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 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).

=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:	1.10
Committed:	Fri Jul 12 04:16:18 2013 UTC (10 years, 10 months ago) by root
Branch:	MAIN
CVS Tags:	rel-2_25
Changes since 1.9:	+4 -0 lines
Log Message:	* empty log message *
#	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 it's 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 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	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 checksum
126	itself is 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 \| 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	a sliding window of 512 packets/sequence numbers to detect reordering,
149	duplication and replay attacks.
150
151	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	=head2 The authentication protocol
156
157	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
161	A host establishes a simplex connection by sending the other node an RSA
162	encrypted challenge containing a random challenge (consisting of the
163	encryption and authentication keys to use when sending packets, more
164	random data and PKCS1_OAEP padding) and a random 16 byte "challenge-id"
165	(used to detect duplicate auth packets). The destination node will respond
166	by replying with an (unencrypted) hash of the decrypted challenge, which
167	will authenticate that node. The destination node will also set the
168	outgoing encryption parameters as given in the packet.
169
170	When the source node receives a correct auth reply (by verifying the
171	hash and the id, which will expire after 120 seconds), it will start to
172	accept data packets from the destination node.
173
174	This means that a node can only initiate a simplex connection, telling the
175	other side the key it has to use when it sends packets. The challenge
176	reply is only used to set the current IP address of the other side and
177	protocol parameters.
178
179	This protocol is completely symmetric, so to be able to send packets the
180	destination node must send a challenge in the exact same way as already
181	described (so, in essence, two simplex connections are created per node
182	pair).
183
184	=head2 Retrying
185
186	When there is no response to an auth request, the node will send auth
187	requests in bursts with an exponential back-off. After some time it will
188	resort to PING packets, which are very small (8 bytes + protocol header)
189	and lightweight (no RSA operations required). A node that receives ping
190	requests from an unconnected peer will respond by trying to create a
191	connection.
192
193	In addition to the exponential back-off, there is a global rate-limit on
194	a per-IP base. It allows long bursts but will limit total packet rate to
195	something like one control packet every ten seconds, to avoid accidental
196	floods due to protocol problems (like a RSA key file mismatch between two
197	nodes).
198
199	The intervals between retries are limited by the C<max-retry>
200	configuration value. A node with C<connect> = C<always> will always retry,
201	a node with C<connect> = C<ondemand> will only try (and re-try) to connect
202	as long as there are packets in the queue, usually this limits the retry
203	period to C<max-ttl> seconds.
204
205	Sending packets over the VPN will reset the retry intervals as well, which
206	means as long as somebody is trying to send packets to a given node, GVPE
207	will try to connect every few seconds.
208
209	=head2 Routing and Protocol translation
210
211	The GVPE routing algorithm is easy: there isn't much routing to speak
212	of: When routing packets to another node, GVPE tries the following
213	options, in order:
214
215	=over 4
216
217	=item If the two nodes should be able to reach each other directly (common
218	protocol, port known), then GVPE will send the packet directly to the
219	other node.
220
221	=item If this isn't possible (e.g. because the node doesn't have a
222	C<hostname> or known port), but the nodes speak a common protocol and a
223	router is available, then GVPE will ask a router to "mediate" between both
224	nodes (see below).
225
226	=item If a direct connection isn't possible (no common protocols) or
227	forbidden (C<deny-direct>) and there are any routers, then GVPE will try
228	to send packets to the router with the highest priority that is connected
229	already I<and> is able (as specified by the config file) to connect
230	directly to the target node.
231
232	=item If no such router exists, then GVPE will simply send the packet to
233	the node with the highest priority available.
234
235	=item Failing all that, the packet will be dropped.
236
237	=back
238
239	A host can usually declare itself unreachable directly by setting it's
240	port number(s) to zero. It can declare other hosts as unreachable by using
241	a config-file that disables all protocols for these other hosts. Another
242	option is to disable all protocols on that host in the other config files.
243
244	If two hosts cannot connect to each other because their IP address(es)
245	are not known (such as dial-up hosts), one side will send a I<mediated>
246	connection request to a router (routers must be configured to act as
247	routers!), which will send both the originating and the destination host
248	a connection info request with protocol information and IP address of the
249	other host (if known). Both hosts will then try to establish a direct
250	connection to the other peer, which is usually possible even when both
251	hosts are behind a NAT gateway.
252
253	Routing via other nodes works because the SRCDST field is not encrypted,
254	so the router can just forward the packet to the destination host. Since
255	each host uses it's own private key, the router will not be able to
256	decrypt or encrypt packets, it will just act as a simple router and
257	protocol translator.
258
259