reducing the advertised EDNS UDP packet

I had to set up a PPPoE connection on my Ubuntu server. With the help of the command pppoeconf, it is kind of straigtforward.

However, when browsing some Websites I could see in the log file:

... after reducing the advertised EDNS UDP packet size to 512 octets

And even worse, some Websites were not showing up (ex: finance.yahoo.co.jp).

Solution

There were 2 problems:

1) Problem with IPv6
I had to disable IPv6 (probably a problem with my ISP?) with lines in /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1 

Then run:

sysctl -p /etc/sysctl.conf

2) Problem with packet size
A rule for iptables was automatically created in /etc/ppp/if-up.d/0clampmss. And later on I was flushing all the rules to set my own rules (set in another file). Because I was not re-running 0clampsmss then the rule was not added.
And this rule was for resizing the packet.
So in my own rules, I stopped flushing the table mangle.

And that's it.