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Usually they let DNS resolve normally (because intercepting at this level will break the website even after you've satisfied the captive portal) and intercept a

Usually they let DNS resolve normally (because intercepting at this level will break the website even after you've satisfied the captive portal) and intercept any HTTP connections, responding with a temporary redirect to a controlled domain. Doing this with a mere DNS server will only work if you don't visit the website you initially tried to access for a while after you connect. It's also super easy to bypass; just pick a public DNS server instead of the one the network tries to assign to you. To do this properly either your router or switch/AP need to be configured to do the necessary rewriting as well as maintain a list of authenticated clients. Your best bet to do this with something in the consumer(ish) price range without a custom firmware like OpenWRT is something like a Uni-Fi access point which can handle the captive portal interception itself.

EGreg 10y ago
Can't I use a regular consumer router but hook up a computer on the LAN to do something? Intercept DNS? Intercept HTTP requests like you said? But the problem is what if the user makes https requests? All I want to do is make a system which "takes attendance" via the phones automatically trying to join the local network, and I use the session is to look up the user. People would have accounts where they log in once via the captive portal and then the attendance would happen automatically. Basically I want to make a captive portal with regular routers so I can sell the solution to regular venues. How do these guys do it: https://www.fusionwifi.com/ https://www.fusionwifi.com/ There are a lot of social wifi solutions now
- brainfire 10y ago
  You must be able to MitM something to get this to work. If you can MitM connections on your network just by connecting a client to it, with no particular participation from the router, your network sucks. Your linked site notes that they provide the router hardware.
- johncolanduoni 10y ago
  Think about it this way: if you could do it with a random computer on the network without special cooperation from the router, what would stop someone from connecting to your network with this software you propose and doing the same? As I explained, telling your router to instruct your clients to use a particular DNS server isn't nearly enough. The closest you could get would be to use another computer with 2 network cards as the actual router (running something like PFSense), and set up a consumer WiFi router as an access point instead of a router. This would only work as long as you don't mind the clients being able to connect to the rest of you local network; if that's an issue you really need something like a captive portal aware AP/switch or a VLAN. However this solution doesn't make much sense in the end, as it would be much cheaper to buy one of the cheaper commercial-grade APs (UniFi's are around $80) and use your "regular router" with WiFi disabled than it is to buy a computer for that purpose. Those APs are also better quality than any consumer router you could find, and scale up much better for larger venues. Also TBH if you're not aware of how captive portals work and why it can't be done by a random computer on your network, this is probably not a good business venture for you. This isn't an unsolved (or expensive to solve) problem to begin with.
  - EGreg 10y ago
    I know it isn't an unsolved problem. That's my point. I have the software to run on a local area network and now I just want to explore the right ways to actually set up our own captive portals on those networks. Consider a building or cruise ship with an existing network. It's a much harder sell to say "replace all your routers" or "flush all your routers and install our firmware" than to say "use your existing router and just set up our DNS server on a computer".
    - johncolanduoni 10y ago
      What networks have you seen that have more than one router and use consumer routers? Even medium sized networks often have only one router and multiple switches. You can get a router that can handle a captive portal and push a million packets per second for around $100. How little are you going to charge for your solution that it's going to be cheaper than that?

EGreg 10y ago

Can't I use a regular consumer router but hook up a computer on the LAN to do something? Intercept DNS? Intercept HTTP requests like you said? But the problem is what if the user makes https requests? All I want to do is make a system which "takes attendance" via the phones automatically trying to join the local network, and I use the session is to look up the user. People would have accounts where they log in once via the captive portal and then the attendance would happen automatically. Basically I want to make a captive portal with regular routers so I can sell the solution to regular venues. How do these guys do it: https://www.fusionwifi.com/ https://www.fusionwifi.com/ There are a lot of social wifi solutions now

brainfire 10y ago
You must be able to MitM something to get this to work. If you can MitM connections on your network just by connecting a client to it, with no particular participation from the router, your network sucks. Your linked site notes that they provide the router hardware.
johncolanduoni 10y ago
Think about it this way: if you could do it with a random computer on the network without special cooperation from the router, what would stop someone from connecting to your network with this software you propose and doing the same? As I explained, telling your router to instruct your clients to use a particular DNS server isn't nearly enough. The closest you could get would be to use another computer with 2 network cards as the actual router (running something like PFSense), and set up a consumer WiFi router as an access point instead of a router. This would only work as long as you don't mind the clients being able to connect to the rest of you local network; if that's an issue you really need something like a captive portal aware AP/switch or a VLAN. However this solution doesn't make much sense in the end, as it would be much cheaper to buy one of the cheaper commercial-grade APs (UniFi's are around $80) and use your "regular router" with WiFi disabled than it is to buy a computer for that purpose. Those APs are also better quality than any consumer router you could find, and scale up much better for larger venues. Also TBH if you're not aware of how captive portals work and why it can't be done by a random computer on your network, this is probably not a good business venture for you. This isn't an unsolved (or expensive to solve) problem to begin with.
- EGreg 10y ago
  I know it isn't an unsolved problem. That's my point. I have the software to run on a local area network and now I just want to explore the right ways to actually set up our own captive portals on those networks. Consider a building or cruise ship with an existing network. It's a much harder sell to say "replace all your routers" or "flush all your routers and install our firmware" than to say "use your existing router and just set up our DNS server on a computer".
  - johncolanduoni 10y ago
    What networks have you seen that have more than one router and use consumer routers? Even medium sized networks often have only one router and multiple switches. You can get a router that can handle a captive portal and push a million packets per second for around $100. How little are you going to charge for your solution that it's going to be cheaper than that?