example for jordan

Change-Id: I28400c94605110dcbac394fc7bfb61797c1e4b82
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlknobs/controlknobs.go

@@ -52,6 +52,8 @@ type Knobs struct {
 	// DisableDNSForwarderTCPRetries is whether the DNS forwarder should
 	// skip retrying truncated queries over TCP.
 	DisableDNSForwarderTCPRetries atomic.Bool
+
+	SilentDisco atomic.Bool
 }
 
 // UpdateFromNodeAttributes updates k (if non-nil) based on the provided self
@@ -74,6 +76,7 @@ func (k *Knobs) UpdateFromNodeAttributes(selfNodeAttrs []tailcfg.NodeCapability,
 		forceBackgroundSTUN           = has(tailcfg.NodeAttrDebugForceBackgroundSTUN)
 		peerMTUEnable                 = has(tailcfg.NodeAttrPeerMTUEnable)
 		dnsForwarderDisableTCPRetries = has(tailcfg.NodeAttrDNSForwarderDisableTCPRetries)
+		silentDisco                   = has(tailcfg.NodeAttrSilentDisco)
 	)
 
 	if has(tailcfg.NodeAttrOneCGNATEnable) {
@@ -91,6 +94,7 @@ func (k *Knobs) UpdateFromNodeAttributes(selfNodeAttrs []tailcfg.NodeCapability,
 	k.DisableDeltaUpdates.Store(disableDeltaUpdates)
 	k.PeerMTUEnable.Store(peerMTUEnable)
 	k.DisableDNSForwarderTCPRetries.Store(dnsForwarderDisableTCPRetries)
+	k.SilentDisco.Store(silentDisco)
 }
 
 // AsDebugJSON returns k as something that can be marshalled with json.Marshal

ipn/ipnlocal/local.go

@@ -4348,6 +4348,8 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	}
 	b.capFileSharing = fs
 
+	b.magicConn().SetSilentDisco(b.ControlKnobs().SilentDisco.Load())
+
 	b.setDebugLogsByCapabilityLocked(nm)
 
 	// See the netns package for documentation on what this capability does.

tailcfg/tailcfg.go

@@ -2126,6 +2126,8 @@ type Oauth2Token struct {
 	// fixed port.
 	NodeAttrRandomizeClientPort NodeCapability = "randomize-client-port"
 
+	NodeAttrSilentDisco NodeCapability = "silent-disco"
+
 	// NodeAttrOneCGNATEnable makes the client prefer one big CGNAT /10 route
 	// rather than a /32 per peer. At most one of this or
 	// NodeAttrOneCGNATDisable may be set; if neither are, it's automatic.

wgengine/magicsock/magicsock.go

@@ -139,6 +139,8 @@ type Conn struct {
 	// logging.
 	noV4, noV6 atomic.Bool
 
+	silentDiscoOn atomic.Bool // whether silent disco is enabled
+
 	// noV4Send is whether IPv4 UDP is known to be unable to transmit
 	// at all. This could happen if the socket is in an invalid state
 	// (as can happen on darwin after a network link status change).
@@ -1801,6 +1803,14 @@ func (c *Conn) debugFlagsLocked() (f debugFlags) {
 	return
 }
 
+func (c *Conn) SetSilentDisco(v bool) {
+	old := c.silentDiscoOn.Swap(v)
+	if old == v {
+		return
+	}
+	// ....
+}
+
 // SetNetworkMap is called when the control client gets a new network
 // map from the control server. It must always be non-nil.
 //