dimkr/dohli

// this file is part of dohli.
//
// Copyright (c) 2020 Dima Krasner
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// stub is a local DNS server that resolves using DoH.
package main

import (
    "bytes"
    "context"
    "flag"
    "io/ioutil"
    "log"
    "net"
    "net/http"
    "time"

    "github.com/dimkr/dohli/pkg/cache"
    "github.com/dimkr/dohli/pkg/dns"
    "golang.org/x/net/dns/dnsmessage"
)

const (
    packetSize       = 512
    dnsHeaderSize    = 12
    fallbackTTL      = 60
    resolvingTimeout = 5 * time.Second
)

var server string

func resolve(ctx context.Context, request []byte) []byte {
    post, err := http.NewRequestWithContext(ctx, "POST", server, bytes.NewBuffer(request))
    if err != nil {
        log.Println("Resolving failed: ", err)
        return nil
    }
    post.Header.Set("Content-Type", "application/dns-message")

    response, err := http.DefaultClient.Do(post)
    if err != nil {
        log.Println("Resolving failed: ", err)
        return nil
    }

    defer response.Body.Close()

    if response.StatusCode != http.StatusOK {
        log.Println("Resolving failed: ", response.Status)
        return nil
    }

    if response.ContentLength != -1 && response.ContentLength < dnsHeaderSize {
        return nil
    }

    buf, err := ioutil.ReadAll(response.Body)
    if err != nil {
        log.Println("Resolving failed: ", err)
        return nil
    }

    if len(buf) < dnsHeaderSize {
        return nil
    }

    return buf
}

func main() {
    flag.StringVar(&server, "server", "https://dohli.herokuapp.com/dns-query", "DoH server to use")
    flag.Parse()

    l, err := net.ListenPacket("udp4", ":53")
    if err != nil {
        panic(err)
    }

    cache, err := cache.OpenCache(&cache.MemoryBackend{})
    if err != nil {
        panic(err)
    }

    for {
        buf := make([]byte, packetSize)

        len, addr, err := l.ReadFrom(buf)
        if err != nil {
            continue
        }

        go func() {
            var p dnsmessage.Parser

            if _, err := p.Start(buf[:len]); err != nil {
                return
            }

            question, err := p.Question()
            if err != nil {
                return
            }

            domain := question.Name.String()

            ctx, cancel := context.WithTimeout(context.Background(), resolvingTimeout)
            defer cancel()

            if cached := cache.Get(ctx, domain, question.Type); cached != nil {
                // we join the first two bytes of the request (the ID) with the
                // cached response, which has a different ID
                l.WriteTo(append(buf[:2], cached[2:]...), addr)
                return
            }

            if response := resolve(ctx, buf[:len]); response != nil {
                l.WriteTo(response, addr)

                ttl := dns.GetShortestTTL(response)
                if ttl == 0 {
                    ttl = fallbackTTL
                }

                cache.Set(ctx, domain, question.Type, response, ttl)
            }
        }()
    }
}