control/controlclient: only use a single DNS label as the hostname.

Fixes #971

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlclient/direct.go

@@ -46,6 +46,7 @@ import (
 	"tailscale.com/types/opt"
 	"tailscale.com/types/persist"
 	"tailscale.com/types/wgkey"
+	"tailscale.com/util/dnsname"
 	"tailscale.com/util/systemd"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/filter"
@@ -179,6 +180,7 @@ var osVersion func() string // non-nil on some platforms
 
 func NewHostinfo() *tailcfg.Hostinfo {
 	hostname, _ := os.Hostname()
+	hostname = dnsname.FirstLabel(hostname)
 	var osv string
 	if osVersion != nil {
 		osv = osVersion()

util/dnsname/dnsname.go

@@ -223,6 +223,14 @@ func NumLabels(hostname string) int {
 	return strings.Count(hostname, ".")
 }
 
+// FirstLabel returns the first DNS label of hostname.
+func FirstLabel(hostname string) string {
+	if i := strings.IndexByte(hostname, '.'); i != -1 {
+		return hostname[:i]
+	}
+	return hostname
+}
+
 var separators = map[byte]bool{
 	' ': true,
 	'.': true,