Add traffic plugin

This allows for advanced loadbalancing and maybe geoIP loadbalancing.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/traffic/README.md

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+# traffic
+
+## Name
+
+*traffic* - handout addresses according to assignments.
+
+## Description
+
+The *traffic* plugin is a load balancer that allows traffic steering, weighted responses and
+draining of endpoints. Endpoints are IP:port pairs. *Traffic* works as an overlay on top of other
+plugins, it does not mandate any storage by itself.
+
+*Traffic* receives (via gRPC?) *assignments* that define the weight of the endpoints in services.
+The plugin takes care of handing out responses that adhere to these assignments. Assignments will
+need to be updated frequently, without new updates *traffic* will hand out responses according to
+the last received assignment. When there are no assignments for a service name (yet), the responses
+will also be modified (see below).
+
+An assignment covers a "service name", which is a domain name. For each service a number of backends
+are expected. A backend is defined as an IP:port pair Each backend comes with a integer indicating
+it relative weight. A zero means the backend exists, but should not be handed out (drain it).
+
+*Traffic* will load balance A and AAAA queries. known to the plugin. It will return precisely one
+record in a response, which is the optimal record according to the assignments and previously handed
+out responses. If a service should be load balanced, but no assignment can be found a random record
+from the *answer section* will be choosen.
+
+Every message that is handled by the *traffic* plugin will have all it's TTLs set to 5 seconds,
+any authority section is removed and all RRSIGs are removed from it.
+
+The *traffic* plugin has no notion of draining, drop overload and anything that advanced, *it just
+acts upon assignments*. This is means that if a backend goes down and *traffic* has not seen a new
+assignment yet, it will still include this backend in responses.
+
+## Syntax
+
+~~~
+traffic
+~~~
+
+This enables traffic load balancing for all (sub-)domains named in the server block.
+
+## Examples
+
+~~~ corefile
+example.org {
+    traffic
+    forward . 10.12.13.14
+}
+~~~
+
+This will add load balancing for domains under example.org; the upstream information comes from
+10.12.13.14; depending on received assignments, replies will be let through as-is or are load balanced.
+
+## Assignments
+
+Assignments are given in protobuf format, but here is an example in YAML conveying the same
+information. This is an example assignment for the service "www.example.org".
+
+~~~ yaml
+assignments:
+    - service: www.example.org
+        - backend: 192.168.1.1:443
+            assign: 4
+          backend: 192.168.1.2:443
+            assign: 6
+          backend: 192.168.1.3:443
+            assign: 0
+~~~
+
+This particular one has 3 backends, one of which is to be drained (192.168.1.3). the two remaining
+ones have a non zero weighted assignment. We use "Weighted Random Selection" to select a backend:
+
+* Add up all the weights for all the items in the list (here 8).
+* Pick a number at random between 1 and the sum of the weights.
+* Iterate over the items
+* For the current item, subtract the item's weight from the random number.
+* If less or zero pick this item, other continue with the next item.
+
+On seeing a query for a service, *traffic* will track the reply. When it returns with an answer
+*traffic* will rewrite it (and discard of any RRSIGs). Using the assignments the answer section will
+be rewritten as such:
+
+* A backend will be picked using the algorithm from above.
+* The TTL on the response will be 5s for all included records.
+* According to previous responses for this service and the relative weights of each backends the
+  best backend will be put in the response.
+* If after the selection *no* backends are available an NODATA response will be sent. An SOA
+  record will be synthesised, and a low TTL (and negative TTL) of 5 seconds will be set.
+
+TTL rewriting always? TODO.
+Authority section will be removed.
+If no assignment, randomly pick an address
+other types then A and AAAA, like SRV - do the same selection.
+
+## Bugs
+
+This plugin does not play nice with DNSSEC - if the backend returns signatures with the answer; they
+will be stripped. You can optionally sign responses on the fly by using the *dnssec* plugin.
+
+## Also See
+
+This is a [post on weighted random
+selection](https://medium.com/@peterkellyonline/weighted-random-selection-3ff222917eb6).
+
+## TODO
+
+Should we add source address information (geographical load balancing) to the assignment? This can
+be handled be having each backend specify an optional source range there this record should be used.
+For IPv4 this must a /24 for IPv6 a /64.
+
+Other points that require more attention:
+
+* deleting assignments?
+* last known good assignment (esp with deleting assignments)?