diff --git a/config/config.go b/config/config.go
index 8622994..03f3969 100644
--- a/config/config.go
+++ b/config/config.go
@@ -2,6 +2,8 @@ package config
 
 import (
 	"fmt"
+	"math"
+	"net"
 	"reflect"
 
 	"github.com/google/go-cmp/cmp"
@@ -231,3 +233,42 @@ func storeStructConfigValues(cvs []Value, v reflect.Value) error {
 type Unmarshaler interface {
 	UnmarshalConfig(Block) error
 }
+
+// Port represents a port number in the configuration. When a configuration is
+// converted to Go types using Unmarshal, it implements special conversion
+// rules:
+// If the config option is a numeric value, it is ensured to be in the range
+// [1–65535]. If the config option is a string, it is converted to a port
+// number using "net".LookupPort (using "tcp" as network).
+type Port int
+
+// UnmarshalConfig converts b to a port number.
+func (p *Port) UnmarshalConfig(b Block) error {
+	if len(b.Values) != 1 || len(b.Children) != 0 {
+		return fmt.Errorf("option %q has to be a single scalar value", b.Key)
+	}
+
+	v := b.Values[0]
+	if f, ok := v.Number(); ok {
+		if math.IsNaN(f) {
+			return fmt.Errorf("the value of the %q option (%v) is invalid", b.Key, f)
+		}
+		if f < 1 || f > math.MaxUint16 {
+			return fmt.Errorf("the value of the %q option (%v) is out of range", b.Key, f)
+		}
+		*p = Port(f)
+		return nil
+	}
+
+	if !v.IsString() {
+		return fmt.Errorf("the value of the %q option must be a number or a string", b.Key)
+	}
+
+	port, err := net.LookupPort("tcp", v.String())
+	if err != nil {
+		return fmt.Errorf("%s: %w", b.Key, err)
+	}
+
+	*p = Port(port)
+	return nil
+}
diff --git a/config/config_test.go b/config/config_test.go
index 58b40f1..3adf3b6 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"math"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -356,6 +357,126 @@ func TestConfig_Unmarshal(t *testing.T) {
 			}{},
 			wantErr: true,
 		},
+		{
+			name: "port numeric success",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{Float64Value(80)},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			want: &struct {
+				Args string
+				Port Port
+			}{
+				Args: "test",
+				Port: Port(80),
+			},
+		},
+		{
+			name: "port out of range",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{Float64Value(1<<48)},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			wantErr: true,
+		},
+		{
+			name: "port not a number",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{Float64Value(math.NaN())},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			wantErr: true,
+		},
+		{
+			name: "port invalid type",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{BoolValue(true)},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			wantErr: true,
+		},
+		{
+			name: "port string success",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{StringValue("http")},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			want: &struct {
+				Args string
+				Port Port
+			}{
+				Args: "test",
+				Port: Port(80),
+			},
+		},
+		{
+			name: "port string failure",
+			src: Block{
+				Key:    "Plugin",
+				Values: []Value{StringValue("test")},
+				Children: []Block{
+					{
+						Key:    "Port",
+						Values: []Value{StringValue("--- invalid ---")},
+					},
+				},
+			},
+			dst: &struct {
+				Args string
+				Port Port
+			}{},
+			wantErr: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {