Understanding Clash TUN Mode Mechanics

When we talk about Clash TUN mode, we are referring to a virtual network interface at the layer 3 (IP) level. Unlike traditional system proxy settings that rely on environment variables or manual application configuration, TUN mode creates a virtual network card that intercepts all outgoing traffic from the operating system. This is the foundation of transparent proxying, ensuring that even applications without proxy support—such as terminal tools, command-line utilities, and certain games—are correctly routed through Clash.

In a typical setup, the Clash core (like Mihomo or the premium core) creates a utun device. The OS routing table is then modified to send packets destined for the internet through this interface. However, simply redirecting packets isn't enough. The core must decide whether to proxy, direct, or block each packet based on its destination. This is where the complexity begins, especially concerning the Domain Name System (DNS).

ℹ Pro Tip: TUN mode is often superior to TAP mode because it works at the IP layer, avoiding the overhead of Ethernet frame encapsulation and providing better compatibility with modern VPN frameworks on macOS and Windows.

The DNS Leak Problem in TUN Mode

A DNS leak occurs when your computer sends DNS queries to your ISP's or a public DNS server (like 8.8.8.8) outside of the encrypted proxy tunnel. Even if your actual data traffic is proxied, an observer (like your ISP or a local network administrator) can see exactly which websites you are visiting by monitoring these unencrypted DNS requests.

In TUN mode, DNS leaks often happen because the operating system's resolver is still configured to use the local network's DNS servers. When an app requests example.com, the OS sends a UDP packet to port 53. If Clash isn't explicitly configured to hijack these port 53 requests on the TUN interface, the packet might bypass the proxy logic entirely or be resolved locally before Clash can intercept the resulting IP traffic.

To fix this, Clash must become the primary DNS resolver for the entire system. This involves setting up a dns block in your YAML configuration that listens on a specific port and tells the TUN interface to redirect all DNS traffic to Clash's internal DNS engine.

Deep Dive into Fake-IP Logic

Clash primarily uses two DNS modes: redir-host and fake-ip. While redir-host is more traditional, Fake-IP is the recommended standard for TUN mode in 2026.

In fake-ip mode, when an application asks for the IP of google.com, Clash immediately returns a "fake" IP address from a reserved range (usually 198.18.0.1/16). The application then sends its data packets to this fake IP. Clash maintains a mapping table: 198.18.0.5 -> google.com. When it receives a packet for that IP, it looks up the domain, applies its routing rules, and initiates the proxy connection using the original domain name.

Illustrative Fake-IP Configuration

dns:
  enable: true
  listen: 0.0.0.0:1053
  ipv6: false
  enhanced-mode: fake-ip
  fake-ip-range: 198.18.0.1/16
  nameserver:
    - 119.29.29.29
    - 223.5.5.5
  fallback:
    - 8.8.8.8
    - 1.1.1.1

The main advantage of fake-ip is speed. The application doesn't have to wait for a real DNS resolution to finish before it starts sending data. It also prevents DNS leaks by ensuring the real resolution happens inside the proxy core or on the remote proxy server.

Configuring for Zero DNS Leaks

To achieve a zero-leak status, you must ensure that your tun configuration is tightly integrated with your dns settings. A common mistake is leaving the system DNS settings pointed at a local router.

The auto-route and auto-detect-interface settings are crucial. On Windows and macOS, these allow Clash to automatically manage the routing table. Furthermore, utilizing dns-hijack ensures that any stray DNS packets are forced into the Clash DNS engine.

Advanced TUN Configuration for 2026

tun:
  enable: true
  stack: system # or gvisor/mixed
  dns-hijack:
    - "any:53"
    - "tcp://any:53"
  auto-route: true
  auto-detect-interface: true
  strict-route: true # Crucial for preventing leaks

Setting strict-route: true is the ultimate defense. It forces all traffic into the TUN interface and prevents the OS from using "default" routes that might bypass the proxy. This is particularly important on Windows, where the networking stack often tries to be "helpful" by using multiple interfaces simultaneously.

Traffic Sniffing and IPv6 Considerations

In 2026, many applications use hardcoded IP addresses or custom protocols that bypass traditional DNS. This is where sniffing comes in. By enabling sniffing: true, Clash can inspect the TLS Client Hello packets or HTTP headers to find the actual domain name even if the application didn't perform a DNS lookup.

Sniffing is the "secret sauce" for modern transparent proxies. It allows Clash to apply domain-based rules to traffic that arrives as raw IP addresses, which is common in mobile apps and modern web frameworks.

IPv6 is another major source of leaks. Most proxy providers do not fully support IPv6, or the local network's IPv6 routing might bypass the IPv4-only TUN interface. The safest approach for most users is to set ipv6: false in both the global and DNS settings of Clash, effectively disabling IPv6 to ensure all traffic is forced through the controlled IPv4 tunnel.

⚠ Compliance Notice: Always comply with local laws and regulations regarding network usage. This guide is intended for technical education on Clash routing and DNS security and does not encourage unauthorized access or illegal activities.

Conclusion and Next Steps

Configuring Clash TUN mode is a journey of fine-tuning. By combining fake-ip, strict-route, and dns-hijack, you can create a robust, leak-proof environment that protects your privacy and simplifies your proxy usage. Remember that a "perfect" config for one network might need adjustments for another, especially when dealing with public Wi-Fi or corporate firewalls.

→ Download Clash V.CORE now to start implementing these advanced configurations and take full control of your network traffic.