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 | ||
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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 | ||
63 | ** VPN-level configuration | |
64 | ||
65 | At a high level we just want to be able to indicate which groups of | |
66 | users can claim ownership of which ranges of IP addresses. Assuming | |
67 | these users (or their representatives) all have accounts on a single | |
68 | machine, we can automate the submission of keys and other information | |
69 | to make up a 'sites' file for the entire VPN. | |
70 | ||
71 | The distributed 'sites' file should be in a more restricted format | |
72 | than the secnet configuration file, to prevent attackers who manage to | |
73 | distribute bogus sites files from taking over their victim's machines. | |
74 | ||
75 | The distributed 'sites' file is read one line at a time. Each line | |
76 | consists of a keyword followed by other information. It defines a | |
77 | number of VPNs; within each VPN it defines a number of locations; | |
78 | within each location it defines a number of sites. These VPNs, | |
79 | locations and sites are turned into a secnet.conf file fragment using | |
80 | a script. | |
81 | ||
82 | Some keywords are valid at any 'level' of the distributed 'sites' | |
83 | file, indicating defaults. | |
84 | ||
85 | The keywords are: | |
86 | ||
87 | vpn n: we are now declaring information to do with VPN 'n'. Must come first. | |
88 | ||
89 | location n: we are now declaring information for location 'n'. | |
90 | ||
91 | site n: we are now declaring information for site 'n'. | |
92 | endsite: we're finished declaring information for the current site | |
93 | ||
94 | restrict-nets a b c ...: restrict the allowable 'networks' for the current | |
95 | level to those in this list. | |
96 | end-definitions: prevent definition of further vpns and locations, and | |
97 | modification of defaults at VPN level | |
98 | ||
99 | dh x y: the current VPN uses the specified group; x=modulus, y=generator | |
100 | ||
101 | hash x: which hash function to use. Valid options are 'md5' and 'sha1'. | |
102 | ||
103 | admin n: administrator email address for current level | |
104 | ||
105 | key-lifetime n | |
106 | setup-retries n | |
107 | setup-timeout n | |
108 | wait-time n | |
109 | renegotiate-time n | |
110 | ||
111 | address a b: a=dnsname, b=port | |
112 | networks a b c ... | |
113 | pubkey x y z: x=keylen, y=encryption key, z=modulus | |
114 | mobile: declare this to be a 'mobile' site | |
115 | ||
b2a56f7c SE |
116 | ** Logging etc. |
117 | ||
118 | There are several possible ways of running secnet: | |
119 | ||
120 | 'reporting' only: --version, --help, etc. command line options and the | |
121 | --just-check-config mode. | |
122 | ||
123 | 'normal' run: perform setup in the foreground, and then background. | |
124 | ||
125 | 'failed' run: setup in the foreground, and terminate with an error | |
126 | before going to background. | |
127 | ||
128 | 'reporting' modes should never output anything except to stdout/stderr. | |
129 | 'normal' and 'failed' runs output to stdout/stderr before | |
130 | backgrounding, then thereafter output only to log destinations. | |
131 | ||
2fe58dfd SE |
132 | ** Protocols |
133 | ||
134 | *** Protocol environment: | |
135 | ||
136 | Each gateway machine serves a particular, well-known set of private IP | |
137 | addresses (i.e. the agreement over which addresses it serves is | |
138 | outside the scope of this discussion). Each gateway machine has an IP | |
139 | address on the interconnecting network (usually the Internet), which | |
140 | may be dynamically allocated and may change at any point. | |
141 | ||
142 | Each gateway knows the RSA public keys of the other gateways with | |
143 | which it wishes to communicate. The mechanism by which this happens is | |
144 | outside the scope of this discussion. There exists a means by which | |
145 | each gateway can look up the probable IP address of any other. | |
146 | ||
147 | *** Protocol goals: | |
148 | ||
149 | The ultimate goal of the protocol is for the originating gateway | |
150 | machine to be able to forward packets from its section of the private | |
151 | network to the appropriate gateway machine for the destination | |
152 | machine, in such a way that it can be sure that the packets are being | |
153 | sent to the correct destination machine, the destination machine can | |
154 | be sure that the source of the packets is the originating gateway | |
155 | machine, and the contents of the packets cannot be understood other | |
156 | than by the two communicating gateways. | |
157 | ||
158 | XXX not sure about the address-change stuff; leave it out of the first | |
159 | version of the protocol. From experience, IP addresses seem to be | |
160 | quite stable so the feature doesn't gain us much. | |
161 | ||
162 | **** Protocol sub-goal 1: establish a shared key | |
163 | ||
164 | Definitions: | |
165 | ||
166 | A is the originating gateway machine | |
167 | B is the destination gateway machine | |
168 | PK_A is the public RSA key of A | |
169 | PK_B is the public RSA key of B | |
170 | PK_A^-1 is the private RSA key of A | |
171 | PK_B^-1 is the private RSA key of B | |
172 | x is the fresh private DH key of A | |
173 | y is the fresh private DH key of B | |
174 | k is g^xy mod m | |
175 | g and m are generator and modulus for Diffie-Hellman | |
176 | nA is a nonce generated by A | |
177 | nB is a nonce generated by B | |
178 | iA is an index generated by A, to be used in packets sent from B to A | |
179 | iB is an index generated by B, to be used in packets sent from A to B | |
180 | i? is appropriate index for receiver | |
181 | ||
182 | Note that 'i' may be re-used from one session to the next, whereas 'n' | |
183 | is always fresh. | |
184 | ||
185 | Messages: | |
186 | ||
baa06aeb | 187 | 1) A->B: *,iA,msg1,A,B,protorange-A,nA |
2fe58dfd | 188 | |
baa06aeb | 189 | 2) B->A: iA,iB,msg2,B,A,chosen-protocol,nB,nA |
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190 | |
191 | (The order of B and A reverses in alternate messages so that the same | |
192 | code can be used to construct them...) | |
193 | ||
baa06aeb | 194 | 3) A->B: {iB,iA,msg3,A,B,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1 |
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195 | |
196 | If message 1 was a replay then A will not generate message 3, because | |
197 | it doesn't recognise nA. | |
198 | ||
199 | If message 2 was from an attacker then B will not generate message 4, | |
200 | because it doesn't recognise nB. | |
201 | ||
baa06aeb SE |
202 | If an attacker is trying to manipulate the chosen protocol, B can spot |
203 | this when it sees A's message 3. | |
204 | ||
205 | 4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,nB,nA,g^y mod m}_PK_B^-1 | |
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206 | |
207 | At this point, A and B share a key, k. B must keep retransmitting | |
208 | message 4 until it receives a packet encrypted using key k. | |
209 | ||
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210 | A can abandon the exchange if the chosen protocol is not the one that |
211 | it would have chosen knowing the acceptable protocol ranges of A and | |
212 | B. | |
213 | ||
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214 | 5) A: iB,iA,msg5,(ping/msg5)_k |
215 | ||
216 | 6) B: iA,iB,msg6,(pong/msg6)_k | |
217 | ||
218 | (Note that these are encrypted using the same transform that's used | |
219 | for normal traffic, so they include sequence number, MAC, etc.) | |
220 | ||
221 | The ping and pong messages can be used by either end of the tunnel at | |
222 | any time, but using msg0 as the unencrypted message type indicator. | |
223 | ||
224 | **** Protocol sub-goal 2: end the use of a shared key | |
225 | ||
226 | 7) i?,i?,msg0,(end-session/msg7,A,B)_k | |
227 | ||
228 | This message can be sent by either party. Once sent, k can be | |
229 | forgotten. Once received and checked, k can be forgotten. No need to | |
230 | retransmit or confirm reception. It is suggested that this message be | |
231 | sent when a key times out, or the tunnel is forcibly terminated for | |
232 | some reason. | |
233 | ||
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234 | XXX not yet implemented. |
235 | ||
2fe58dfd SE |
236 | 8) i?,i?,NAK/msg8 |
237 | ||
238 | If the link-layer can't work out what to do with a packet (session has | |
239 | gone away, etc.) it can transmit a NAK back to the sender. The sender | |
240 | can then try to verify whether the session is alive by sending ping | |
241 | packets, and forget the key if it isn't. Potential denial-of-service | |
242 | if the attacker can stop the ping/pong packets getting through (the | |
243 | key will be forgotten and another key setup must take place), but if | |
244 | they can delete packets then we've lost anyway... | |
245 | ||
246 | The attacker can of course forge NAKs since they aren't protected. But | |
247 | if they can only forge packets then they won't be able to stop the | |
248 | ping/pong working. Trust in NAKs can be rate-limited... | |
249 | ||
974d0468 SE |
250 | Alternative idea (which is actually implemented): if you receive a |
251 | packet you can't decode, because there's no key established, then | |
252 | initiate key setup... | |
253 | ||
254 | Keepalives are probably a good idea. | |
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255 | |
256 | **** Protocol sub-goal 3: send a packet | |
257 | ||
258 | 9) i?,i?,msg0,(send-packet/msg9,packet)_k |