Address validation protocol

This section documents how the GNUnet transport service validates connections with other peers. It is a high-level description of the protocol necessary to understand the details of the implementation. It should be noted that when we talk about PING and PONG messages in this section, we refer to transport-level PING and PONG messages, which are different from core-level PING and PONG messages (both in implementation and function).

The goal of transport-level address validation is to minimize the chances of a successful man-in-the-middle attack against GNUnet peers on the transport level. Such an attack would not allow the adversary to decrypt the P2P transmissions, but a successful attacker could at least measure traffic volumes and latencies (raising the adversaries capabilities by those of a global passive adversary in the worst case). The scenarios we are concerned about is an attacker, Mallory, giving a HELLO to Alice that claims to be for Bob, but contains Mallory’s IP address instead of Bobs (for some transport). Mallory would then forward the traffic to Bob (by initiating a connection to Bob and claiming to be Alice). As a further complication, the scheme has to work even if say Alice is behind a NAT without traversal support and hence has no address of her own (and thus Alice must always initiate the connection to Bob).

An additional constraint is that HELLO messages do not contain a cryptographic signature since other peers must be able to edit (i.e. remove) addresses from the HELLO at any time (this was not true in GNUnet 0.8.x). A basic assumption is that each peer knows the set of possible network addresses that it might be reachable under (so for example, the external IP address of the NAT plus the LAN address(es) with the respective ports).

The solution is the following. If Alice wants to validate that a given address for Bob is valid (i.e. is actually established directly with the intended target), she sends a PING message over that connection to Bob. Note that in this case, Alice initiated the connection so only Alice knows which address was used for sure (Alice may be behind NAT, so whatever address Bob sees may not be an address Alice knows she has). Bob checks that the address given in the PING is actually one of Bob’s addresses (ie: does not belong to Mallory), and if it is, sends back a PONG (with a signature that says that Bob owns/uses the address from the PING). Alice checks the signature and is happy if it is valid and the address in the PONG is the address Alice used. This is similar to the 0.8.x protocol where the HELLO contained a signature from Bob for each address used by Bob. Here, the purpose code for the signature is GNUNET_SIGNATURE_PURPOSE_TRANSPORT_PONG_OWN. After this, Alice will remember Bob’s address and consider the address valid for a while (12h in the current implementation). Note that after this exchange, Alice only considers Bob’s address to be valid, the connection itself is not considered ‘established’. In particular, Alice may have many addresses for Bob that Alice considers valid.

The PONG message is protected with a nonce/challenge against replay attacks (replay) and uses an expiration time for the signature (but those are almost implementation details).

NAT library .. _NAT-library:

NAT library

The goal of the GNUnet NAT library is to provide a general-purpose API for NAT traversal without third-party support. So protocols that involve contacting a third peer to help establish a connection between two peers are outside of the scope of this API. That does not mean that GNUnet doesn’t support involving a third peer (we can do this with the distance-vector transport or using application-level protocols), it just means that the NAT API is not concerned with this possibility. The API is written so that it will work for IPv6-NAT in the future as well as current IPv4-NAT. Furthermore, the NAT API is always used, even for peers that are not behind NAT — in that case, the mapping provided is simply the identity.

NAT traversal is initiated by calling GNUNET_NAT_register. Given a set of addresses that the peer has locally bound to (TCP or UDP), the NAT library will return (via callback) a (possibly longer) list of addresses the peer might be reachable under. Internally, depending on the configuration, the NAT library will try to punch a hole (using UPnP) or just "know" that the NAT was manually punched and generate the respective external IP address (the one that should be globally visible) based on the given information.

The NAT library also supports ICMP-based NAT traversal. Here, the other peer can request connection-reversal by this peer (in this special case, the peer is even allowed to configure a port number of zero). If the NAT library detects a connection-reversal request, it returns the respective target address to the client as well. It should be noted that connection-reversal is currently only intended for TCP, so other plugins must pass NULL for the reversal callback. Naturally, the NAT library also supports requesting connection reversal from a remote peer (GNUNET_NAT_run_client).

Once initialized, the NAT handle can be used to test if a given address is possibly a valid address for this peer (GNUNET_NAT_test_address). This is used for validating our addresses when generating PONGs.

Finally, the NAT library contains an API to test if our NAT configuration is correct. Using GNUNET_NAT_test_start before binding to the respective port, the NAT library can be used to test if the configuration works. The test function act as a local client, initialize the NAT traversal and then contact a gnunet-nat-server (running by default on and ask for a connection to be established. This way, it is easy to test if the current NAT configuration is valid.

Distance-Vector plugin

The Distance Vector (DV) transport is a transport mechanism that allows peers to act as relays for each other, thereby connecting peers that would otherwise be unable to connect. This gives a larger connection set to applications that may work better with more peers to choose from (for example, File Sharing and/or DHT).

The Distance Vector transport essentially has two functions. The first is "gossiping" connection information about more distant peers to directly connected peers. The second is taking messages intended for non-directly connected peers and encapsulating them in a DV wrapper that contains the required information for routing the message through forwarding peers. Via gossiping, optimal routes through the known DV neighborhood are discovered and utilized and the message encapsulation provides some benefits in addition to simply getting the message from the correct source to the proper destination.

The gossiping function of DV provides an up to date routing table of peers that are available up to some number of hops. We call this a fisheye view of the network (like a fish, nearby objects are known while more distant ones unknown). Gossip messages are sent only to directly connected peers, but they are sent about other knowns peers within the "fisheye distance". Whenever two peers connect, they immediately gossip to each other about their appropriate other neighbors. They also gossip about the newly connected peer to previously connected neighbors. In order to keep the routing tables up to date, disconnect notifications are propagated as gossip as well (because disconnects may not be sent/received, timeouts are also used remove stagnant routing table entries).

Routing of messages via DV is straightforward. When the DV transport is notified of a message destined for a non-direct neighbor, the appropriate forwarding peer is selected, and the base message is encapsulated in a DV message which contains information about the initial peer and the intended recipient. At each forwarding hop, the initial peer is validated (the forwarding peer ensures that it has the initial peer in its neighborhood, otherwise the message is dropped). Next the base message is re-encapsulated in a new DV message for the next hop in the forwarding chain (or delivered to the current peer, if it has arrived at the destination).

Assume a three peer network with peers Alice, Bob and Carol. Assume that

Alice <-> Bob and Bob <-> Carol

are direct (e.g. over TCP or UDP transports) connections, but that Alice cannot directly connect to Carol. This may be the case due to NAT or firewall restrictions, or perhaps based on one of the peers respective configurations. If the Distance Vector transport is enabled on all three peers, it will automatically discover (from the gossip protocol) that Alice and Carol can connect via Bob and provide a "virtual" Alice <-> Carol connection. Routing between Alice and Carol happens as follows; Alice creates a message destined for Carol and notifies the DV transport about it. The DV transport at Alice looks up Carol in the routing table and finds that the message must be sent through Bob for Carol. The message is encapsulated setting Alice as the initiator and Carol as the destination and sent to Bob. Bob receives the messages, verifies that both Alice and Carol are known to Bob, and re-wraps the message in a new DV message for Carol. The DV transport at Carol receives this message, unwraps the original message, and delivers it to Carol as though it came directly from Alice.

SMTP plugin .. _SMTP-plugin:

SMTP plugin



This section describes the new SMTP transport plugin for GNUnet as it exists in the 0.7.x and 0.8.x branch. SMTP support is currently not available in GNUnet 0.9.x. This page also describes the transport layer abstraction (as it existed in 0.7.x and 0.8.x) in more detail and gives some benchmarking results. The performance results presented are quite old and maybe outdated at this point. For the readers in the year 2019, you will notice by the mention of version 0.7, 0.8, and 0.9 that this section has to be taken with your usual grain of salt and be updated eventually.

  • Why use SMTP for a peer-to-peer transport?

  • SMTPHow does it work?

  • How do I configure my peer?

  • How do I test if it works?

  • How fast is it?

  • Is there any additional documentation?

Why use SMTP for a peer-to-peer transport?

There are many reasons why one would not want to use SMTP:

  • SMTP is using more bandwidth than TCP, UDP or HTTP

  • SMTP has a much higher latency.

  • SMTP requires significantly more computation (encoding and decoding time) for the peers.

  • SMTP is significantly more complicated to configure.

  • SMTP may be abused by tricking GNUnet into sending mail to non-participating third parties.

So why would anybody want to use SMTP?

  • SMTP can be used to contact peers behind NAT boxes (in virtual private networks).

  • SMTP can be used to circumvent policies that limit or prohibit peer-to-peer traffic by masking as "legitimate" traffic.

  • SMTP uses E-mail addresses which are independent of a specific IP, which can be useful to address peers that use dynamic IP addresses.

  • SMTP can be used to initiate a connection (e.g. initial address exchange) and peers can then negotiate the use of a more efficient protocol (e.g. TCP) for the actual communication.

In summary, SMTP can for example be used to send a message to a peer behind a NAT box that has a dynamic IP to tell the peer to establish a TCP connection to a peer outside of the private network. Even an extraordinary overhead for this first message would be irrelevant in this type of situation.

How does it work?

When a GNUnet peer needs to send a message to another GNUnet peer that has advertised (only) an SMTP transport address, GNUnet base64-encodes the message and sends it in an E-mail to the advertised address. The advertisement contains a filter which is placed in the E-mail header, such that the receiving host can filter the tagged E-mails and forward it to the GNUnet peer process. The filter can be specified individually by each peer and be changed over time. This makes it impossible to censor GNUnet E-mail messages by searching for a generic filter.

How do I configure my peer?

First, you need to configure procmail to filter your inbound E-mail for GNUnet traffic. The GNUnet messages must be delivered into a pipe, for example /tmp/gnunet.smtp. You also need to define a filter that is used by procmail to detect GNUnet messages. You are free to choose whichever filter you like, but you should make sure that it does not occur in your other E-mail. In our example, we will use X-mailer: GNUnet. The ~/.procmailrc configuration file then looks like this:

* ^X-mailer: GNUnet
# where do you want your other e-mail delivered to
# (default: /var/spool/mail/)
:0: /var/spool/mail/

After adding this file, first make sure that your regular E-mail still works (e.g. by sending an E-mail to yourself). Then edit the GNUnet configuration. In the section SMTP you need to specify your E-mail address under EMAIL, your mail server (for outgoing mail) under SERVER, the filter (X-mailer: GNUnet in the example) under FILTER and the name of the pipe under PIPE. The completed section could then look like this:

"X-mailer: GNUnet" PIPE = /tmp/gnunet.smtp


set highlighting for this code block properly.

Finally, you need to add smtp to the list of TRANSPORTS in the GNUNETD section. GNUnet peers will use the E-mail address that you specified to contact your peer until the advertisement times out. Thus, if you are not sure if everything works properly or if you are not planning to be online for a long time, you may want to configure this timeout to be short, e.g. just one hour. For this, set HELLOEXPIRES to 1 in the GNUNETD section.

This should be it, but you may probably want to test it first.

How do I test if it works?

Any transport can be subjected to some rudimentary tests using the gnunet-transport-check tool. The tool sends a message to the local node via the transport and checks that a valid message is received. While this test does not involve other peers and can not check if firewalls or other network obstacles prohibit proper operation, this is a great testcase for the SMTP transport since it tests pretty much nearly all of the functionality.

gnunet-transport-check should only be used without running gnunetd at the same time. By default, gnunet-transport-check tests all transports that are specified in the configuration file. But you can specifically test SMTP by giving the option --transport=smtp.

Note that this test always checks if a transport can receive and send. While you can configure most transports to only receive or only send messages, this test will only work if you have configured the transport to send and receive messages.

How fast is it?

We have measured the performance of the UDP, TCP and SMTP transport layer directly and when used from an application using the GNUnet core. Measuring just the transport layer gives the better view of the actual overhead of the protocol, whereas evaluating the transport from the application puts the overhead into perspective from a practical point of view.

The loopback measurements of the SMTP transport were performed on three different machines spanning a range of modern SMTP configurations. We used a PIII-800 running RedHat 7.3 with the Purdue Computer Science configuration which includes filters for spam. We also used a Xenon 2 GHZ with a vanilla RedHat 8.0 sendmail configuration. Furthermore, we used qmail on a PIII-1000 running Sorcerer GNU Linux (SGL). The numbers for UDP and TCP are provided using the SGL configuration. The qmail benchmark uses qmail’s internal filtering whereas the sendmail benchmarks relies on procmail to filter and deliver the mail. We used the transport layer to send a message of b bytes (excluding transport protocol headers) directly to the local machine. This way, network latency and packet loss on the wire have no impact on the timings. n messages were sent sequentially over the transport layer, sending message i+1 after the i-th message was received. All messages were sent over the same connection and the time to establish the connection was not taken into account since this overhead is minuscule in practice — as long as a connection is used for a significant number of messages.




SMTP (Purdue s endmail)

SMTP (RH 8.0)

SMTP (SGL qmail)

11 bytes

31 ms

55 ms

781 s

77 s

24 s

407 bytes

37 ms

62 ms

789 s

78 s

25 s

1,221 bytes

46 ms

73 ms

804 s

78 s

25 s

The benchmarks show that UDP and TCP are, as expected, both significantly faster compared with any of the SMTP services. Among the SMTP implementations, there can be significant differences depending on the SMTP configuration. Filtering with an external tool like procmail that needs to re-parse its configuration for each mail can be very expensive. Applying spam filters can also significantly impact the performance of the underlying SMTP implementation. The microbenchmark shows that SMTP can be a viable solution for initiating peer-to-peer sessions: a couple of seconds to connect to a peer are probably not even going to be noticed by users. The next benchmark measures the possible throughput for a transport. Throughput can be measured by sending multiple messages in parallel and measuring packet loss. Note that not only UDP but also the TCP transport can actually loose messages since the TCP implementation drops messages if the write to the socket would block. While the SMTP protocol never drops messages itself, it is often so slow that only a fraction of the messages can be sent and received in the given time-bounds. For this benchmark we report the message loss after allowing t time for sending m messages. If messages were not sent (or received) after an overall timeout of t, they were considered lost. The benchmark was performed using two Xeon 2 GHZ machines running RedHat 8.0 with sendmail. The machines were connected with a direct 100 MBit Ethernet connection. Figures udp1200, tcp1200 and smtp-MTUs show that the throughput for messages of size 1,200 octets is 2,343 kbps, 3,310 kbps and 6 kbps for UDP, TCP and SMTP respectively. The high per-message overhead of SMTP can be improved by increasing the MTU, for example, an MTU of 12,000 octets improves the throughput to 13 kbps as figure smtp-MTUs shows. Our research paper [Transport2014] has some more details on the benchmarking results.

Bluetooth plugin .. _Bluetooth-plugin:

Bluetooth plugin

This page describes the new Bluetooth transport plugin for GNUnet. The plugin is still in the testing stage so don’t expect it to work perfectly. If you have any questions or problems just post them here or ask on the IRC channel.

  • What do I need to use the Bluetooth plugin transport?

  • BluetoothHow does it work?

  • What possible errors should I be aware of?

  • How do I configure my peer?

  • How can I test it?

What do I need to use the Bluetooth plugin transport?

If you are a GNU/Linux user and you want to use the Bluetooth transport plugin you should install the BlueZ development libraries (if they aren’t already installed). For instructions about how to install the libraries you should check out the BlueZ site ( If you don’t know if you have the necessary libraries, don’t worry, just run the GNUnet configure script and you will be able to see a notification at the end which will warn you if you don’t have the necessary libraries.


Change to unique title?

How does it work2?

The Bluetooth transport plugin uses virtually the same code as the WLAN plugin and only the helper binary is different. The helper takes a single argument, which represents the interface name and is specified in the configuration file. Here are the basic steps that are followed by the helper binary used on GNU/Linux:

  • it verifies if the name corresponds to a Bluetooth interface name

  • it verifies if the interface is up (if it is not, it tries to bring it up)

  • it tries to enable the page and inquiry scan in order to make the device discoverable and to accept incoming connection requests The above operations require root access so you should start the transport plugin with root privileges.

  • it finds an available port number and registers a SDP service which will be used to find out on which port number is the server listening on and switch the socket in listening mode

  • it sends a HELLO message with its address

  • finally it forwards traffic from the reading sockets to the STDOUT and from the STDIN to the writing socket

Once in a while the device will make an inquiry scan to discover the nearby devices and it will send them randomly HELLO messages for peer discovery.

What possible errors should I be aware of?

This section is dedicated for GNU/Linux users

Well there are many ways in which things could go wrong but I will try to present some tools that you could use to debug and some scenarios.

  • bluetoothd -n -d : use this command to enable logging in the foreground and to print the logging messages

  • hciconfig: can be used to configure the Bluetooth devices. If you run it without any arguments it will print information about the state of the interfaces. So if you receive an error that the device couldn’t be brought up you should try to bring it manually and to see if it works (use hciconfig -a hciX up). If you can’t and the Bluetooth address has the form 00:00:00:00:00:00 it means that there is something wrong with the D-Bus daemon or with the Bluetooth daemon. Use bluetoothd tool to see the logs

  • sdptool can be used to control and interrogate SDP servers. If you encounter problems regarding the SDP server (like the SDP server is down) you should check out if the D-Bus daemon is running correctly and to see if the Bluetooth daemon started correctly(use bluetoothd tool). Also, sometimes the SDP service could work but somehow the device couldn’t register its service. Use sdptool browse [dev-address] to see if the service is registered. There should be a service with the name of the interface and GNUnet as provider.

  • hcitool : another useful tool which can be used to configure the device and to send some particular commands to it.

  • hcidump : could be used for low level debugging


Fix name/referencing now that we’re using Sphinx.

How do I configure my peer2?

On GNU/Linux, you just have to be sure that the interface name corresponds to the one that you want to use. Use the hciconfig tool to check that. By default it is set to hci0 but you can change it.

A basic configuration looks like this:

# Name of the interface (typically hciX)
# Real hardware, no testing

In order to use the Bluetooth transport plugin when the transport service is started, you must add the plugin name to the default transport service plugins list. For example:

[transport] ...  PLUGINS = dns bluetooth ...

If you want to use only the Bluetooth plugin set PLUGINS = bluetooth

On Windows, you cannot specify which device to use. The only thing that you should do is to add bluetooth on the plugins list of the transport service.

How can I test it?

If you have two Bluetooth devices on the same machine and you are using GNU/Linux you must:

  • create two different file configuration (one which will use the first interface (hci0) and the other which will use the second interface (hci1)). Let’s name them peer1.conf and peer2.conf.

  • run gnunet-peerinfo -c peerX.conf -s in order to generate the peers private keys. The X must be replace with 1 or 2.

  • run gnunet-arm -c peerX.conf -s -i=transport in order to start the transport service. (Make sure that you have "bluetooth" on the transport plugins list if the Bluetooth transport service doesn’t start.)

  • run gnunet-peerinfo -c peer1.conf -s to get the first peer’s ID. If you already know your peer ID (you saved it from the first command), this can be skipped.

  • run gnunet-transport -c peer2.conf -p=PEER1_ID -s to start sending data for benchmarking to the other peer.

This scenario will try to connect the second peer to the first one and then start sending data for benchmarking.

If you have two different machines and your configuration files are good you can use the same scenario presented on the beginning of this section.

Another way to test the plugin functionality is to create your own application which will use the GNUnet framework with the Bluetooth transport service.

The implementation of the Bluetooth transport plugin

This page describes the implementation of the Bluetooth transport plugin.

First I want to remind you that the Bluetooth transport plugin uses virtually the same code as the WLAN plugin and only the helper binary is different. Also the scope of the helper binary from the Bluetooth transport plugin is the same as the one used for the WLAN transport plugin: it accesses the interface and then it forwards traffic in both directions between the Bluetooth interface and stdin/stdout of the process involved.

The Bluetooth plugin transport could be used both on GNU/Linux and Windows platforms.

  • Linux functionality

  • Pending Features

Linux functionality

In order to implement the plugin functionality on GNU/Linux I used the BlueZ stack. For the communication with the other devices I used the RFCOMM protocol. Also I used the HCI protocol to gain some control over the device. The helper binary takes a single argument (the name of the Bluetooth interface) and is separated in two stages:


‘THE INITIALIZATION’ should be in bigger letters or stand out, not starting a new section?


  • first, it checks if we have root privileges (Remember that we need to have root privileges in order to be able to bring the interface up if it is down or to change its state.).

  • second, it verifies if the interface with the given name exists.

    If the interface with that name exists and it is a Bluetooth interface:

  • it creates a RFCOMM socket which will be used for listening and call the open_device method

    On the open_device method:

    • creates a HCI socket used to send control events to the device

    • searches for the device ID using the interface name

    • saves the device MAC address

    • checks if the interface is down and tries to bring it UP

    • checks if the interface is in discoverable mode and tries to make it discoverable

    • closes the HCI socket and binds the RFCOMM one

    • switches the RFCOMM socket in listening mode

    • registers the SDP service (the service will be used by the other devices to get the port on which this device is listening on)

  • drops the root privileges

    If the interface is not a Bluetooth interface the helper exits with a suitable error


The helper binary uses a list where it saves all the connected neighbour devices (neighbours.devices) and two buffers (write_pout and write_std). The first message which is send is a control message with the device’s MAC address in order to announce the peer presence to the neighbours. Here are a short description of what happens in the main loop:

  • Every time when it receives something from the STDIN it processes the data and saves the message in the first buffer (write_pout). When it has something in the buffer, it gets the destination address from the buffer, searches the destination address in the list (if there is no connection with that device, it creates a new one and saves it to the list) and sends the message.

  • Every time when it receives something on the listening socket it accepts the connection and saves the socket on a list with the reading sockets.

  • Every time when it receives something from a reading socket it parses the message, verifies the CRC and saves it in the write_std buffer in order to be sent later to the STDOUT.

So in the main loop we use the select function to wait until one of the file descriptor saved in one of the two file descriptors sets used is ready to use. The first set (rfds) represents the reading set and it could contain the list with the reading sockets, the STDIN file descriptor or the listening socket. The second set (wfds) is the writing set and it could contain the sending socket or the STDOUT file descriptor. After the select function returns, we check which file descriptor is ready to use and we do what is supposed to do on that kind of event. For example: if it is the listening socket then we accept a new connection and save the socket in the reading list; if it is the STDOUT file descriptor, then we write to STDOUT the message from the write_std buffer.

To find out on which port a device is listening on we connect to the local SDP server and search the registered service for that device.

You should be aware of the fact that if the device fails to connect to another one when trying to send a message it will attempt one more time. If it fails again, then it skips the message. Also you should know that the transport Bluetooth plugin has support forbroadcast messages.

Details about the broadcast implementation

First I want to point out that the broadcast functionality for the CONTROL messages is not implemented in a conventional way. Since the inquiry scan time is too big and it will take some time to send a message to all the discoverable devices I decided to tackle the problem in a different way. Here is how I did it:

  • If it is the first time when I have to broadcast a message I make an inquiry scan and save all the devices’ addresses to a vector.

  • After the inquiry scan ends I take the first address from the list and I try to connect to it. If it fails, I try to connect to the next one. If it succeeds, I save the socket to a list and send the message to the device.

  • When I have to broadcast another message, first I search on the list for a new device which I’m not connected to. If there is no new device on the list I go to the beginning of the list and send the message to the old devices. After 5 cycles I make a new inquiry scan to check out if there are new discoverable devices and save them to the list. If there are no new discoverable devices I reset the cycling counter and go again through the old list and send messages to the devices saved in it.


  • every time when I have a broadcast message I look up on the list for a new device and send the message to it

  • if I reached the end of the list for 5 times and I’m connected to all the devices from the list I make a new inquiry scan. The number of the list’s cycles after an inquiry scan could be increased by redefining the MAX_LOOPS variable

  • when there are no new devices I send messages to the old ones.

Doing so, the broadcast control messages will reach the devices but with delay.

NOTICE: When I have to send a message to a certain device first I check on the broadcast list to see if we are connected to that device. If not we try to connect to it and in case of success we save the address and the socket on the list. If we are already connected to that device we simply use the socket.

Pending features

  • Implement a testcase for the helper : The testcase consists of a program which emulates the plugin and uses the helper. It will simulate connections, disconnections and data transfers.

If you have a new idea about a feature of the plugin or suggestions about how I could improve the implementation you are welcome to comment or to contact me.

WLAN plugin

This section documents how the wlan transport plugin works. Parts which are not implemented yet or could be better implemented are described at the end.