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 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 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:	1.5
Committed:	Thu Apr 21 21:13:59 2005 UTC (19 years, 1 month ago) by pcg
Branch:	MAIN
CVS Tags:	rel-2_01, rel-2_0
Changes since 1.4:	+1 -1 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	1.3	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	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			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	pcg	1.2	=head2 ICMP
36
37	pcg	1.3	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	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			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	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			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	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			compiletime and doesn't change. The same structure is used for all
118	pcg	1.2	transort 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			node ids (12 bits each). The protocol does not yet scale well beyond 30+
134	pcg	1.2	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	pcg	1.1
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	pcg	1.2	a sliding window of 512 packets/sequence numbers to detect reordering,
152			duplication and reply attacks.
153	pcg	1.1
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	pcg	1.2	reply is only used to set the current IP address of the other side and
176			protocol parameters.
177	pcg	1.1
178	pcg	1.2	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	pcg	1.1
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	pcg	1.2	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	pcg	1.1
191			In addition to the exponential backoff, there is a global rate-limit on
192	pcg	1.2	a per-IP base. It allows long bursts but will limit total packet rate to
193	pcg	1.1	something like one control packet every ten seconds, to avoid accidental
194	pcg	1.2	floods due to protocol problems (like a RSA key file mismatch between two
195	pcg	1.1	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