Add fuzzing infrastructure (#1118)

Fix file/fuzz.go build and docs in Makefile.fuzz
Each plugin can add a fuzz.go to join the fuzzing craze.
pkg/fuzz/do.go could be made a lot smarter, but is probably good enough
for starters.

$ make -f Makefile.fuzz <plugin>

will build with go-fuzz-build and then execute a go-fuzz run. Each
plugin's fuzz run uses a per-plugin directory to store the fuzz data.

Makefile.fuzz

+# Makefile for fuzzing
+#
+# Use go-fuzz and needs the tools installed. For each fuzz.go in a plugin's directory
+# you can start the fuzzing with: make -f Makefile.fuzz <plugin>
+# e.g.
+#
+# 	make -f Makefile.fuzz proxy
+# 
+# Each plugin that wants to join the fuzzing fray only needs to add a fuzz.go that calls
+# the plugins's ServeDNS and used the plugin/pkg/fuzz for the Do function.
+#
+# Installing go-fuzz
+#$ go get github.com/dvyukov/go-fuzz/go-fuzz
+#$ go get github.com/dvyukov/go-fuzz/go-fuzz-build
+
+REPO:="github.com/coredns/coredns/plugin"
+
+FUZZ:=$(dir $(wildcard plugin/*/fuzz.go)) # plugin/cache/
+PLUGINS:=$(foreach f,$(FUZZ),$(subst plugin, ,$(f:/=))) # > /cache
+PLUGINS:=$(foreach f,$(PLUGINS),$(subst /, ,$(f))) # > cache
+
+.PHONY: echo
+echo:
+	@echo fuzz targets: $(PLUGINS)
+
+.PHONY: $(PLUGINS)
+$(PLUGINS): echo
+	go-fuzz-build $(REPO)/$(@)
+	go-fuzz -bin=./$(@)-fuzz.zip -workdir=fuzz/$(@)
+
+.PHONY: clean
+clean:
+	rm *-fuzz.zip

plugin/cache/fuzz.go

+package cache
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+)
+
+// Fuzz fuzzes cache.
+func Fuzz(data []byte) int {
+	c := &Cache{pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxNTTL, prefetch: 0, duration: 1 * time.Minute}
+
+	return fuzz.Do(c, data)
+}

plugin/file/fuzz.go

+package file
+
+import (
+	"strings"
+
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+	"github.com/coredns/coredns/plugin/test"
+)
+
+// Fuzz fuzzes file.
+func Fuzz(data []byte) int {
+	name := "miek.nl."
+	zone, _ := Parse(strings.NewReader(fuzzMiekNL), name, "stdin", 0)
+	f := File{Next: test.ErrorHandler(), Zones: Zones{Z: map[string]*Zone{name: zone}, Names: []string{name}}}
+
+	return fuzz.Do(f, data)
+}
+
+const fuzzMiekNL = `
+$TTL    30M
+$ORIGIN miek.nl.
+@       IN      SOA     linode.atoom.net. miek.miek.nl. (
+                             1282630057 ; Serial
+                             4H         ; Refresh
+                             1H         ; Retry
+                             7D         ; Expire
+                             4H )       ; Negative Cache TTL
+                IN      NS      linode.atoom.net.
+                IN      NS      ns-ext.nlnetlabs.nl.
+                IN      NS      omval.tednet.nl.
+                IN      NS      ext.ns.whyscream.net.
+
+                IN      MX      1  aspmx.l.google.com.
+                IN      MX      5  alt1.aspmx.l.google.com.
+                IN      MX      5  alt2.aspmx.l.google.com.
+                IN      MX      10 aspmx2.googlemail.com.
+                IN      MX      10 aspmx3.googlemail.com.
+
+		IN      A       139.162.196.78
+		IN      AAAA    2a01:7e00::f03c:91ff:fef1:6735
+
+a               IN      A       139.162.196.78
+                IN      AAAA    2a01:7e00::f03c:91ff:fef1:6735
+www             IN      CNAME   a
+archive         IN      CNAME   a
+
+srv		IN	SRV     10 10 8080 a.miek.nl.
+mx		IN	MX      10 a.miek.nl.`

plugin/pkg/fuzz/do.go

+package fuzz
+
+import (
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Do will fuzz p - used by gofuzz. See Maefile.fuzz for comments and context.
+func Do(p plugin.Handler, data []byte) int {
+	ctx := context.TODO()
+	ret := 1
+	r := new(dns.Msg)
+	if err := r.Unpack(data); err != nil {
+		ret = 0
+	}
+
+	if _, err := p.ServeDNS(ctx, &test.ResponseWriter{}, r); err != nil {
+		ret = 1
+	}
+
+	return ret
+}

plugin/proxy/fuzz.go

+package proxy
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+
+	"github.com/mholt/caddy"
+)
+
+// Fuzz fuzzes proxy.
+func Fuzz(data []byte) int {
+	c := caddy.NewTestController("dns", "proxy . 8.8.8.8:53")
+	up, err := NewStaticUpstreams(&c.Dispenser)
+	if err != nil {
+		return 0
+	}
+	p := &Proxy{Upstreams: &up}
+
+	return fuzz.Do(p, data)
+}

plugin/rewrite/fuzz.go

+package rewrite
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+
+	"github.com/mholt/caddy"
+)
+
+// Fuzz fuzzes rewrite.
+func Fuzz(data []byte) int {
+	c := caddy.NewTestController("dns", "rewrite edns0 subnet set 24 56")
+	rules, err := rewriteParse(c)
+	if err != nil {
+		return 0
+	}
+	r := Rewrite{Rules: rules}
+
+	return fuzz.Do(r, data)
+}