Re: [tlug] Re: Absurd DNS failures

["Norman Diamond" <ndiamond@example.com>]

"J. David Beutel" <jdb@example.com> wrote:

> > On Thu, 3 Jul 2003, Norman Diamond wrote:
> > >
> > > How could this get success for www.asia.microsoft.com but timeouts for
> > > plain microsoft.com?
> >
> > Different packet length?  I'd see if different MTU settings have any
> > effect.  (But sorry, offhand I don't know how to do that.)
>
> P.S.  See http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/mtu-issues.html

Even though my situation has nothing to do with masquerading (it is a single
machine dialling up directly through a modem), this turned out to be
relevant.  But first a digression.  The cited page is confusing as to
whether the most frequently suitable setting would be 1492 or 1460 or 1412,
and actually the cited page is worse than that.  When masquerading is
involved, the page says that the machine which is connected MUST have MTU of
1500, but also says that the MTU is limited to at most 1492 if the
connection is PPPoE, and also says that some ISPs have quirks which limit
the MTU to 1460.  The MUST setting of 1500 seems to be MUST AND MUST NOT.

Anyway, since masquerading isn't involved, I set both MTU and MRU 1500.  And
it worked.

So where the Linux Network Administrators Guide (NAG) recommends an MTU and
MRU of 296 for dialled PPP connections, the NAG seems to be pretty much out
to lunch.

SuSE's default was 1492.  I'll bet it will work most of the time, though
your cited TLUG page implies that there might be occasional failures (i.e.
when the must and must not of 1500 is not actually set, there will still be
occasional servers that will not cooperate with 1492 or whatever).  So when
I obeyed the NAG and changed it from 1492 to 296, it failed more frequently.

I wonder if akadns.net filters out the type of incoming ICMP packet that
tries to ask akadns.net to reduce its outgoing MTU for clients who have
obeyed the NAG.  If so, then akadns.net's replies will never reach those
clients, and those clients will never be able to resolve the ordinary names
of microsoft.com and yahoo.com.