[yuch]

Hi I have an Asus RT-AC56U running : DD-WRT v3.0-r47608 (10/28/21) BS build on stock settings.

The router has USB ports and I use iPhone USB tethering connected to the router for my Internet. Ie I share the mobile data plan across the house. I use a iPhone 6 running iOs 12.4.7.

A Unifi 6 AP is connected to LAN port 1 of the router and devices connect wirelessly through the AP.

After I updated the firmware for the iOS 14 fix (because I occasionally use another iPhone for tethering), I encountered an issue, where a laptop connecting wirelessly to the AP would crash the Internet for all devices. This typically happens when Chrome is opened on that laptop with multiple tabs. I’ve also encountered this issue to a lesser extent with another laptop.

After the Internet crashes I am still able to access the dd-wrt admin page. Typically unplugging and plugging the iPhone back into the router will resolve the issue (until the laptop in question connects again). I’ve also found that changing TCP congestion rule from westwood to vega helps marginally (in that it takes longer before the Internet crashes again) but does not resolve the issue definitively.

I would be grateful if someone could suggest what areas I should be looking at to troubleshoot this problem.

[the-joker]

Hello @yuch

Well, because Apple, would be my guess. You yourself said it was after the IOS update, so I would really try to bark up that tree.

However when you say the internet for all devices is killed, I presume you mean WAN connection is killed, not the WiFi. (Because sometimes when the WiFi dies here the person shouting at me, says "the internet is gone" and really Its not, the two are separate subjects, so I have to ask exactly what it is you mean, WAN or WiFi.

If the WAN, then its likely the iPhone really or DD-WRT since it is where you are tethered, or if the WiFi its the Unifi 6 AP for the WiFi because you aren't connected to the RT-AC56U WiFi.

You can also, start removing possible culprits from that list, e.g. connect to RT-AC56U WiFi and try to reproduce the issue. So at least you know for sure if the issues is related to the AP or the Asus.

You can even create a standalone HotSpot from the iPhone and connect directly using the clients that cause the issue, to that and try to reproduce the issue, while not even including Asus or Unifi into the mix.

At least you have some food for thought where to look next, this is how would troubleshoot the issue, and work backwards form there until you found the pint of failure.

For my money though, its really the update that's to blame, maybe it cant cope with all those requests and doesn't handle it graciously.

I have moved the thread to Advanced Networking subforum, I feel this is a more appropriate spot for this mixed network devices related question.

Gotta love Apple... not

[yuch]

Thanks! Indeed, am suspecting the Apple or at least how it interacts with the AC56U. As you say, it is the WAN that goes down and not the Wi-Fi generally. I’ve also tried using the AC56U Wi-Fi without the Unifi with the same result. The Apple hotspot by itself works fine (and has been my temporary workaround).

I had a few potential issues shortlisted:

- I think I am double-NAT-ed as the iPhone WAN IP shows up differently from my public IP. But I feel like there is no real way to resolve this even if this was the problem since I can’t do anything on the Apple side.

- I thought the iPhone could be messing up DHCP. Whenever the internet fails, I see a lot of DHCP activity in the logs(extracted below) but I’m not sure what to make of it.

Nov 25 18:58:48 DD-WRT daemon.info dnsmasq-dhcp[6865]: DHCPDISCOVER(br0) XXXX
Nov 25 18:58:48 DD-WRT daemon.info dnsmasq-dhcp[6865]: DHCPOFFER(br0) 192.168.1.111 XXXX
Nov 25 18:58:48 DD-WRT daemon.info dnsmasq-dhcp[6865]: DHCPREQUEST(br0) 192.168.1.111 XXXX
Nov 25 18:58:48 DD-WRT daemon.info dnsmasq-dhcp[6865]: DHCPACK(br0) 192.168.1.111 XXXX

[the-joker]

You could try https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=78029

FYI that came from https://forum.dd-wrt.com/phpBB2/viewtopic.php?p=115456which has a different solution.

[kernel-panic69]

I don't think you can do that successfully, but it is worth a try. You would likely have to turn off DHCP, DNS, and let the iPhone hand out the IP addresses, etc. I think there might be other areas to investigate within the router's configuration... for example, is there an MTU setting associated with iPhone tethering?

[Per Yngve Berg]

Check Status->Router->Network if all available network connections are used.

Every station on the network will make a connection entry for every server they communicate with. So a station with a lot of tabs can create over 100 connections.

[yuch]

Thanks all!

The MTU fix appears to work - for now. For some reason though, I have to set it pretty low (now at 1260) to achieve the balance between speed and stability.

[kernel-panic69]

1200 is what was recommended (for Android) in the wiki; 4096 is more than likely plenty (which should be the default?). Glad you made progress in the correct direction!