feat(dns): implement IPv6 preference with latency testing and caching

- Enable IPv6 support by default (ipv6 = true)
- Add IP preference system with multiple modes:
  * standard: prefer IPv6, fallback to IPv4
  * fastest: test all IPs concurrently, pick lowest latency
  * ipv6: force IPv6 only
  * ipv4: force IPv4 only
- Implement preference cache separate from DNS cache
- Add resolver_preference.go with thread-safe cache
- Replace resolveStrictIPv6 with resolveWithPreference
- Introduce concurrent DNS queries (AAAA + A parallel)
- Add latency testing with configurable timeout (500ms default)
- Add cache TTL (216000 seconds = 60h default)
- Add comprehensive unit tests and benchmarks
- Update config.default.toml with new [preference] section
- Update sample config.toml with bilingual documentation
- Fix internal/config/defaults.go to use correct ruleslib symbols
- Fix gendefaults tool compatibility
- All tests passing, builds successfully

Author: xihale

go.sum

@@ -33,6 +33,8 @@ github.com/spf13/pflag v1.0.10 h1:4EBh2KAYBwaONj6b2Ye1GiHfwjqyROoF4RwYO+vPwFk=
 github.com/spf13/pflag v1.0.10/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+github.com/xihale/snirect-shared v1.1.1 h1:k6pKyYbABqNrYNy4EVO8amJZSfS2YI1svO993Q0ZZvM=
+github.com/xihale/snirect-shared v1.1.1/go.mod h1:eT47oR1HH5PX/omD/RO8KPQ3D8cGoOuMWHrAbuBVq+U=
 go.uber.org/mock v0.6.0 h1:hyF9dfmbgIX5EfOdasqLsWD6xqpNZlXblLB/Dbnwv3Y=
 go.uber.org/mock v0.6.0/go.mod h1:KiVJ4BqZJaMj4svdfmHM0AUx4NJYO8ZNpPnZn1Z+BBU=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=

internal/config/config.default.toml

@@ -3,7 +3,7 @@
 
 set_proxy = true
 ca_install = "auto"
-ipv6 = false
+ipv6 = true
 check_hostname = true
 ecs = ""
 
@@ -31,3 +31,25 @@ logfile = ""
 address = "127.0.0.1"
 port = 7654
 pac_host = "127.0.0.1"
+
+[preference]
+# Mode: standard (current), fastest (test both), ipv6 (force v6), ipv4 (force v4)
+mode = "standard"
+
+# When mode="fastest", whether to actually test latency or just use first
+enable_testing = true
+
+# Timeout for each connection test in milliseconds
+test_timeout_ms = 500
+
+# Test IPs in parallel (true) or sequentially (false)
+test_parallel = true
+
+# Maximum number of IPs to test per query (to avoid too many goroutines)
+max_test_ips = 10
+
+# Preference cache TTL in seconds (0 = use DNS TTL / 2)
+cache_ttl = 216000
+
+# Maximum number of entries in preference cache (0 = unlimited)
+cache_size = 5000

internal/config/config.go

@@ -80,6 +80,36 @@ type Config struct {
 
 	// Server contains local proxy server settings.
 	Server ServerConfig `toml:"server"`
+
+	// Preference contains DNS IP preference settings.
+	Preference PreferenceConfig `toml:"preference"`
+}
+
+// IPPreferenceMode defines how IP selection works when both IPv6 and IPv4 are available.
+type IPPreferenceMode string
+
+const (
+	IPPreferenceStandard IPPreferenceMode = "standard" // Current behavior: prefer v6 if enabled, then first
+	IPPreferenceFastest  IPPreferenceMode = "fastest"  // Test both, use lowest latency (cached)
+	IPPreferenceIPv6     IPPreferenceMode = "ipv6"     // Always prefer IPv6 if available
+	IPPreferenceIPv4     IPPreferenceMode = "ipv4"     // Always prefer IPv4
+)
+
+// PreferenceConfig contains DNS IP preference settings.
+type PreferenceConfig struct {
+	Mode          IPPreferenceMode `toml:"mode"`           // Selection mode
+	EnableTesting bool             `toml:"enable_testing"` // Whether to do latency testing (for fastest mode)
+	// TestTimeoutMs is the timeout for each connection test in milliseconds.
+	// When testing IPs, we dial with this timeout and measure connection establishment time.
+	TestTimeoutMs int `toml:"test_timeout_ms"`
+	// TestParallel determines whether to test all IPs in parallel (true) or sequentially (false).
+	TestParallel bool `toml:"test_parallel"`
+	// MaxTestIPs limits the number of IPs to test per query to avoid resource exhaustion.
+	MaxTestIPs int `toml:"max_test_ips"`
+	// CacheTTL is the preference cache TTL in seconds. 0 means use DNS TTL / 2.
+	CacheTTL int `toml:"cache_ttl"`
+	// CacheSize limits the number of entries in the preference cache. 0 = unlimited.
+	CacheSize int `toml:"cache_size"`
 }
 
 // TimeoutConfig contains timeout settings in seconds.

internal/config/config.toml

@@ -87,6 +87,65 @@
 # 引导 DNS 服务器，用于解析上述加密 DNS 服务器自身的域名。
 # bootstrap_dns = ["tls://223.5.5.5"]
 
+# [DNS IP Preference]
+# Controls how Snirect selects between IPv6 and IPv4 addresses when both are available.
+# Requires ipv6 = true to function (IPv6 must be enabled).
+#
+# DNS IP 优选策略
+# 控制 Snirect 在有 IPv6 和 IPv4 地址时如何选择。
+# 需要先开启 ipv6 = true 才会生效。
+[preference]
+# Mode: standard, fastest, ipv6, ipv4
+#   standard - (Default) Prefer IPv6 if available, otherwise first available
+#   fastest  - Test connection latency to all IPs and use the fastest (cached)
+#   ipv6     - Always prefer IPv6 when available (no testing)
+#   ipv4     - Force IPv4 only
+#
+# 模式: standard (标准), fastest (最快), ipv6 (强制v6), ipv4 (强制v4)
+mode = "standard"
+
+# When mode="fastest", whether to actually test latency.
+# If false, behaves like "standard" (no testing overhead).
+#
+# 在 fastest 模式下是否实际进行延迟测试。
+# 若为 false，则退化为 standard 模式（无测试开销）。
+enable_testing = true
+
+# Timeout for each connection test in milliseconds.
+# Lower = faster fallback, higher = more accurate.
+# 每个连接测试的超时时间（毫秒）。
+test_timeout_ms = 500
+
+# Test multiple IPs in parallel (true) or sequentially (false).
+# Parallel is faster but uses more resources.
+#
+# 并行 (true) 或 顺序 (false) 测试多个 IP。
+# 并行更快但占用更多资源。
+test_parallel = true
+
+# Maximum number of IPs to test per query.
+# Set to conserve resources when many IPs are available (e.g., CDNs).
+#
+# 每次查询最多测试的 IP 数量。
+# 当可用 IP 很多时（如 CDN）可限制资源消耗。
+max_test_ips = 10
+
+# Preference cache TTL in seconds.
+# 0 = automatically use half of the DNS record's TTL.
+# Shorter TTL = adapt to network changes faster.
+#
+# 优选结果缓存时间（秒）。
+# 0 = 自动使用 DNS 记录 TTL 的一半。
+# 较短的 TTL 能更快适应网络变化。
+cache_ttl = 300
+
+# Maximum entries in preference cache (0 = unlimited).
+# PrefCache is separate from DNS cache and usually much smaller.
+#
+# 优选缓存的最大条目数 (0 = 不限制)。
+# 优选缓存独立于 DNS 缓存，通常小得多。
+cache_size = 5000
+
 # [Timeout Settings]
 # Timeouts in seconds.
 #

internal/config/defaults.go

@@ -9,12 +9,13 @@ import ruleslib "github.com/xihale/snirect-shared/rules"
 //go:embed config.default.toml
 var DefaultConfigTOML string
 
-var DefaultRulesTOML = ruleslib.FetchedRulesTOML
+var DefaultRulesTOML = ruleslib.DefaultRulesTOML
 
 //go:embed config.toml
 var SampleConfigTOML string
 
-var SampleRulesTOML = ruleslib.UserRulesTOML
+// SampleRulesTOML is not available in the shared rules package; leave empty.
+var SampleRulesTOML = ""
 
 //go:embed pac
 var DefaultPAC string

internal/config/tools/convert_rules/main.go

@@ -84,7 +84,7 @@ func run() error {
 		return fmt.Errorf("failed to parse config: %w", err)
 	}
 
-	rulesData := []byte(ruleslib.FetchedRulesTOML)
+	rulesData := []byte(ruleslib.DefaultRulesTOML)
 
 	var rules Rules
 	if err := toml.Unmarshal(rulesData, &rules); err != nil {