initial commit

.gitignore

@@ -0,0 +1,2 @@
+go.mod
+go.sum

ResponseWrapper.go

@@ -0,0 +1,45 @@
+package overide
+
+import (
+	"net"
+
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/miekg/dns"
+)
+
+var log = clog.NewWithPlugin("overide")
+
+type Rule struct {
+	Search   net.IP
+	Override net.IP
+}
+
+type ResponseWrapper struct {
+	dns.ResponseWriter
+	Rules []Rule
+}
+
+func (r *ResponseWrapper) WriteMsg(res *dns.Msg) error {
+	for _, r := range r.Rules {
+		for _, rr := range res.Answer {
+			if rr.Header().Rrtype == dns.TypeA {
+				a := rr.(*dns.A)
+				if a.A.Equal(r.Search) {
+					a.A = r.Override
+				}
+			} else if rr.Header().Rrtype == dns.TypeAAAA {
+				a := rr.(*dns.AAAA)
+				if a.AAAA.Equal(r.Search) {
+					a.AAAA = r.Override
+				}
+			}
+		}
+	}
+
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+func (r *ResponseWrapper) Write(buf []byte) (int, error) {
+	log.Warning("ResponseWrapper called with Write: not ensuring overide")
+	return r.ResponseWriter.Write(buf)
+}

override.go

@@ -0,0 +1,20 @@
+package overide
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/miekg/dns"
+)
+
+type Override struct {
+	Next  plugin.Handler
+	Rules []Rule
+}
+
+func (o Override) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	rw := &ResponseWrapper{ResponseWriter: w, Rules: o.Rules}
+	return plugin.NextOrFailure(o.Name(), o.Next, ctx, rw, r)
+}
+
+func (o Override) Name() string { return "override" }

setup.go

@@ -0,0 +1,40 @@
+package overide
+
+import (
+	"net"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+)
+
+func init() {
+	plugin.Register("override", setup)
+}
+
+func setup(c *caddy.Controller) error {
+	c.Next()
+
+	rules, err := parseRules(c)
+
+	if err != nil {
+		return plugin.Error("override", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Override{Next: next, Rules: rules}
+	})
+
+	return nil
+}
+
+func parseRules(c *caddy.Controller) ([]Rule, error) {
+	var rules []Rule
+
+	rules = append(rules, Rule{
+		Search:   net.ParseIP("45.9.63.244"),
+		Override: net.ParseIP("10.0.0.1"),
+	})
+
+	return rules, nil
+}