Added environment variable expansion for tunnel Host field in config

internal/config/config.go

@@ -69,6 +69,17 @@ func Load() (*Config, error) {
 		}
 	}
 
+	// Expand environment variables for a pre-defined set of fields
+	expand := func(s string) string { return os.Expand(s, expandWithDefault) }
+	for i := range cfg.Tunnels {
+		t := &cfg.Tunnels[i]
+		t.Host = expand(t.Host)
+		t.User = expand(t.User)
+		t.IdentityFile = expand(t.IdentityFile)
+		t.LocalAddress = tunnel.StringOrInt(expand(t.LocalAddress.String()))
+		t.RemoteAddress = tunnel.StringOrInt(expand(t.RemoteAddress.String()))
+	}
+
 	// Create a map of tunnel names to tunnel pointers for easy lookup later
 	m, err := buildTunnelsMap(cfg.Tunnels)
 	if err != nil {
@@ -127,3 +138,16 @@ func specialPrefix(s string) bool {
 func containsGlob(s string) bool {
 	return strings.ContainsAny(s, "*?[")
 }
+
+// expandWithDefault resolves an environment variable reference, supporting
+// the ${VAR:-default} syntax. If the variable is unset or empty and a default
+// is provided after ":-", the default value is returned.
+func expandWithDefault(key string) string {
+	if varName, defaultVal, found := strings.Cut(key, ":-"); found {
+		if val := os.Getenv(varName); val != "" {
+			return val
+		}
+		return defaultVal
+	}
+	return os.Getenv(key)
+}

internal/config/config_test.go

@@ -19,3 +19,84 @@ func TestSpecialPrefixEmpty(t *testing.T) {
 		t.Error(`specialPrefix("") = true, want false`)
 	}
 }
+
+func loadFixture(t *testing.T, path string) *Config {
+	orig := Path
+	t.Cleanup(func() { Path = orig })
+	Path = path
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+	return cfg
+}
+
+func TestExpandEnvVars(t *testing.T) {
+	t.Setenv("TEST_HOST", "example.com")
+	t.Setenv("TEST_USER", "alice")
+	t.Setenv("TEST_IDENTITY", "/keys/id_ed25519")
+	t.Setenv("TEST_LOCAL", "9000")
+	t.Setenv("TEST_REMOTE", "localhost:8080")
+
+	cfg := loadFixture(t, "../../test/testdata/config/expand/vars.toml")
+
+	tun := cfg.Tunnels[0]
+	if tun.Host != "example.com" {
+		t.Errorf("Host = %q, want %q", tun.Host, "example.com")
+	}
+	if tun.User != "alice" {
+		t.Errorf("User = %q, want %q", tun.User, "alice")
+	}
+	if tun.IdentityFile != "/keys/id_ed25519" {
+		t.Errorf("IdentityFile = %q, want %q", tun.IdentityFile, "/keys/id_ed25519")
+	}
+	if tun.LocalAddress.String() != "9000" {
+		t.Errorf("LocalAddress = %q, want %q", tun.LocalAddress.String(), "9000")
+	}
+	if tun.RemoteAddress.String() != "localhost:8080" {
+		t.Errorf("RemoteAddress = %q, want %q", tun.RemoteAddress.String(), "localhost:8080")
+	}
+}
+
+func TestExpandDefault(t *testing.T) {
+	t.Setenv("TEST_SET", "real")
+	t.Setenv("TEST_EMPTY", "")
+	// TEST_UNSET is unset
+
+	cfg := loadFixture(t, "../../test/testdata/config/expand/defaults.toml")
+
+	cases := map[string]string{
+		"set":   "real",
+		"empty": "fallback",
+		"unset": "fallback",
+	}
+	for name, want := range cases {
+		if got := cfg.TunnelsMap[name].Host; got != want {
+			t.Errorf("tunnel %q Host = %q, want %q", name, got, want)
+		}
+	}
+}
+
+func TestExpandUnsetIsEmpty(t *testing.T) {
+	// A referenced-but-unset variable with no default expands to ""
+	cfg := loadFixture(t, "../../test/testdata/config/expand/unset.toml")
+
+	if got := cfg.Tunnels[0].Host; got != "" {
+		t.Errorf("Host = %q, want empty string", got)
+	}
+}
+
+func TestExpandFieldsNotExpanded(t *testing.T) {
+	// Name and Group are identifiers and are intentionally not expanded
+	t.Setenv("TEST_VAR", "expanded")
+
+	cfg := loadFixture(t, "../../test/testdata/config/expand/literal_fields.toml")
+
+	tun := cfg.Tunnels[0]
+	if tun.Name != "tun_$TEST_VAR" {
+		t.Errorf("Name = %q, want it left literal", tun.Name)
+	}
+	if tun.Group != "grp_$TEST_VAR" {
+		t.Errorf("Group = %q, want it left literal", tun.Group)
+	}
+}

test/testdata/config/expand/defaults.toml

@@ -0,0 +1,12 @@
+# ${VAR:-fallback} syntax: TEST_SET is set, TEST_EMPTY is empty, TEST_UNSET is unset
+[[tunnels]]
+name = "set"
+host = "${TEST_SET:-fallback}"
+
+[[tunnels]]
+name = "empty"
+host = "${TEST_EMPTY:-fallback}"
+
+[[tunnels]]
+name = "unset"
+host = "${TEST_UNSET:-fallback}"

test/testdata/config/expand/literal_fields.toml

@@ -0,0 +1,5 @@
+# name and group are identifiers and are NOT expanded, so $TEST_VAR stays literal
+[[tunnels]]
+name = "tun_$TEST_VAR"
+group = "grp_$TEST_VAR"
+host = "example.com"

test/testdata/config/expand/unset.toml

@@ -0,0 +1,4 @@
+# TEST_NOPE is unset and has no default, so it should expand to an empty string
+[[tunnels]]
+name = "tun"
+host = "${TEST_NOPE}"

test/testdata/config/expand/vars.toml

@@ -0,0 +1,8 @@
+# Expanded against TEST_HOST, TEST_USER, TEST_IDENTITY, TEST_LOCAL, TEST_REMOTE
+[[tunnels]]
+name = "tun"
+host = "${TEST_HOST}"
+user = "$TEST_USER"
+identity = "${TEST_IDENTITY}"
+local = "${TEST_LOCAL}"
+remote = "${TEST_REMOTE}"