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# Content
1 =head1 The GNU-VPE Protocols
2
3 =head1 Overview
4
5 GVPE can make use of a number of protocols. One of them is the GNU VPE
6 protocol which is used to authenticate tunnels and send encrypted data
7 packets. This protocol is described in more detail the second part of this
8 document.
9
10 The first part of this document describes the transport protocols which
11 are used by GVPE to send its data packets over the network.
12
13 =head1 PART 1: Transport protocols
14
15 GVPE offers a wide range of transport protocols that can be used to
16 interchange data between nodes. Protocols differ in their overhead, speed,
17 reliability, and robustness.
18
19 The following sections describe each transport protocol in more
20 detail. They are sorted by overhead/efficiency, the most efficient
21 transport is listed first:
22
23 =head2 RAW IP
24
25 This protocol is the best choice, performance-wise, as the minimum
26 overhead per packet is only 38 bytes.
27
28 It works by sending the VPN payload using raw IP frames (using the
29 protocol set by C<ip-proto>).
30
31 Using raw IP frames has the drawback that many firewalls block "unknown"
32 protocols, so this transport only works if you have full IP connectivity
33 between nodes.
34
35 =head2 ICMP
36
37 This protocol offers very low overhead (minimum 42 bytes), and can
38 sometimes tunnel through firewalls when other protocols can not.
39
40 It works by prepending an ICMP header with type C<icmp-type> and a code
41 of C<255>. The default C<icmp-type> is C<echo-reply>, so the resulting
42 packets look like echo replies, which looks rather strange to network
43 administrators.
44
45 This transport should only be used if other transports (i.e. raw IP) are
46 not available or undesirable (due to their overhead).
47
48 =head2 UDP
49
50 This is a good general choice for the transport protocol as UDP packets
51 tunnel well through most firewalls and routers, and the overhead per
52 packet is moderate (minimum 58 bytes).
53
54 It should be used if RAW IP is not available.
55
56 =head2 TCP
57
58 This protocol is a very bad choice, as it not only has high overhead (more
59 than 60 bytes), but the transport also retries on its own, which leads
60 to congestion when the link has moderate packet loss (as both the TCP
61 transport and the tunneled traffic will retry, increasing congestion more
62 and more). It also has high latency and is quite inefficient.
63
64 It's only useful when tunneling through firewalls that block better
65 protocols. If a node doesn't have direct internet access but a HTTP proxy
66 that supports the CONNECT method it can be used to tunnel through a web
67 proxy. For this to work, the C<tcp-port> should be C<443> (C<https>), as
68 most proxies do not allow connections to other ports.
69
70 It is an abuse of the usage a proxy was designed for, so make sure you are
71 allowed to use it for GVPE.
72
73 This protocol also has server and client sides. If the C<tcp-port> is
74 set to zero, other nodes cannot connect to this node directly. If the
75 C<tcp-port> is non-zero, the node can act both as a client as well as a
76 server.
77
78 =head2 DNS
79
80 B<WARNING:> Parsing and generating DNS packets is rather tricky. The code
81 almost certainly contains buffer overflows and other, likely exploitable,
82 bugs. You have been warned.
83
84 This is the worst choice of transport protocol with respect to overhead
85 (overhead can be 2-3 times higher than the transferred data), and latency
86 (which can be many seconds). Some DNS servers might not be prepared to
87 handle the traffic and drop or corrupt packets. The client also has to
88 constantly poll the server for data, so the client will constantly create
89 traffic even if it doesn't need to transport packets.
90
91 In addition, the same problems as the TCP transport also plague this
92 protocol.
93
94 Its only use is to tunnel through firewalls that do not allow direct
95 internet access. Similar to using a HTTP proxy (as the TCP transport
96 does), it uses a local DNS server/forwarder (given by the C<dns-forw-host>
97 configuration value) as a proxy to send and receive data as a client,
98 and an C<NS> record pointing to the GVPE server (as given by the
99 C<dns-hostname> directive).
100
101 The only good side of this protocol is that it can tunnel through most
102 firewalls mostly undetected, iff the local DNS server/forwarder is sane
103 (which is true for most routers, wireless LAN gateways and nameservers).
104
105 Fine-tuning needs to be done by editing C<src/vpn_dns.C> directly.
106
107 =head1 PART 2: The GNU VPE protocol
108
109 This section, unfortunately, is not yet finished, although the protocol
110 is stable (until bugs in the cryptography are found, which will likely
111 completely change the following description). Nevertheless, it should give
112 you some overview over the protocol.
113
114 =head2 Anatomy of a VPN packet
115
116 The exact layout and field lengths of a VPN packet is determined at
117 compile time and doesn't change. The same structure is used for all
118 transport protocols, be it RAWIP or TCP.
119
120 +------+------+--------+------+
121 | HMAC | TYPE | SRCDST | DATA |
122 +------+------+--------+------+
123
124 The HMAC field is present in all packets, even if not used (e.g. in auth
125 request packets), in which case it is set to all zeroes. The MAC itself is
126 calculated over the TYPE, SRCDST and DATA fields in all cases.
127
128 The TYPE field is a single byte and determines the purpose of the packet
129 (e.g. RESET, COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE,
130 CONNECT REQUEST/INFO etc.).
131
132 SRCDST is a three byte field which contains the source and destination
133 node IDs (12 bits each).
134
135 The DATA portion differs between each packet type, naturally, and is the
136 only part that can be encrypted. Data packets contain more fields, as
137 shown:
138
139 +------+------+--------+-------+------+
140 | HMAC | TYPE | SRCDST | SEQNO | DATA |
141 +------+------+--------+-------+------+
142
143 SEQNO is a 32-bit sequence number. It is negotiated at every connection
144 initialization and starts at some random 31 bit value. GVPE currently uses
145 a sliding window of 512 packets/sequence numbers to detect reordering,
146 duplication and replay attacks.
147
148 The encryption is done on SEQNO+DATA in CTR mode with IV generated from
149 the seqno (for AES: seqno || seqno || seqno || (u32)0), which ensures
150 uniqueness for a given key.
151
152 =head2 The authentication/key exchange protocol
153
154 Before nodes can exchange packets, they need to establish authenticity of
155 the other side and a key. Every node has a private RSA key and the public
156 RSA keys of all other nodes.
157
158 When a node wants to establish a connection to another node, it sends an
159 RSA-OEAP-encrypted challenge and an ECDH (curve25519) key. The other node
160 replies with its own ECDH key and a HKDF of the challenge and both ECDH
161 keys to prove its identity.
162
163 The remote node enganges in exactly the same protocol. When both nodes
164 have exchanged their challenge and verified the response, they calculate a
165 cipher key and a HMAC key and start exchanging data packets.
166
167 In detail, the challenge consist of:
168
169 RSA-OAEP (SEQNO MAC CIPHER SALT EXTRA-AUTH) ECDH1
170
171 That is, it encrypts (with the public key of the remote node) an initial
172 sequence number for data packets, key material for the HMAC key, key
173 material for the cipher key, a salt used by the HKDF (as shown later) and
174 some extra random bytes that are unused except for authentication. It also
175 sends the public key of a curve25519 exchange.
176
177 The remote node decrypts the RSA data, generates its own ECDH key (ECDH2),
178 and replies with:
179
180 HKDF-Expand (HKDF-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2
181
182 That is, it extracts from the decrypted RSA challenge, using its ECDH
183 key as salt, and then expands using the requesting node's ECDH1 key. The
184 resulting hash is returned as a proof that the node could decrypt the RSA
185 challenge data, together with the ECDH key.
186
187 After both nodes have done this to each other, they calculate the shared
188 ECDH secret, cipher and HMAC keys for the session (each node generates two
189 cipher and HMAC keys, one for sending and one for receiving).
190
191 The HMAC key for sending is generated as follow:
192
193 HMAC_KEY = HKDF-Expand (HKDF-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE)
194
195 It extracts from MAC and ECDH_SECRET using the I<remote> SALT, then
196 expands using a static info string.
197
198 The cipher key is generated in the same way, except using the CIPHER part
199 of the original challenge.
200
201 The result of this process is to authenticate each node to the other
202 node, while exchanging keys using both RSA and ECDH, the latter providing
203 perfect forward secrecy.
204
205 The protocol has been overdesigned where this was possible without
206 increasing implementation complexity, in an attempt to protect against
207 implementation or protocol failures. For example, if the ECDH challenge
208 was found to be flawed, perfect forward secrecy would be lost, but the
209 data would likely still be protected. Likewise, standard algorithms and
210 implementations are used where possible.
211
212 =head2 Retrying
213
214 When there is no response to an auth request, the node will send auth
215 requests in bursts with an exponential back-off. After some time it will
216 resort to PING packets, which are very small (8 bytes + protocol header)
217 and lightweight (no RSA operations required). A node that receives ping
218 requests from an unconnected peer will respond by trying to create a
219 connection.
220
221 In addition to the exponential back-off, there is a global rate-limit on
222 a per-IP base. It allows long bursts but will limit total packet rate to
223 something like one control packet every ten seconds, to avoid accidental
224 floods due to protocol problems (like a RSA key file mismatch between two
225 nodes).
226
227 The intervals between retries are limited by the C<max-retry>
228 configuration value. A node with C<connect> = C<always> will always retry,
229 a node with C<connect> = C<ondemand> will only try (and re-try) to connect
230 as long as there are packets in the queue, usually this limits the retry
231 period to C<max-ttl> seconds.
232
233 Sending packets over the VPN will reset the retry intervals as well, which
234 means as long as somebody is trying to send packets to a given node, GVPE
235 will try to connect every few seconds.
236
237 =head2 Routing and Protocol translation
238
239 The GVPE routing algorithm is easy: there isn't much routing to speak
240 of: When routing packets to another node, GVPE tries the following
241 options, in order:
242
243 =over 4
244
245 =item If the two nodes should be able to reach each other directly (common
246 protocol, port known), then GVPE will send the packet directly to the
247 other node.
248
249 =item If this isn't possible (e.g. because the node doesn't have a
250 C<hostname> or known port), but the nodes speak a common protocol and a
251 router is available, then GVPE will ask a router to "mediate" between both
252 nodes (see below).
253
254 =item If a direct connection isn't possible (no common protocols) or
255 forbidden (C<deny-direct>) and there are any routers, then GVPE will try
256 to send packets to the router with the highest priority that is connected
257 already I<and> is able (as specified by the config file) to connect
258 directly to the target node.
259
260 =item If no such router exists, then GVPE will simply send the packet to
261 the node with the highest priority available.
262
263 =item Failing all that, the packet will be dropped.
264
265 =back
266
267 A host can usually declare itself unreachable directly by setting its
268 port number(s) to zero. It can declare other hosts as unreachable by using
269 a config-file that disables all protocols for these other hosts. Another
270 option is to disable all protocols on that host in the other config files.
271
272 If two hosts cannot connect to each other because their IP address(es)
273 are not known (such as dial-up hosts), one side will send a I<mediated>
274 connection request to a router (routers must be configured to act as
275 routers!), which will send both the originating and the destination host
276 a connection info request with protocol information and IP address of the
277 other host (if known). Both hosts will then try to establish a direct
278 connection to the other peer, which is usually possible even when both
279 hosts are behind a NAT gateway.
280
281 Routing via other nodes works because the SRCDST field is not encrypted,
282 so the router can just forward the packet to the destination host. Since
283 each host uses its own private key, the router will not be able to
284 decrypt or encrypt packets, it will just act as a simple router and
285 protocol translator.
286
287