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.\" ======================================================================== |
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.\" |
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.IX Title "VPE.PROTOCOL 7" |
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.TH VPE.PROTOCOL 7 "2003-10-14" "1.2" "Virtual Private Ethernet" |
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.SH "The VPE Protocol" |
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.IX Header "The VPE Protocol" |
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.Sh "Anatomy of a \s-1VPN\s0 packet" |
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.IX Subsection "Anatomy of a VPN packet" |
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The exact layout and field lengths of a \s-1VPN\s0 packet is determined at |
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compiletime and doesn't change. The same structure is used for all |
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protocols, be it rawip or tcp. |
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.PP |
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.Vb 3 |
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\& +------+------+--------+------+ |
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\& | HMAC | TYPE | SRCDST | DATA | |
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\& +------+------+--------+------+ |
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.Ve |
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.PP |
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The \s-1HMAC\s0 field is present in all packets, even if not used (e.g. in auth |
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request packets), in which case it is set to all zeroes. The checksum |
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itself is over the \s-1TYPE\s0, \s-1SRCDST\s0 and \s-1DATA\s0 fields in all cases. |
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.PP |
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The \s-1TYPE\s0 field is a single byte and determines the purpose of the packet |
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(e.g. \s-1RESET\s0, \s-1COMPRESSED/UNCOMPRESSED\s0 \s-1DATA\s0, \s-1PING\s0, \s-1AUTH\s0 \s-1REQUEST/RESPONSE\s0, |
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\&\s-1CONNECT\s0 \s-1REQUEST/INFO\s0 etc.). |
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.PP |
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\&\s-1SRCDST\s0 is a three byte field which contains the source and destination |
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node ids (12 bits each). The protocol does not yet scale well beyond 30+ |
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hosts, since all hosts connect to each other on startup. But if restarts |
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are rare or tolerable and most connections are on demand, larger networks |
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are possible. |
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.PP |
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The \s-1DATA\s0 portion differs between each packet type, naturally, and is the |
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only part that can be encrypted. Data packets contain more fields, as |
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shown: |
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.PP |
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.Vb 3 |
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\& +------+------+--------+------+-------+------+ |
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\& | HMAC | TYPE | SRCDST | RAND | SEQNO | DATA | |
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\& +------+------+--------+------+-------+------+ |
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.Ve |
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.PP |
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\&\s-1RAND\s0 is a sequence of fully random bytes, used to increase the entropy of |
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the data for encryption purposes. |
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.PP |
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\&\s-1SEQNO\s0 is a 32\-bit sequence number. It is negotiated at every connection |
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initialization and starts at some random 31 bit value. \s-1VPE\s0 currently uses |
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a sliding window of 512 packets to detect reordering, duplication and |
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reply attacks. |
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.Sh "The authentification protocol" |
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.IX Subsection "The authentification protocol" |
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Before hosts can exchange packets, they need to establish authenticity of |
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the other side and a key. Every host has a private \s-1RSA\s0 key and the public |
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\&\s-1RSA\s0 keys of all other hosts. |
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.PP |
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A host establishes a simplex connection by sending the other host a |
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\&\s-1RSA\s0 encrypted challenge containing a random challenge (consisting of |
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the encryption key to use when sending packets, more random data and |
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\&\s-1PKCS1_OAEP\s0 padding) and a random 16 byte \*(L"challenge\-id\*(R" (used to detect |
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duplicate auth packets). The destination host will respond by replying |
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with an (unencrypted) \s-1RIPEMD160\s0 hash of the decrypted challenge, which |
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will authentify that host. The destination host will also set the outgoing |
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encryption parameters as given in the packet. |
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.PP |
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When the source host receives a correct auth reply (by verifying the |
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hash and the id, which will expire after 120 seconds), it will start to |
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accept data packets from the destination host. |
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.PP |
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This means that a host can only initate a simplex connection, telling the |
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other side the key it has to use when it sends packets. The challenge |
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reply is only used to set the current \s-1IP\s0 address and protocol parameters. |
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.PP |
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The protocol here is completely symmetric, so to be able to send packets |
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the destination host must send a challenge in the exact same way as |
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already described (so, in essence, two simplex connections are created per |
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host pair). |
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.Sh "Retrying" |
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.IX Subsection "Retrying" |
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When there is no response to an auth request, the host will send auth |
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requests in bursts with an exponential backoff. After some time it will |
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resort to \s-1PING\s0 packets, which are very small (8 byte) and lightweight (no |
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\&\s-1RSA\s0 operations). A host that receives ping requests from an unconnected |
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peer will respond by trying to create a connection. |
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.PP |
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In addition to the exponential backoff, there is a global rate-limit on |
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a per-ip base. It allows long bursts but will limit total packet rate to |
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something like one control packet every ten seconds, to avoid accidental |
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floods due to protocol problems (like a rsa key file mismatch between two |
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hosts). |
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.Sh "Routing and Protocol translation" |
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.IX Subsection "Routing and Protocol translation" |
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The vpe routing algorithm is easy: there isn't any routing. Vped always |
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tries to establish direct connections, if the protocol abilities of the |
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two hosts allow it. |
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.PP |
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If the two hosts should be able to reach each other (common protocol, ip |
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and port all known), but cannot (network down), then there will be no |
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connection, point. |
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.PP |
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A host can usually declare itself unreachable directly by setting it's |
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port number(s) to zero. It can declare other hosts as unreachable by using |
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a config-file that disables all protocols for these other hosts. |
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.PP |
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If two hosts cannot connect to each other because their \s-1IP\s0 address(es) |
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are not known (such as dialup hosts), one side will send a connection |
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request to a router (routers must be configured to act as routers!), which |
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will send both the originating and the destination host a connection info |
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request with protocol information and \s-1IP\s0 address of the other host (if |
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known). Both hosts will then try to establish a connection to the other |
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peer, which is usually possible even when both hosts are behind a \s-1NAT\s0 |
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gateway. |
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.PP |
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If the hosts cannot reach each other because they have no common protocol, |
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the originator instead use the router with highest priority and matching |
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protocol as peer. Since the \s-1SRCDST\s0 field is not encrypted, the router host |
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can just forward the packet to the destination host. Since each host uses |
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it's own private key, the router will not be able to decrypt or encrypt |
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packets, it will just act as a simple router and protocol translator. |
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.PP |
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When no router is connected, the host will aggressively try to connect to |
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all routers, and if a router is asked for an unconnected host it will try |
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to ask another router to establish the connection. |
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.PP |
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\&... more not yet written about the details of the routing, please bug me |
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\&... |
