edns.go

// Package edns provides function useful for adding/inspecting OPT records to/in messages.
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
// reason this response should not contain a question RR in the question section.
func Version(req *dns.Msg) (*dns.Msg, error) {
	opt := req.IsEdns0()
	if opt == nil {
		return nil, nil
	}
	if opt.Version() == 0 {
		return nil, nil
	}
	m := new(dns.Msg)
	m.SetReply(req)
	// zero out question section, wtf.
	m.Question = nil

	o := new(dns.OPT)
	o.Hdr.Name = "."
	o.Hdr.Rrtype = dns.TypeOPT
	o.SetVersion(0)
	m.Rcode = dns.RcodeBadVers
	o.SetExtendedRcode(dns.RcodeBadVers)
	m.Extra = []dns.RR{o}

	return m, errors.New("EDNS0 BADVERS")
}

// Size returns a normalized size based on proto.
func Size(proto string, size uint16) uint16 {
	if proto == "tcp" {
		return dns.MaxMsgSize
	}
	if size < dns.MinMsgSize {
		return dns.MinMsgSize
	}
	return size
}

/*

The below wasn't from the edns package

*/

// SetSizeAndDo adds an OPT record that the reflects the intent from request.
func SetSizeAndDo(req, m *dns.Msg) *dns.OPT {
	o := req.IsEdns0()
	if o == nil {
		return nil
	}

	if mo := m.IsEdns0(); mo != nil {
		mo.Hdr.Name = "."
		mo.Hdr.Rrtype = dns.TypeOPT
		mo.SetVersion(0)
		mo.SetUDPSize(o.UDPSize())
		mo.Hdr.Ttl &= 0xff00 // clear flags

		// Assume if the message m has options set, they are OK and represent what an upstream can do.

		if o.Do() {
			mo.SetDo()
		}
		return mo
	}

	// Reuse the request's OPT record and tack it to m.
	o.Hdr.Name = "."
	o.Hdr.Rrtype = dns.TypeOPT
	o.SetVersion(0)
	o.Hdr.Ttl &= 0xff00 // clear flags

	if len(o.Option) > 0 {
		o.Option = SupportedOptions(o.Option)
	}

	m.Extra = append(m.Extra, o)
	return o
}

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.EDNS0COOKIE:
			fallthrough
		case dns.EDNS0SUBNET:
			supported = append(supported, opt)
		default:
			if SupportedOption(code) {
				supported = append(supported, opt)

			}
		}
	}
	return supported
}