Cisco CWA Redirect Failure: 14 Tactical Scenarios and Fixes

Macro Phase 4: Name Resolution, Portal Access & Certificates

Scenario 9: ISE FQDN DNS Resolution Failure

• The Symptom: The client gets the redirect URL pushed into its session table, but the browser displays a DNS_PROBE_FINISHED_NXDOMAIN block page.

• The Root Cause: The guest endpoint cannot translate the Fully Qualified Domain Name (FQDN) of the ISE Policy Service Node (PSN) into a routable IP address. This occurs if public DNS options (like 8.8.8.8) are handed out to guests via DHCP, but the ISE FQDN is only defined on private, internal corporate name servers.

• The Solution: Ensure the guest DNS scope can resolve the ISE FQDN, or map a publicly resolvable FQDN for your ISE PSN nodes.

🛑 Note: If you deploy a single public FQDN pointing to multiple ISE nodes behind a Load Balancer, ensure the Load Balancer uses strict Sticky Sessions / Source IP Persistence. If a client’s 3-way handshake hits PSN-01 but the next packet shifts to PSN-02, the TCP state machine resets instantly.

Scenario 10: Client Unable to Open Portal (TCP Timeout)

• The Symptom: DNS correctly resolves the FQDN to the proper IP address, but the guest browser eventually times out with a “Connection Timed Out” error.

• The Root Cause: The initial TCP SYN packet targeting the captive portal port (typically 8443) cannot reach the ISE node. This is usually caused by missing entries in the redirect ACL (forgetting to add a secondary or tertiary PSN node, causing the WLC to loop the traffic back to itself) or upstream security firewalls blocking port 8443.

• The Solution: Update the Redirect ACL to explicitly pass traffic destined for all potential ISE PSN nodes.

ip access-list extended WebAuth_Redirect_ACL
 deny ip any host <ISE-PSN1-IP>
 deny ip host <ISE-PSN1-IP> any
 deny ip any host <ISE-PSN2-IP>
 deny ip host <ISE-PSN2-IP> any
 deny udp any any eq domain
 permit ip any any

Scenario 11: Untrusted Certificate Warning (The HTTPS Intercept Trap)

• The Symptom: When a guest client connects to the SSID and attempts to navigate to a secure HTTPS website (e.g., google.com) to kick off the redirection flow, their browser immediately drops into a hard stop, presenting an intimidating “Your connection is not private” (NET::ERR_CERT_AUTHORITY_INVALID) security alert page.

• The Root Cause: The Catalyst 9800 WLC is aggressively attempting to execute Layer 3 interception on encrypted TLS traffic. Because the WLC does not own—and cannot cryptographically impersonate—external third-party domains on the internet, the client’s web browser flags this redirection behavior as an active Man-in-the-Middle (MITM) attack. The browser drops a hard security block on the execution of the page to maintain TLS session integrity.

• The Solution: Disable HTTPS interception entirely within the WLC’s global web authentication parameter map. This architectural shift forces the network infrastructure to ignore encrypted HTTPS requests for redirection and rely exclusively on unencrypted HTTP requests. Modern mobile and desktop operating systems generate these unencrypted queries in the background via automated connectivity checks (such as Apple’s captive.apple.com or Microsoft’s msftconnecttest.com). Dropping HTTPS interception allows the native OS Captive Network Assistant (CNA) engine to trigger cleanly, surfacing your login portal instantly without generating browser-level security alerts.

conf t
  parameter-map type webauth global
   no intercept-https-enable
   webauth-http-enable
   secure-webauth-disable
exit

🛑 Note: The Public Certificate Fallacy

Purchasing and installing a publicly signed SSL/TLS certificate (e.g., DigiCert or Let’s Encrypt) on your Catalyst 9800 WLC will not fix HTTPS interception errors. A public certificate is cryptographically restricted; it only validates the exact Fully Qualified Domain Name (FQDN) assigned to your controller’s portal interface. It explicitly lacks the authorization to sign or spoof traffic belonging to other secure websites on the internet.

Furthermore, because modern enterprise web destinations utilize HTTP Strict Transport Security (HSTS), mobile operating systems will strictly refuse to grant any “Proceed Anyway” bypass options to the end user if a certificate mismatch is detected. Turning off HTTPS interception entirely remains the only standard, enterprise-grade resolution path.

 

The Future of WebAuth: Moving Beyond HTTP Intercepts

Traditionally, captive portals rely on “intercepting” HTTP traffic to redirect users—a method that is becoming increasingly difficult as the web moves toward total encryption.

RFC 8910 introduces a more modern approach: Captive-Portal Identification in DHCP and Router Advertisements (RAs). Instead of waiting for a browser to trigger a redirect, the network explicitly tells the client it is behind a portal via DHCP options or IPv6 RAs. This provides a cleaner, more reliable user experience and aligns with the industry’s shift toward secure-by-default networking.

For the full technical details, check out the official RFC here: Captive-Portal Identification in DHCP and Router Advertisements (RAs)

Scenario 12: ISE Portal TLS Handshake Loop (Internal CA Block)

• The Symptom: The device redirects to the authentic ISE portal page, but the browser drops into a hard-lock loop with no option to click “Proceed anyway”.

• The Root Cause: ISE is utilizing a web portal certificate signed by a corporate internal Certificate Authority (CA). Guest devices do not carry this private root trust anchor in their local stores. Furthermore, if the domain is protected by HTTP Strict Transport Security (HSTS), modern mobile operating systems will immediately block page rendering with zero bypass paths available.

• The Solution: Deploy a publicly trusted certificate (e.g., DigiCert, Let’s Encrypt) for all guest-facing portals. Ensure the entire certificate chain (including all intermediate certificates) is bundled and uploaded to ISE.