Xray-docs-next/docs/en/document/level-1/fallbacks-lv1.md

Brief Analysis of the "Fallback" Feature

While using Xray, you’ve probably heard about the "fallback" feature countless times. This article will briefly explain the logic and usage of this function.

1. Revisiting the Fallback in "Plain Language"

If you’ve used the Xray configuration from "Plain Language" and completed the HTTP to HTTPS redirection optimization, then you already have a simple fallback mechanism based on the VLESS protocol:

{
  "inbounds": [
    {
      "port": 443,
      "protocol": "vless",
      "settings": {
        "clients": [
          // ... ...
        ],
        "decryption": "none",
        "fallbacks": [
          {
            "dest": 8080 // Default fallback to the anti-probing proxy
          }
        ]
      },
      "streamSettings": {
        // ... ...
      }
    }
  ]
}

How can we explain this configuration in simpler terms?

1. Xray listens on port 443 for inbound traffic Xray is responsible for handling the HTTPS traffic on port 443.

2. The inbound protocol is vless Only vless protocol traffic is processed by Xray.

   ::: warning Note: The VLESS protocol was originally developed for Xray and v2fly to introduce the fallback feature and reduce redundant checks/encryption. (Currently, the trojan protocol in Xray also fully supports fallback functionality.) :::

3. Fallback destination is port 8080 Xray processes vless traffic and forwards it accordingly. Non-vless traffic is forwarded to port 8080.

   ::: warning Question: Why do we use plurals like inbounds and fallbacks in the config?

   Answer: While the configuration file uses plurals to indicate support for multiple elements (like multiple inbound ports or fallbacks), the explanation here only covers one example, so I used the singular form. :::

4. Traffic forwarded to port 8080 is handled by subsequent programs In the "Plain Language" example, traffic forwarded to port 8080 is processed by Nginx to display a website.

5. Summary of the simple fallback:

   graph LR;
   W(External HTTP:80 request) --> N80(HTTP:80)
   subgraph Nginx External Listener
   N80 -.- N301(301 Redirect) -.- N443(HTTPS:443)
   end
   N443 --> X(Xray listens on 443) .- X1{Inbound Check}
   X1 --> |VLESS traffic| X2(Xray Internal Rules)
   X2 --> O(Xray Outbounds)
   X1 ==> |Non-VLESS traffic| N8080(Nginx:8080)
   N8080:::nginxclass ==> H(index.html)
   classDef nginxclass fill:#FFFFDE
   

2. Understanding Fallbacks (WHAT, HOW v1)

From the above example, you can understand what a fallback is (WHAT) and how it works (HOW). In simple terms:

1. The fallback happens after traffic reaches the Xray listening port.
2. It’s based on characteristics like the protocol type.
3. The traffic is forwarded to a specific port.
4. The traffic at the fallback port is handled by subsequent programs.

3. Why Use a Fallback (WHY v1)

Initially, fallbacks were implemented to defend against active probing.

Active Probing: In simple terms, this refers to external entities sending specific network requests to determine if a server is running proxy tools like Xray, v2fly, or Shadowsocks. If identified, the server may face interference or be blocked.

Fallbacks help prevent identification by forwarding probing traffic to programs like Nginx, masking the server's real functionality.

4. The Full Power of Fallbacks (WHAT, WHY, HOW v2)

As the VLESS protocol evolved, fallbacks became more flexible. By utilizing information in the initial data packet (like path or alpn), fallbacks can now be multi-layered and support various protocols and features.

Modern fallbacks in Xray are:

• Secure: Resistant to active probing.
• Efficient: Minimal performance loss.
• Flexible: Capable of multi-protocol and feature-based traffic routing.

5. Multi-layer Fallback Example and Analysis

Below is an example configuration of Xray’s fallback mechanism, listening on port 443:

{
  "port": 443,
  "protocol": "vless",
  "settings": {
    "clients": [
      {
        "id": "", // Your UUID
        "flow": "xtls-rprx-vision",
        "level": 0,
        "email": "love@example.com"
      }
    ],
    "decryption": "none",
    "fallbacks": [
      {
        "dest": 1310, // Default fallback to Xray's Trojan protocol
        "xver": 1
      },
      {
        "path": "/websocket",
        "dest": 1234,
        "xver": 1
      },
      {
        "path": "/vmesstcp",
        "dest": 2345,
        "xver": 1
      },
      {
        "path": "/vmessws",
        "dest": 3456,
        "xver": 1
      }
    ]
  },
  "streamSettings": {
    "network": "tcp",
    "security": "tls",
    "tlsSettings": {
      "alpn": ["http/1.1"],
      "certificates": [
        {
          "certificateFile": "/path/to/fullchain.crt", // Your certificate path
          "keyFile": "/path/to/private.key" // Your private key path
        }
      ]
    }
  }
}

This configuration shows multi-layer fallbacks, each handling different traffic types based on the path provided.

6. Conclusion

This article has provided an overview of the powerful fallback feature in Xray. With its flexibility and security benefits, it’s an essential part of any secure proxy setup.