support ExternalHostname which has precedence over IPs

CONFIG.md

@@ -1,12 +1,26 @@
 Explanation of each field:
 
     {
+        "ExternalHostname": "",
+
+The `ExternalHostname` is used for the client config server `Endpoint` if
+defined. It has precedence over `ExternalIP` and `ExternalIP6`.
+
+
         "ExternalIP": "198.51.100.2",
         "ExternalIP6": "2001:0db8:85a3:0000:0000:8a2e:0370:7334",
 
-This is the external IP that will be the value of Endpoint for the server peer
+This is the external IPv4 and IPv6 that will be the value of Endpoint for the
-in client configs. It is automatically detected by opening a socket or using an
+server peer in client configs. It is automatically detected by opening a socket
-external IP discovery service -- the first to give a valid public IP will win.
+or using an external IP discovery service -- the first to give a valid public
+IP will win.
+
+When generating configs, the `ExternalHostname` has precendence for the server
+`Endpoint`, followed by `ExternalIP` (IPv4) and `ExternalIP6` (IPv6) The IPs are
+discovered automatically on init. Define an `ExternalHostname` if you're using
+dynamic DNS, want to change IPs without updating configs, or want wireguard to
+be able to choose between IPv4/IPv6. It is only possible to specify one
+Endpoint per peer entry in wireguard.
 
 
         "ListenPort": 51820,

README.md

@@ -60,6 +60,7 @@ Main (automatically generated) configuration example:
 
 
     {
+        "ExternalHostname": "",
         "ExternalIP": "198.51.100.2",
         "ExternalIP6": "2001:0db8:85a3:0000:0000:8a2e:0370:7334",
         "ListenPort": 51820,

add.go

@@ -22,11 +22,7 @@ DNS={{ .DsnetConfig.DNS }}
 [Peer]
 PublicKey={{ .DsnetConfig.PrivateKey.PublicKey.Key }}
 PresharedKey={{ .Peer.PresharedKey.Key }}
-{{ if gt (.DsnetConfig.ExternalIP | len) 0 -}}
+Endpoint={{ .Endpoint }}:{{ .DsnetConfig.ListenPort }}
-Endpoint={{ .DsnetConfig.ExternalIP }}:{{ .DsnetConfig.ListenPort }}
-{{ else -}}
-Endpoint={{ .DsnetConfig.ExternalIP6 }}:{{ .DsnetConfig.ListenPort }}
-{{ end -}}
 PersistentKeepalive={{ .Keepalive }}
 {{ if gt (.DsnetConfig.Network.IPNet.IP | len) 0 -}}
 AllowedIPs={{ .DsnetConfig.Network }}
@@ -54,11 +50,7 @@ set interfaces wireguard {{ .Wgif }} description {{ .DsnetConfig.InterfaceName }
 #set service dns forwarding name-server {{ .DsnetConfig.DNS }}
 {{ end }}
 
-{{ if gt (.DsnetConfig.ExternalIP | len) 0 -}}
+set interfaces wireguard {{ .Wgif }} peer {{ .DsnetConfig.PrivateKey.PublicKey.Key }} endpoint {{ .Endpoint }}:{{ .DsnetConfig.ListenPort }}
-set interfaces wireguard {{ .Wgif }} peer {{ .DsnetConfig.PrivateKey.PublicKey.Key }} endpoint {{ .DsnetConfig.ExternalIP }}:{{ .DsnetConfig.ListenPort }}
-{{ else -}}
-set interfaces wireguard {{ .Wgif }} peer {{ .DsnetConfig.PrivateKey.PublicKey.Key }} endpoint {{ .DsnetConfig.ExternalIP6 }}:{{ .DsnetConfig.ListenPort }}
-{{ end -}}
 set interfaces wireguard {{ .Wgif }} peer {{ .DsnetConfig.PrivateKey.PublicKey.Key }} persistent-keepalive {{ .Keepalive }}
 set interfaces wireguard {{ .Wgif }} peer {{ .DsnetConfig.PrivateKey.PublicKey.Key }} preshared-key {{ .Peer.PresharedKey.Key }}
 {{ if gt (.DsnetConfig.Network.IPNet.IP | len) 0 -}}
@@ -151,6 +143,19 @@ func PrintPeerCfg(peer PeerConfig, conf *DsnetConfig) {
 		wgifSeed += int(b)
 	}
 
+	// See DsnetConfig type for explanation
+	var endpoint string
+
+	if conf.ExternalHostname != "" {
+		endpoint = conf.ExternalHostname
+	} else if len(conf.ExternalIP) > 0 {
+		endpoint = conf.ExternalIP.String()
+	} else if len(conf.ExternalIP6) > 0 {
+		endpoint = conf.ExternalIP6.String()
+	} else {
+		ExitFail("Config does not contain ExternalIP, ExternalIP6 or ExternalHostname")
+	}
+
 	t := template.Must(template.New("peerConf").Parse(peerConf))
 	err := t.Execute(os.Stdout, map[string]interface{}{
 		"Peer":        peer,
@@ -162,6 +167,7 @@ func PrintPeerCfg(peer PeerConfig, conf *DsnetConfig) {
 		// deterministically choosing one in this range will probably allow use
 		// of the config without a colliding interface name
 		"Wgif":     fmt.Sprintf("wg%d", wgifSeed%999),
+		"Endpoint": endpoint,
 	})
 	check(err)
 }

configtypes.go

@@ -34,11 +34,18 @@ type PeerConfig struct {
 }
 
 type DsnetConfig struct {
-	// domain to append to hostnames. Relies on separate DNS server for
+	// When generating configs, the ExternalHostname has precendence for the
-	// resolution. Informational only.
+	// server Endpoint, followed by ExternalIP (IPv4) and ExternalIP6 (IPv6)
+	// The IPs are discovered automatically on init. Define an ExternalHostname
+	// if you're using dynamic DNS, want to change IPs without updating
+	// configs, or want wireguard to be able to choose between IPv4/IPv6. It is
+	// only possible to specify one Endpoint per peer entry in wireguard.
+	ExternalHostname string
 	ExternalIP       net.IP
 	ExternalIP6      net.IP
 	ListenPort       int `validate:"gte=1024,lte=65535"`
+	// domain to append to hostnames. Relies on separate DNS server for
+	// resolution. Informational only.
 	Domain        string `validate:"required,gte=1,lte=255"`
 	InterfaceName string `validate:"required,gte=1,lte=255"`
 	// IP network from which to allocate automatic sequential addresses
@@ -76,8 +83,8 @@ func MustLoadDsnetConfig() *DsnetConfig {
 	err = validator.New().Struct(conf)
 	check(err)
 
-	if len(conf.ExternalIP) == 0 && len(conf.ExternalIP6) == 0 {
+	if conf.ExternalHostname == "" && len(conf.ExternalIP) == 0 && len(conf.ExternalIP6) == 0 {
-		ExitFail("Config does not contain ExternalIP or ExternalIP6")
+		ExitFail("Config does not contain ExternalIP, ExternalIP6 or ExternalHostname")
 	}
 
 	return &conf