ubuntu gateway not converting mtu?

Question

linux

Greetings, there is an ubuntu 16 software router, it has 2 network cards, one for the local network, the other for the Internet, unfortunately ppppoe. In the pppoe settings, mtu 1492 is installed. The problem is that most sites on the Internet cannot be opened for clients on the local network, although everything works on the gateway. If I manually reduce the mtu for each client to 1492, then everything opens for the client, I suppose the gateway does not convert client mtu? the rule in the firewall helps:

$ip -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

I would like to understand why the gateway behaves this way and why this rule works?

Answer 1

https://habrahabr.ru/post/136871/