feat(healthcheck): combination of ICMP and TCP+TLS checks (#2923)

- New option: `HEALTH_ICMP_TARGET_IP` defaults to `0.0.0.0` meaning use the VPN server public IP address.
- Options removed: `HEALTH_VPN_INITIAL_DURATION` and `HEALTH_VPN_ADDITIONAL_DURATION` - times and retries are handpicked and hardcoded.
- Less aggressive checks and less false positive detection

internal/configuration/settings/health.go

@@ -2,6 +2,7 @@ package settings
 
 import (
 	"fmt"
+	"net/netip"
 	"os"
 	"time"
 
@@ -24,16 +25,13 @@ type Health struct {
 	// HTTP server. It defaults to 500 milliseconds.
 	ReadTimeout time.Duration
 	// TargetAddress is the address (host or host:port)
-	// to TCP dial to periodically for the health check.
+	// to TCP TLS dial to periodically for the health check.
 	// It cannot be the empty string in the internal state.
 	TargetAddress string
-	// SuccessWait is the duration to wait to re-run the
-	// healthcheck after a successful healthcheck.
-	// It defaults to 5 seconds and cannot be zero in
-	// the internal state.
-	SuccessWait time.Duration
-	// VPN has health settings specific to the VPN loop.
-	VPN HealthyWait
+	// ICMPTargetIP is the IP address to use for ICMP echo requests
+	// in the health checker. It can be set to an unspecified address
+	// such that the VPN server IP is used, which is also the default behavior.
+	ICMPTargetIP netip.Addr
 }
 
 func (h Health) Validate() (err error) {
@@ -42,11 +40,6 @@ func (h Health) Validate() (err error) {
 		return fmt.Errorf("server listening address is not valid: %w", err)
 	}
 
-	err = h.VPN.validate()
-	if err != nil {
-		return fmt.Errorf("health VPN settings: %w", err)
-	}
-
 	return nil
 }
 
@@ -56,8 +49,7 @@ func (h *Health) copy() (copied Health) {
 		ReadHeaderTimeout: h.ReadHeaderTimeout,
 		ReadTimeout:       h.ReadTimeout,
 		TargetAddress:     h.TargetAddress,
-		SuccessWait:       h.SuccessWait,
-		VPN:               h.VPN.copy(),
+		ICMPTargetIP:      h.ICMPTargetIP,
 	}
 }
 
@@ -69,8 +61,7 @@ func (h *Health) OverrideWith(other Health) {
 	h.ReadHeaderTimeout = gosettings.OverrideWithComparable(h.ReadHeaderTimeout, other.ReadHeaderTimeout)
 	h.ReadTimeout = gosettings.OverrideWithComparable(h.ReadTimeout, other.ReadTimeout)
 	h.TargetAddress = gosettings.OverrideWithComparable(h.TargetAddress, other.TargetAddress)
-	h.SuccessWait = gosettings.OverrideWithComparable(h.SuccessWait, other.SuccessWait)
-	h.VPN.overrideWith(other.VPN)
+	h.ICMPTargetIP = gosettings.OverrideWithComparable(h.ICMPTargetIP, other.ICMPTargetIP)
 }
 
 func (h *Health) SetDefaults() {
@@ -80,9 +71,7 @@ func (h *Health) SetDefaults() {
 	const defaultReadTimeout = 500 * time.Millisecond
 	h.ReadTimeout = gosettings.DefaultComparable(h.ReadTimeout, defaultReadTimeout)
 	h.TargetAddress = gosettings.DefaultComparable(h.TargetAddress, "cloudflare.com:443")
-	const defaultSuccessWait = 5 * time.Second
-	h.SuccessWait = gosettings.DefaultComparable(h.SuccessWait, defaultSuccessWait)
-	h.VPN.setDefaults()
+	h.ICMPTargetIP = gosettings.DefaultComparable(h.ICMPTargetIP, netip.IPv4Unspecified()) // use the VPN server IP
 }
 
 func (h Health) String() string {
@@ -93,10 +82,11 @@ func (h Health) toLinesNode() (node *gotree.Node) {
 	node = gotree.New("Health settings:")
 	node.Appendf("Server listening address: %s", h.ServerAddress)
 	node.Appendf("Target address: %s", h.TargetAddress)
-	node.Appendf("Duration to wait after success: %s", h.SuccessWait)
-	node.Appendf("Read header timeout: %s", h.ReadHeaderTimeout)
-	node.Appendf("Read timeout: %s", h.ReadTimeout)
-	node.AppendNode(h.VPN.toLinesNode("VPN"))
+	icmpTarget := "VPN server IP"
+	if !h.ICMPTargetIP.IsUnspecified() {
+		icmpTarget = h.ICMPTargetIP.String()
+	}
+	node.Appendf("ICMP target IP: %s", icmpTarget)
 	return node
 }
 
@@ -104,16 +94,9 @@ func (h *Health) Read(r *reader.Reader) (err error) {
 	h.ServerAddress = r.String("HEALTH_SERVER_ADDRESS")
 	h.TargetAddress = r.String("HEALTH_TARGET_ADDRESS",
 		reader.RetroKeys("HEALTH_ADDRESS_TO_PING"))
-
-	h.SuccessWait, err = r.Duration("HEALTH_SUCCESS_WAIT_DURATION")
+	h.ICMPTargetIP, err = r.NetipAddr("HEALTH_ICMP_TARGET_IP")
 	if err != nil {
 		return err
 	}
-
-	err = h.VPN.read(r)
-	if err != nil {
-		return fmt.Errorf("VPN health settings: %w", err)
-	}
-
 	return nil
 }