| 1 | * Design of new, multi-subnet secnet protocol |
| 2 | |
| 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. |
| 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 | |
| 15 | [The original plan] |
| 16 | |
| 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 | |
| 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 | [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 | |
| 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 | |
| 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 | |
| 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 | |
| 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 | |
| 201 | Messages: |
| 202 | |
| 203 | 1) A->B: *,iA,msg1,A,B,protorange-A,nA |
| 204 | |
| 205 | 2) B->A: iA,iB,msg2,B,A,chosen-protocol,nB,nA |
| 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 | |
| 210 | 3) A->B: {iB,iA,msg3,A,B,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1 |
| 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 | |
| 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 |
| 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 | |
| 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 | |
| 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 | |
| 250 | 8) i?,i?,NAK (encoded as zero) |
| 251 | |
| 252 | If the link-layer can't work out what to do with a packet (session has |
| 253 | gone away, etc.) it can transmit a NAK back to the sender. The sender |
| 254 | can then try to verify whether the session is alive by sending ping |
| 255 | packets, and forget the key if it isn't. Potential denial-of-service |
| 256 | if the attacker can stop the ping/pong packets getting through (the |
| 257 | key will be forgotten and another key setup must take place), but if |
| 258 | they can delete packets then we've lost anyway... |
| 259 | |
| 260 | The attacker can of course forge NAKs since they aren't protected. But |
| 261 | if they can only forge packets then they won't be able to stop the |
| 262 | ping/pong working. Trust in NAKs can be rate-limited... |
| 263 | |
| 264 | Alternative idea (which is actually implemented): if you receive a |
| 265 | packet you can't decode, because there's no key established, then |
| 266 | initiate key setup... |
| 267 | |
| 268 | Keepalives are probably a good idea. |
| 269 | |
| 270 | **** Protocol sub-goal 3: send a packet |
| 271 | |
| 272 | 9) i?,i?,msg0,(send-packet/msg9,packet)_k |
| 273 | |
| 274 | Some messages may take a long time to prepare (software modexp on slow |
| 275 | machines); this is a "please wait" message to indicate that a message |
| 276 | is in preparation. |
| 277 | |
| 278 | 10) i?,i?,msg8,A,B,nA,nB,msg? |