gvpe/doc/gvpe.protocol.7

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.\" ========================================================================
.\"
.IX Title "GVPE.PROTOCOL 7"
.TH GVPE.PROTOCOL 7 "2013-07-19" "2.25" "GNU Virtual Private Ethernet"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "The GNU-VPE Protocols"
.IX Header "The GNU-VPE Protocols"
.SH "Overview"
.IX Header "Overview"
\&\s-1GVPE\s0 can make use of a number of protocols. One of them is the \s-1GNU\s0 \s-1VPE\s0
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.
.PP
The first part of this document describes the transport protocols which
are used by \s-1GVPE\s0 to send it's data packets over the network.
.SH "PART 1: Transport protocols"
.IX Header "PART 1: Transport protocols"
\&\s-1GVPE\s0 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.
.PP
The following sections describe each transport protocol in more
detail. They are sorted by overhead/efficiency, the most efficient
transport is listed first:
.SS "\s-1RAW\s0 \s-1IP\s0"
.IX Subsection "RAW IP"
This protocol is the best choice, performance-wise, as the minimum
overhead per packet is only 38 bytes.
.PP
It works by sending the \s-1VPN\s0 payload using raw \s-1IP\s0 frames (using the
protocol set by \f(CW\*(C`ip\-proto\*(C'\fR).
.PP
Using raw \s-1IP\s0 frames has the drawback that many firewalls block \*(L"unknown\*(R"
protocols, so this transport only works if you have full \s-1IP\s0 connectivity
between nodes.
.SS "\s-1ICMP\s0"
.IX Subsection "ICMP"
This protocol offers very low overhead (minimum 42 bytes), and can
sometimes tunnel through firewalls when other protocols can not.
.PP
It works by prepending an \s-1ICMP\s0 header with type \f(CW\*(C`icmp\-type\*(C'\fR and a code
of \f(CW255\fR. The default \f(CW\*(C`icmp\-type\*(C'\fR is \f(CW\*(C`echo\-reply\*(C'\fR, so the resulting
packets look like echo replies, which looks rather strange to network
administrators.
.PP
This transport should only be used if other transports (i.e. raw \s-1IP\s0) are
not available or undesirable (due to their overhead).
.SS "\s-1UDP\s0"
.IX Subsection "UDP"
This is a good general choice for the transport protocol as \s-1UDP\s0 packets
tunnel well through most firewalls and routers, and the overhead per
packet is moderate (minimum 58 bytes).
.PP
It should be used if \s-1RAW\s0 \s-1IP\s0 is not available.
.SS "\s-1TCP\s0"
.IX Subsection "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 \s-1TCP\s0
transport and the tunneled traffic will retry, increasing congestion more
and more). It also has high latency and is quite inefficient.
.PP
It's only useful when tunneling through firewalls that block better
protocols. If a node doesn't have direct internet access but a \s-1HTTP\s0 proxy
that supports the \s-1CONNECT\s0 method it can be used to tunnel through a web
proxy. For this to work, the \f(CW\*(C`tcp\-port\*(C'\fR should be \f(CW443\fR (\f(CW\*(C`https\*(C'\fR), as
most proxies do not allow connections to other ports.
.PP
It is an abuse of the usage a proxy was designed for, so make sure you are
allowed to use it for \s-1GVPE\s0.
.PP
This protocol also has server and client sides. If the \f(CW\*(C`tcp\-port\*(C'\fR is
set to zero, other nodes cannot connect to this node directly. If the
\&\f(CW\*(C`tcp\-port\*(C'\fR is non-zero, the node can act both as a client as well as a
server.
.SS "\s-1DNS\s0"
.IX Subsection "DNS"
\&\fB\s-1WARNING:\s0\fR Parsing and generating \s-1DNS\s0 packets is rather tricky. The code
almost certainly contains buffer overflows and other, likely exploitable,
bugs. You have been warned.
.PP
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 \s-1DNS\s0 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.
.PP
In addition, the same problems as the \s-1TCP\s0 transport also plague this
protocol.
.PP
Its only use is to tunnel through firewalls that do not allow direct
internet access. Similar to using a \s-1HTTP\s0 proxy (as the \s-1TCP\s0 transport
does), it uses a local \s-1DNS\s0 server/forwarder (given by the \f(CW\*(C`dns\-forw\-host\*(C'\fR
configuration value) as a proxy to send and receive data as a client,
and an \f(CW\*(C`NS\*(C'\fR record pointing to the \s-1GVPE\s0 server (as given by the
\&\f(CW\*(C`dns\-hostname\*(C'\fR directive).
.PP
The only good side of this protocol is that it can tunnel through most
firewalls mostly undetected, iff the local \s-1DNS\s0 server/forwarder is sane
(which is true for most routers, wireless \s-1LAN\s0 gateways and nameservers).
.PP
Fine-tuning needs to be done by editing \f(CW\*(C`src/vpn_dns.C\*(C'\fR directly.
.SH "PART 2: The GNU VPE protocol"
.IX Header "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.
.SS "Anatomy of a \s-1VPN\s0 packet"
.IX Subsection "Anatomy of a VPN packet"
The exact layout and field lengths of a \s-1VPN\s0 packet is determined at
compile time and doesn't change. The same structure is used for all
transport protocols, be it \s-1RAWIP\s0 or \s-1TCP\s0.
.PP
.Vb 3
\& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+
\& | HMAC | TYPE | SRCDST | DATA |
\& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+
.Ve
.PP
The \s-1HMAC\s0 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 \s-1TYPE\s0, \s-1SRCDST\s0 and \s-1DATA\s0 fields in all cases.
.PP
The \s-1TYPE\s0 field is a single byte and determines the purpose of the packet
(e.g. \s-1RESET\s0, \s-1COMPRESSED/UNCOMPRESSED\s0 \s-1DATA\s0, \s-1PING\s0, \s-1AUTH\s0 \s-1REQUEST/RESPONSE\s0,
\&\s-1CONNECT\s0 \s-1REQUEST/INFO\s0 etc.).
.PP
\&\s-1SRCDST\s0 is a three byte field which contains the source and destination
node IDs (12 bits each).
.PP
The \s-1DATA\s0 portion differs between each packet type, naturally, and is the
only part that can be encrypted. Data packets contain more fields, as
shown:
.PP
.Vb 3
\& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-+\-\-\-\-\-\-+
\& | HMAC | TYPE | SRCDST | RAND | SEQNO | DATA |
\& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-+\-\-\-\-\-\-+
.Ve
.PP
\&\s-1RAND\s0 is a sequence of fully random bytes, used to increase the entropy of
the data for encryption purposes.
.PP
\&\s-1SEQNO\s0 is a 32\-bit sequence number. It is negotiated at every connection
initialization and starts at some random 31 bit value. \s-1GVPE\s0 currently uses
a sliding window of 512 packets/sequence numbers to detect reordering,
duplication and replay attacks.
.PP
The encryption is done on \s-1RAND+SEQNO+DATA\s0 in \s-1CBC\s0 mode with zero \s-1IV\s0 (or,
equivalently, the \s-1IV\s0 is \s-1RAND+SEQNO\s0, encrypted with the block cipher,
unless \s-1RAND\s0 size is decreased or increased over the default value).
.PP
The random prefix itself is generated by using \s-1AES\s0 in \s-1CTR\s0 mode with a
random key and starting value, which should make them unpredictable even
before encrypting them again. The sequence number additionally ensures
that the \s-1IV\s0 is unique.
.SS "The authentication/key exchange protocol"
.IX Subsection "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 \s-1RSA\s0 key and the public
\&\s-1RSA\s0 keys of all other nodes.
.PP
When a node wants to establish a connection to another node, it sends an
RSA-OEAP-encrypted challenge and an \s-1ECDH\s0 key. The other node replies with
it's own \s-1ECDH\s0 key and a \s-1HKDF\s0 of the challange and both \s-1ECDH\s0 keys to proof
it's identity.
.PP
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 \s-1HMAC\s0 key and start exchanging data packets.
.PP
In detail, the challenge consist of:
.PP
.Vb 1
\&  RSA\-OAEP (SEQNO MAC CIPHER SALT EXTRA\-AUTH) ECDH1
.Ve
.PP
That is, it encrypts (with the public key of the remote node) an initial
sequence number for data packets, key material for the \s-1HMAC\s0 key, key
material for the cipher key, a salt used by the \s-1HKDF\s0 (as shown later) and
some extra random bytes that are unused except for authentication. It also
sends the public key of a curve25519 exchange.
.PP
The remote node decrypts the \s-1RSA\s0 data, generates it's own \s-1ECDH\s0 key (\s-1ECDH2\s0), and
replies with:
.PP
.Vb 1
\&  HKDF\-Expand (HKDF\-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2
.Ve
.PP
That is, it extracts from the decrypted \s-1RSA\s0 challenge, using it's \s-1ECDH\s0
key as salt, and then expands using the requesting node's \s-1ECDH1\s0 key. The
resulting has is returned as a proof that the node could decrypt the \s-1RSA\s0
challenge data, together with the \s-1ECDH\s0 key.
.PP
After both nodes have done this to each other, they calculate the shared
\&\s-1ECDH\s0 secrets, cipher and \s-1HMAC\s0 keys for the session (each
node generates two cipher and \s-1HMAC\s0 keys, one for sending and one for
receiving).
.PP
The \s-1HMAC\s0 key for sending is generated as follow:
.PP
.Vb 1
\&   HMAC_KEY = HKDF\-Expand (HKDF\-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE)
.Ve
.PP
It extracts from \s-1MAC\s0 and \s-1ECDH_SECRET\s0 using the \fIremote\fR \s-1SALT\s0, then
expands using a static info string.
.PP
The cipher key is generated in the same way, except using the \s-1CIPHER\s0 part
of the original challenge.
.PP
The result of this process is to authenticate each node to the other
node, while exchanging keys using both \s-1RSA\s0 and \s-1ECDH\s0, the latter providing
perfect forward secrecy.
.PP
The protocol has been overdesigned where this was possible without
increasing implementation complexity, in an attempt to protect against
implementation or protocol failures. For example, if the \s-1ECDH\s0 challenge
was found to be flawed, perfect forward secrecy would be lost, but
the data would still be protected. Likewise, standard algorithms and
implementations are used where possible.
.SS "Retrying"
.IX Subsection "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 \s-1PING\s0 packets, which are very small (8 bytes + protocol header)
and lightweight (no \s-1RSA\s0 operations required). A node that receives ping
requests from an unconnected peer will respond by trying to create a
connection.
.PP
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 \s-1RSA\s0 key file mismatch between two
nodes).
.PP
The intervals between retries are limited by the \f(CW\*(C`max\-retry\*(C'\fR
configuration value. A node with \f(CW\*(C`connect\*(C'\fR = \f(CW\*(C`always\*(C'\fR will always retry,
a node with \f(CW\*(C`connect\*(C'\fR = \f(CW\*(C`ondemand\*(C'\fR will only try (and re-try) to connect
as long as there are packets in the queue, usually this limits the retry
period to \f(CW\*(C`max\-ttl\*(C'\fR seconds.
.PP
Sending packets over the \s-1VPN\s0 will reset the retry intervals as well, which
means as long as somebody is trying to send packets to a given node, \s-1GVPE\s0
will try to connect every few seconds.
.SS "Routing and Protocol translation"
.IX Subsection "Routing and Protocol translation"
The \s-1GVPE\s0 routing algorithm is easy: there isn't much routing to speak
of: When routing packets to another node, \s-1GVPE\s0 tries the following
options, in order:
.IP "If the two nodes should be able to reach each other directly (common protocol, port known), then \s-1GVPE\s0 will send the packet directly to the other node." 4
.IX 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."
.PD 0
.ie n .IP "If this isn't possible (e.g. because the node doesn't have a \*(C`hostname\*(C' or known port), but the nodes speak a common protocol and a router is available, then \s-1GVPE\s0 will ask a router to ""mediate"" between both nodes (see below)." 4
.el .IP "If this isn't possible (e.g. because the node doesn't have a \f(CW\*(C`hostname\*(C'\fR or known port), but the nodes speak a common protocol and a router is available, then \s-1GVPE\s0 will ask a router to ``mediate'' between both nodes (see below)." 4
.IX Item "If this isn't possible (e.g. because the node doesn't have a 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)."
.ie n .IP "If a direct connection isn't possible (no common protocols) or forbidden (\*(C`deny\-direct\*(C') and there are any routers, then \s-1GVPE\s0 will try to send packets to the router with the highest priority that is connected already \fIand\fR is able (as specified by the config file) to connect directly to the target node." 4
.el .IP "If a direct connection isn't possible (no common protocols) or forbidden (\f(CW\*(C`deny\-direct\*(C'\fR) and there are any routers, then \s-1GVPE\s0 will try to send packets to the router with the highest priority that is connected already \fIand\fR is able (as specified by the config file) to connect directly to the target node." 4
.IX Item "If a direct connection isn't possible (no common protocols) or forbidden (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 and is able (as specified by the config file) to connect directly to the target node."
.IP "If no such router exists, then \s-1GVPE\s0 will simply send the packet to the node with the highest priority available." 4
.IX Item "If no such router exists, then GVPE will simply send the packet to the node with the highest priority available."
.IP "Failing all that, the packet will be dropped." 4
.IX Item "Failing all that, the packet will be dropped."
.PD
.PP
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.
.PP
If two hosts cannot connect to each other because their \s-1IP\s0 address(es)
are not known (such as dial-up hosts), one side will send a \fImediated\fR
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 \s-1IP\s0 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 \s-1NAT\s0 gateway.
.PP
Routing via other nodes works because the \s-1SRCDST\s0 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:	1.13
Committed:	Fri Sep 20 11:57:03 2013 UTC (10 years, 8 months ago) by root
Branch:	MAIN
Changes since 1.12:	+69 -26 lines
Log Message:	* empty log message *