Add new --ednsopt argument to add arbitrary EDNS option

To add a new edns option with code 4242:

$ echo -n "This is a binary string" | base64
VGhpcyBpcyBhIGJpbmFyeSBzdHJpbmc=

./dnsproxy -u 1.1.1.1 --ednsopt "4242:VGhpcyBpcyBhIGJpbmFyeSBzdHJpbmc="

main.go

@@ -2,11 +2,14 @@ package main
 
 import (
 	"crypto/tls"
+	"encoding/base64"
 	"fmt"
 	"io/ioutil"
 	"net"
 	"os"
 	"os/signal"
+	"strconv"
+	"strings"
 	"syscall"
 	"time"
 
@@ -100,6 +103,9 @@ type Options struct {
 	// Use Custom EDNS Client Address
 	EDNSAddr string `long:"edns-addr" description:"Send EDNS Client Address"`
 
+	// Add user-specific EDNS exstension
+	EDNSOpt []string `long:"ednsopt" description:"List of EDNS extensions to send along with the DNS query (ex: 8:deadbeaf)"`
+
 	// Other settings and options
 	// --
 
@@ -218,6 +224,27 @@ func createProxyConfig(options Options) proxy.Config {
 		}
 	}
 
+	if len(options.EDNSOpt) > 0 {
+		config.EDNSOpts = make(map[uint16][]byte)
+		for _, s := range options.EDNSOpt {
+			kv := strings.Split(s, ":")
+			if len(kv) != 2 {
+				log.Fatalf("parse error for '%s', --ednsopt must be of the form: option-code:base64encodeddata (ex: --ednsopt 8:SGVsbG8gd29ybGQK)", s)
+			} else {
+				r, err := strconv.ParseUint(kv[0], 10, 16)
+				if err != nil {
+					log.Fatalf("parse error for '%s', --ednsopt option-code must be a valid 16 bits numbers (between 0 and 65535)", kv[0])
+				}
+				optionCode := uint16(r)
+				data, err := base64.StdEncoding.DecodeString(kv[1])
+				if err != nil {
+					log.Fatalf("parse error for '%s', --ednsopt data must be a valid base64 encoded string: (%s)", kv[1], err)
+				}
+				config.EDNSOpts[optionCode] = data
+			}
+		}
+	}
+
 	if options.Fallbacks != nil {
 		fallbacks := []upstream.Upstream{}
 		for i, f := range options.Fallbacks {

proxy/config.go

@@ -72,6 +72,9 @@ type Config struct {
 	EnableEDNSClientSubnet bool
 	EDNSAddr               net.IP // ECS IP used in request
 
+	// List of raw EDNS options
+	EDNSOpts map[uint16][]byte
+
 	// Cache settings
 	// --
 

proxy/proxy.go

@@ -258,6 +258,10 @@ func (p *Proxy) Resolve(d *DNSContext) error {
 		p.processECS(d)
 	}
 
+	if p.Config.EDNSOpts != nil {
+		p.processEDNSOpts(d)
+	}
+
 	if p.replyFromCache(d) {
 		return nil
 	}
@@ -349,3 +353,30 @@ func (p *Proxy) processECS(d *DNSContext) {
 	d.ecsReqIP = ip
 	d.ecsReqMask = mask
 }
+
+// Set EDNSOpts
+func (p *Proxy) processEDNSOpts(d *DNSContext) {
+	var o (*dns.OPT) = nil
+	for _, ex := range d.Req.Extra {
+		if ex.Header().Rrtype == dns.TypeOPT {
+			o = ex.(*dns.OPT)
+			break
+		}
+	}
+
+	if o == nil { // OPT Record does not already exist, create it
+		o = new(dns.OPT)
+		o.SetUDPSize(4096)
+		o.Hdr.Name = "."
+		o.Hdr.Rrtype = dns.TypeOPT
+	}
+
+	for k, v := range p.Config.EDNSOpts {
+		e := new(dns.EDNS0_LOCAL)
+		e.Code = k
+		e.Data = v
+		o.Option = append(o.Option, e)
+	}
+
+	d.Req.Extra = append(d.Req.Extra, o)
+}