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133 | .\" ======================================================================== |
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134 | .\" |
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135 | .IX Title "GVPE.PROTOCOL 7" |
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136 | .TH GVPE.PROTOCOL 7 "2015-10-31" "2.25" "GNU Virtual Private Ethernet" |
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137 | .\" For nroff, turn off justification. Always turn off hyphenation; it makes |
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138 | .\" way too many mistakes in technical documents. |
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139 | .if n .ad l |
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140 | .nh |
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141 | .SH "The GNU-VPE Protocols" |
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142 | .IX Header "The GNU-VPE Protocols" |
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143 | .SH "Overview" |
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144 | .IX Header "Overview" |
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145 | \&\s-1GVPE\s0 can make use of a number of protocols. One of them is the \s-1GNU VPE\s0 |
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146 | protocol which is used to authenticate tunnels and send encrypted data |
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147 | packets. This protocol is described in more detail the second part of this |
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148 | document. |
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149 | .PP |
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150 | The first part of this document describes the transport protocols which |
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151 | are used by \s-1GVPE\s0 to send its data packets over the network. |
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152 | .SH "PART 1: Transport protocols" |
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153 | .IX Header "PART 1: Transport protocols" |
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154 | \&\s-1GVPE\s0 offers a wide range of transport protocols that can be used to |
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155 | interchange data between nodes. Protocols differ in their overhead, speed, |
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156 | reliability, and robustness. |
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157 | .PP |
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158 | The following sections describe each transport protocol in more |
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159 | detail. They are sorted by overhead/efficiency, the most efficient |
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160 | transport is listed first: |
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161 | .SS "\s-1RAW IP\s0" |
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162 | .IX Subsection "RAW IP" |
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163 | This protocol is the best choice, performance-wise, as the minimum |
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164 | overhead per packet is only 38 bytes. |
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165 | .PP |
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166 | It works by sending the \s-1VPN\s0 payload using raw \s-1IP\s0 frames (using the |
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167 | protocol set by \f(CW\*(C`ip\-proto\*(C'\fR). |
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168 | .PP |
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169 | Using raw \s-1IP\s0 frames has the drawback that many firewalls block \*(L"unknown\*(R" |
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170 | protocols, so this transport only works if you have full \s-1IP\s0 connectivity |
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171 | between nodes. |
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172 | .SS "\s-1ICMP\s0" |
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173 | .IX Subsection "ICMP" |
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174 | This protocol offers very low overhead (minimum 42 bytes), and can |
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175 | sometimes tunnel through firewalls when other protocols can not. |
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176 | .PP |
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177 | It works by prepending an \s-1ICMP\s0 header with type \f(CW\*(C`icmp\-type\*(C'\fR and a code |
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178 | of \f(CW255\fR. The default \f(CW\*(C`icmp\-type\*(C'\fR is \f(CW\*(C`echo\-reply\*(C'\fR, so the resulting |
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179 | packets look like echo replies, which looks rather strange to network |
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180 | administrators. |
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181 | .PP |
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182 | This transport should only be used if other transports (i.e. raw \s-1IP\s0) are |
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183 | not available or undesirable (due to their overhead). |
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184 | .SS "\s-1UDP\s0" |
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185 | .IX Subsection "UDP" |
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186 | This is a good general choice for the transport protocol as \s-1UDP\s0 packets |
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187 | tunnel well through most firewalls and routers, and the overhead per |
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188 | packet is moderate (minimum 58 bytes). |
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189 | .PP |
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190 | It should be used if \s-1RAW IP\s0 is not available. |
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191 | .SS "\s-1TCP\s0" |
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192 | .IX Subsection "TCP" |
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193 | This protocol is a very bad choice, as it not only has high overhead (more |
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194 | than 60 bytes), but the transport also retries on its own, which leads |
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195 | to congestion when the link has moderate packet loss (as both the \s-1TCP\s0 |
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196 | transport and the tunneled traffic will retry, increasing congestion more |
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197 | and more). It also has high latency and is quite inefficient. |
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198 | .PP |
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199 | It's only useful when tunneling through firewalls that block better |
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200 | protocols. If a node doesn't have direct internet access but a \s-1HTTP\s0 proxy |
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201 | that supports the \s-1CONNECT\s0 method it can be used to tunnel through a web |
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202 | proxy. For this to work, the \f(CW\*(C`tcp\-port\*(C'\fR should be \f(CW443\fR (\f(CW\*(C`https\*(C'\fR), as |
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203 | most proxies do not allow connections to other ports. |
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204 | .PP |
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205 | It is an abuse of the usage a proxy was designed for, so make sure you are |
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206 | allowed to use it for \s-1GVPE.\s0 |
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207 | .PP |
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208 | This protocol also has server and client sides. If the \f(CW\*(C`tcp\-port\*(C'\fR is |
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209 | set to zero, other nodes cannot connect to this node directly. If the |
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210 | \&\f(CW\*(C`tcp\-port\*(C'\fR is non-zero, the node can act both as a client as well as a |
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211 | server. |
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212 | .SS "\s-1DNS\s0" |
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213 | .IX Subsection "DNS" |
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214 | \&\fB\s-1WARNING:\s0\fR Parsing and generating \s-1DNS\s0 packets is rather tricky. The code |
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215 | almost certainly contains buffer overflows and other, likely exploitable, |
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216 | bugs. You have been warned. |
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217 | .PP |
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218 | This is the worst choice of transport protocol with respect to overhead |
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219 | (overhead can be 2\-3 times higher than the transferred data), and latency |
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220 | (which can be many seconds). Some \s-1DNS\s0 servers might not be prepared to |
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221 | handle the traffic and drop or corrupt packets. The client also has to |
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222 | constantly poll the server for data, so the client will constantly create |
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223 | traffic even if it doesn't need to transport packets. |
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224 | .PP |
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225 | In addition, the same problems as the \s-1TCP\s0 transport also plague this |
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226 | protocol. |
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227 | .PP |
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228 | Its only use is to tunnel through firewalls that do not allow direct |
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229 | internet access. Similar to using a \s-1HTTP\s0 proxy (as the \s-1TCP\s0 transport |
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230 | does), it uses a local \s-1DNS\s0 server/forwarder (given by the \f(CW\*(C`dns\-forw\-host\*(C'\fR |
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231 | configuration value) as a proxy to send and receive data as a client, |
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232 | and an \f(CW\*(C`NS\*(C'\fR record pointing to the \s-1GVPE\s0 server (as given by the |
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233 | \&\f(CW\*(C`dns\-hostname\*(C'\fR directive). |
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234 | .PP |
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235 | The only good side of this protocol is that it can tunnel through most |
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236 | firewalls mostly undetected, iff the local \s-1DNS\s0 server/forwarder is sane |
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237 | (which is true for most routers, wireless \s-1LAN\s0 gateways and nameservers). |
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238 | .PP |
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239 | Fine-tuning needs to be done by editing \f(CW\*(C`src/vpn_dns.C\*(C'\fR directly. |
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240 | .SH "PART 2: The GNU VPE protocol" |
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241 | .IX Header "PART 2: The GNU VPE protocol" |
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242 | This section, unfortunately, is not yet finished, although the protocol |
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243 | is stable (until bugs in the cryptography are found, which will likely |
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244 | completely change the following description). Nevertheless, it should give |
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245 | you some overview over the protocol. |
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246 | .SS "Anatomy of a \s-1VPN\s0 packet" |
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247 | .IX Subsection "Anatomy of a VPN packet" |
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248 | The exact layout and field lengths of a \s-1VPN\s0 packet is determined at |
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249 | compile time and doesn't change. The same structure is used for all |
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250 | transport protocols, be it \s-1RAWIP\s0 or \s-1TCP.\s0 |
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251 | .PP |
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252 | .Vb 3 |
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253 | \& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+ |
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254 | \& | HMAC | TYPE | SRCDST | DATA | |
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255 | \& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-+ |
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256 | .Ve |
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257 | .PP |
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258 | The \s-1HMAC\s0 field is present in all packets, even if not used (e.g. in auth |
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259 | request packets), in which case it is set to all zeroes. The \s-1MAC\s0 itself is |
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260 | calculated over the \s-1TYPE, SRCDST\s0 and \s-1DATA\s0 fields in all cases. |
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261 | .PP |
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262 | The \s-1TYPE\s0 field is a single byte and determines the purpose of the packet |
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263 | (e.g. \s-1RESET, COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE, |
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264 | CONNECT REQUEST/INFO\s0 etc.). |
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265 | .PP |
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266 | \&\s-1SRCDST\s0 is a three byte field which contains the source and destination |
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267 | node IDs (12 bits each). |
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268 | .PP |
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269 | The \s-1DATA\s0 portion differs between each packet type, naturally, and is the |
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270 | only part that can be encrypted. Data packets contain more fields, as |
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271 | shown: |
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272 | .PP |
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273 | .Vb 3 |
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274 | \& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-\-+\-\-\-\-\-\-+ |
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275 | \& | HMAC | TYPE | SRCDST | SEQNO | DATA | |
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276 | \& +\-\-\-\-\-\-+\-\-\-\-\-\-+\-\-\-\-\-\-\-\-+\-\-\-\-\-\-\-+\-\-\-\-\-\-+ |
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277 | .Ve |
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278 | .PP |
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279 | \&\s-1SEQNO\s0 is a 32\-bit sequence number. It is negotiated at every connection |
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280 | initialization and starts at some random 31 bit value. \s-1GVPE\s0 currently uses |
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281 | a sliding window of 512 packets/sequence numbers to detect reordering, |
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282 | duplication and replay attacks. |
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283 | .PP |
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284 | The encryption is done on \s-1SEQNO+DATA\s0 in \s-1CTR\s0 mode with \s-1IV\s0 generated from |
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285 | the seqno (for \s-1AES:\s0 seqno || seqno || seqno || (u32)0), which ensures |
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286 | uniqueness for a given key. |
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287 | .SS "The authentication/key exchange protocol" |
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288 | .IX Subsection "The authentication/key exchange protocol" |
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289 | Before nodes can exchange packets, they need to establish authenticity of |
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290 | the other side and a key. Every node has a private \s-1RSA\s0 key and the public |
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291 | \&\s-1RSA\s0 keys of all other nodes. |
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292 | .PP |
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293 | When a node wants to establish a connection to another node, it sends an |
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294 | RSA-OEAP-encrypted challenge and an \s-1ECDH \s0(curve25519) key. The other node |
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295 | replies with its own \s-1ECDH\s0 key and a \s-1HKDF\s0 of the challenge and both \s-1ECDH\s0 |
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296 | keys to prove its identity. |
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297 | .PP |
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298 | The remote node enganges in exactly the same protocol. When both nodes |
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299 | have exchanged their challenge and verified the response, they calculate a |
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300 | cipher key and a \s-1HMAC\s0 key and start exchanging data packets. |
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301 | .PP |
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302 | In detail, the challenge consist of: |
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303 | .PP |
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304 | .Vb 1 |
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305 | \& RSA\-OAEP (SEQNO MAC CIPHER SALT EXTRA\-AUTH) ECDH1 |
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306 | .Ve |
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307 | .PP |
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308 | That is, it encrypts (with the public key of the remote node) an initial |
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309 | sequence number for data packets, key material for the \s-1HMAC\s0 key, key |
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310 | material for the cipher key, a salt used by the \s-1HKDF \s0(as shown later) and |
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311 | some extra random bytes that are unused except for authentication. It also |
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312 | sends the public key of a curve25519 exchange. |
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313 | .PP |
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314 | The remote node decrypts the \s-1RSA\s0 data, generates its own \s-1ECDH\s0 key (\s-1ECDH2\s0), |
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315 | and replies with: |
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316 | .PP |
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317 | .Vb 1 |
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318 | \& HKDF\-Expand (HKDF\-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2 |
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319 | .Ve |
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320 | .PP |
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321 | That is, it extracts from the decrypted \s-1RSA\s0 challenge, using its \s-1ECDH\s0 |
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322 | key as salt, and then expands using the requesting node's \s-1ECDH1\s0 key. The |
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323 | resulting hash is returned as a proof that the node could decrypt the \s-1RSA\s0 |
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324 | challenge data, together with the \s-1ECDH\s0 key. |
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325 | .PP |
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326 | After both nodes have done this to each other, they calculate the shared |
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327 | \&\s-1ECDH\s0 secret, cipher and \s-1HMAC\s0 keys for the session (each node generates two |
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328 | cipher and \s-1HMAC\s0 keys, one for sending and one for receiving). |
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329 | .PP |
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330 | The \s-1HMAC\s0 key for sending is generated as follow: |
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331 | .PP |
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332 | .Vb 1 |
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333 | \& HMAC_KEY = HKDF\-Expand (HKDF\-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE) |
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334 | .Ve |
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335 | .PP |
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336 | It extracts from \s-1MAC\s0 and \s-1ECDH_SECRET\s0 using the \fIremote\fR \s-1SALT,\s0 then |
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337 | expands using a static info string. |
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338 | .PP |
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339 | The cipher key is generated in the same way, except using the \s-1CIPHER\s0 part |
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340 | of the original challenge. |
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341 | .PP |
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342 | The result of this process is to authenticate each node to the other |
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343 | node, while exchanging keys using both \s-1RSA\s0 and \s-1ECDH,\s0 the latter providing |
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344 | perfect forward secrecy. |
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345 | .PP |
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346 | The protocol has been overdesigned where this was possible without |
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347 | increasing implementation complexity, in an attempt to protect against |
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348 | implementation or protocol failures. For example, if the \s-1ECDH\s0 challenge |
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349 | was found to be flawed, perfect forward secrecy would be lost, but the |
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350 | data would likely still be protected. Likewise, standard algorithms and |
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351 | implementations are used where possible. |
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352 | .SS "Retrying" |
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353 | .IX Subsection "Retrying" |
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354 | When there is no response to an auth request, the node will send auth |
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355 | requests in bursts with an exponential back-off. After some time it will |
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356 | resort to \s-1PING\s0 packets, which are very small (8 bytes + protocol header) |
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357 | and lightweight (no \s-1RSA\s0 operations required). A node that receives ping |
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358 | requests from an unconnected peer will respond by trying to create a |
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359 | connection. |
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360 | .PP |
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361 | In addition to the exponential back-off, there is a global rate-limit on |
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362 | a per-IP base. It allows long bursts but will limit total packet rate to |
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363 | something like one control packet every ten seconds, to avoid accidental |
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364 | floods due to protocol problems (like a \s-1RSA\s0 key file mismatch between two |
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365 | nodes). |
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366 | .PP |
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367 | The intervals between retries are limited by the \f(CW\*(C`max\-retry\*(C'\fR |
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368 | configuration value. A node with \f(CW\*(C`connect\*(C'\fR = \f(CW\*(C`always\*(C'\fR will always retry, |
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369 | a node with \f(CW\*(C`connect\*(C'\fR = \f(CW\*(C`ondemand\*(C'\fR will only try (and re-try) to connect |
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370 | as long as there are packets in the queue, usually this limits the retry |
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371 | period to \f(CW\*(C`max\-ttl\*(C'\fR seconds. |
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372 | .PP |
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373 | Sending packets over the \s-1VPN\s0 will reset the retry intervals as well, which |
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374 | means as long as somebody is trying to send packets to a given node, \s-1GVPE\s0 |
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375 | will try to connect every few seconds. |
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376 | .SS "Routing and Protocol translation" |
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377 | .IX Subsection "Routing and Protocol translation" |
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378 | The \s-1GVPE\s0 routing algorithm is easy: there isn't much routing to speak |
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379 | of: When routing packets to another node, \s-1GVPE\s0 tries the following |
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380 | options, in order: |
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381 | .IP "If the two nodes should be able to reach each other directly (common protocol, port known), then \s-1GVPE\s0 will send the packet directly to the other node." 4 |
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382 | .IX Item "If the two nodes should be able to reach each other directly (common protocol, port known), then GVPE will send the packet directly to the other node." |
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383 | .PD 0 |
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384 | .ie n .IP "If this isn't possible (e.g. because the node doesn't have a \*(C`hostname\*(C' or known port), but the nodes speak a common protocol and a router is available, then \s-1GVPE\s0 will ask a router to ""mediate"" between both nodes (see below)." 4 |
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385 | .el .IP "If this isn't possible (e.g. because the node doesn't have a \f(CW\*(C`hostname\*(C'\fR or known port), but the nodes speak a common protocol and a router is available, then \s-1GVPE\s0 will ask a router to ``mediate'' between both nodes (see below)." 4 |
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386 | .IX Item "If this isn't possible (e.g. because the node doesn't have a hostname or known port), but the nodes speak a common protocol and a router is available, then GVPE will ask a router to mediate between both nodes (see below)." |
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387 | .ie n .IP "If a direct connection isn't possible (no common protocols) or forbidden (\*(C`deny\-direct\*(C') and there are any routers, then \s-1GVPE\s0 will try to send packets to the router with the highest priority that is connected already \fIand\fR is able (as specified by the config file) to connect directly to the target node." 4 |
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388 | .el .IP "If a direct connection isn't possible (no common protocols) or forbidden (\f(CW\*(C`deny\-direct\*(C'\fR) and there are any routers, then \s-1GVPE\s0 will try to send packets to the router with the highest priority that is connected already \fIand\fR is able (as specified by the config file) to connect directly to the target node." 4 |
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389 | .IX Item "If a direct connection isn't possible (no common protocols) or forbidden (deny-direct) and there are any routers, then GVPE will try to send packets to the router with the highest priority that is connected already and is able (as specified by the config file) to connect directly to the target node." |
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390 | .IP "If no such router exists, then \s-1GVPE\s0 will simply send the packet to the node with the highest priority available." 4 |
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391 | .IX Item "If no such router exists, then GVPE will simply send the packet to the node with the highest priority available." |
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392 | .IP "Failing all that, the packet will be dropped." 4 |
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393 | .IX Item "Failing all that, the packet will be dropped." |
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394 | .PD |
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395 | .PP |
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396 | A host can usually declare itself unreachable directly by setting its |
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397 | port number(s) to zero. It can declare other hosts as unreachable by using |
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398 | a config-file that disables all protocols for these other hosts. Another |
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399 | option is to disable all protocols on that host in the other config files. |
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400 | .PP |
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401 | If two hosts cannot connect to each other because their \s-1IP\s0 address(es) |
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402 | are not known (such as dial-up hosts), one side will send a \fImediated\fR |
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403 | connection request to a router (routers must be configured to act as |
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404 | routers!), which will send both the originating and the destination host |
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405 | a connection info request with protocol information and \s-1IP\s0 address of the |
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406 | other host (if known). Both hosts will then try to establish a direct |
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407 | connection to the other peer, which is usually possible even when both |
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408 | hosts are behind a \s-1NAT\s0 gateway. |
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409 | .PP |
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410 | Routing via other nodes works because the \s-1SRCDST\s0 field is not encrypted, |
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411 | so the router can just forward the packet to the destination host. Since |
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412 | each host uses its own private key, the router will not be able to |
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413 | decrypt or encrypt packets, it will just act as a simple router and |
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414 | protocol translator. |
