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: 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<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 cannot.

It works by prepending a 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
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<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 (and C<tcp-port>
zero cannot be used). 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.

Most configuration needs to be done by editing C<src/vpn_dns.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<dns-forw-host>
configuration value) as a proxy to send and receive data as a client,
and a 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 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
...

Revision:	1.3
Committed:	Thu Mar 17 22:24:31 2005 UTC (19 years, 2 months ago) by pcg
Branch:	MAIN
CVS Tags:	rel-1_8
Changes since 1.2:	+82 -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: Tansport protocols
14
15	GVPE offers a range of transport protocols that can be used to interchange
16	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	transprot 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 cannot.
39
40	It works by prepending a 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	admins.
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 set
74	to zero, other nodes cannot connect to this node directly (and C<tcp-port>
75	zero cannot be used). If the C<tcp-port> is non-zero, the node can act
76	both as a client as well as a 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	Most configuration needs to be done by editing C<src/vpn_dns.C> directly.
95
96	It's only use is to tunnel through firewalls that do not allow direct
97	internet access. Similar to using a HTTP proxy (as the TCP transport
98	does), it uses a local DNS server/forwarder (given by the C<dns-forw-host>
99	configuration value) as a proxy to send and receive data as a client,
100	and a C<NS> record pointing to the GVPE server (as given by the
101	C<dns-hostname> directive).
102
103	The only good side of this protocol is that it can tunnel through most
104	firewalls undetected, iff the local DNS server/forwarder is sane (which is
105	true for most routers, wlan gateways and nameservers).
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	compiletime and doesn't change. The same structure is used for all
118	transort 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). The protocol does not yet scale well beyond 30+
134	hosts, since all hosts must connect to each other once on startup. But if
135	restarts are rare or tolerable and most connections are on demand, much
136	larger networks are feasible.
137
138	The DATA portion differs between each packet type, naturally, and is the
139	only part that can be encrypted. Data packets contain more fields, as
140	shown:
141
142	+------+------+--------+------+-------+------+
143	\| HMAC \| TYPE \| SRCDST \| RAND \| SEQNO \| DATA \|
144	+------+------+--------+------+-------+------+
145
146	RAND is a sequence of fully random bytes, used to increase the entropy of
147	the data for encryption purposes.
148
149	SEQNO is a 32-bit sequence number. It is negotiated at every connection
150	initialization and starts at some random 31 bit value. VPE currently uses
151	a sliding window of 512 packets/sequence numbers to detect reordering,
152	duplication and reply attacks.
153
154	=head2 The authentification protocol
155
156	Before hosts can exchange packets, they need to establish authenticity of
157	the other side and a key. Every host has a private RSA key and the public
158	RSA keys of all other hosts.
159
160	A host establishes a simplex connection by sending the other host a
161	RSA encrypted challenge containing a random challenge (consisting of
162	the encryption key to use when sending packets, more random data and
163	PKCS1_OAEP padding) and a random 16 byte "challenge-id" (used to detect
164	duplicate auth packets). The destination host will respond by replying
165	with an (unencrypted) RIPEMD160 hash of the decrypted challenge, which
166	will authentify that host. The destination host will also set the outgoing
167	encryption parameters as given in the packet.
168
169	When the source host receives a correct auth reply (by verifying the
170	hash and the id, which will expire after 120 seconds), it will start to
171	accept data packets from the destination host.
172
173	This means that a host can only initate a simplex connection, telling the
174	other side the key it has to use when it sends packets. The challenge
175	reply is only used to set the current IP address of the other side and
176	protocol parameters.
177
178	This protocol is completely symmetric, so to be able to send packets the
179	destination host must send a challenge in the exact same way as already
180	described (so, in essence, two simplex connections are created per host
181	pair).
182
183	=head2 Retrying
184
185	When there is no response to an auth request, the host will send auth
186	requests in bursts with an exponential backoff. After some time it will
187	resort to PING packets, which are very small (8 bytes) and lightweight
188	(no RSA operations required). A host that receives ping requests from an
189	unconnected peer will respond by trying to create a connection.
190
191	In addition to the exponential backoff, there is a global rate-limit on
192	a per-IP base. It allows long bursts but will limit total packet rate to
193	something like one control packet every ten seconds, to avoid accidental
194	floods due to protocol problems (like a RSA key file mismatch between two
195	hosts).
196
197	=head2 Routing and Protocol translation
198
199	The gvpe routing algorithm is easy: there isn't any routing. GVPE always
200	tries to establish direct connections, if the protocol abilities of the
201	two hosts allow it.
202
203	If the two hosts should be able to reach each other (common protocol, ip
204	and port all known), but cannot (network down), then there will be no
205	connection, point.
206
207	A host can usually declare itself unreachable directly by setting it's
208	port number(s) to zero. It can declare other hosts as unreachable by using
209	a config-file that disables all protocols for these other hosts.
210
211	If two hosts cannot connect to each other because their IP address(es)
212	are not known (such as dialup hosts), one side will send a connection
213	request to a router (routers must be configured to act as routers!), which
214	will send both the originating and the destination host a connection info
215	request with protocol information and IP address of the other host (if
216	known). Both hosts will then try to establish a connection to the other
217	peer, which is usually possible even when both hosts are behind a NAT
218	gateway.
219
220	If the hosts cannot reach each other because they have no common protocol,
221	the originator instead use the router with highest priority and matching
222	protocol as peer. Since the SRCDST field is not encrypted, the router host
223	can just forward the packet to the destination host. Since each host uses
224	it's own private key, the router will not be able to decrypt or encrypt
225	packets, it will just act as a simple router and protocol translator.
226
227	When no router is connected, the host will aggressively try to connect to
228	all routers, and if a router is asked for an unconnected host it will try
229	to ask another router to establish the connection.
230
231	... more not yet written about the details of the routing, please bug me
232	...
233