gvpe/doc/gvpe.protocol.7.pod

=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: 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:

=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<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.

=head2 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 C<icmp-type> and a code
of C<255>. The default C<icmp-type> is C<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).

=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<tcp-port> should be C<443> (C<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 C<tcp-port> is
set to zero, other nodes cannot connect to this node directly. If the
C<tcp-port> is non-zero, the node can act both as a client as well as a
server.

=head2 DNS

B<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 C<dns-forw-host>
configuration value) as a proxy to send and receive data as a client,
and an C<NS> record pointing to the GVPE server (as given by the
C<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 C<src/vpn_dns.C> directly.

=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
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).

=head2 The authentication/key exchange 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.

When a node wants to establish a connection to another node, it sends an
RSA-OEAP-encrypted challenge and an ECDH key. The other node replies with
it's own ECDH key and a HKDF of the challange and both ECDH keys to proof
it's identity.

The remote node enganges in exactly the same protocol. When both nodes
have exchanged their challenge and verified the response, they calculate a
cipher key and a HMAC key and start exchanging data packets.

In detail, the challenge consist of:

  RSA-OAEP (SEQNO MAC CIPHER SALT EXTRA-AUTH) ECDH1

That is, it encrypts (with the public key of the remote node) an initial
sequence number for data packets, key material for the HMAC key, key
material for the cipher key, a salt used by the HKDF (as shown later) and
some extra random bytes that are unused except for authentication. It also
sends the public key of a curve25519 exchange.

The remote node decrypts the RSA data, generates it's own ECDH key (ECDH2), and
replies with:

  HKDF-Expand (HKDF-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2

That is, it extracts from the decrypted RSA challenge, using it's ECDH
key as salt, and then expands using the requesting node's ECDH1 key. The
resulting has is returned as a proof that the node could decrypt the RSA
challenge data, together with the ECDH key.

After both nodes have done this to each other, they calculate the shared
ECDH secrets, cipher and HMAC keys for the session (each
node generates two cipher and HMAC keys, one for sending and one for
receiving).

The HMAC key for sending is generated as follow:

   HMAC_KEY = HKDF-Expand (HKDF-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE)

It extracts from MAC and ECDH_SECRET using the I<remote> SALT, then
expands using a static info string.

The cipher key is generated in the same way, except using the CIPHER part
of the original challenge.

The result of this process is to authenticate each node to the other
node, while exchanging keys using both RSA and ECDH, the latter providing
perfect forward secrecy.

=head2 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 C<max-retry>
configuration value. A node with C<connect> = C<always> will always retry,
a node with C<connect> = C<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 C<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.

=head2 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:

=over 4

=item 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.

=item If this isn't possible (e.g. because the node doesn't have a
C<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).

=item If a direct connection isn't possible (no common protocols) or
forbidden (C<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 I<and> is able (as specified by the config file) to connect
directly to the target node.

=item If no such router exists, then GVPE will simply send the packet to
the node with the highest priority available.

=item Failing all that, the packet will be dropped.

=back

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 I<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.


Revision:	1.11
Committed:	Fri Jul 19 21:01:16 2013 UTC (10 years, 10 months ago) by root
Branch:	MAIN
Changes since 1.10:	+48 -23 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	pcg	1.2	=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 it's data packets over the network.
12
13	pcg	1.5	=head1 PART 1: Transport protocols
14	pcg	1.2
15	pcg	1.6	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	pcg	1.3	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	pcg	1.4	transport is listed first:
22	pcg	1.3
23	pcg	1.2	=head2 RAW IP
24
25	pcg	1.3	This protocol is the best choice, performance-wise, as the minimum
26			overhead per packet is only 38 bytes.
27
28	pcg	1.7	It works by sending the VPN payload using raw IP frames (using the
29	pcg	1.3	protocol set by C<ip-proto>).
30
31	pcg	1.7	Using raw IP frames has the drawback that many firewalls block "unknown"
32	pcg	1.3	protocols, so this transport only works if you have full IP connectivity
33			between nodes.
34
35	pcg	1.2	=head2 ICMP
36
37	pcg	1.3	This protocol offers very low overhead (minimum 42 bytes), and can
38	pcg	1.6	sometimes tunnel through firewalls when other protocols can not.
39	pcg	1.3
40	pcg	1.6	It works by prepending an ICMP header with type C<icmp-type> and a code
41	pcg	1.3	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	pcg	1.7	administrators.
44	pcg	1.3
45	pcg	1.7	This transport should only be used if other transports (i.e. raw IP) are
46	pcg	1.3	not available or undesirable (due to their overhead).
47
48	pcg	1.2	=head2 UDP
49
50	pcg	1.3	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	pcg	1.2	=head2 TCP
57
58	pcg	1.3	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 it's 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	pcg	1.6	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	pcg	1.3
78	pcg	1.2	=head2 DNS
79
80	pcg	1.3	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	root	1.8	Its only use is to tunnel through firewalls that do not allow direct
95	pcg	1.3	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	pcg	1.6	and an C<NS> record pointing to the GVPE server (as given by the
99	pcg	1.3	C<dns-hostname> directive).
100
101			The only good side of this protocol is that it can tunnel through most
102	pcg	1.6	firewalls mostly undetected, iff the local DNS server/forwarder is sane
103	pcg	1.7	(which is true for most routers, wireless LAN gateways and nameservers).
104	pcg	1.6
105	pcg	1.7	Fine-tuning needs to be done by editing C<src/vpn_dns.C> directly.
106	pcg	1.3
107	pcg	1.2	=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	pcg	1.1
114			=head2 Anatomy of a VPN packet
115
116			The exact layout and field lengths of a VPN packet is determined at
117	pcg	1.7	compile time and doesn't change. The same structure is used for all
118			transport protocols, be it RAWIP or TCP.
119	pcg	1.1
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 checksum
126	pcg	1.2	itself is calculated over the TYPE, SRCDST and DATA fields in all cases.
127	pcg	1.1
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	pcg	1.6	node IDs (12 bits each).
134	pcg	1.1
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 \| RAND \| SEQNO \| DATA \|
141			+------+------+--------+------+-------+------+
142
143			RAND is a sequence of fully random bytes, used to increase the entropy of
144			the data for encryption purposes.
145
146			SEQNO is a 32-bit sequence number. It is negotiated at every connection
147			initialization and starts at some random 31 bit value. VPE currently uses
148	pcg	1.2	a sliding window of 512 packets/sequence numbers to detect reordering,
149	pcg	1.6	duplication and replay attacks.
150	pcg	1.1
151	root	1.10	The encryption is done on RAND+SEQNO+DATA in CBC mode with zero IV (or,
152			equivalently, the IV is RAND+SEQNO, encrypted with the block cipher,
153			unless RAND size is decreased or increased over the default value).
154
155	root	1.11	=head2 The authentication/key exchange protocol
156	pcg	1.1
157	pcg	1.7	Before nodes can exchange packets, they need to establish authenticity of
158			the other side and a key. Every node has a private RSA key and the public
159			RSA keys of all other nodes.
160	pcg	1.1
161	root	1.11	When a node wants to establish a connection to another node, it sends an
162			RSA-OEAP-encrypted challenge and an ECDH key. The other node replies with
163			it's own ECDH key and a HKDF of the challange and both ECDH keys to proof
164			it's identity.
165
166			The remote node enganges in exactly the same protocol. When both nodes
167			have exchanged their challenge and verified the response, they calculate a
168			cipher key and a HMAC key and start exchanging data packets.
169
170			In detail, the challenge consist of:
171
172			RSA-OAEP (SEQNO MAC CIPHER SALT EXTRA-AUTH) ECDH1
173
174			That is, it encrypts (with the public key of the remote node) an initial
175			sequence number for data packets, key material for the HMAC key, key
176			material for the cipher key, a salt used by the HKDF (as shown later) and
177			some extra random bytes that are unused except for authentication. It also
178			sends the public key of a curve25519 exchange.
179
180			The remote node decrypts the RSA data, generates it's own ECDH key (ECDH2), and
181			replies with:
182
183			HKDF-Expand (HKDF-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2
184
185			That is, it extracts from the decrypted RSA challenge, using it's ECDH
186			key as salt, and then expands using the requesting node's ECDH1 key. The
187			resulting has is returned as a proof that the node could decrypt the RSA
188			challenge data, together with the ECDH key.
189
190			After both nodes have done this to each other, they calculate the shared
191			ECDH secrets, cipher and HMAC keys for the session (each
192			node generates two cipher and HMAC keys, one for sending and one for
193			receiving).
194
195			The HMAC key for sending is generated as follow:
196
197			HMAC_KEY = HKDF-Expand (HKDF-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE)
198
199			It extracts from MAC and ECDH_SECRET using the I<remote> SALT, then
200			expands using a static info string.
201
202			The cipher key is generated in the same way, except using the CIPHER part
203			of the original challenge.
204
205			The result of this process is to authenticate each node to the other
206			node, while exchanging keys using both RSA and ECDH, the latter providing
207			perfect forward secrecy.
208	pcg	1.1
209			=head2 Retrying
210
211	pcg	1.7	When there is no response to an auth request, the node will send auth
212			requests in bursts with an exponential back-off. After some time it will
213	pcg	1.6	resort to PING packets, which are very small (8 bytes + protocol header)
214	pcg	1.7	and lightweight (no RSA operations required). A node that receives ping
215	pcg	1.6	requests from an unconnected peer will respond by trying to create a
216			connection.
217	pcg	1.1
218	pcg	1.7	In addition to the exponential back-off, there is a global rate-limit on
219	pcg	1.2	a per-IP base. It allows long bursts but will limit total packet rate to
220	pcg	1.1	something like one control packet every ten seconds, to avoid accidental
221	pcg	1.2	floods due to protocol problems (like a RSA key file mismatch between two
222	pcg	1.7	nodes).
223	pcg	1.1
224	pcg	1.6	The intervals between retries are limited by the C<max-retry>
225			configuration value. A node with C<connect> = C<always> will always retry,
226			a node with C<connect> = C<ondemand> will only try (and re-try) to connect
227			as long as there are packets in the queue, usually this limits the retry
228			period to C<max-ttl> seconds.
229
230			Sending packets over the VPN will reset the retry intervals as well, which
231			means as long as somebody is trying to send packets to a given node, GVPE
232			will try to connect every few seconds.
233
234	pcg	1.1	=head2 Routing and Protocol translation
235
236	pcg	1.6	The GVPE routing algorithm is easy: there isn't much routing to speak
237	root	1.9	of: When routing packets to another node, GVPE tries the following
238	pcg	1.6	options, in order:
239
240			=over 4
241
242	pcg	1.7	=item If the two nodes should be able to reach each other directly (common
243	pcg	1.6	protocol, port known), then GVPE will send the packet directly to the
244			other node.
245
246			=item If this isn't possible (e.g. because the node doesn't have a
247			C<hostname> or known port), but the nodes speak a common protocol and a
248			router is available, then GVPE will ask a router to "mediate" between both
249			nodes (see below).
250
251			=item If a direct connection isn't possible (no common protocols) or
252			forbidden (C<deny-direct>) and there are any routers, then GVPE will try
253			to send packets to the router with the highest priority that is connected
254			already I<and> is able (as specified by the config file) to connect
255			directly to the target node.
256
257			=item If no such router exists, then GVPE will simply send the packet to
258			the node with the highest priority available.
259
260			=item Failing all that, the packet will be dropped.
261
262			=back
263	pcg	1.1
264			A host can usually declare itself unreachable directly by setting it's
265			port number(s) to zero. It can declare other hosts as unreachable by using
266	pcg	1.6	a config-file that disables all protocols for these other hosts. Another
267			option is to disable all protocols on that host in the other config files.
268	pcg	1.1
269			If two hosts cannot connect to each other because their IP address(es)
270	pcg	1.7	are not known (such as dial-up hosts), one side will send a I<mediated>
271	pcg	1.6	connection request to a router (routers must be configured to act as
272			routers!), which will send both the originating and the destination host
273			a connection info request with protocol information and IP address of the
274			other host (if known). Both hosts will then try to establish a direct
275			connection to the other peer, which is usually possible even when both
276			hosts are behind a NAT gateway.
277
278			Routing via other nodes works because the SRCDST field is not encrypted,
279			so the router can just forward the packet to the destination host. Since
280			each host uses it's own private key, the router will not be able to
281			decrypt or encrypt packets, it will just act as a simple router and
282			protocol translator.
283	pcg	1.1
284