1 * Design of new, multi-subnet secnet protocol
3 Like the first (1995/6) version, we're tunnelling IP packets inside
4 UDP packets. To defeat various restrictions which may be imposed on us
5 by network providers (like the prohibition of incoming TCP
6 connections) we're sticking with UDP for everything this time,
7 including key setup. This means we have to handle retries, etc.
9 Other new features include being able to deal with subnets hidden
10 behind changing 'real' IP addresses, and the ability to choose
11 algorithms and keys per pair of communicating sites.
13 ** Configuration and structure
17 The network is made up from a number of 'sites'. These are collections
18 of machines with private IP addresses. The new secnet code runs on
19 machines which have interfaces on the private site network and some
20 way of accessing the 'real' internet.
22 Each end of a tunnel is identified by a name. Often it will be
23 convenient for every gateway machine to use the same name for each
24 tunnel endpoint, but this is not vital. Individual tunnels are
25 identified by their two endpoint names.
29 It appears that people want to be able to use secnet on mobile
30 machines like laptops as well as to interconnect sites. In particular,
31 they want to be able to use their laptop in three situations:
33 1) connected to their internal LAN by a cable; no tunnel involved
34 2) connected via wireless, using a tunnel to protect traffic
35 3) connected to some other network, using a tunnel to access the
38 They want the laptop to keep the same IP address all the time.
42 Case (2) requires that the laptop run a copy of secnet, and have a
43 tunnel configured between it and the main internal LAN default
44 gateway. secnet must support the concept of a 'soft' tunnel where it
45 adds a route and causes the gateway to do proxy-ARP when the tunnel is
46 up, and removes the route again when the tunnel is down.
48 The usual prohibition of packets coming in from one tunnel and going
49 out another must be relaxed in this case (in particular, the
50 destination address of packets from these 'mobile station' tunnels may
51 be another tunnel as well as the host).
53 (Quick sanity check: if chiark's secnet address was in
54 192.168.73.0/24, would this work properly? Yes, because there will be
55 an explicit route to it, and proxy ARP will be done for it. Do we want
56 packets from the chiark tunnel to be able to go out along other
57 routes? No. So, spotting a 'local' address in a remote site's list of
58 networks isn't sufficient to switch on routing for a site. We need an
59 explicit option. NB packets may be routed if the source OR the
60 destination is marked as allowing routing [otherwise packets couldn't
61 get back from eg. chiark to a laptop at greenend]).
65 secnet sites are configured to grant access to particular IP address
66 ranges to the holder of a particular public key. The key can certify
67 other keys, which will then be permitted to use a subrange of the IP
68 address range of the certifying key.
70 This means that secnet won't know in advance (i.e. at configuration
71 time) how many tunnels it might be required to support, so we have to
72 be able to create them (and routes, and so on) on the fly.
74 ** VPN-level configuration
76 At a high level we just want to be able to indicate which groups of
77 users can claim ownership of which ranges of IP addresses. Assuming
78 these users (or their representatives) all have accounts on a single
79 machine, we can automate the submission of keys and other information
80 to make up a 'sites' file for the entire VPN.
82 The distributed 'sites' file should be in a more restricted format
83 than the secnet configuration file, to prevent attackers who manage to
84 distribute bogus sites files from taking over their victim's machines.
86 The distributed 'sites' file is read one line at a time. Each line
87 consists of a keyword followed by other information. It defines a
88 number of VPNs; within each VPN it defines a number of locations;
89 within each location it defines a number of sites. These VPNs,
90 locations and sites are turned into a secnet.conf file fragment using
93 Some keywords are valid at any 'level' of the distributed 'sites'
94 file, indicating defaults.
98 vpn n: we are now declaring information to do with VPN 'n'. Must come first.
100 location n: we are now declaring information for location 'n'.
102 site n: we are now declaring information for site 'n'.
103 endsite: we're finished declaring information for the current site
105 restrict-nets a b c ...: restrict the allowable 'networks' for the current
106 level to those in this list.
107 end-definitions: prevent definition of further vpns and locations, and
108 modification of defaults at VPN level
110 dh x y: the current VPN uses the specified group; x=modulus, y=generator
112 hash x: which hash function to use. Valid options are 'md5' and 'sha1'.
114 admin n: administrator email address for current level
122 address a b: a=dnsname, b=port
124 pubkey x y z: x=keylen, y=encryption key, z=modulus
125 mobile: declare this to be a 'mobile' site
129 There are several possible ways of running secnet:
131 'reporting' only: --version, --help, etc. command line options and the
132 --just-check-config mode.
134 'normal' run: perform setup in the foreground, and then background.
136 'failed' run: setup in the foreground, and terminate with an error
137 before going to background.
139 'reporting' modes should never output anything except to stdout/stderr.
140 'normal' and 'failed' runs output to stdout/stderr before
141 backgrounding, then thereafter output only to log destinations.
143 ** Site long-term keys
145 We use authenticated DH. Sites identify themselves to each other
146 using long-term signing keys.
148 These signing keys may be for a variety of algorithms. (An algorithm
149 specifies completely how to do a signature and verification.)
151 Each site may have several keys. This helps support key rollover and
152 algorithm agility. Several keys of different algorithms can form a
153 key group. Usually a key group consists of keys generated at the same
154 time. A key is identified by a 4-byte group id (invented by its
155 publisher and opaque) plus a 1-byte algorithm id (defined by the
156 protocol spec for each algorithm).
158 Keys are published in key sets. A key set is a collection of key
159 groups (including older keys as well as newer ones) published at a
160 particular time. Key sets have their own 4-byte ids; these are
161 invented by the publisher but are ordered using sequence number
162 arithmetic. This allows reliers to favour new sets over old ones.
164 Within each key set, some groups may be marked as `fallback'. This
165 means a group that should be tolerated by a relier only if the relier
166 doesn't support any non-fallback keys.
168 Keys within groups, and groups within sets, are ordered (by the
169 publisher of the set), from most to least preferred.
171 When deciding which public keys to accept, a relier should:
172 Process each group within the key set.
173 Discard unknown algorithms.
174 Choose a preferred algorithm:
175 Earliest in the group
176 (or local config could have algorithm prefererence).
177 Discard empty groups.
178 Discard unneeded fallback groups:
179 If any (non-empty) non-fallback groups found, discard all
180 fallback groups. Otherwise there are only fallback groups;
181 discard all but first group in the set.
182 Discard any keys exceeding limit on number of keys honoured:
184 Discard keys later in the set
185 In wire protocol, offer the resulting subset of keyids to
186 the peer and a allow the signer to select which key to use
189 In configuration and key management, long-term private and public keys
190 are octet strings. Private keys are generally stored in disk files,
191 one key per file. The octet string for a private key must identify
192 the algorithm (although actually this is wrong and are going to change
193 it later).. The octet string for a public key need not identify the
194 algorithm; when it's loaded the algorithm will be known from context.
196 The group id 00000000 is special. It should contain only one key,
197 algorithm 00. Key 0000000000 refers to the rsa1 key promulgated
198 before the key rollover/advertisement protocols, or the key which
199 should be used by sites running old software.
201 The key set id 00000000 is special and is considered older than all
202 othere key sets (ie this is an exception to the sequence number
203 arithmetic). It is the implied key set id of the rsa1 key
204 promulgated before the key rollover/advertisement protocols.
206 The algorithm 00 is special and refers to the old rsa1 signature
207 protocol but unusually does not identify the hash function. The hash
208 function is conventional and must be specified out of band. In known
209 existing installations it is SHA-1.
213 *** Protocol environment:
215 Each gateway machine serves a particular, well-known set of private IP
216 addresses (i.e. the agreement over which addresses it serves is
217 outside the scope of this discussion). Each gateway machine has an IP
218 address on the interconnecting network (usually the Internet), which
219 may be dynamically allocated and may change at any point.
221 Each gateway knows the RSA public keys of the other gateways with
222 which it wishes to communicate. The mechanism by which this happens is
223 outside the scope of this discussion. There exists a means by which
224 each gateway can look up the probable IP address of any other.
228 The ultimate goal of the protocol is for the originating gateway
229 machine to be able to forward packets from its section of the private
230 network to the appropriate gateway machine for the destination
231 machine, in such a way that it can be sure that the packets are being
232 sent to the correct destination machine, the destination machine can
233 be sure that the source of the packets is the originating gateway
234 machine, and the contents of the packets cannot be understood other
235 than by the two communicating gateways.
237 XXX not sure about the address-change stuff; leave it out of the first
238 version of the protocol. From experience, IP addresses seem to be
239 quite stable so the feature doesn't gain us much.
241 **** Protocol sub-goal 1: establish a shared key
245 A is the originating gateway machine name
246 B is the destination gateway machine name
247 A+ and B+ are the names with optional additional data, see below
248 PK_A is the public RSA key of A
249 PK_B is the public RSA key of B
250 PK_A^-1 is the private RSA key of A
251 PK_B^-1 is the private RSA key of B
252 x is the fresh private DH key of A
253 y is the fresh private DH key of B
255 g and m are generator and modulus for Diffie-Hellman
256 nA is a nonce generated by A
257 nB is a nonce generated by B
258 iA is an index generated by A, to be used in packets sent from B to A
259 iB is an index generated by B, to be used in packets sent from A to B
260 i? is appropriate index for receiver
262 Note that 'i' may be re-used from one session to the next, whereas 'n'
265 The optional additional data after the sender's name consists of some
266 initial subset of the following list of items:
267 * A 32-bit integer with a set of capability flags, representing the
268 abilities of the sender.
269 * In MSG3/MSG4: a 16-bit integer being the sender's MTU, or zero.
270 (In other messages: nothing.) See below.
271 * More data which is yet to be defined and which must be ignored
273 The optional additional data after the receiver's name is not
274 currently used. If any is seen, it must be ignored.
276 Capability flag bits must be in one the following two categories:
278 1. Early capability flags must be advertised in MSG1 or MSG2, as
279 applicable. If MSG3 or MSG4 advertise any "early" capability bits,
280 MSG1 or MSG3 (as applicable) must have advertised them too. Sadly,
281 advertising an early capability flag will produce MSG1s which are
282 not understood by versions of secnet which predate the capability
285 2. Late capability flags are advertised in MSG2 or MSG3, as
286 applicable. They may also appear in MSG1, but this is not
287 guaranteed. MSG4 must advertise the same set as MSG2.
289 Currently, the low 16 bits are allocated for negotiating bulk-crypto
290 transforms. Bits 8 to 15 are used by Secnet as default capability
291 numbers for the various kinds of transform closures: bit 8 is for the
292 original CBCMAC-based transform, and bit 9 for the new EAX transform;
293 bits 10 to 15 are reserved for future expansion. The the low eight bits
294 are reserved for local use, e.g., to allow migration from one set of
295 parameters for a particular transform to a different, incompatible set
296 of parameters for the same transform. Bit 31, if advertised by both
297 ends, indicates that a mobile end gets priority in case of crossed MSG1.
298 The remaining bits have not yet been assigned a purpose.
300 Whether a capability number is early depends on its meaning, rather than
301 being a static property of its number. That said, the mobile-end-gets
302 priority bit (31) is always sent as an `early' capability bit.
307 In older versions of secnet, secnet was not capable of fragmentation
308 or sending ICMP Frag Needed. Administrators were expected to configure
309 consistent MTUs across the network.
311 It is still the case in the current version that the MTUs need to be
312 configured reasonably coherently across the network: the allocated
313 buffer sizes must be sufficient to cope with packets from all other
316 However, provided the buffers are sufficient, all packets will be
317 processed properly: a secnet receiving a packet larger than the
318 applicable MTU for its delivery will either fragment it, or reject it
319 with ICMP Frag Needed.
321 The MTU additional data field allows secnet to advertise an MTU to the
322 peer. This allows the sending end to handle overlarge packets, before
323 they are transmitted across the underlying public network. This can
324 therefore be used to work around underlying network braindamage
325 affecting large packets.
327 If the MTU additional data field is zero or not present, then the peer
328 should use locally-configured MTU information (normally, its local
329 netlink MTU) instead.
331 If it is nonzero, the peer may send packets up to the advertised size
332 (and if that size is bigger than the peer's administratively
333 configured size, the advertiser promises that its buffers can handle
334 such a large packet).
336 A secnet instance should not assume that just because it has
337 advertised an mtu which is lower than usual for the vpn, the peer will
338 honour it, unless the administrator knows that the peers are
339 sufficiently modern to understand the mtu advertisement option. So
340 secnet will still accept packets which exceed the link MTU (whether
341 negotiated or assumed).
346 1) A->B: i*,iA,msg1,A+,B+,nA
348 i* must be encoded as 0. (However, it is permitted for a site to use
349 zero as its "index" for another site.)
351 2) B->A: iA,iB,msg2,B+,A+,nB,nA
353 (The order of B and A reverses in alternate messages so that the same
354 code can be used to construct them...)
356 3) A->B: {iB,iA,msg3,A+,B+,[chosen-transform],nA,nB,g^x mod m}_PK_A^-1
358 If message 1 was a replay then A will not generate message 3, because
359 it doesn't recognise nA.
361 If message 2 was from an attacker then B will not generate message 4,
362 because it doesn't recognise nB.
364 4) B->A: {iA,iB,msg4,B+,A+,nB,nA,g^y mod m}_PK_B^-1
366 At this point, A and B share a key, k. B must keep retransmitting
367 message 4 until it receives a packet encrypted using key k.
369 5) A: iB,iA,msg5,(ping/msg5)_k
371 6) B: iA,iB,msg6,(pong/msg6)_k
373 (Note that these are encrypted using the same transform that's used
374 for normal traffic, so they include sequence number, MAC, etc.)
376 The ping and pong messages can be used by either end of the tunnel at
377 any time, but using msg0 as the unencrypted message type indicator.
379 **** Protocol sub-goal 2: end the use of a shared key
381 7) i?,i?,msg0,(end-session/msg7,A,B)_k
383 This message can be sent by either party. Once sent, k can be
384 forgotten. Once received and checked, k can be forgotten. No need to
385 retransmit or confirm reception. It is suggested that this message be
386 sent when a key times out, or the tunnel is forcibly terminated for
389 **** Protocol sub-goal 3: send a packet
391 8) i?,i?,msg0,(send-packet/msg9,packet)_k
395 9) i?,i?,NAK (NAK is encoded as zero)
397 If the link-layer can't work out what to do with a packet (session has
398 gone away, etc.) it can transmit a NAK back to the sender.
400 This can alert the sender to the situation where the sender has a key
401 but the receiver doesn't (eg because it has been restarted). The
402 sender, on receiving the NAK, will try to initiate a key exchange.
404 Forged (or overly delayed) NAKs can cause wasted resources due to
405 spurious key exchange initiation, but there is a limit on this because
406 of the key exchange retry timeout.
408 10) i?,i?,msg8,A,B,nA,nB,msg?
410 This is an obsolete form of NAK packet which is not sent by any even
411 vaguely recent version of secnet. (In fact, there is no evidence in
412 the git history of it ever being sent.)
414 This message number is reserved.
418 Sent in response to a NAK from B to A. Requests that B initiates a
419 key exchange with A, if B is willing and lacks a transport key for A.
420 (If B doesn't have A's address configured, implicitly supplies A's
423 This is necessary because if one end of a link (B) is restarted while
424 a key exchange is in progress, the following bad state can persist:
425 the non-restarted end (A) thinks that the key is still valid and keeps
426 sending packets, but B either doesn't realise that a key exchange with
427 A is necessary or (if A is a mobile site) doesn't know A's public IP
430 Normally in these circumstances B would send NAKs to A, causing A to
431 initiate a key exchange. However if A and B were already in the
432 middle of a key exchange then A will not want to try another one until
433 the first one has timed out ("setup-time" x "setup-retries") and then
434 the key exchange retry timeout ("wait-time") has elapsed.
436 However if B's setup has timed out, B would be willing to participate
437 in a key exchange initiated by A, if A could be induced to do so.
438 This is the purpose of the PROD packet.
440 We send no more PRODs than we would want to send data packets, to
441 avoid a traffic amplification attack. We also send them only in state
442 WAIT, as in other states we wouldn't respond favourably. And we only
443 honour them if we don't already have a key.
445 With PROD, the period of broken communication due to a key exchange
446 interrupted by a restart is limited to the key exchange total
447 retransmission timeout, rather than also including the key exchange