How Nodogsplash (NDS) works

A wireless router, typically running OpenWrt or some other Linux distribution, has two or more interfaces; NDS manages one of them. This will typically be br-lan, the bridge to both the wireless and wired LAN; or could be for example wlan0 if you wanted NDS to work just on the wireless interface.

Summary of Operation

By default, NDS blocks everything, but intercepts port 80 requests.

An initial port 80 request will be generated on a client device, either by the user manually browsing to an http web page, or automatically by the client device’s built in Captive Portal Detection (CPD).

As soon as this initial port 80 request is received, NDS will redirect the client to a “splash” page.

This splash page is either one of the two standard options or a custom configuration provided by the user (See FAS, PreAuth).

The user of the client device will then be expected to complete some actions on the splash page, such as accepting terms of service, entering a username and password etc.

Once the user on the client device has successfully completed the splash page actions, that page then links directly back to NDS.

For security, NDS expects to receive the same valid token it allocated when the client issued its initial port 80 request. If the token received is valid, NDS then “authenticates” the client device, allowing access to the Internet.

Post authentication processing extensions may be added to NDS (See BinAuth). Once NDS has received a valid token it calls a Binauth script.

If the BinAuth script returns positively (ie return code 0), NDS then “authenticates” the client device, allowing access to the Internet.

In FAS secure modes are provided (levels 1, 2 and 3), where the client token and other required variables are kept securely hidden from the Client, ensuring verification cannot be bypassed.


FAS and Binauth can be enabled together. This can give great flexibility with FAS providing authentication and Binauth providing post authentication processing closely linked to NDS.

Captive Portal Detection (CPD)

All modern mobile devices, most desktop operating systems and most browsers now have a CPD process that automatically issues a port 80 request on connection to a network. NDS detects this and serves a special “splash” web page to the connecting client device.

The port 80 html request made by the client CPD can be one of many vendor specific URLs.

Typical CPD URLs used are, for example:

  • Plus many more

It is important to remember that CPD is designed primarily for mobile devices to automatically detect the presence of a portal and to trigger the login page, without having to resort to breaking SSL/TLS security by requiring the portal to redirect port 443 for example.

Just about all current CPD implementations work very well but some compromises are necessary depending on the application.

The vast majority of devices attaching to a typical Captive Portal are mobile devices. CPD works well giving the initial login page.

For a typical guest wifi, eg a coffee shop, bar, club, hotel etc., a device connects, the Internet is accessed for a while, then the user takes the device out of range.

When taken out of range, a typical mobile device begins periodically polling the wireless spectrum for SSIDs that it knows about to try to obtain a connection again, subject to timeouts to preserve battery life.

Most Captive Portals have a session duration limit (NDS included).

If a previously logged in device returns to within the coverage of the portal, the previously used SSID is recognised and CPD is triggered and tests for an Internet connection in the normal way. Within the session duration limit of the portal, the Internet connection will be established, if the session has expired, the splash page will be displayed again.

Early mobile device implementations of CPD used to poll their detection URL at regular intervals, typically around 30 to 300 seconds. This would trigger the Portal splash page quite quickly if the device stayed in range and the session limit had been reached.

However it was very quickly realised that this polling kept the WiFi on the device enabled continuously having a very negative effect on battery life, so this polling whilst connected was either increased to a very long interval or removed all together (depending on vendor) to preserve battery charge. As most mobile devices come and go into and out of range, this is not an issue.

A common issue raised is:

My devices show the splash page when they first connect, but when the authorization expires, they just announce there is no internet connection. I have to make them “forget” the wireless network to see the splash page again. Is this how is it supposed to work?

The workaround is as described in the issue, or even just manually disconnecting or turning WiFi off and on will simulate a “going out of range”, initialising an immediate trigger of the CPD. One or any combination of these workarounds should work, again depending on the particular vendor’s implementation of CPD.

In contrast, most laptop/desktop operating systems, and browser versions for these still implement CPD polling whilst online as battery considerations are not so important.

For example, Gnome desktop has its own built in CPD browser with a default interval of 300 seconds. Firefox also defaults to something like 300 seconds. Windows 10 is similar.

This IS how it is supposed to work, but does involve some compromises.

The best solution is to set the session timeout to a value greater than the expected length of time a client device is likely to be present. Experience shows a limit of 24 hours covers most situations eg bars, clubs, coffee shops, motels etc. If for example an hotel has guests regularly staying for a few days, then increase the session timeout as required.

Staff at the venue could have their devices added to the Trusted List if appropriate, but experience shows, it is better not to do this as they very soon learn what to do and can help guests who encounter the issue. (Anything that reduces support calls is good!)

Packet filtering

Nodogsplash considers four kinds of packets coming into the router over the managed interface. Each packet is one of these kinds:

  1. Blocked, if the MAC mechanism is block, and the source MAC address of the packet matches one listed in the BlockedMACList; or if the MAC mechanism is allow, and source MAC address of the packet does not match one listed in the AllowedMACList or the TrustedMACList. These packets are dropped.
  2. Trusted, if the source MAC address of the packet matches one listed in the TrustedMACList. By default, these packets are accepted and routed to all destination addresses and ports. If desired, this behavior can be customized by FirewallRuleSet trusted-users and FirewallRuleSet trusted-users-to-router lists in the nodogsplash.conf configuration file, or by the EmptyRuleSetPolicy trusted-users EmptyRuleSetPolicy trusted-users-to-router directives.
  3. Authenticated, if the packet’s IP and MAC source addresses have gone through the nodogsplash authentication process and has not yet expired. These packets are accepted and routed to a limited set of addresses and ports (see FirewallRuleSet authenticated-users and FirewallRuleSet users-to-router in the nodogsplash.conf configuration file).
  4. Preauthenticated. Any other packet. These packets are accepted and routed to a limited set of addresses and ports (see FirewallRuleSet preauthenticated-users and FirewallRuleSet users-to-router in the nodogsplash.conf configuration file). Any other packet is dropped, except that a packet for destination port 80 at any address is redirected to port 2050 on the router, where nodogsplash’s built in libhttpd-based web server is listening. This begins the ‘authentication’ process. The server will serve a splash page back to the source IP address of the packet. The user clicking the appropriate link on the splash page will complete the process, causing future packets from this IP/MAC address to be marked as Authenticated until the inactive or forced timeout is reached, and its packets revert to being Preauthenticated.

Nodogsplash implements these actions by inserting rules in the router’s iptables mangle PREROUTING chain to mark packets, and by inserting rules in the nat PREROUTING, filter INPUT and filter FORWARD chains which match on those marks.

Because it inserts its rules at the beginning of existing chains, nodogsplash should be insensitive to most typical existing firewall configurations.

Traffic control

Data rate control on an IP connection basis can be achieved using Smart Queue Management (SQM) configured separately, with NDS being fully compatible.

It should be noted that while setup options and BinAuth do accept traffic/quota settings, these values currently have no effect and are reserved for future development.