Captive Portal Fails to Load Properly or Returns 404

Captive Portal Fails to Load Properly or Returns 404

This article describes how to fix the issue whereby captive portal fails to load, is returned only partially without the user/pass fields or returns a 404 error. This also mitigates issues with slow access to a Mobile Access Gateway on wireless or lossy networks.
The mechanism responsible for the problems is “SACK” – an acronym for Selective ACKknowledgment. The “SACK-permitted” option and “SACK” option alter the acknowledgment behavior of TCP:


The SACK-permitted option is offered to the remote end during TCP setup as an option to an opening SYN packet. The SACK option permits selective acknowledgment of permitted data. The default TCP acknowledgment behavior is to acknowledge the highest sequence number of in-order bytes. This default behavior is prone to cause unnecessary retransmission of data, which can exacerbate a congestion condition that may have been the cause of the original packet loss.
A packet capture on the client or gateway will typically show many retransmitted packets and ends with the client eventually sending RST packets to the gateway.


The SACK option allows the receiver to modify the acknowledgment field to describe noncontinuous blocks of received data so that the sender can retransmit only what is missing at the receivers end.


Disable Selective-ACK by setting the value of cpas_tcp_do_sack kernel parameter to “0” (zero).
To Disable Selective-ACK on-the-fly, run:
[Expert@gateway]# fw ctl set int cpas_tcp_do_sack 0
Check that the value was accepted:
[Expert@gateway]# fw ctl get int cpas_tcp_do_sack
To disable the parameter permanently:
Edit the $FWDIR/boot/modules/fwkern.conf file using Vi editor to add a line with the format “parameter_name=value", in this case:
Reboot the gateway after any changes to the file$FWDIR/boot/modules/fwkern.conf.


  • Disabling Selective-ACK can impact networking throughput when client/server are using large TCP Windows and there is packet loss between these hosts.
  • In Cluster, perform the procedure on all cluster members.


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