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, 31 insertions, 0 deletions

diff --git a/request/edns0.go b/request/edns0.go
new file mode 100644
index 000000000..89eb6b468
--- /dev/null
+++ b/request/edns0.go
@@ -0,0 +1,31 @@
+package request
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/edns"
+
+	"github.com/miekg/dns"
+)
+
+func supportedOptions(o []dns.EDNS0) []dns.EDNS0 {
+	var supported = make([]dns.EDNS0, 0, 3)
+	// For as long as possible try avoid looking up in the map, because that need an Rlock.
+	for _, opt := range o {
+		switch code := opt.Option(); code {
+		case dns.EDNS0NSID:
+			fallthrough
+		case dns.EDNS0EXPIRE:
+			fallthrough
+		case dns.EDNS0COOKIE:
+			fallthrough
+		case dns.EDNS0TCPKEEPALIVE:
+			fallthrough
+		case dns.EDNS0PADDING:
+			supported = append(supported, opt)
+		default:
+			if edns.SupportedOption(code) {
+				supported = append(supported, opt)
+			}
+		}
+	}
+	return supported
+}