=head1 The GNU-VPE Protocols =head1 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. =head1 PART 1: Tansport protocols GVPE offers a 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 transprot is listed first: =head2 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 C). 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. =head2 ICMP This protocol offers very low overhead (minimum 42 bytes), and can sometimes tunnel through firewalls when other protocols cannot. It works by prepending a ICMP header with type C and a code of C<255>. The default C is C, so the resulting packets look like echo replies, which looks rather strange to network admins. This transport should only be used if other transports (i.e. raw ip) are not available or undesirable (due to their overhead). =head2 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. =head2 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 C should be C<443> (C), 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 C is set to zero, other nodes cannot connect to this node directly (and C zero cannot be used). If the C is non-zero, the node can act both as a client as well as a server. =head2 DNS B 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. Most configuration needs to be done by editing C directly. It's 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 C configuration value) as a proxy to send and receive data as a client, and a C record pointing to the GVPE server (as given by the C directive). The only good side of this protocol is that it can tunnel through most firewalls undetected, iff the local DNS server/forwarder is sane (which is true for most routers, wlan gateways and nameservers). =head1 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. =head2 Anatomy of a VPN packet The exact layout and field lengths of a VPN packet is determined at compiletime and doesn't change. The same structure is used for all transort 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 protocol does not yet scale well beyond 30+ hosts, since all hosts must connect to each other once on startup. But if restarts are rare or tolerable and most connections are on demand, much larger networks are feasible. 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 reply attacks. =head2 The authentification protocol Before hosts can exchange packets, they need to establish authenticity of the other side and a key. Every host has a private RSA key and the public RSA keys of all other hosts. A host establishes a simplex connection by sending the other host a RSA encrypted challenge containing a random challenge (consisting of the encryption key 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 host will respond by replying with an (unencrypted) RIPEMD160 hash of the decrypted challenge, which will authentify that host. The destination host will also set the outgoing encryption parameters as given in the packet. When the source host 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 host. This means that a host can only initate 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 host must send a challenge in the exact same way as already described (so, in essence, two simplex connections are created per host pair). =head2 Retrying When there is no response to an auth request, the host will send auth requests in bursts with an exponential backoff. After some time it will resort to PING packets, which are very small (8 bytes) and lightweight (no RSA operations required). A host that receives ping requests from an unconnected peer will respond by trying to create a connection. In addition to the exponential backoff, 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 hosts). =head2 Routing and Protocol translation The gvpe routing algorithm is easy: there isn't any routing. GVPE always tries to establish direct connections, if the protocol abilities of the two hosts allow it. If the two hosts should be able to reach each other (common protocol, ip and port all known), but cannot (network down), then there will be no connection, point. 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. If two hosts cannot connect to each other because their IP address(es) are not known (such as dialup hosts), one side will send a 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 connection to the other peer, which is usually possible even when both hosts are behind a NAT gateway. If the hosts cannot reach each other because they have no common protocol, the originator instead use the router with highest priority and matching protocol as peer. Since the SRCDST field is not encrypted, the router host 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. When no router is connected, the host will aggressively try to connect to all routers, and if a router is asked for an unconnected host it will try to ask another router to establish the connection. ... more not yet written about the details of the routing, please bug me ...