Re: Path MTU discovery broken / advmss

[Michael Tremer <michael.tremer@ipfire.org>]

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Hi,

let's discuss this on the forums then since Arne already answered
there:
  http://forum.ipfire.org/viewtopic.php?f=27&t=14783

-Michael

On Fri, 2015-08-28 at 16:23 +0000, John P Poet wrote:
> Hi,
> 
> I hope it is okay if I ask a configuration question here.
> 
> A couple of weeks ago, my internet dropped and I noticed that my DSL
> modem was rebooting.  When it came back up I had a new public IP and
> I could no longer load most web pages.  Other than google, the ones
> which would load were very slow.  I was finally able to get a page to
> load which suggested this behaviour could be caused by a MTU problem.
>   I tried lowering the MTU on one of my machines from 1500 to 1492
> and the problem was solved.
> 
> I then went to all of my various computers and changed the MTU to
> 1492, except my iPad.  Apple being Apple, does not allow users the
> ability to change things like MTU.  This means that I can still not
> get to most web pages, etc, on my iPad.
> 
> Further searching brought up pages like:
> https://blog.cloudflare.com/path-mtu-discovery-in-practice/
> http://rtoodtoo.net/path-mtu-ip-fragmentation-and-mss/
> 
> Given the info on those pages, I have tried to set advmss on my
> router to 1452, 1200, 1024, etc.  But no matter what I do, I end up
> killing the routing completely.  I have tried doing things like:
> 
> # echo 1 > /proc/sys/net/ipv4/tcp_mtu_probing
> # echo 1024 > /proc/sys/net/ipv4/tcp_base_mss
> or
> # ip route change 0.0.0.0/0 dev red0 advmss 1024
> Obviously, those are not the correct way to get ipfire to set mss to
> a lower value.  I am hoping someone can tell me the correct way to
> work around a broken "path MTU discovery" issue.
> 
> Thanks,
> 
> John
> 

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