fix(health): use TCP dialing instead of ping

- `HEALTH_TARGET_ADDRESS` to replace `HEALTH_ADDRESS_TO_PING`
- Remove `github.com/go-ping/ping` dependency
- Dial TCP the target address, appending `:443` if port is not set

internal/configuration/settings/health.go

@@ -15,10 +15,10 @@ type Health struct {
 	// for the health check server.
 	// It cannot be the empty string in the internal state.
 	ServerAddress string
-	// AddressToPing is the IP address or domain name to
-	// ping periodically for the health check.
+	// TargetAddress is the address (host or host:port)
+	// to TCP dial to periodically for the health check.
 	// It cannot be the empty string in the internal state.
-	AddressToPing string
+	TargetAddress string
 	VPN           HealthyWait
 }
 
@@ -41,7 +41,7 @@ func (h Health) Validate() (err error) {
 func (h *Health) copy() (copied Health) {
 	return Health{
 		ServerAddress: h.ServerAddress,
-		AddressToPing: h.AddressToPing,
+		TargetAddress: h.TargetAddress,
 		VPN:           h.VPN.copy(),
 	}
 }
@@ -50,7 +50,7 @@ func (h *Health) copy() (copied Health) {
 // unset field of the receiver settings object.
 func (h *Health) MergeWith(other Health) {
 	h.ServerAddress = helpers.MergeWithString(h.ServerAddress, other.ServerAddress)
-	h.AddressToPing = helpers.MergeWithString(h.AddressToPing, other.AddressToPing)
+	h.TargetAddress = helpers.MergeWithString(h.TargetAddress, other.TargetAddress)
 	h.VPN.mergeWith(other.VPN)
 }
 
@@ -59,13 +59,13 @@ func (h *Health) MergeWith(other Health) {
 // settings.
 func (h *Health) OverrideWith(other Health) {
 	h.ServerAddress = helpers.OverrideWithString(h.ServerAddress, other.ServerAddress)
-	h.AddressToPing = helpers.OverrideWithString(h.AddressToPing, other.AddressToPing)
+	h.TargetAddress = helpers.OverrideWithString(h.TargetAddress, other.TargetAddress)
 	h.VPN.overrideWith(other.VPN)
 }
 
 func (h *Health) SetDefaults() {
 	h.ServerAddress = helpers.DefaultString(h.ServerAddress, "127.0.0.1:9999")
-	h.AddressToPing = helpers.DefaultString(h.AddressToPing, "github.com")
+	h.TargetAddress = helpers.DefaultString(h.TargetAddress, "github.com:443")
 	h.VPN.setDefaults()
 }
 
@@ -76,7 +76,7 @@ func (h Health) String() string {
 func (h Health) toLinesNode() (node *gotree.Node) {
 	node = gotree.New("Health settings:")
 	node.Appendf("Server listening address: %s", h.ServerAddress)
-	node.Appendf("Address to ping: %s", h.AddressToPing)
+	node.Appendf("Target address: %s", h.TargetAddress)
 	node.AppendNode(h.VPN.toLinesNode("VPN"))
 	return node
 }

internal/configuration/settings/settings_test.go

@@ -66,7 +66,7 @@ func Test_Settings_String(t *testing.T) {
 |   └── Log level: INFO
 ├── Health settings:
 |   ├── Server listening address: 127.0.0.1:9999
-|   ├── Address to ping: github.com
+|   ├── Target address: github.com:443
 |   └── VPN wait durations:
 |       ├── Initial duration: 6s
 |       └── Additional duration: 5s

internal/configuration/sources/env/health.go

@@ -10,7 +10,7 @@ import (
 
 func (r *Reader) ReadHealth() (health settings.Health, err error) {
 	health.ServerAddress = os.Getenv("HEALTH_SERVER_ADDRESS")
-	health.AddressToPing = os.Getenv("HEALTH_ADDRESS_TO_PING")
+	_, health.TargetAddress = r.getEnvWithRetro("HEALTH_TARGET_ADDRESS", "HEALTH_ADDRESS_TO_PING")
 
 	health.VPN.Initial, err = r.readDurationWithRetro(
 		"HEALTH_VPN_DURATION_INITIAL",