Create DNS resolve tool

x/examples/resolve/README.md

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+# Domain Resolution Tool
+
+The `resolve` tool lets you resolve domain names with custom DNS resolvers and using configurable transports.
+
+Usage:
+
+```txt
+Usage: resolve [flags...] <domain>
+  -resolver string
+        The address of the recursive DNS resolver to use in host:port format. If the port is missing, it's assumed to be 53
+  -tcp
+        Force TCP when querying the DNS resolver
+  -transport string
+        The transport for the connection to the recursive DNS resolver
+  -type string
+        The type of the query (A, AAAA, CNAME, NS or TXT). (default "A")
+  -v    Enable debug output
+```
+
+Lookup the IPv4 for `www.rferl.org` using the system resolver:
+
+```console
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve www.rferl.org     
+104.102.138.8
+```
+
+Use `-type aaaa` to lookup the IPv6:
+
+```console
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type aaaa www.rferl.org
+2600:141b:1c00:1098::1317
+2600:141b:1c00:10a1::1317
+```
+
+Use `-resolver` to specify which resolver to use. In this case we use Google's Public DNS:
+
+```console
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -resolver 8.8.8.8 www.rferl.org
+104.102.138.83
+```
+
+It's possible to specify a proxy to connect to the resolver using the `-transport` flag. This is very helpful for experimentation. In the example below, we resolve via a remote proxy in Russia. When using a remote server, you must also specify the resolver to use. Note in the example output how the domain is blocked with a CNAME to `fz139.ttk.ru`
+
+```console
+$ KEY=ss://[REDACTED OUTLINE KEY]
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -transport "$KEY" -resolver 8.8.8.8 www.rferl.org
+fz139.ttk.ru.
+```
+
+Using Quad9 in the Russian server bypasses the blocking:
+
+```console
+$ KEY=ss://[REDACTED OUTLINE KEY]
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -transport "$KEY" -resolver 9.9.9.9 www.rferl.org
+e4887.dscb.akamaiedge.net.
+```
+
+It's possible to specify non-standard ports. For example, OpenDNS supports port 443:
+
+```console
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -resolver 208.67.222.222:443 www.rferl.org
+e4887.dscb.akamaiedge.net.
+```
+
+However, it seems UDP on alternate ports is blocked in our remote test proxy:
+
+```console
+$ KEY=ss://[REDACTED OUTLINE KEY]
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -transport "$KEY" -resolver 208.67.222.222:443 www.rferl.org
+2023/10/13 19:04:18 Failed to lookup CNAME: lookup www.rferl.org on 208.67.222.222:443: could not create PacketConn: could not connect to endpoint: dial udp [REDACTED ADDRESS]: i/o timeout
+exit status 1
+```
+
+By forcing TCP with the `-tcp` flag, you can make it work again:
+
+```console
+$ KEY=ss://[REDACTED OUTLINE KEY]
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -transport "$KEY" -resolver 208.67.222.222:443 -tcp www.rferl.org
+e4887.dscb.akamaiedge.net.
+```
+
+Forcing TCP lets you use stream fragmentation. In this example, we split the first 20 bytes:
+
+```console
+$ go run github.com/Jigsaw-Code/outline-sdk/x/examples/resolve -type CNAME -tcp -transport "split:20" -resolver 208.67.222.222:443 www.rferl.org
+e4887.dscb.akamaiedge.net.
+```
+
+You can see that the domain name in the query got split:
+
+![image](https://github.com/Jigsaw-Code/outline-sdk/assets/113565/195bfa95-6d35-40ef-84e0-b1d6e690bb84)

x/examples/resolve/main.go

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+// Copyright 2023 Jigsaw Operations LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"os"
+	"path"
+	"strings"
+	"time"
+
+	"github.com/Jigsaw-Code/outline-sdk/transport"
+	"github.com/Jigsaw-Code/outline-sdk/x/config"
+)
+
+var debugLog log.Logger = *log.New(io.Discard, "", 0)
+
+func init() {
+	flag.Usage = func() {
+		fmt.Fprintf(flag.CommandLine.Output(), "Usage: %s [flags...] <domain>\n", path.Base(os.Args[0]))
+		flag.PrintDefaults()
+	}
+}
+
+func cleanDNSError(err error, resolverAddr string) error {
+	dnsErr := &net.DNSError{}
+	if resolverAddr != "" && errors.As(err, &dnsErr) {
+		dnsErr.Server = resolverAddr
+		return dnsErr
+	}
+	return err
+}
+
+func main() {
+	verboseFlag := flag.Bool("v", false, "Enable debug output")
+	typeFlag := flag.String("type", "A", "The type of the query (A, AAAA, CNAME, NS or TXT).")
+	resolverFlag := flag.String("resolver", "", "The address of the recursive DNS resolver to use in host:port format. If the port is missing, it's assumed to be 53")
+	transportFlag := flag.String("transport", "", "The transport for the connection to the recursive DNS resolver")
+	tcpFlag := flag.Bool("tcp", false, "Force TCP when querying the DNS resolver")
+
+	flag.Parse()
+	if *verboseFlag {
+		debugLog = *log.New(os.Stderr, "[DEBUG] ", log.LstdFlags|log.Lmicroseconds|log.Lshortfile)
+	}
+
+	domain := strings.TrimSpace(flag.Arg(0))
+	if domain == "" {
+		log.Fatal("Need to pass the domain to resolve in the command-line")
+	}
+
+	resolverAddr := *resolverFlag
+	if resolverAddr != "" && !strings.Contains(resolverAddr, ":") {
+		resolverAddr = net.JoinHostPort(resolverAddr, "53")
+	}
+
+	var err error
+	var packetDialer transport.PacketDialer
+	if !*tcpFlag {
+		packetDialer, err = config.NewPacketDialer(*transportFlag)
+		if err != nil {
+			log.Fatalf("Could not create packet dialer: %v", err)
+		}
+	}
+	streamDialer, err := config.NewStreamDialer(*transportFlag)
+	if err != nil {
+		log.Fatalf("Could not create stream dialer: %v", err)
+	}
+
+	resolver := net.Resolver{PreferGo: true}
+	resolver.Dial = func(ctx context.Context, network, sysResolverAddr string) (net.Conn, error) {
+		dialAddr := sysResolverAddr
+		if resolverAddr != "" {
+			dialAddr = resolverAddr
+		}
+		if strings.HasPrefix(network, "tcp") || *tcpFlag {
+			debugLog.Printf("Dial TCP: %v", dialAddr)
+			return streamDialer.Dial(ctx, dialAddr)
+		}
+		debugLog.Printf("Dial UDP: %v", dialAddr)
+		return packetDialer.Dial(ctx, dialAddr)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	switch strings.ToUpper(*typeFlag) {
+	case "A":
+		ips, err := resolver.LookupIP(ctx, "ip4", domain)
+		err = cleanDNSError(err, resolverAddr)
+		if err != nil {
+			log.Fatalf("Failed to lookup IPs: %v", err)
+		}
+		for _, ip := range ips {
+			fmt.Println(ip.String())
+		}
+	case "AAAA":
+		ips, err := resolver.LookupIP(ctx, "ip6", domain)
+		err = cleanDNSError(err, resolverAddr)
+		if err != nil {
+			log.Fatalf("Failed to lookup IPs: %v", err)
+		}
+		for _, ip := range ips {
+			fmt.Println(ip.String())
+		}
+	case "CNAME":
+		cname, err := resolver.LookupCNAME(ctx, domain)
+		err = cleanDNSError(err, resolverAddr)
+		if err != nil {
+			log.Fatalf("Failed to lookup CNAME: %v", err)
+		}
+		fmt.Println(cname)
+	case "NS":
+		nss, err := resolver.LookupNS(ctx, domain)
+		err = cleanDNSError(err, resolverAddr)
+		if err != nil {
+			log.Fatalf("Failed to lookup NS: %v", err)
+		}
+		for _, ns := range nss {
+			fmt.Println(ns.Host)
+		}
+	case "TXT":
+		lines, err := resolver.LookupTXT(ctx, domain)
+		err = cleanDNSError(err, resolverAddr)
+		if err != nil {
+			log.Fatalf("Failed to lookup NS: %v", err)
+		}
+		for _, line := range lines {
+			fmt.Println(line)
+		}
+	default:
+		log.Fatalf("Invalid query type %v", *typeFlag)
+	}
+}