Merge pull request #52 from /issues/51-dns-probe

cmd/run: failed dns probe for any reason should not be fatal
alphasoc · Feb 15, 2022 · 8303298 · 8303298
2 parents f650b95 + 1eee537
commit 8303298
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Showing 2 changed files with 16 additions and 10 deletions.
diff --git a/cmd/run/print.go b/cmd/run/print.go
@@ -22,6 +22,9 @@ func printMsg(s *Simulation, msg string) {
 }
 
 func printWelcome(ip, dnsIntfIP string) {
+	if dnsIntfIP == "" {
+		dnsIntfIP = "UNKNOWN, system defaults will be used"
+	}
 	fmt.Printf(`
 AlphaSOC Network Flight Simulator™ %s (https://github.com/alphasoc/flightsim)
 The address of the network interface for IP traffic is %s

diff --git a/cmd/run/run.go b/cmd/run/run.go
@@ -327,8 +327,8 @@ const (
 )
 
 // getDefaultDNSIntf runs a DNS probe using default system resolver and returns the IP of
-// the interface used and an error.  Thanks @tg.
-func getDefaultDNSIntf() (string, error) {
+// the interface used, or an empty string.  Thanks @tg.
+func getDefaultDNSIntf() string {
 	timeout := 10 * time.Second
 	ctx, cancel := context.WithTimeout(context.Background(), timeout)
 	defer cancel()
@@ -345,31 +345,34 @@ func getDefaultDNSIntf() (string, error) {
 	}
 	_, err := r.LookupHost(ctx, "alphasoc.com")
 	if err != nil {
-		return "", err
+		return ""
+	}
+	// In cases where r.Dial() is not invoked, defaultDNSServer will be "", so don't bother
+	// continuing with detection (ie. on Windows).
+	if defaultDNSServer == "" {
+		return ""
 	}
 	conn, err := net.DialTimeout("udp", defaultDNSServer, timeout)
 	if err != nil {
-		return "", err
+		return ""
 	}
 	dnsIntfIP, _, err := net.SplitHostPort(conn.LocalAddr().String())
 	if err != nil {
-		return "", err
+		return ""
 	}
-	return dnsIntfIP, nil
+	return dnsIntfIP
 }
 
 func run(sims []*Simulation, bind simulator.BindAddr, size int) error {
 	// If user override on iface, both IP and DNS traffic will flow through bind.Addr.
 	// NOTE: not passing the DNS server to printWelcome(), as it may be confusing in cases
 	// where there are multiple nameservers configured (ie. resolver errors will carry
 	// the address of the last queried nameserver).
-	defaultDNSIntfIP, err := getDefaultDNSIntf()
-	if err != nil {
-		return fmt.Errorf("Failed DNS probe: %v", err)
-	}
+	defaultDNSIntfIP := getDefaultDNSIntf()
 	if bind.UserSet {
 		printWelcome(bind.String(), bind.String())
 	} else {
+		// NOTE: defaultDNSIntfIP _may_ be "".
 		printWelcome(bind.String(), defaultDNSIntfIP)
 	}
 	printHeader()