Robert Greenwalt | 47e4ceb | 2012-03-26 15:36:57 -0700 | [diff] [blame] | 1 | Private DNS |
| 2 | |
| 3 | Summary |
| 4 | |
| 5 | Private DNS is an extension to standard Wide Area Bonjour that allows |
| 6 | for secure, encrypted, and authorized communications. Private data sent |
| 7 | from a client to a DNS server is encrypted using Transport Layer |
| 8 | Security (TLS), ensuring that the data is hidden from prying eyes, and |
| 9 | contains Transaction Signatures (TSIG), so the server can authorize the |
| 10 | request. TSIGs are typically associated with Dynamic Updates; we are |
| 11 | using them for standard and long-lived queries as well. Private DNS also |
| 12 | protects Dynamic Updates from eavesdropping, by wrapping the update in a |
| 13 | TLS communication channel if the server has been configured appropriately. |
| 14 | |
| 15 | Architectural Overview |
| 16 | |
| 17 | mDNSResponder has been modified to automatically issue a private query |
| 18 | when necessary. After receiving an NXDOMAIN error, mDNSResponder checks |
| 19 | in the system keychain to see if the user has a DNS query key (TSIG key) |
| 20 | for the name in question, or for a parent of that name. If a suitable |
| 21 | key is found, mDNSResponder looks up the zone data associated with the |
| 22 | name of the question. After determining the correct name server, |
| 23 | mDNSResponder looks up an additional SRV record "_dns-private._tcp". If |
| 24 | it finds this record, mDNSResponder will re-issue the query privately. |
| 25 | If either there is no _dns-private._tcp record, or there is no secret |
| 26 | key, the call fails as it initially did, with an NXDOMAIN error. |
| 27 | |
| 28 | Once the secret key is found and the SRV record is looked up, mDNSResponder |
| 29 | opens a TLS connection to the server on the port specified in the SRV |
| 30 | record just looked up. After the connection succeeds, mDNSResponder |
| 31 | can proceed to use that communication channel to make requests of |
| 32 | the server. Every private packet must also have a TSIG record; |
| 33 | the DNS server uses this TSIG record to allow access to its data. |
| 34 | |
| 35 | When setting up a long-lived query over TCP (with or without TLS) |
| 36 | TCP's standard three-way handshake makes the full four-packet LLQ setup |
| 37 | exchange described in <http://files.dns-sd.org/draft-sekar-dns-llq.txt> |
| 38 | unnecessary. Instead, when connecting over TCP, the client simply sends |
| 39 | a setup message and expects to receive ACK + Answers. The setup message |
| 40 | sent is formatted as described in the LLQ document, however there is |
| 41 | an additional TSIG' resource record added to the end of it. The TSIG |
| 42 | resource records looks and acts exactly as it does in a secure update. |
| 43 | So when the server receives an LLQ (or a standard query), it looks to |
| 44 | see if the zone that is being referenced is public or private. If it's |
| 45 | private, then it makes sure that the client is authorized to query that |
| 46 | zone (by using the TSIG signature) and returns the appropriate data. |
| 47 | When a zone is configured as private, the server will do this type of |
| 48 | authorization checking for every query except those queries that are |
| 49 | looking for SOA and NS records. |
| 50 | |
| 51 | Implementation Issues |
| 52 | |
| 53 | dnsextd |
| 54 | |
| 55 | dnsextd has been modified to behave much like a DNS firewall. The "real" |
| 56 | DNS server is configured to listen on non-standard ports on the loopback |
| 57 | interface. dnsextd then listens on the standard DNS ports (TCP/UDP port |
| 58 | 53) and intercepts all DNS traffic. It is responsible for determining |
| 59 | what zone a DNS request is associated with, determining whether the |
| 60 | client is allowed access to that zone, and returning the appropriate |
| 61 | information back to the caller. If the packet is allowed access, dnsextd |
| 62 | forwards the request to the "real" nameserver, and returns the result to |
| 63 | the caller. |
| 64 | |
| 65 | It was tempting to use BIND9's facility for configuring TSIG enabled |
| 66 | queries while doing this work. However after proceeding down that path, |
| 67 | enough subtle interaction problems were found that it was not practical |
| 68 | to pursue this direction, so instead dnsextd does all TSIG processing |
| 69 | for queries itself. It does continue to use BIND9 for processing TSIG |
| 70 | enabled dynamic updates, though one minor downside with this is that |
| 71 | there are two configuration files (named.conf or dnsextd.conf) that have |
| 72 | the same secret key information. That seems redundant and error-prone, |
| 73 | and moving all TSIG processing for both queries and updates into dnsextd |
| 74 | would fix this. |
| 75 | |
| 76 | All private LLQ operations are TSIG-enabled and sent over a secure |
| 77 | encrypted TLS channel. To accommodate service providers who don't want |
| 78 | to have to keep open a large number of TLS connections to a large number |
| 79 | of client machines, the server has the option of dropping the TLS |
| 80 | connection after initial LLQ setup and sending subsequent events and |
| 81 | refreshes using unencrypted UDP packets. This results in less load on |
| 82 | the server, at the cost of slightly lower security (LLQs can only be set |
| 83 | up by an authorized client, but once set up, subsequent change event |
| 84 | packets sent over unencrypted UDP could be observed by an eavesdropper). |
| 85 | A potential solution to this deficiency might be in using DTLS, which is |
| 86 | a protocol based on TLS that is capable of securing datagram traffic. |
| 87 | More investigation needs to be done to see if DTLS is suitable for |
| 88 | private DNS. |
| 89 | |
| 90 | It was necessary to relax one of the checks that dnsextd performs during |
| 91 | processing of an LLQ refresh. Prior to these changes, dnsextd would |
| 92 | verify that the refresh request came from the same entity that setup the |
| 93 | LLQ by comparing both the IP Address and port number of the request |
| 94 | packet with the IP Address and port number of the setup packet. Because |
| 95 | of the preceding issue, a refresh request might be sent over two |
| 96 | different sockets. While their IP addresses would be the same, their |
| 97 | port numbers could potentially differ. This check has been modified to |
| 98 | only check that the IP addresses match. |
| 99 | |
| 100 | When setting up a semi-private LLQ (where the request and initial answer |
| 101 | set is sent over TLS/TCP, but subsequent change events are sent over |
| 102 | unencrypted UDP), dnsextd uses the port number of the client's TCP |
| 103 | socket to determine the UDP event port number. While this eliminates the |
| 104 | need to pass the UDP event port number in the LLQ setup request |
| 105 | (obviating a potential data mismatch error), I think it does more harm |
| 106 | than good, for three reasons: |
| 107 | |
| 108 | 1) We are relying that all the routers out there implement the Port |
| 109 | Mapping Protocol spec correctly. |
| 110 | |
| 111 | 2) Upon setup every LLQ must NAT map two ports. Upon tear down every LLQ |
| 112 | must tear down two NAT mappings. |
| 113 | |
| 114 | 3) Every LLQ opens up two sockets (TCP and UDP), rather than just the |
| 115 | one TCP socket. |
| 116 | |
| 117 | All of this just to avoid sending two bytes in the LLQ setup packet |
| 118 | doesn't seem logical. The approach also necessitates creating an |
| 119 | additional UDP socket for every private LLQ, port mapping both the TCP |
| 120 | socket as well as the UDP socket, and moderately increasing the |
| 121 | complexity and efficiency of the code. Because of this we plan to allow |
| 122 | the LLQ setup packet to specify a different UDP port for change event |
| 123 | packets. This will allow mDNSResponder to receive all UDP change event |
| 124 | packets on a single UDP port, instead of a different one for each LLQ. |
| 125 | |
| 126 | Currently, dnsextd is buggy on multi-homed hosts. If it receives a |
| 127 | packet on interface 2, it will reply on interface 1 causing an error in |
| 128 | the client program. |
| 129 | |
| 130 | dnsextd doesn't fully process all of its option parameters. |
| 131 | Specifically, it doesn't process the keywords: "listen-on", |
| 132 | "nameserver", "private", and "llq". It defaults to expecting the "real" |
| 133 | nameserver to be listening on 127.0.0.1:5030. |
| 134 | |
| 135 | |
| 136 | mDNSResponder |
| 137 | |
| 138 | Currently, mDNSResponder attempts to issue private queries for all |
| 139 | queries that initially result in an NXDOMAIN error. This behavior might |
| 140 | be modified in future versions, however it seems patently incorrect to |
| 141 | do this for reverse name lookups. The code that attempts to get the zone |
| 142 | data associated with the name will never find the zone for a reverse |
| 143 | name lookup, and so will issue a number of wasteful DNS queries. |
| 144 | |
| 145 | mDNSResponder doesn't handle SERV_FULL or STATIC return codes after |
| 146 | setting up an LLQ over TCP. This isn't a terrible problem right now, |
| 147 | because dnsextd doesn't ever return them, but this should be fixed so |
| 148 | that mDNSResponder will work when talking to other servers that do |
| 149 | return these error codes. |
| 150 | |
| 151 | |
| 152 | Configuration: |
| 153 | |
| 154 | Sample named.conf: |
| 155 | |
| 156 | // |
| 157 | // Include keys file |
| 158 | // |
| 159 | include "/etc/rndc.key"; |
| 160 | // Declares control channels to be used by the rndc utility. |
| 161 | // |
| 162 | // It is recommended that 127.0.0.1 be the only address used. |
| 163 | // This also allows non-privileged users on the local host to manage |
| 164 | // your name server. |
| 165 | |
| 166 | // |
| 167 | // Default controls |
| 168 | // |
| 169 | controls |
| 170 | { |
| 171 | inet 127.0.0.1 port 54 allow { any; } keys { "rndc-key"; }; |
| 172 | }; |
| 173 | |
| 174 | options |
| 175 | { |
| 176 | directory "/var/named"; |
| 177 | /* |
| 178 | * If there is a firewall between you and nameservers you want |
| 179 | * to talk to, you might need to uncomment the query-source |
| 180 | * directive below. Previous versions of BIND always asked |
| 181 | * questions using port 53, but BIND 8.1 uses an unprivileged |
| 182 | * port by default. |
| 183 | */ |
| 184 | |
| 185 | forwarders |
| 186 | { |
| 187 | 65.23.128.2; |
| 188 | 65.23.128.3; |
| 189 | }; |
| 190 | |
| 191 | listen-on port 5030 { 127.0.0.1; }; |
| 192 | recursion true; |
| 193 | }; |
| 194 | |
| 195 | // |
| 196 | // a caching only nameserver config |
| 197 | // |
| 198 | zone "." IN |
| 199 | { |
| 200 | type hint; |
| 201 | file "named.ca"; |
| 202 | }; |
| 203 | |
| 204 | zone "localhost" IN |
| 205 | { |
| 206 | type master; |
| 207 | file "localhost.zone"; |
| 208 | allow-update { none; }; |
| 209 | }; |
| 210 | |
| 211 | zone "0.0.127.in-addr.arpa" IN |
| 212 | { |
| 213 | type master; |
| 214 | file "named.local"; |
| 215 | allow-update { none; }; |
| 216 | }; |
| 217 | |
| 218 | zone "hungrywolf.org." in |
| 219 | { |
| 220 | type master; |
| 221 | file "db.hungrywolf.org"; |
| 222 | allow-update { key hungrywolf.org.; }; |
| 223 | }; |
| 224 | |
| 225 | zone "157.23.65.in-addr.arpa" IN |
| 226 | { |
| 227 | file "db.65.23.157"; |
| 228 | type master; |
| 229 | }; |
| 230 | |
| 231 | zone "100.255.17.in-addr.arpa" IN |
| 232 | { |
| 233 | file "db.17.255.100"; |
| 234 | type master; |
| 235 | }; |
| 236 | |
| 237 | zone "66.6.24.in-addr.arpa" IN |
| 238 | { |
| 239 | file "db.24.6.66"; |
| 240 | type master; |
| 241 | }; |
| 242 | |
| 243 | key hungrywolf.org. |
| 244 | { |
| 245 | algorithm hmac-md5; |
| 246 | secret "c8LWr16K6ju6KMO5zT6Tyg=="; |
| 247 | }; |
| 248 | |
| 249 | logging |
| 250 | { |
| 251 | category default { _default_log; }; |
| 252 | |
| 253 | channel _default_log |
| 254 | { |
| 255 | file "/Library/Logs/named.log"; |
| 256 | severity info; |
| 257 | print-time yes; |
| 258 | }; |
| 259 | }; |
| 260 | |
| 261 | |
| 262 | Sample dnsextd.conf: |
| 263 | |
| 264 | options { }; |
| 265 | |
| 266 | key "hungrywolf.org." |
| 267 | { |
| 268 | secret "c8LWr16K6ju6KMO5zT6Tyg=="; |
| 269 | }; |
| 270 | |
| 271 | zone "hungrywolf.org." |
| 272 | { |
| 273 | type private; |
| 274 | allow-query { key hungrywolf.org.; }; |
| 275 | }; |