Use cached discovered records on temporary discovery failure

When client run in endpoint discovery mode (-discover flag), it caches
SRV records lookup result on success; if in next time when client starts
with the same discover argument it temporarily fails to get SRV records
(i.e. network is down), it will use previously cached results instead of
failing.

This makes case or running client as a service on a laptop that change
networks often or may be offline more reliable.

go.mod

@@ -10,3 +10,5 @@ require (
 )
 
 replace github.com/armon/go-socks5 => github.com/artyom/go-socks5 v0.0.0-20171215124554-5ab49e6379c2
+
+go 1.13

main.go

@@ -72,6 +72,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
@@ -84,6 +85,7 @@ import (
 	"net"
 	"net/http"
 	"os"
+	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
@@ -159,6 +161,13 @@ func runClient(args clientArgs, servers []string) error {
 			return r.LookupSRV(ctx, service, proto, name)
 		}
 		cname, recs, err := lookup("tlstun", "tcp", args.Discover)
+		if te, ok := err.(interface{ Temporary() bool }); ok && te.Temporary() {
+			if cached := loadDiscoverCache(args.Discover); len(cached) > 0 {
+				log.Printf("using cached server list, temporary discover failure: %v", err)
+				servers = cached
+				goto hasServers
+			}
+		}
 		if err != nil {
 			return err
 		}
@@ -170,7 +179,9 @@ func runClient(args clientArgs, servers []string) error {
 			servers = append(servers, net.JoinHostPort(rec.Target,
 				strconv.FormatUint(uint64(rec.Port), 10)))
 		}
+		saveDiscoverCache(args.Discover, servers)
 	}
+hasServers:
 	for _, addr := range servers {
 		host, port, err := net.SplitHostPort(addr)
 		if err != nil {
@@ -639,6 +650,31 @@ func (sr socksResolver) Resolve(ctx context.Context, name string) (context.Conte
 	return ctx, ips[0].IP, nil
 }
 
+func loadDiscoverCache(name string) []string {
+	dir, err := os.UserCacheDir()
+	if err != nil {
+		return nil
+	}
+	file := filepath.Join(dir, "tlstun",
+		fmt.Sprintf("%x", sha256.Sum256([]byte(name))))
+	b, err := ioutil.ReadFile(file)
+	if err != nil {
+		return nil
+	}
+	return strings.Split(string(b), "\n")
+}
+
+func saveDiscoverCache(name string, vals []string) {
+	dir, err := os.UserCacheDir()
+	if err != nil {
+		return
+	}
+	file := filepath.Join(dir, "tlstun",
+		fmt.Sprintf("%x", sha256.Sum256([]byte(name))))
+	_ = os.MkdirAll(filepath.Dir(file), 0700)
+	_ = ioutil.WriteFile(file, []byte(strings.Join(vals, "\n")), 0600)
+}
+
 func usageMain(serverFlags, clientFlags *flag.FlagSet) {
 	fmt.Fprint(os.Stderr, usageHead)
 	fmt.Fprint(os.Stderr, "\nTo run client:\n\n")