Add secondary support

Allow specifying a primary server and retrieve the zone's content.

Add tests and an Expired bool to zone struct, to stop server zones

that are expired. The zone is retrieved on Startup, no updates of

changed content are done. We also don't respond to notifies yet.

middleware/file/README.md

@@ -1,9 +1,9 @@
 # file
 
-`file` enabled reading zone data from a RFC-1035 styled file.
+`file` enables serving zone data from a RFC-1035 styled file.
 
-The etcd middleware makes extensive use of the proxy middleware to forward and query
-other servers in the network.
+The file middleware is used for "old-style" DNS server. It serves from a preloaded file that exists
+on disk.
 
 ## Syntax
 
@@ -17,15 +17,26 @@ file dbfile [zones...]
 
 If you want to round robin A and AAAA responses look at the `loadbalance` middleware.
 
-TSIG key configuration is TODO; directive format will change.
+TSIG key configuration is TODO; directive format for transfer will probably be extended with
+TSIG key information, something like `transfer out [address] key [name] [base64]`
 
 ~~~
 file dbfile [zones... ] {
     transfer out [address...]
+    transfer to [address]
 }
 ~~~
 
-* `transfer` enable zone transfers, for now only `transfer out` does something. It enables outgoing
-  zone transfers when defined.
+* `transfer` enables zone transfers. It may be specified multiples times. *To* or *from* signals
+    the direction. Address must be denoted in CIDR notation (127.0.0.1/32 etc.). The special
+    wildcard "*" means: the entire internet.
 
 ## Examples
+
+Load the `miek.nl` zone from `miek.nl.signed` and allow transfers to the internet.
+
+~~~
+file miek.nl.signed miek.nl {
+    transfer to *
+}
+~~~