dnscrypt-proxy/dnscrypt-proxy/sources.go

package main

import (
	"bytes"
	"fmt"
	"math/rand"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"time"

	"github.com/dchest/safefile"

	"github.com/jedisct1/dlog"
	"github.com/jedisct1/go-dnsstamps"
	"github.com/jedisct1/go-minisign"
)

type SourceFormat int

const (
	SourceFormatV2 = iota
)

const (
	DefaultPrefetchDelay    time.Duration = 24 * time.Hour
	MinimumPrefetchInterval time.Duration = 10 * time.Minute
)

type Source struct {
	name                    string
	urls                    []*url.URL
	format                  SourceFormat
	bin                     []byte
	minisignKey             *minisign.PublicKey
	cacheFile               string
	cacheTTL, prefetchDelay time.Duration
	refresh                 time.Time
	prefix                  string
}

func (source *Source) checkSignature(bin, sig []byte) (err error) {
	var signature minisign.Signature
	if signature, err = minisign.DecodeSignature(string(sig)); err == nil {
		_, err = source.minisignKey.Verify(bin, signature)
	}
	return err
}

// timeNow() can be replaced by tests to provide a static value
var timeNow = time.Now

func (source *Source) fetchFromCache(now time.Time) (ttl time.Duration, err error) {
	var bin, sig []byte
	if bin, err = os.ReadFile(source.cacheFile); err != nil {
		return
	}
	if sig, err = os.ReadFile(source.cacheFile + ".minisig"); err != nil {
		return
	}
	if err = source.checkSignature(bin, sig); err != nil {
		return
	}
	source.bin = bin
	var fi os.FileInfo
	if fi, err = os.Stat(source.cacheFile); err != nil {
		return
	}
	if elapsed := now.Sub(fi.ModTime()); elapsed < source.cacheTTL {
		ttl = source.prefetchDelay - elapsed
		dlog.Debugf("Source [%s] cache file [%s] is still fresh, next update: %v", source.name, source.cacheFile, ttl)
	} else {
		dlog.Debugf("Source [%s] cache file [%s] needs to be refreshed", source.name, source.cacheFile)
	}
	return
}

func writeSource(f string, bin, sig []byte) (err error) {
	var fSrc, fSig *safefile.File
	if fSrc, err = safefile.Create(f, 0o644); err != nil {
		return
	}
	defer fSrc.Close()
	if fSig, err = safefile.Create(f+".minisig", 0o644); err != nil {
		return
	}
	defer fSig.Close()
	if _, err = fSrc.Write(bin); err != nil {
		return
	}
	if _, err = fSig.Write(sig); err != nil {
		return
	}
	if err = fSrc.Commit(); err != nil {
		return
	}
	return fSig.Commit()
}

func (source *Source) updateCache(bin, sig []byte, now time.Time) {
	f := source.cacheFile
	var writeErr error // an error writing cache isn't fatal
	defer func() {
		source.bin = bin
		if writeErr == nil {
			return
		}
		if absPath, absErr := filepath.Abs(f); absErr == nil {
			f = absPath
		}
		dlog.Warnf("%s: %s", f, writeErr)
	}()
	if !bytes.Equal(source.bin, bin) {
		if writeErr = writeSource(f, bin, sig); writeErr != nil {
			return
		}
	}
	writeErr = os.Chtimes(f, now, now)
}

func (source *Source) parseURLs(urls []string) {
	for _, urlStr := range urls {
		if srcURL, err := url.Parse(urlStr); err != nil {
			dlog.Warnf("Source [%s] failed to parse URL [%s]", source.name, urlStr)
		} else {
			source.urls = append(source.urls, srcURL)
		}
	}
}

func fetchFromURL(xTransport *XTransport, u *url.URL) (bin []byte, err error) {
	bin, _, _, _, err = xTransport.Get(u, "", DefaultTimeout)
	return bin, err
}

func (source *Source) fetchWithCache(xTransport *XTransport, now time.Time) (ttl time.Duration, err error) {
	if ttl, err = source.fetchFromCache(now); err != nil {
		if len(source.urls) == 0 {
			dlog.Errorf("Source [%s] cache file [%s] not present and no valid URL", source.name, source.cacheFile)
			return
		}
		dlog.Debugf("Source [%s] cache file [%s] not present", source.name, source.cacheFile)
	}
	if len(source.urls) > 0 {
		defer func() {
			source.refresh = now.Add(ttl)
		}()
	}
	if len(source.urls) == 0 || ttl > 0 {
		return
	}
	ttl = MinimumPrefetchInterval
	var bin, sig []byte
	for _, srcURL := range source.urls {
		dlog.Infof("Source [%s] loading from URL [%s]", source.name, srcURL)
		sigURL := &url.URL{}
		*sigURL = *srcURL // deep copy to avoid parsing twice
		sigURL.Path += ".minisig"
		if bin, err = fetchFromURL(xTransport, srcURL); err != nil {
			dlog.Debugf("Source [%s] failed to download from URL [%s]", source.name, srcURL)
			continue
		}
		if sig, err = fetchFromURL(xTransport, sigURL); err != nil {
			dlog.Debugf("Source [%s] failed to download signature from URL [%s]", source.name, sigURL)
			continue
		}
		if err = source.checkSignature(bin, sig); err == nil {
			break // valid signature
		} // above err check inverted to make use of implicit continue
		dlog.Debugf("Source [%s] failed signature check using URL [%s]", source.name, srcURL)
	}
	if err != nil {
		return
	}
	source.updateCache(bin, sig, now)
	ttl = source.prefetchDelay
	return
}

// NewSource loads a new source using the given cacheFile and urls, ensuring it has a valid signature
func NewSource(
	name string,
	xTransport *XTransport,
	urls []string,
	minisignKeyStr string,
	cacheFile string,
	formatStr string,
	refreshDelay time.Duration,
	prefix string,
) (source *Source, err error) {
	if refreshDelay < DefaultPrefetchDelay {
		refreshDelay = DefaultPrefetchDelay
	}
	source = &Source{
		name:          name,
		urls:          []*url.URL{},
		cacheFile:     cacheFile,
		cacheTTL:      refreshDelay,
		prefetchDelay: DefaultPrefetchDelay,
		prefix:        prefix,
	}
	if formatStr == "v2" {
		source.format = SourceFormatV2
	} else {
		return source, fmt.Errorf("Unsupported source format: [%s]", formatStr)
	}
	if minisignKey, err := minisign.NewPublicKey(minisignKeyStr); err == nil {
		source.minisignKey = &minisignKey
	} else {
		return source, err
	}
	source.parseURLs(urls)
	if _, err = source.fetchWithCache(xTransport, timeNow()); err == nil {
		dlog.Noticef("Source [%s] loaded", name)
	}
	return
}

// PrefetchSources downloads latest versions of given sources, ensuring they have a valid signature before caching
func PrefetchSources(xTransport *XTransport, sources []*Source) time.Duration {
	now := timeNow()
	interval := MinimumPrefetchInterval
	for _, source := range sources {
		if source.refresh.IsZero() || source.refresh.After(now) {
			continue
		}
		dlog.Debugf("Prefetching [%s]", source.name)
		if delay, err := source.fetchWithCache(xTransport, now); err != nil {
			dlog.Infof("Prefetching [%s] failed: %v, will retry in %v", source.name, err, interval)
		} else {
			dlog.Debugf("Prefetching [%s] succeeded, next update in %v min", source.name, delay)
			if delay >= MinimumPrefetchInterval && (interval == MinimumPrefetchInterval || interval > delay) {
				interval = delay
			}
		}
	}
	return interval
}

func (source *Source) Parse() ([]RegisteredServer, error) {
	if source.format == SourceFormatV2 {
		return source.parseV2()
	}
	dlog.Fatal("Unexpected source format")
	return []RegisteredServer{}, nil
}

func (source *Source) parseV2() ([]RegisteredServer, error) {
	var registeredServers []RegisteredServer
	var stampErrs []string
	appendStampErr := func(format string, a ...interface{}) {
		stampErr := fmt.Sprintf(format, a...)
		stampErrs = append(stampErrs, stampErr)
		dlog.Warn(stampErr)
	}
	in := string(source.bin)
	parts := strings.Split(in, "## ")
	if len(parts) < 2 {
		return registeredServers, fmt.Errorf("Invalid format for source at [%v]", source.urls)
	}
	parts = parts[1:]
	for _, part := range parts {
		part = strings.TrimSpace(part)
		subparts := strings.Split(part, "\n")
		if len(subparts) < 2 {
			return registeredServers, fmt.Errorf("Invalid format for source at [%v]", source.urls)
		}
		name := strings.TrimSpace(subparts[0])
		if len(name) == 0 {
			return registeredServers, fmt.Errorf("Invalid format for source at [%v]", source.urls)
		}
		subparts = subparts[1:]
		name = source.prefix + name
		var stampStr, description string
		stampStrs := make([]string, 0)
		for _, subpart := range subparts {
			subpart = strings.TrimSpace(subpart)
			if strings.HasPrefix(subpart, "sdns:") && len(subpart) >= 6 {
				stampStrs = append(stampStrs, subpart)
				continue
			} else if len(subpart) == 0 || strings.HasPrefix(subpart, "//") {
				continue
			}
			if len(description) > 0 {
				description += "\n"
			}
			description += subpart
		}
		stampStrsLen := len(stampStrs)
		if stampStrsLen <= 0 {
			appendStampErr("Missing stamp for server [%s]", name)
			continue
		} else if stampStrsLen > 1 {
			rand.Shuffle(stampStrsLen, func(i, j int) { stampStrs[i], stampStrs[j] = stampStrs[j], stampStrs[i] })
		}
		var stamp dnsstamps.ServerStamp
		var err error
		for _, stampStr = range stampStrs {
			stamp, err = dnsstamps.NewServerStampFromString(stampStr)
			if err == nil {
				break
			}
			appendStampErr("Invalid or unsupported stamp [%v]: %s", stampStr, err.Error())
		}
		if err != nil {
			continue
		}
		registeredServer := RegisteredServer{
			name: name, stamp: stamp, description: description,
		}
		dlog.Debugf("Registered [%s] with stamp [%s]", name, stamp.String())
		registeredServers = append(registeredServers, registeredServer)
	}
	if len(stampErrs) > 0 {
		return registeredServers, fmt.Errorf("%s", strings.Join(stampErrs, ", "))
	}
	return registeredServers, nil
}