fix: add missing EnvName, document Strategy invariant, improve rcode display

- Add EnvName to four new deprecation constants so users can suppress
  warnings via ENABLE_DEPRECATED_* environment variables
- Add comment explaining why applyDNSRouteOptions skips Strategy
- Use dns.RcodeToString in DNSResponseRCodeItem.String() for readability
- Remove redundant Fqdn(FqdnToDomain(domain)) round-trip

dns/router.go

@@ -410,6 +410,9 @@ func (r *Router) matchDNS(ctx context.Context, rules []adapter.DNSRule, allowFak
 }
 
 func (r *Router) applyDNSRouteOptions(options *adapter.DNSQueryOptions, routeOptions R.RuleActionDNSRouteOptions) {
+	// Strategy is intentionally skipped here. A non-default DNS rule action strategy
+	// forces legacy mode via resolveLegacyDNSMode, so this path is only reachable
+	// when strategy remains at its default value.
 	if routeOptions.DisableCache {
 		options.DisableCache = true
 	}
@@ -658,7 +661,7 @@ func (r *Router) lookupWithRulesType(ctx context.Context, rules []adapter.DNSRul
 			RecursionDesired: true,
 		},
 		Question: []mDNS.Question{{
-			Name:   mDNS.Fqdn(FqdnToDomain(domain)),
+			Name:   mDNS.Fqdn(domain),
 			Qtype:  qType,
 			Qclass: mDNS.ClassINET,
 		}},

experimental/deprecated/constants.go

@@ -98,6 +98,7 @@ var OptionIPAcceptAny = Note{
 	Description:       "`ip_accept_any` in DNS rules",
 	DeprecatedVersion: "1.14.0",
 	ScheduledVersion:  "1.16.0",
+	EnvName:           "DNS_RULE_IP_ACCEPT_ANY",
 	MigrationLink:     "https://sing-box.sagernet.org/configuration/dns/rule/",
 }
 
@@ -106,6 +107,7 @@ var OptionRuleSetIPCIDRAcceptEmpty = Note{
 	Description:       "`rule_set_ip_cidr_accept_empty` in DNS rules",
 	DeprecatedVersion: "1.14.0",
 	ScheduledVersion:  "1.16.0",
+	EnvName:           "DNS_RULE_RULE_SET_IP_CIDR_ACCEPT_EMPTY",
 	MigrationLink:     "https://sing-box.sagernet.org/configuration/dns/rule/",
 }
 
@@ -114,6 +116,7 @@ var OptionLegacyDNSAddressFilter = Note{
 	Description:       "legacy address filter DNS rule items",
 	DeprecatedVersion: "1.14.0",
 	ScheduledVersion:  "1.16.0",
+	EnvName:           "LEGACY_DNS_ADDRESS_FILTER",
 	MigrationLink:     "https://sing-box.sagernet.org/configuration/dns/rule/",
 }
 
@@ -122,6 +125,7 @@ var OptionLegacyDNSRuleStrategy = Note{
 	Description:       "`strategy` in DNS rule actions",
 	DeprecatedVersion: "1.14.0",
 	ScheduledVersion:  "1.16.0",
+	EnvName:           "LEGACY_DNS_RULE_STRATEGY",
 	MigrationLink:     "https://sing-box.sagernet.org/configuration/dns/rule_action/",
 }
 

route/rule/rule_item_response_rcode.go

@@ -1,6 +1,7 @@
 package rule
 
 import (
+	"github.com/miekg/dns"
 	"github.com/sagernet/sing-box/adapter"
 	F "github.com/sagernet/sing/common/format"
 )
@@ -20,5 +21,5 @@ func (r *DNSResponseRCodeItem) Match(metadata *adapter.InboundContext) bool {
 }
 
 func (r *DNSResponseRCodeItem) String() string {
-	return F.ToString("response_rcode=", r.rcode)
+	return F.ToString("response_rcode=", dns.RcodeToString[r.rcode])
 }