Implement chaos middleware

This allows for CH TXT queries that return some information about
the server and/or the authors (or whatever you put in there).

middleware/chaos/chaos.go

@@ -0,0 +1,50 @@
+package chaos
+
+import (
+	"os"
+
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+type Chaos struct {
+	Next    middleware.Handler
+	Version string
+	Authors map[string]bool // get randomization for free \o/
+}
+
+func (c Chaos) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := middleware.State{W: w, Req: r}
+	if state.QClass() != dns.ClassINET || state.QType() != dns.TypeTXT {
+		return c.Next.ServeDNS(ctx, w, r)
+	}
+	m := new(dns.Msg)
+	hdr := dns.RR_Header{Name: state.QName(), Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
+	switch state.Name() {
+	default:
+		return c.Next.ServeDNS(ctx, w, r)
+	case "authors.bind.":
+		for a, _ := range c.Authors {
+			m.Answer = append(m.Answer, &dns.TXT{Hdr: hdr, Txt: []string{trim(a)}})
+		}
+	case "version.bind.", "version.server.":
+		m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{trim(c.Version)}}}
+	case "hostname.bind.", "id.server.":
+		hostname, err := os.Hostname()
+		if err != nil {
+			hostname = "localhost"
+		}
+		m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{trim(hostname)}}}
+	}
+	w.WriteMsg(m)
+	return 0, nil
+}
+
+func trim(s string) string {
+	if len(s) < 256 {
+		return s
+	}
+	return s[:255]
+}