cmd/tailscale,ipn,net/netutil: remove rp_filter strict mode warnings (#18863)

PR #18860 adds firewall rules in the mangle table to save outbound packet
marks to conntrack and restore them on reply packets before the routing
decision. When reply packets have their marks restored, the kernel uses
the correct routing table (based on the mark) and the packets pass the
rp_filter check.

This makes the risk check and reverse path filtering warnings unnecessary.

Updates #3310
Fixes tailscale/corp#37846

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

ipn/localapi/localapi.go

@@ -28,7 +28,6 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/feature"
 	"tailscale.com/feature/buildfeatures"
-	"tailscale.com/health/healthmsg"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnauth"
@@ -100,9 +99,6 @@ func init() {
 		Register("check-udp-gro-forwarding", (*Handler).serveCheckUDPGROForwarding)
 		Register("set-udp-gro-forwarding", (*Handler).serveSetUDPGROForwarding)
 	}
-	if buildfeatures.HasUseExitNode && runtime.GOOS == "linux" {
-		Register("check-reverse-path-filtering", (*Handler).serveCheckReversePathFiltering)
-	}
 	if buildfeatures.HasClientMetrics {
 		Register("upload-client-metrics", (*Handler).serveUploadClientMetrics)
 	}
@@ -780,32 +776,6 @@ func (h *Handler) serveCheckSOMarkInUse(w http.ResponseWriter, r *http.Request)
 	})
 }
 
-func (h *Handler) serveCheckReversePathFiltering(w http.ResponseWriter, r *http.Request) {
-	if !h.PermitRead {
-		http.Error(w, "reverse path filtering check access denied", http.StatusForbidden)
-		return
-	}
-	var warning string
-
-	state := h.b.Sys().NetMon.Get().InterfaceState()
-	warn, err := netutil.CheckReversePathFiltering(state)
-	if err == nil && len(warn) > 0 {
-		var msg strings.Builder
-		msg.WriteString(healthmsg.WarnExitNodeUsage + ":\n")
-		for _, w := range warn {
-			msg.WriteString("- " + w + "\n")
-		}
-		msg.WriteString(healthmsg.DisableRPFilter)
-		warning = msg.String()
-	}
-	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(struct {
-		Warning string
-	}{
-		Warning: warning,
-	})
-}
-
 func (h *Handler) serveCheckUDPGROForwarding(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitRead {
 		http.Error(w, "UDP GRO forwarding check access denied", http.StatusForbidden)