net/dnsfallback: cache most recent DERP map on disk (#5545)

This is especially helpful as we launch newer DERPs over time, and older
clients have progressively out-of-date static DERP maps baked in. After
this, as long as the client has successfully connected once, it'll cache
the most recent DERP map it knows about.

Resolves an in-code comment from @bradfitz

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

ipn/ipnlocal/local.go

@@ -34,6 +34,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/ipn/policy"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/tsaddr"
@@ -731,6 +732,9 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 		b.e.SetNetworkMap(st.NetMap)
 		b.e.SetDERPMap(st.NetMap.DERPMap)
 
+		// Update our cached DERP map
+		dnsfallback.UpdateCache(st.NetMap.DERPMap)
+
 		b.send(ipn.Notify{NetMap: st.NetMap})
 	}
 	if st.URL != "" {

ipn/ipnserver/server.go

@@ -37,6 +37,7 @@ import (
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/localapi"
 	"tailscale.com/logtail/backoff"
+	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/netstat"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/tsdial"
@@ -786,6 +787,8 @@ func New(logf logger.Logf, logid string, store ipn.StateStore, eng wgengine.Engi
 			b.SetTailnetKeyAuthority(authority, storage)
 			logf("tka initialized at head %x", authority.Head())
 		}
+
+		dnsfallback.SetCachePath(filepath.Join(root, "derpmap.cached.json"))
 	} else {
 		logf("network-lock unavailable; no state directory")
 	}