
Disconnecting from PEERSTORE
When the connection to the PEERSTORE service is no longer needed, disconnect using the following function:
void
GNUNET_PEERSTORE_disconnect (struct GNUNET_PEERSTORE_Handle *h,
int sync_first);
If the sync_first flag is set to GNUNET_YES, the API will delay
the disconnection until all store requests are received by the PEERSTORE
service. Otherwise, it will disconnect immediately.
Using the DHT
The DHT allows to store data so other peers in the P2P network can access it and retrieve data stored by any peers in the network. This section will explain how to use the DHT. Of course, the first thing to do is to connect to the DHT service:
dht_handle = GNUNET_DHT_connect (cfg, parallel_requests);
The second parameter indicates how many requests in parallel to expect. It is not a hard limit, but a good approximation will make the DHT more efficient.
Storing data in the DHT
Since the DHT is a dynamic environment (peers join and leave frequently) the data that we put in the DHT does not stay there indefinitely. It is important to “refresh” the data periodically by simply storing it again, in order to make sure other peers can access it.
The put API call offers a callback to signal that the PUT request has been sent. This does not guarantee that the data is accessible to others peers, or even that is has been stored, only that the service has requested to a neighboring peer the retransmission of the PUT request towards its final destination. Currently there is no feedback about whether or not the data has been successfully stored or where it has been stored. In order to improve the availablilty of the data and to compensate for possible errors, peers leaving and other unfavorable events, just make several PUT requests!
message_sent_cont (void *cls,
const struct GNUNET_SCHEDULER_TaskContext *tc)
{
// Request has left local node
}
struct GNUNET_DHT_PutHandle *
GNUNET_DHT_put (struct GNUNET_DHT_Handle *handle,
const struct GNUNET_HashCode *key,
uint32_t desired_replication_level,
enum GNUNET_DHT_RouteOption options,
enum GNUNET_BLOCK_Type type,
size_t size,
const void *data,
struct GNUNET_TIME_Absolute exp,
struct GNUNET_TIME_Relative timeout,
GNUNET_DHT_PutContinuation cont, void *cont_cls)
Exercise: Store a value in the DHT periodically to make sure it is
available over time. You might consider using the function
GNUNET\_SCHEDULER\_add\_delayed and call GNUNET\_DHT\_put from
inside a helper function.
Obtaining data from the DHT
As we saw in the previous example, the DHT works in an asynchronous
mode. Each request to the DHT is executed “in the background” and the
API calls return immediately. In order to receive results from the DHT,
the API provides a callback. Once started, the request runs in the
service, the service will try to get as many results as possible
(filtering out duplicates) until the timeout expires or we explicitly
stop the request. It is possible to give a “forever” timeout with
GNUNET\_TIME\_UNIT\_FOREVER\_REL.
If we give a route option GNUNET\_DHT\_RO\_RECORD\_ROUTE the
callback will get a list of all the peers the data has travelled, both
on the PUT path and on the GET path.
static void
get_result_iterator (void *cls, struct GNUNET_TIME_Absolute expiration,
const struct GNUNET_HashCode *key,
const struct GNUNET_PeerIdentity *get_path,
unsigned int get_path_length,
const struct GNUNET_PeerIdentity *put_path,
unsigned int put_path_length,
enum GNUNET_BLOCK_Type type, size_t size,
const void *data)
{
// Optionally:
GNUNET_DHT_get_stop (get_handle);
}
get_handle =
GNUNET_DHT_get_start (dht_handle,
block_type,
&key,
replication,
GNUNET_DHT_RO_NONE,
NULL,
0,
&get_result_iterator,
cls)
Exercise: Store a value in the DHT and after a while retrieve it. Show
the IDs of all the peers the requests have gone through. In order to
convert a peer ID to a string, use the function GNUNET\_i2s. Pay
attention to the route option parameters in both calls!
Implementing a block plugin
In order to store data in the DHT, it is necessary to provide a block plugin. The DHT uses the block plugin to ensure that only well-formed requests and replies are transmitted over the network.
The block plugin should be put in a file plugin\_block\_SERVICE.c in
the service’s respective directory. The mandatory functions that need to
be implemented for a block plugin are described in the following
sections.
Validating requests and replies
The evaluate function should validate a reply or a request. It returns a
GNUNET\_BLOCK\_EvaluationResult, which is an enumeration. All
possible answers are in gnunet\_block\_lib.h. The function will be
called with a reply\_block argument of NULL for requests. Note
that depending on how evaluate is called, only some of the possible
return values are valid. The specific meaning of the xquery argument
is application-specific. Applications that do not use an extended query
should check that the xquery\_size is zero. The block group is
typically used to filter duplicate replies.
static enum GNUNET_BLOCK_EvaluationResult
block_plugin_SERVICE_evaluate (void *cls,
enum GNUNET_BLOCK_Type type,
struct GNUNET_BlockGroup *bg,
const GNUNET_HashCode *query,
const void *xquery,
size_t xquery_size,
const void *reply_block,
size_t reply_block_size)
{
// Verify type, block and bg
}
Note that it is mandatory to detect duplicate replies in this function
and return the respective status code. Duplicate detection is typically
done using the Bloom filter block group provided by
libgnunetblockgroup.so. Failure to do so may cause replies to circle
in the network.
Deriving a key from a reply
The DHT can operate more efficiently if it is possible to derive a key
from the value of the corresponding block. The get\_key function is
used to obtain the key of a block — for example, by means of hashing.
If deriving the key is not possible, the function should simply return
GNUNET\_SYSERR (the DHT will still work just fine with such blocks).
static int
block_plugin_SERVICE_get_key (void *cls, enum GNUNET_BLOCK_Type type,
const void *block, size_t block_size,
struct GNUNET_HashCode *key)
{
// Store the key in the key argument, return GNUNET_OK on success.
}
Initialization of the plugin
The plugin is realized as a shared C library. The library must export an initialization function which should initialize the plugin. The initialization function specifies what block types the plugin cares about and returns a struct with the functions that are to be used for validation and obtaining keys (the ones just defined above).
void *
libgnunet_plugin_block_SERVICE_init (void *cls)
{
static enum GNUNET_BLOCK_Type types[] =
{
GNUNET_BLOCK_TYPE_SERVICE_BLOCKYPE,
GNUNET_BLOCK_TYPE_ANY
};
struct GNUNET_BLOCK_PluginFunctions *api;
api = GNUNET_new (struct GNUNET_BLOCK_PluginFunctions);
api->evaluate = &block_plugin_SERICE_evaluate;
api->get_key = &block_plugin_SERVICE_get_key;
api->types = types;
return api;
}
Shutdown of the plugin
Following GNUnet’s general plugin API concept, the plugin must export a second function for cleaning up. It usually does very little.
void *
libgnunet_plugin_block_SERVICE_done (void *cls)
{
struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
GNUNET_free (api);
return NULL;
}
Integration of the plugin with the build system
In order to compile the plugin, the Makefile.am file for the service
SERVICE should contain a rule similar to this:
plugindir = $(libdir)/gnunet
plugin_LTLIBRARIES = \
libgnunet_plugin_block_ext.la
libgnunet_plugin_block_ext_la_SOURCES = \
plugin_block_ext.c
libgnunet_plugin_block_ext_la_LIBADD = \
$(prefix)/lib/libgnunethello.la \
$(prefix)/lib/libgnunetblock.la \
$(prefix)/lib/libgnunetutil.la
libgnunet_plugin_block_ext_la_LDFLAGS = \
$(GN_PLUGIN_LDFLAGS)
libgnunet_plugin_block_ext_la_DEPENDENCIES = \
$(prefix)/lib/libgnunetblock.la
Exercise: Write a block plugin that accepts all queries and all replies but prints information about queries and replies when the respective validation hooks are called.
Monitoring the DHT
It is possible to monitor the functioning of the local DHT service. When monitoring the DHT, the service will alert the monitoring program of any events, both started locally or received for routing from another peer. The are three different types of events possible: a GET request, a PUT request or a response (a reply to a GET).
Since the different events have different associated data, the API gets 3 different callbacks (one for each message type) and optional type and key parameters, to allow for filtering of messages. When an event happens, the appropriate callback is called with all the information about the event.
static void
get_callback (void *cls,
enum GNUNET_DHT_RouteOption options,
enum GNUNET_BLOCK_Type type,
uint32_t hop_count,
uint32_t desired_replication_level,
unsigned int path_length,
const struct GNUNET_PeerIdentity *path,
const struct GNUNET_HashCode * key)
{
}
static void
get_resp_callback (void *cls,
enum GNUNET_BLOCK_Type type,
const struct GNUNET_PeerIdentity *get_path,
unsigned int get_path_length,
const struct GNUNET_PeerIdentity *put_path,
unsigned int put_path_length,
struct GNUNET_TIME_Absolute exp,
const struct GNUNET_HashCode * key,
const void *data,
size_t size)
{
}
static void
put_callback (void *cls,
enum GNUNET_DHT_RouteOption options,
enum GNUNET_BLOCK_Type type,
uint32_t hop_count,
uint32_t desired_replication_level,
unsigned int path_length,
const struct GNUNET_PeerIdentity *path,
struct GNUNET_TIME_Absolute exp,
const struct GNUNET_HashCode * key,
const void *data,
size_t size)
{
}
monitor_handle = GNUNET_DHT_monitor_start (dht_handle,
block_type,
key,
&get_callback,
&get_resp_callback,
&put_callback,
cls);
Debugging with gnunet-arm
Even if services are managed by gnunet-arm, you can start them with
gdb or valgrind. For example, you could add the following lines
to your configuration file to start the DHT service in a gdb session
in a fresh xterm:
[dht]
PREFIX=xterm -e gdb --args
Alternatively, you can stop a service that was started via ARM and run it manually:
$ gnunet-arm -k dht
$ gdb --args gnunet-service-dht -L DEBUG
$ valgrind gnunet-service-dht -L DEBUG
Assuming other services are well-written, they will automatically re-integrate the restarted service with the peer.
GNUnet provides a powerful logging mechanism providing log levels
ERROR, WARNING, INFO and DEBUG. The current log level is
configured using the $GNUNET_FORCE_LOG environmental variable. The
DEBUG level is only available if --enable-logging=verbose was
used when running configure. More details about logging can be found
under https://docs.gnunet.org/#Logging.
You should also probably enable the creation of core files, by setting
ulimit, and echo’ing 1 into
/proc/sys/kernel/core\_uses\_pid. Then you can investigate the core
dumps with gdb, which is often the fastest method to find simple
errors.
Exercise: Add a memory leak to your service and obtain a trace pointing
to the leak using valgrind while running the service from
gnunet-service-arm.
GNU Free Documentation License
license, GNU Free Documentation License Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
http://fsf.org/
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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
RELICENSING
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.