Provided by: gvpe_2.25-2_amd64 
The GNU-VPE Protocols
Overview
GVPE can make use of a number of protocols. One of them is the GNU VPE protocol which is used to
authenticate tunnels and send encrypted data packets. This protocol is described in more detail the
second part of this document.
The first part of this document describes the transport protocols which are used by GVPE to send it's
data packets over the network.
PART 1: Transport protocols
GVPE offers a wide range of transport protocols that can be used to interchange data between nodes.
Protocols differ in their overhead, speed, reliability, and robustness.
The following sections describe each transport protocol in more detail. They are sorted by
overhead/efficiency, the most efficient transport is listed first:
RAW IP
This protocol is the best choice, performance-wise, as the minimum overhead per packet is only 38 bytes.
It works by sending the VPN payload using raw IP frames (using the protocol set by ip-proto).
Using raw IP frames has the drawback that many firewalls block "unknown" protocols, so this transport
only works if you have full IP connectivity between nodes.
ICMP
This protocol offers very low overhead (minimum 42 bytes), and can sometimes tunnel through firewalls
when other protocols can not.
It works by prepending an ICMP header with type icmp-type and a code of 255. The default icmp-type is
echo-reply, so the resulting packets look like echo replies, which looks rather strange to network
administrators.
This transport should only be used if other transports (i.e. raw IP) are not available or undesirable
(due to their overhead).
UDP
This is a good general choice for the transport protocol as UDP packets tunnel well through most
firewalls and routers, and the overhead per packet is moderate (minimum 58 bytes).
It should be used if RAW IP is not available.
TCP
This protocol is a very bad choice, as it not only has high overhead (more than 60 bytes), but the
transport also retries on it's own, which leads to congestion when the link has moderate packet loss (as
both the TCP transport and the tunneled traffic will retry, increasing congestion more and more). It also
has high latency and is quite inefficient.
It's only useful when tunneling through firewalls that block better protocols. If a node doesn't have
direct internet access but a HTTP proxy that supports the CONNECT method it can be used to tunnel through
a web proxy. For this to work, the tcp-port should be 443 (https), as most proxies do not allow
connections to other ports.
It is an abuse of the usage a proxy was designed for, so make sure you are allowed to use it for GVPE.
This protocol also has server and client sides. If the tcp-port is set to zero, other nodes cannot
connect to this node directly. If the tcp-port is non-zero, the node can act both as a client as well as
a server.
DNS
WARNING: Parsing and generating DNS packets is rather tricky. The code almost certainly contains buffer
overflows and other, likely exploitable, bugs. You have been warned.
This is the worst choice of transport protocol with respect to overhead (overhead can be 2-3 times higher
than the transferred data), and latency (which can be many seconds). Some DNS servers might not be
prepared to handle the traffic and drop or corrupt packets. The client also has to constantly poll the
server for data, so the client will constantly create traffic even if it doesn't need to transport
packets.
In addition, the same problems as the TCP transport also plague this protocol.
Its only use is to tunnel through firewalls that do not allow direct internet access. Similar to using a
HTTP proxy (as the TCP transport does), it uses a local DNS server/forwarder (given by the dns-forw-host
configuration value) as a proxy to send and receive data as a client, and an NS record pointing to the
GVPE server (as given by the dns-hostname directive).
The only good side of this protocol is that it can tunnel through most firewalls mostly undetected, iff
the local DNS server/forwarder is sane (which is true for most routers, wireless LAN gateways and
nameservers).
Fine-tuning needs to be done by editing src/vpn_dns.C directly.
PART 2: The GNU VPE protocol
This section, unfortunately, is not yet finished, although the protocol is stable (until bugs in the
cryptography are found, which will likely completely change the following description). Nevertheless, it
should give you some overview over the protocol.
Anatomy of a VPN packet
The exact layout and field lengths of a VPN packet is determined at compile time and doesn't change. The
same structure is used for all transport protocols, be it RAWIP or TCP.
+------+------+--------+------+
| HMAC | TYPE | SRCDST | DATA |
+------+------+--------+------+
The HMAC field is present in all packets, even if not used (e.g. in auth request packets), in which case
it is set to all zeroes. The checksum itself is calculated over the TYPE, SRCDST and DATA fields in all
cases.
The TYPE field is a single byte and determines the purpose of the packet (e.g. RESET,
COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE, CONNECT REQUEST/INFO etc.).
SRCDST is a three byte field which contains the source and destination node IDs (12 bits each).
The DATA portion differs between each packet type, naturally, and is the only part that can be encrypted.
Data packets contain more fields, as shown:
+------+------+--------+------+-------+------+
| HMAC | TYPE | SRCDST | RAND | SEQNO | DATA |
+------+------+--------+------+-------+------+
RAND is a sequence of fully random bytes, used to increase the entropy of the data for encryption
purposes.
SEQNO is a 32-bit sequence number. It is negotiated at every connection initialization and starts at some
random 31 bit value. VPE currently uses a sliding window of 512 packets/sequence numbers to detect
reordering, duplication and replay attacks.
The encryption is done on RAND+SEQNO+DATA in CBC mode with zero IV (or, equivalently, the IV is
RAND+SEQNO, encrypted with the block cipher, unless RAND size is decreased or increased over the default
value).
The authentication protocol
Before nodes can exchange packets, they need to establish authenticity of the other side and a key. Every
node has a private RSA key and the public RSA keys of all other nodes.
A host establishes a simplex connection by sending the other node an RSA encrypted challenge containing a
random challenge (consisting of the encryption and authentication keys to use when sending packets, more
random data and PKCS1_OAEP padding) and a random 16 byte "challenge-id" (used to detect duplicate auth
packets). The destination node will respond by replying with an (unencrypted) hash of the decrypted
challenge, which will authenticate that node. The destination node will also set the outgoing encryption
parameters as given in the packet.
When the source node receives a correct auth reply (by verifying the hash and the id, which will expire
after 120 seconds), it will start to accept data packets from the destination node.
This means that a node can only initiate a simplex connection, telling the other side the key it has to
use when it sends packets. The challenge reply is only used to set the current IP address of the other
side and protocol parameters.
This protocol is completely symmetric, so to be able to send packets the destination node must send a
challenge in the exact same way as already described (so, in essence, two simplex connections are created
per node pair).
Retrying
When there is no response to an auth request, the node will send auth requests in bursts with an
exponential back-off. After some time it will resort to PING packets, which are very small (8 bytes +
protocol header) and lightweight (no RSA operations required). A node that receives ping requests from an
unconnected peer will respond by trying to create a connection.
In addition to the exponential back-off, there is a global rate-limit on a per-IP base. It allows long
bursts but will limit total packet rate to something like one control packet every ten seconds, to avoid
accidental floods due to protocol problems (like a RSA key file mismatch between two nodes).
The intervals between retries are limited by the max-retry configuration value. A node with connect =
always will always retry, a node with connect = ondemand will only try (and re-try) to connect as long as
there are packets in the queue, usually this limits the retry period to max-ttl seconds.
Sending packets over the VPN will reset the retry intervals as well, which means as long as somebody is
trying to send packets to a given node, GVPE will try to connect every few seconds.
Routing and Protocol translation
The GVPE routing algorithm is easy: there isn't much routing to speak of: When routing packets to another
node, GVPE tries the following options, in order:
If the two nodes should be able to reach each other directly (common protocol, port known), then GVPE
will send the packet directly to the other node.
If this isn't possible (e.g. because the node doesn't have a hostname or known port), but the nodes speak
a common protocol and a router is available, then GVPE will ask a router to "mediate" between both nodes
(see below).
If a direct connection isn't possible (no common protocols) or forbidden (deny-direct) and there are any
routers, then GVPE will try to send packets to the router with the highest priority that is connected
already and is able (as specified by the config file) to connect directly to the target node.
If no such router exists, then GVPE will simply send the packet to the node with the highest priority
available.
Failing all that, the packet will be dropped.
A host can usually declare itself unreachable directly by setting it's port number(s) to zero. It can
declare other hosts as unreachable by using a config-file that disables all protocols for these other
hosts. Another option is to disable all protocols on that host in the other config files.
If two hosts cannot connect to each other because their IP address(es) are not known (such as dial-up
hosts), one side will send a mediated connection request to a router (routers must be configured to act
as routers!), which will send both the originating and the destination host a connection info request
with protocol information and IP address of the other host (if known). Both hosts will then try to
establish a direct connection to the other peer, which is usually possible even when both hosts are
behind a NAT gateway.
Routing via other nodes works because the SRCDST field is not encrypted, so the router can just forward
the packet to the destination host. Since each host uses it's own private key, the router will not be
able to decrypt or encrypt packets, it will just act as a simple router and protocol translator.