Add $OPTION directive and "fallthrough" option

Add the `$OPTION` directive to allow configuring the plugin.

Implement an option, "fallthrough" which causes the plugin to continue to the next plugin in the chain when there is no match, instead of always returning NXDOMAIN.

Signed-Off-By: Dan Fuhry <dan@fuhry.com>

Author: fuhry

README.md

@@ -33,6 +33,7 @@ This plugin can only be used once per Server Block.
 
 ~~~
 records [ZONES...] {
+    $OPTION [OPTION]
     [INLINE]
 }
 ~~~
@@ -42,6 +43,9 @@ records [ZONES...] {
 * **INLINE** the resource record that are to be served. These must be specified as the text
    represenation (as specifed in RFC 1035) of the record. See the examples below. Each record must
    be on a single line.
+* **OPTION** is a configuration option for the plugin. The following options are supported:
+  * `fallthrough`: When no matching record is found, instead of returning NXDOMAIN, the plugin will
+    call to the next plugin in the chain.
 
 If domain name in **INLINE** are not fully qualifed each of the **ZONES** are used as the origin and
 added to the names.
@@ -73,6 +77,22 @@ RFC 1035 zone file and everything after it will be ignored, hence the need for q
 }
 ~~~
 
+Override the record for `example.com`, without overriding anything else. Subdomains, like
+`foo.example.com`, will continue to be resolved normally (the `forward` plugin, in this case).
+
+~~~
+. {
+	records . {
+		$OPTION fallthrough
+		example.com.	300	IN	A	127.0.0.1
+	}
+
+	forward . 192.168.0.1 {
+		except example.com
+	}
+}
+~~~
+
 ## Bugs
 
 DNSSEC, nor wildcards are implemented. The lookup algorithm is pretty basic. Future enhancements

records.go

@@ -13,6 +13,7 @@ import (
 type Records struct {
 	origins []string // for easy matching, these strings are the index in the map m.
 	m       map[string][]dns.RR
+	fallthru bool
 
 	Next plugin.Handler
 }
@@ -48,6 +49,9 @@ func (re *Records) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 
 	// handle NXDOMAIN, NODATA and normal response here.
 	if nxdomain {
+		if re.fallthru {
+			return plugin.NextOrFailure(re.Name(), re.Next, ctx, w, r)
+		}
 		m.Rcode = dns.RcodeNameError
 		if soa != nil {
 			m.Ns = []dns.RR{soa}

setup.go

@@ -1,6 +1,7 @@
 package records
 
 import (
+	"fmt"
 	"strings"
 
 	"github.com/coredns/caddy"
@@ -57,9 +58,26 @@ func recordsParse(c *caddy.Controller) (*Records, error) {
 		// c.Val() +  c.RemainingArgs() is the record we need to parse (for each zone given; now tracked in re.origins). When parsing
 		// the record we just set the ORIGIN to the correct value and magic will happen. If no origin we set it to "."
 
+	parseBlocks:
 		for c.NextBlock() {
-			s := c.Val() + " "
-			s += strings.Join(c.RemainingArgs(), " ")
+			s := c.Val()
+			if s == "$OPTION" {
+				if !c.NextArg() {
+					return nil, fmt.Errorf("parsing block failed: $OPTION missing argument")
+				}
+				opt := c.Val()
+				switch opt {
+				case "fallthrough":
+					re.fallthru = true
+				default:
+					return nil, fmt.Errorf("parsing block failed: unknown option: %q", opt)
+				}
+				if len(c.RemainingArgs()) > 0 {
+					return nil, fmt.Errorf("parsing block failed: extra arguments after option %q", opt)
+				}
+				continue parseBlocks
+			}
+			s += " " + strings.Join(c.RemainingArgs(), " ")
 			for _, o := range re.origins {
 				rr, err := dns.NewRR("$ORIGIN " + o + "\n" + s + "\n")
 				if err != nil {