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DNS Exfiltration

httpjail prevents DNS exfiltration attacks by intercepting all DNS queries in isolated environments.

The Attack

Malicious code can exfiltrate sensitive data by encoding it in DNS queries:

  • secret-data.attacker.com
  • env-var-contents.evil.com
  • api-key-12345.tunnel.io

These queries reach public DNS servers even when HTTP/HTTPS traffic is blocked.

How Protection Works

In Linux strong mode, httpjail:

  1. Intercepts all DNS queries from the jailed process
  2. Returns dummy response (6.6.6.6) for every query
  3. Prevents external DNS access - queries never reach public resolvers
  4. Maintains HTTP/HTTPS functionality through transparent proxy redirection

Traffic Flow

sequenceDiagram
    participant J as Jailed Process
    participant S as Jail Server
    participant D as Public DNS Resolvers

    Note over J,D: DNS Exfiltration Attempt
    J->>S: DNS Query: secret-data.attacker.com
    S-->>J: Response: 6.6.6.6 (dummy)
    Note over S,D: ❌ Query never reaches public resolvers

    Note over J,D: Blocked HTTP Flow
    J->>S: HTTP GET http://blocked.com
    Note over S: Rule evaluation: denied
    S-->>J: 403 Forbidden
    Note over S,D: ❌ No DNS resolution needed

    Note over J,D: Allowed HTTP Flow
    J->>S: HTTP GET http://example.com
    Note over S: Rule evaluation: allowed
    S->>D: DNS Query: example.com (only if needed)
    D-->>S: Real IP address
    S->>S: Forward to upstream server
    S-->>J: HTTP response
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The diagram shows three scenarios:

  1. DNS Exfiltration Prevention: All DNS queries receive dummy response, never reaching public resolvers
  2. Blocked HTTP Traffic: Requests denied by rules without any DNS resolution
  3. Allowed HTTP Traffic: Only when rules permit, httpjail performs actual DNS resolution

Platform Support

  • Linux (Strong Mode): Full DNS interception and protection
  • macOS (Weak Mode): No DNS interception - applications resolve normally
  • Windows: Planned

Why 6.6.6.6?

The choice of 6.6.6.6 is arbitrary - any non-lookback IP would work.