To understand how DNS tunneling can bypass captive portals, it’s crucial to grasp how captive portals and DNS queries function. Here’s a step-by-step explanation of the process, addressing how DNS requests can be sent without bypassing the captive portal.

Captive Portals and DNS Traffic

Captive portals intercept HTTP and HTTPS requests from users and redirect them to a login page where authentication is required. However, they typically allow DNS traffic to pass through unimpeded, as DNS is necessary for resolving domain names.

DNS Tunneling Mechanics

DNS tunneling works by encapsulating data within DNS queries and responses. Since DNS traffic is generally allowed through captive portals, this method can exploit that allowance. Here's a detailed breakdown:

1. Setup DNS Tunneling Server and Client:
   • Server: Set up a DNS tunneling server on an external machine you control (e.g., using tools like iodine or dnscat2). The server listens for DNS queries containing tunneled data.
   • Client: Install a DNS tunneling client on your device. Configure it to communicate with the server using a specific subdomain (e.g., tunnel.example.com).
2. Initial DNS Request:
   • When connected to the captive portal’s Wi-Fi, your device needs to resolve domain names (e.g., to display the login page). The captive portal allows DNS queries to pass through.
   • The DNS tunneling client sends DNS queries to tunnel.example.com, embedding data within these queries.
3. Data Encapsulation:
   • The data to be tunneled (e.g., HTTP requests) is broken down into small chunks and embedded within the payload of DNS queries.
   • The DNS queries are sent to the external DNS server (your tunneling server).
4. Server Processing:
   • The DNS tunneling server receives the queries, extracts the data, and processes the request.
   • If the client requests internet access, the server fetches the required data from the internet.
5. Response Transmission:
   • The server encapsulates the response data within DNS responses and sends them back to the client.
   • The client reassembles the data from the DNS responses and provides it to the application requiring internet access.

Example Workflow

1. Setting Up the Server:

• On your external server (e.g., example.com), set up iodine
• sudo iodine -f example.com

2. Configuring the Client:

• On your laptop connected to the captive portal, run
• iodine -f -P password example.com

3. Sending DNS Queries:

• The client sends DNS queries to example.com, with data embedded in the query payload.
• These queries pass through the captive portal since DNS traffic is allowed.

4. Server Processing:

• The server decodes the queries, processes them, and encapsulates the response in DNS replies.

5. Receiving Responses:

• The client receives the DNS responses, decodes them, and reassembles the original data.

Practical Use Case

Scenario: You are at an airport with a captive portal. You haven't logged in, but you need to access a website.

1. Initial Connection:
   • Connect to the Wi-Fi. Your device can resolve domain names, but HTTP/HTTPS traffic is blocked until you authenticate.
2. Running DNS Tunneling Client:
   • Run the DNS tunneling client configured to communicate with your external server.
3. DNS Queries:
   • The client sends DNS queries to example.com, carrying the data for the website you want to access.
   • The queries are allowed through by the captive portal.
4. Server Fetches Data:
   • The external server receives the DNS queries, extracts the data request, fetches the website data, and encapsulates it in DNS responses.
5. Receiving Data:
   • The DNS responses pass through the captive portal, are received by your client, and the data is reassembled, providing you access to the website.

Detecting and Preventing DNS Tunneling

Detecting and preventing DNS tunneling involves monitoring DNS traffic for unusual patterns and implementing security measures to block such activity. Here’s how to detect and prevent DNS tunneling:

Detection

1. Analyze DNS Query Patterns:
   • High Volume of DNS Queries: DNS tunneling typically generates a high volume of DNS queries to a single domain or a set of domains. Monitor for an unusually high number of DNS queries from a single source.
   • Unusual Query Types: Standard DNS queries (A, AAAA, CNAME) are common. However, DNS tunneling often uses TXT, NULL, or other uncommon query types. Monitor for an unusual spike in these types of queries.
2. Examine DNS Query Payloads:
   • Long Domain Names: DNS tunneling often involves long and complex domain names as they encapsulate data within the subdomains. Monitor for unusually long domain names in DNS queries.
   • Pattern Recognition: Look for repeated patterns or base64-encoded data in domain names, which might indicate data encapsulation.
3. Monitor DNS Traffic Volume:
   • Volume Anomalies: Normal DNS queries generate a predictable amount of traffic. DNS tunneling can cause a significant increase in DNS traffic volume. Set up baselines for normal DNS traffic and alert on deviations.
4. Frequency Analysis:
   • High Frequency: DNS tunneling can result in a high frequency of DNS requests to a particular domain. Use frequency analysis to identify domains with an unusually high query rate.
5. DNS Server Logs:
   • Log Analysis: Regularly analyze DNS server logs for any suspicious activity. Correlate logs with other network traffic data to identify potential tunneling.

Prevention

1. DNS Firewall and Security Services:
   • DNS Filtering: Use DNS security services or firewalls (e.g., Cisco Umbrella, Cloudflare Gateway) to block known malicious domains and enforce policies against suspicious domains.
   • Block Uncommon Query Types: Restrict or block uncommon DNS query types (TXT, NULL) that are often used in DNS tunneling.
2. Rate Limiting:
   • Query Rate Limiting: Implement rate limiting on DNS queries to prevent a high volume of requests from a single source. This can help mitigate the impact of DNS tunneling.
3. Inspection and Logging:
   • Deep Packet Inspection (DPI): Use DPI tools to inspect DNS query payloads for suspicious patterns and long domain names.
   • Enhanced Logging: Enable detailed logging on DNS servers and regularly review logs for signs of tunneling.
4. DNS Response Size Limiting:
   • Limit Response Sizes: Restrict the size of DNS responses to prevent the use of large payloads that can carry tunneled data.
5. Behavioral Analysis:
   • User Behavior Analytics (UBA): Implement UBA to detect anomalous behavior associated with DNS tunneling. This includes monitoring user and system behavior for unusual patterns.
6. Network Segmentation:
   • Isolate Critical Systems: Segment your network to isolate critical systems and prevent direct access to DNS servers from untrusted or guest networks.

Conclusion

DNS tunneling leverages the fact that DNS traffic is typically allowed through captive portals. By encapsulating non-DNS data within DNS queries and responses, it is possible to bypass the restrictions imposed by captive portals. This method can be highly effective, though it requires control over an external DNS server and appropriate tunneling software on the client device.