Remove client metadata from AnyTLS requests by default

docs/configuration/outbound/anytls.md

@@ -17,6 +17,7 @@ icon: material/new-box
   "idle_session_check_interval": "30s",
   "idle_session_timeout": "30s",
   "min_idle_session": 5,
+  "client_metadata": "",
   "tls": {},
 
   ... // Dial Fields
@@ -55,6 +56,12 @@ In the check, close sessions that have been idle for longer than this. Default:
 
 In the check, at least the first `n` idle sessions are kept open. Default value: `n`=0
 
+#### client_metadata
+
+!!! question "Since sing-box 1.13.16"
+
+Check [AnyTLS client metadata](/manual/misc/anytls-client-metadata/).
+
 #### tls
 
 ==Required==

docs/configuration/outbound/anytls.zh.md

@@ -17,6 +17,7 @@ icon: material/new-box
   "idle_session_check_interval": "30s",
   "idle_session_timeout": "30s",
   "min_idle_session": 5,
+  "client_metadata": "",
   "tls": {},
 
   ... // 拨号字段
@@ -55,6 +56,12 @@ AnyTLS 密码。
 
 在检查中，至少前 `n` 个空闲会话保持打开状态。默认值：`n`=0
 
+#### client_metadata
+
+!!! question "自 sing-box 1.13.16 起"
+
+参阅 [AnyTLS 客户端元数据](/zh/manual/misc/anytls-client-metadata/)。
+
 #### tls
 
 ==必填==

docs/manual/misc/anytls-client-metadata.md

@@ -0,0 +1,67 @@
+---
+icon: material/incognito
+---
+
+# AnyTLS client metadata
+
+The AnyTLS protocol has a design flaw: its settings frame requires the client
+to send its software name and version to the server, and the protocol
+specification requires that clients not disguise this information.
+
+This field serves no protocol purpose — AnyTLS already has a separate version
+field for compatibility negotiation, and the open-source server implementation
+does not use client metadata. However, the field allows vendors to collect and track client types, and for
+platform-specific clients, potentially infer private information such as the operating system type and version range —
+something that should not, and is not expected by users to, appear in an
+anti-censorship protocol. We have received reports that
+commercial proxy providers use this information to identify and block
+connections from the official library provided by AnyTLS for sing-box
+integration, reportedly because abusive users connect to their servers with
+sing-box or with clients using the same official library. This indicates that
+client metadata is being collected and used for discrimination in practice.
+
+The protocol specification states that "disguising it has no value." We
+disagree: the situation is analogous to browsers implementing TLS ECH GREASE —
+without it, privacy-protecting clients can be fingerprinted and treated
+differently.
+
+## Status
+
+### 2025-02-20
+
+We merged the
+[pull request adding this protocol](https://github.com/SagerNet/sing-box/pull/2615).
+Since the metadata was fixed at `sing-anytls/<library version>` in the
+implementation provided for our use, and we did not carefully review the
+protocol specification and other implementations, we wrongly believed that it
+was not private information.
+
+### 2025-04-05
+
+The protocol document
+[added](https://github.com/anytls/anytls-go/commit/8812aae7ab29dd88bb89067b9ca676e2e7e29171)
+the requirement that third-party implementations fill in the real software
+name and version, claiming that "disguising it has no value".
+
+### 2026-07-18
+
+A [pull request submitted to sing-box](https://github.com/SagerNet/sing-box/pull/4311)
+was found to additionally upload the `sing-box` name and the actual version;
+the change was subsequently reverted and was never released.
+
+### 2026-08-03
+
+sing-box 1.13.16 and 1.14.0-beta.5 have been released; the client metadata in
+AnyTLS requests is now empty by default. For compatibility, the
+[client_metadata](/configuration/outbound/anytls/#client_metadata) outbound
+option allows users to set a custom value.
+
+Since the open-source server implementation does not use this information and
+it has no legitimate use, this is not considered a breaking change.
+
+## Recommendations
+
+We recommend that the AnyTLS protocol remove the client metadata, or replace
+it with an option that is not sent by default and can be customized by the
+user; and that other client implementations also take action, to jointly stop
+statistics collection and discrimination based on client metadata.

docs/manual/misc/anytls-client-metadata.zh.md

@@ -0,0 +1,35 @@
+---
+icon: material/incognito
+---
+
+# AnyTLS 客户端元数据
+
+AnyTLS 协议具有设计缺陷：其 settings 帧要求客户端向服务器发送软件名称和版本，且协议规范要求客户端不得伪装此信息。
+
+此字段不承担协议功能——AnyTLS 已有独立的版本字段用于兼容性协商，且开源服务端实现不使用客户端元数据。然而，此字段使得供应商可以收集并统计客户端类型，对于某些平台特定的客户端，还可能推断出操作系统类型与版本范围等隐私信息，而这不应该，也不是被用户预期的，在一个反审查协议中出现。我们收到报告，有商业代理提供商利用此信息识别和阻止来自 sing-box 使用的、由 AnyTLS 提供的用于 sing-box 集成的官方代码库的连接，据传原因是恶意用户使用 sing-box 或使用相同官方代码库的客户端连接到服务器，这表明客户端元数据在实践中已被用于收集和区别对待。
+
+协议规范称「伪装它没有任何意义」。我们不同意：这类似于浏览器实现 TLS ECH GREASE——如果没有这一机制，保护隐私的客户端会被识别并受到差别对待。
+
+## 状态
+
+### 2025-02-20
+
+我们合并了[添加此协议的 PR](https://github.com/SagerNet/sing-box/pull/2615)。由于在供我们使用的实现中，metadata 被固定在 `sing-anytls/<library version>`，且我们没有仔细审查协议规范和其他实现，我们错误地认为这不是隐私信息。
+
+### 2025-04-05
+
+协议文档[加入](https://github.com/anytls/anytls-go/commit/8812aae7ab29dd88bb89067b9ca676e2e7e29171)了要求第三方实现填写真实软件名称与版本号的条款，并声称「伪装它没有任何意义」。
+
+### 2026-07-18
+
+[向 sing-box 提出的 PR](https://github.com/SagerNet/sing-box/pull/4311) 被发现额外上传了 `sing-box` 和实际版本的字符串，随后此更改被回退，没有发布。
+
+### 2026-08-03
+
+发布了 sing-box 1.13.16 和 1.14.0-beta.5，现在 AnyTLS 请求中的客户端元数据默认为空。出于兼容性考虑，[client_metadata](/zh/configuration/outbound/anytls/#client_metadata) 出站选项允许用户自定义此值。
+
+由于开源服务端实现不使用此信息，且它没有合理用途，这不被视为破坏性更改。
+
+## 建议
+
+我们建议 AnyTLS 协议移除客户端元数据，或将其替换为非默认提供、且用户可以自定义的选项；并建议其他客户端实现也采取行动，共同阻止基于客户端元数据的统计和区别对待。

mkdocs.yml

@@ -75,6 +75,7 @@ nav:
           - Hysteria 2: manual/proxy-protocol/hysteria2.md
       - Misc:
           - TunnelVision: manual/misc/tunnelvision.md
+          - AnyTLS client metadata: manual/misc/anytls-client-metadata.md
   - Configuration:
       - configuration/index.md
       - Log:

option/anytls.go

@@ -22,4 +22,5 @@ type AnyTLSOutboundOptions struct {
 	IdleSessionCheckInterval badoption.Duration `json:"idle_session_check_interval,omitempty"`
 	IdleSessionTimeout       badoption.Duration `json:"idle_session_timeout,omitempty"`
 	MinIdleSession           int                `json:"min_idle_session,omitempty"`
+	ClientMetadata           string             `json:"client_metadata,omitempty"`
 }

protocol/anytls/client_metadata.go

@@ -0,0 +1,64 @@
+package anytls
+
+import (
+	"encoding/binary"
+	"net"
+	"reflect"
+	"strings"
+	"sync"
+	"unsafe"
+
+	"github.com/sagernet/sing/common"
+
+	anytls "github.com/anytls/sing-anytls"
+	"github.com/anytls/sing-anytls/session"
+)
+
+const (
+	commandSettings = 4
+	frameHeaderSize = 7
+)
+
+var (
+	clientSessionField, _   = reflect.TypeFor[anytls.Client]().FieldByName("sessionClient")
+	streamSessionField, _   = reflect.TypeFor[session.Stream]().FieldByName("sess")
+	sessionConnLockField, _ = reflect.TypeFor[session.Session]().FieldByName("connLock")
+	sessionBufferField, _   = reflect.TypeFor[session.Session]().FieldByName("buffer")
+)
+
+func sessionClientOf(client *anytls.Client) *session.Client {
+	return *(**session.Client)(unsafe.Add(unsafe.Pointer(client), clientSessionField.Offset))
+}
+
+func (h *Outbound) rewriteClientMetadata(conn net.Conn) {
+	sess := *(**session.Session)(unsafe.Add(unsafe.Pointer(conn.(*session.Stream)), streamSessionField.Offset))
+	connLock := (*sync.Mutex)(unsafe.Add(unsafe.Pointer(sess), sessionConnLockField.Offset))
+	bufferPointer := (*[]byte)(unsafe.Add(unsafe.Pointer(sess), sessionBufferField.Offset))
+	connLock.Lock()
+	defer connLock.Unlock()
+	buffer := *bufferPointer
+	offset := 0
+	for offset+frameHeaderSize <= len(buffer) {
+		dataLength := int(binary.BigEndian.Uint16(buffer[offset+5 : offset+7]))
+		frameEnd := offset + frameHeaderSize + dataLength
+		if frameEnd > len(buffer) {
+			return
+		}
+		if buffer[offset] == commandSettings {
+			data := []byte(strings.Join(common.Map(strings.Split(string(buffer[offset+frameHeaderSize:frameEnd]), "\n"), func(line string) string {
+				if strings.HasPrefix(line, "client=") {
+					return "client=" + h.clientMetadata
+				}
+				return line
+			}), "\n"))
+			newBuffer := make([]byte, 0, offset+frameHeaderSize+len(data)+len(buffer)-frameEnd)
+			newBuffer = append(newBuffer, buffer[:offset+5]...)
+			newBuffer = binary.BigEndian.AppendUint16(newBuffer, uint16(len(data)))
+			newBuffer = append(newBuffer, data...)
+			newBuffer = append(newBuffer, buffer[frameEnd:]...)
+			*bufferPointer = newBuffer
+			return
+		}
+		offset = frameEnd
+	}
+}

protocol/anytls/outbound.go

@@ -19,6 +19,7 @@ import (
 	"github.com/sagernet/sing/common/uot"
 
 	anytls "github.com/anytls/sing-anytls"
+	"github.com/anytls/sing-anytls/session"
 )
 
 func RegisterOutbound(registry *outbound.Registry) {
@@ -27,12 +28,14 @@ func RegisterOutbound(registry *outbound.Registry) {
 
 type Outbound struct {
 	outbound.Adapter
-	dialer    tls.Dialer
+	dialer         tls.Dialer
-	server    M.Socksaddr
+	server         M.Socksaddr
-	tlsConfig tls.Config
+	tlsConfig      tls.Config
-	client    *anytls.Client
+	clientMetadata string
-	uotClient *uot.Client
+	client         *anytls.Client
-	logger    log.ContextLogger
+	sessionClient  *session.Client
+	uotClient      *uot.Client
+	logger         log.ContextLogger
 }
 
 func NewOutbound(ctx context.Context, router adapter.Router, logger log.ContextLogger, tag string, options option.AnyTLSOutboundOptions) (adapter.Outbound, error) {
@@ -81,14 +84,30 @@ func NewOutbound(ctx context.Context, router adapter.Router, logger log.ContextL
 		return nil, err
 	}
 	outbound.client = client
+	outbound.clientMetadata = options.ClientMetadata
+	outbound.sessionClient = sessionClientOf(client)
 
 	outbound.uotClient = &uot.Client{
-		Dialer:  (anytlsDialer)(client.CreateProxy),
+		Dialer:  (anytlsDialer)(outbound.createProxy),
 		Version: uot.Version,
 	}
 	return outbound, nil
 }
 
+func (h *Outbound) createProxy(ctx context.Context, destination M.Socksaddr) (net.Conn, error) {
+	conn, err := h.sessionClient.CreateStream(ctx)
+	if err != nil {
+		return nil, err
+	}
+	h.rewriteClientMetadata(conn)
+	err = M.SocksaddrSerializer.WriteAddrPort(conn, destination)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+	return conn, nil
+}
+
 type anytlsDialer func(ctx context.Context, destination M.Socksaddr) (net.Conn, error)
 
 func (d anytlsDialer) DialContext(ctx context.Context, network string, destination M.Socksaddr) (net.Conn, error) {
@@ -110,7 +129,7 @@ func (h *Outbound) DialContext(ctx context.Context, network string, destination
 	switch N.NetworkName(network) {
 	case N.NetworkTCP:
 		h.logger.InfoContext(ctx, "outbound connection to ", destination)
-		return h.client.CreateProxy(ctx, destination)
+		return h.createProxy(ctx, destination)
 	case N.NetworkUDP:
 		h.logger.InfoContext(ctx, "outbound UoT packet connection to ", destination)
 		return h.uotClient.DialContext(ctx, network, destination)