Commit | Line | Data |
---|---|---|
974d0468 | 1 | * Design of new, multi-subnet secnet protocol |
2fe58dfd | 2 | |
974d0468 SE |
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, | |
3454dce4 | 7 | including key setup. This means we have to handle retries, etc. |
2fe58dfd SE |
8 | |
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. | |
12 | ||
13 | ** Configuration and structure | |
14 | ||
3454dce4 SE |
15 | [The original plan] |
16 | ||
2fe58dfd SE |
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. | |
21 | ||
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. | |
26 | ||
3454dce4 SE |
27 | [The new plan] |
28 | ||
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: | |
32 | ||
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 | |
36 | internal LAN. | |
37 | ||
38 | They want the laptop to keep the same IP address all the time. | |
39 | ||
40 | Case (1) is simple. | |
41 | ||
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. | |
47 | ||
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). | |
52 | ||
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]). | |
62 | ||
4f5e39ec SE |
63 | [the even newer plan] |
64 | ||
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. | |
69 | ||
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. | |
73 | ||
3454dce4 SE |
74 | ** VPN-level configuration |
75 | ||
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. | |
81 | ||
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. | |
85 | ||
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 | |
91 | a script. | |
92 | ||
93 | Some keywords are valid at any 'level' of the distributed 'sites' | |
94 | file, indicating defaults. | |
95 | ||
96 | The keywords are: | |
97 | ||
98 | vpn n: we are now declaring information to do with VPN 'n'. Must come first. | |
99 | ||
100 | location n: we are now declaring information for location 'n'. | |
101 | ||
102 | site n: we are now declaring information for site 'n'. | |
103 | endsite: we're finished declaring information for the current site | |
104 | ||
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 | |
109 | ||
110 | dh x y: the current VPN uses the specified group; x=modulus, y=generator | |
111 | ||
112 | hash x: which hash function to use. Valid options are 'md5' and 'sha1'. | |
113 | ||
114 | admin n: administrator email address for current level | |
115 | ||
116 | key-lifetime n | |
117 | setup-retries n | |
118 | setup-timeout n | |
119 | wait-time n | |
120 | renegotiate-time n | |
121 | ||
122 | address a b: a=dnsname, b=port | |
123 | networks a b c ... | |
124 | pubkey x y z: x=keylen, y=encryption key, z=modulus | |
125 | mobile: declare this to be a 'mobile' site | |
126 | ||
b2a56f7c SE |
127 | ** Logging etc. |
128 | ||
129 | There are several possible ways of running secnet: | |
130 | ||
131 | 'reporting' only: --version, --help, etc. command line options and the | |
132 | --just-check-config mode. | |
133 | ||
134 | 'normal' run: perform setup in the foreground, and then background. | |
135 | ||
136 | 'failed' run: setup in the foreground, and terminate with an error | |
137 | before going to background. | |
138 | ||
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. | |
142 | ||
2fe58dfd SE |
143 | ** Protocols |
144 | ||
145 | *** Protocol environment: | |
146 | ||
147 | Each gateway machine serves a particular, well-known set of private IP | |
148 | addresses (i.e. the agreement over which addresses it serves is | |
149 | outside the scope of this discussion). Each gateway machine has an IP | |
150 | address on the interconnecting network (usually the Internet), which | |
151 | may be dynamically allocated and may change at any point. | |
152 | ||
153 | Each gateway knows the RSA public keys of the other gateways with | |
154 | which it wishes to communicate. The mechanism by which this happens is | |
155 | outside the scope of this discussion. There exists a means by which | |
156 | each gateway can look up the probable IP address of any other. | |
157 | ||
158 | *** Protocol goals: | |
159 | ||
160 | The ultimate goal of the protocol is for the originating gateway | |
161 | machine to be able to forward packets from its section of the private | |
162 | network to the appropriate gateway machine for the destination | |
163 | machine, in such a way that it can be sure that the packets are being | |
164 | sent to the correct destination machine, the destination machine can | |
165 | be sure that the source of the packets is the originating gateway | |
166 | machine, and the contents of the packets cannot be understood other | |
167 | than by the two communicating gateways. | |
168 | ||
169 | XXX not sure about the address-change stuff; leave it out of the first | |
170 | version of the protocol. From experience, IP addresses seem to be | |
171 | quite stable so the feature doesn't gain us much. | |
172 | ||
173 | **** Protocol sub-goal 1: establish a shared key | |
174 | ||
175 | Definitions: | |
176 | ||
177 | A is the originating gateway machine | |
178 | B is the destination gateway machine | |
179 | PK_A is the public RSA key of A | |
180 | PK_B is the public RSA key of B | |
181 | PK_A^-1 is the private RSA key of A | |
182 | PK_B^-1 is the private RSA key of B | |
183 | x is the fresh private DH key of A | |
184 | y is the fresh private DH key of B | |
185 | k is g^xy mod m | |
186 | g and m are generator and modulus for Diffie-Hellman | |
187 | nA is a nonce generated by A | |
188 | nB is a nonce generated by B | |
189 | iA is an index generated by A, to be used in packets sent from B to A | |
190 | iB is an index generated by B, to be used in packets sent from A to B | |
191 | i? is appropriate index for receiver | |
192 | ||
193 | Note that 'i' may be re-used from one session to the next, whereas 'n' | |
194 | is always fresh. | |
195 | ||
ff05a229 SE |
196 | The protocol version selection stuff is not yet implemented: I'm not |
197 | yet convinced it's a good idea. Instead, the initiator could try | |
198 | using its preferred protocol (which starts with a different magic | |
199 | number) and fall back if there's no reply. | |
200 | ||
2fe58dfd SE |
201 | Messages: |
202 | ||
baa06aeb | 203 | 1) A->B: *,iA,msg1,A,B,protorange-A,nA |
2fe58dfd | 204 | |
baa06aeb | 205 | 2) B->A: iA,iB,msg2,B,A,chosen-protocol,nB,nA |
2fe58dfd SE |
206 | |
207 | (The order of B and A reverses in alternate messages so that the same | |
208 | code can be used to construct them...) | |
209 | ||
baa06aeb | 210 | 3) A->B: {iB,iA,msg3,A,B,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1 |
2fe58dfd SE |
211 | |
212 | If message 1 was a replay then A will not generate message 3, because | |
213 | it doesn't recognise nA. | |
214 | ||
215 | If message 2 was from an attacker then B will not generate message 4, | |
216 | because it doesn't recognise nB. | |
217 | ||
baa06aeb SE |
218 | If an attacker is trying to manipulate the chosen protocol, B can spot |
219 | this when it sees A's message 3. | |
220 | ||
221 | 4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,nB,nA,g^y mod m}_PK_B^-1 | |
2fe58dfd SE |
222 | |
223 | At this point, A and B share a key, k. B must keep retransmitting | |
224 | message 4 until it receives a packet encrypted using key k. | |
225 | ||
baa06aeb SE |
226 | A can abandon the exchange if the chosen protocol is not the one that |
227 | it would have chosen knowing the acceptable protocol ranges of A and | |
228 | B. | |
229 | ||
2fe58dfd SE |
230 | 5) A: iB,iA,msg5,(ping/msg5)_k |
231 | ||
232 | 6) B: iA,iB,msg6,(pong/msg6)_k | |
233 | ||
234 | (Note that these are encrypted using the same transform that's used | |
235 | for normal traffic, so they include sequence number, MAC, etc.) | |
236 | ||
237 | The ping and pong messages can be used by either end of the tunnel at | |
238 | any time, but using msg0 as the unencrypted message type indicator. | |
239 | ||
240 | **** Protocol sub-goal 2: end the use of a shared key | |
241 | ||
242 | 7) i?,i?,msg0,(end-session/msg7,A,B)_k | |
243 | ||
244 | This message can be sent by either party. Once sent, k can be | |
245 | forgotten. Once received and checked, k can be forgotten. No need to | |
246 | retransmit or confirm reception. It is suggested that this message be | |
247 | sent when a key times out, or the tunnel is forcibly terminated for | |
248 | some reason. | |
249 | ||
1e80c220 | 250 | **** Protocol sub-goal 3: send a packet |
2fe58dfd | 251 | |
1e80c220 | 252 | 8) i?,i?,msg0,(send-packet/msg9,packet)_k |
2fe58dfd | 253 | |
1e80c220 | 254 | **** Other messages |
974d0468 | 255 | |
1e80c220 | 256 | 9) i?,i?,NAK (NAK is encoded as zero) |
2fe58dfd | 257 | |
1e80c220 IJ |
258 | If the link-layer can't work out what to do with a packet (session has |
259 | gone away, etc.) it can transmit a NAK back to the sender. | |
2fe58dfd | 260 | |
1e80c220 IJ |
261 | This can alert the sender to the situation where the sender has a key |
262 | but the receiver doesn't (eg because it has been restarted). The | |
263 | sender, on receiving the NAK, will try to initiate a key exchange. | |
4f5e39ec | 264 | |
1e80c220 IJ |
265 | Forged (or overly delayed) NAKs can cause wasted resources due to |
266 | spurious key exchange initiation, but there is a limit on this because | |
267 | of the key exchange retry timeout. | |
4f5e39ec SE |
268 | |
269 | 10) i?,i?,msg8,A,B,nA,nB,msg? | |
1e80c220 IJ |
270 | |
271 | This is an obsolete form of NAK packet which is not sent by any even | |
272 | vaguely recent version of secnet. (In fact, there is no evidence in | |
273 | the git history of it ever being sent.) | |
274 | ||
275 | This message number is reserved. |