[Solved] Slow WAN => LAN but not when going through tun0

I've having a strange issue that I can't figure out what's causing.

Using a WRT1900ACS with stock 18.06.1. I have 100Mbps down from my ISP and can't seem to get over 9Mbps on clients that AREN'T tunneled through OpenVPN.

From OpenWRT router to ISP provided modem, I can get the full 100Mbps running speedtest-cli directly on the router.
But from a wired desktop attached to WRT19000ACS, I can't get over 9-10Mbit.

So I know my connection to ISP modem is fine, because the router can do it by itself, I can also get a full gigabit on client to client on LAN with iperf, so I know the clients and wiring is fine. Once I try and go LAN => WAN, my speeds drop like a rock, but LAN => WAN through an OpenVPN tun0 is also fine.

What gives??

What are your MTU settings? Those symptoms sound as if there might be some fragmentation going on.

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MTU is untouched on all interfaces, so 1500?

Well well, indeed you were right.

Turns out I had MSS clamping set wrong in my firewall config. Thanks for your suggestion!


You're welcome!

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