[FreeNX-kNX] Re: Connection error with ubuntu 10.04 & 10.10

[chris at ccburton.com]

Arnaud Fenioux <afenioux at gmail.com> wrote on 31/12/2010 10:00:45:
> >> Right, the mtu given by the 3G link is equal to 1486
[SNIP]
> ifconfig is fine.
> but a "ping -s XXX -M do" with XXX > 548 (576 with the headers),
> hasn't any answer,

Pinging the Nx server may we presume ???

> although I can see much bigger packets when I dumping while browsing...
[SNIP]
> I mean, it worked for windows, not for Linux event if I set the mtu to 
1500
> >
> >> why did it worked with previous version of ubuntu ( 9.10- 
 server-side
> >> speaking) ?

Something has changed somewhere !! Lets have a look.

> > Sounds like something isn't letting ICMP through, so it is defaulting 
to
> > 1500, and/or something just isn't working, like the 3G driver on 10.04
> > thinks it should be set to 1500 when the link provided is actually 
lower.
> >
> > Try putting the server back up to 1500 and setting the 3G interface
> > to something lower.
> 
> I tried severals values on the 3G interface (500-1500), nothing worked,
> the reason could be because I'm NATed/Proxyfied by my ISP...
> I'm using a 10.x.x.x IP on a "point to point noarp multicast"  link

That's not the issue else the windows connection to NX wouldn't work.

I'd say that two things are broken.

1/ The 3G linux driver is not setting the MTU (it does in windows
        so PMTUD isn't needed)

2/ The NX server is not getting ICMP, so it won't get pings, nor
will it get PMTUD (I doubt only Type3-C4 packets are filtered).

So, on the NX server run your favourite tcpdump :-
        (sudo) tcpdump -vv icmp
then
        ping the NX server from :-
        your 3G 
        your remote ADSL and
        the NX-server segment

This should help spot any ICMP filtering.

Then leaving tcpdump running:-
        connect an NX session
watch for "Frag needed and DF set" as the session connects
and see what sends them.

I predict you won't see any, because something is filtering them
out until you find and remove the filtering.
So
Do you run a firewall on the 10.04 server
or
do you have ICMP disabled on the edge router at the NX
site
(maybe 9.04 does something different from 10.04 - like not having
a bug in PMTUD or maybe even not having PMTUD switched
off as a default (unlikely), or maybe 9.04 automatically reduces
the MTU if connections get interrupted, without waiting for ICMP
to tell it that it needs to)

> 
> Well, happy new year!

And to you too !!

> and BTW, do you have an idea, why 3.4.0 backend is (really) slower
> than the 3.3.0 (even in ADSL) to establish the display connection? ;)

I haven't noticed this difference. Check to see where your delay is
occuring.
Is it part of the NX ssh connection (eg sshd dns reverse lookups) or
is it after the ssh connection is in place, at agent startup ??

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