feat: add DSCP rule for Tproxy UDP packets (#996)

* feat: add `DSCP` rule for Tproxy UDP packets

* fix: fix compatibility issue with non_linux platform

* chore: remove redundant lines for DSCP

rules/common/dscp.go

@@ -0,0 +1,47 @@
+package common
+
+import (
+	"fmt"
+	"strconv"
+
+	C "github.com/metacubex/mihomo/constant"
+)
+
+type DSCP struct {
+	*Base
+	dscp    uint8
+	payload string
+	adapter string
+}
+
+func (d *DSCP) RuleType() C.RuleType {
+	return C.DSCP
+}
+
+func (d *DSCP) Match(metadata *C.Metadata) (bool, string) {
+	return metadata.DSCP == d.dscp, d.adapter
+}
+
+func (d *DSCP) Adapter() string {
+	return d.adapter
+}
+
+func (d *DSCP) Payload() string {
+	return d.payload
+}
+
+func NewDSCP(dscp string, adapter string) (*DSCP, error) {
+	dscpi, err := strconv.Atoi(dscp)
+	if err != nil {
+		return nil, fmt.Errorf("parse DSCP rule fail: %w", err)
+	}
+	if dscpi < 0 || dscpi > 63 {
+		return nil, fmt.Errorf("DSCP couldn't be negative or exceed 63")
+	}
+	return &DSCP{
+		Base:    &Base{},
+		payload: dscp,
+		dscp:    uint8(dscpi),
+		adapter: adapter,
+	}, nil
+}

rules/parser.go

@@ -38,6 +38,8 @@ func ParseRule(tp, payload, target string, params []string, subRules map[string]
 		parsed, parseErr = RC.NewPort(payload, target, C.DstPort)
 	case "IN-PORT":
 		parsed, parseErr = RC.NewPort(payload, target, C.InPort)
+	case "DSCP":
+		parsed, parseErr = RC.NewDSCP(payload, target)
 	case "PROCESS-NAME":
 		parsed, parseErr = RC.NewProcess(payload, target, true)
 	case "PROCESS-PATH":