Add middleware reverse (#452)

middleware/reverse/README.md

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+# reverse
+
+The *reverse* middleware allows CoreDNS to respond dynamic to an PTR request and the related A/AAAA request.
+
+## Syntax
+
+~~~
+reverse NETWORK.. {
+    hostname TEMPLATE
+    [ttl TTL]
+    [fallthrough]
+~~~
+
+* **NETWORK** one or more CIDR formatted networks to respond on.
+* `hostname` inject the ip and zone to an template for the hostname. Defaults to "ip-{ip}.{zone[0]}". See below for template.
+* `ttl` defaults to 60
+* `fallthrough` If zone matches and no record can be generated, pass request to the next middleware.
+
+### Template Syntax
+The template for the hostname is used for generating the PTR for an reverse lookup and matching the forward lookup back to an ip.
+
+#### `{ip}`
+This symbol is **required** to work.
+V4 network replaces the "." with an "-". 10.1.1.1 results in "10-1-1-1"
+V6 network removes the ":" and fills the zeros. "ffff::ffff" results in "ffff000000000000000000000000ffff"
+
+#### `{zone[i]}`
+This symbol is **optional** to use and can be replaced by a fix zone string.
+The zone will be matched by the configured listener on the server block key.
+`i` needs to be replaced to the index of the configured listener zones, starting with 0.
+
+`arpa.:53 domain.com.:8053` will resolve `zone{0}` to `arpa.` and `zone{1}` to `domain.com.`
+
+## Examples
+
+~~~
+# Serve on port 53
+# match arpa. and compute.internal. to resolv reverse and forward lookup 
+.arpa.:53 compute.internal.:53 {
+    # proxy unmatched requests
+    proxy . 8.8.8.8
+
+    # answer requests for IPs in this networks
+    # PTR 1.0.32.10.in-addr.arpa. 3600 ip-10-0-32-1.compute.internal.
+    # A ip-10-0-32-1.compute.internal. 3600 10.0.32.1
+    # v6 is also possible
+    # PTR 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.d.f.ip6.arpa. 3600 ip-fd010000000000000000000000000001.compute.internal.
+    # AAAA ip-fd010000000000000000000000000001.compute.internal. 3600 fd01::1
+    reverse 10.32.0.0/16 fd01::/16 {
+        # template of the ip injection to hostname, zone resolved to compute.internal.
+        hostname ip-{ip}.{zone[1]}
+
+        # set time-to-live of the RR
+        ttl 3600
+
+        # forward unanswered or unmatched requests to proxy
+        # without this flag, requesting A/AAAA records on compute.internal. will end here
+        fallthrough
+    }
+
+    # cache with ttl timeout
+    cache
+}
+~~~
+
+
+~~~
+# Serve on port 53
+# listen only on the specific network
+32.10.in-addr.arpa.arpa.:53 arpa.company.org.:53 {
+
+    reverse 10.32.0.0/16 {
+        # template of the ip injection to hostname, zone resolved to arpa.company.org.
+        hostname "ip-{ip}.v4.{zone[1]}"
+
+        # set time-to-live of the RR
+        ttl 3600
+
+        # fallthrough is not required, v4.arpa.company.org. will be only answered here
+    }
+
+    # cidr closer to the ip wins, so we can overwrite the "default"
+    reverse 10.32.2.0/24 {
+        # its also possible to set fix domain suffix
+        hostname ip-{ip}.fix.arpa.company.org.
+
+        # set time-to-live of the RR
+        ttl 3600
+    }
+
+    # cache with ttl timeout
+    cache
+}
+~~~
+
+
+