Fix EDNS0 compliance (#2357)

* Fix EDNS0 compliance

Do SizeAndDo in the server (ScrubWriter) and remove all uses of this
from the plugins. Also *always* do it. This is to get into compliance
for https://dnsflagday.net/.

The pkg/edns0 now exports the EDNS0 options we understand; this is
exported to allow plugins add things there. The *rewrite* plugin used
this to add custom EDNS0 option codes that the server needs to
understand.

This also needs a new release of miekg/dns because it triggered a
race-condition that was basicly there forever.

See:
* https://github.com/miekg/dns/issues/857
* https://github.com/miekg/dns/pull/859

Running a test instance and pointing the https://ednscomp.isc.org/ednscomp
to it shows the tests are now fixed:

~~~
EDNS Compliance Tester
Checking: 'miek.nl' as at 2018-12-01T17:53:15Z

miek.nl. @147.75.204.203 (drone.coredns.io.): dns=ok edns=ok edns1=ok edns@512=ok ednsopt=ok edns1opt=ok do=ok ednsflags=ok docookie=ok edns512tcp=ok optlist=ok
miek.nl. @2604:1380:2002:a000::1 (drone.coredns.io.): dns=ok edns=ok edns1=ok edns@512=ok ednsopt=ok edns1opt=ok do=ok ednsflags=ok docookie=ok edns512tcp=ok optlist=ok

All Ok
Codes
ok - test passed.
~~~

Signed-off-by: Miek Gieben <miek@miek.nl>

Signed-off-by: Miek Gieben <miek@miek.nl>

* typos in comments

Signed-off-by: Miek Gieben <miek@miek.nl>

1 files changed, 27 insertions, 0 deletions

diff --git a/plugin/pkg/edns/edns.go b/plugin/pkg/edns/edns.go
index 3f0ea5e16..e99fdfb0a 100644
--- a/plugin/pkg/edns/edns.go
+++ b/plugin/pkg/edns/edns.go
@@ -3,10 +3,37 @@ package edns
 
 import (
 	"errors"
+	"sync"
 
 	"github.com/miekg/dns"
 )
 
+var sup = &supported{m: make(map[uint16]struct{})}
+
+type supported struct {
+	m map[uint16]struct{}
+	sync.RWMutex
+}
+
+// SetSupportedOption adds a new supported option the set of EDNS0 options that we support. Plugins typically call
+// this in their setup code to signal support for a new option.
+// By default we support:
+// dns.EDNS0NSID, dns.EDNS0EXPIRE, dns.EDNS0COOKIE, dns.EDNS0TCPKEEPALIVE, dns.EDNS0PADDING. These
+// values are not in this map and checked directly in the server.
+func SetSupportedOption(option uint16) {
+	sup.Lock()
+	sup.m[option] = struct{}{}
+	sup.Unlock()
+}
+
+// SupportedOption returns true if the option code is supported as an extra EDNS0 option.
+func SupportedOption(option uint16) bool {
+	sup.RLock()
+	_, ok := sup.m[option]
+	sup.RUnlock()
+	return ok
+}
+
 // Version checks the EDNS version in the request. If error
 // is nil everything is OK and we can invoke the plugin. If non-nil, the
 // returned Msg is valid to be returned to the client (and should). For some