Daniel Webb wrote:
> On Sat, Apr 08, 2006 at 11:44:29PM -0600, Daniel Webb wrote:
>
>
>>I'm still unsure if this is a problem on my end or gmail's. Why would all
>>other mails go through fine but not to gmail?
>
>
> Here's what I think is happening, and why it was only manifesting with Google:
> Google is filtering ICMP packets used for path MTU discovery and thus breaking
> emails that go over 400 bytes (only with my system). Because the mail server
> had an MTU of 1500, Google was trying to use 1500 byte packets.
>
> See the "Example Path MTU Discovery Failure Scenario" here:
>
> http://www.netheaven.com/pmtu.html
>
> I think that's exactly what's happening.
>
> I'm an amateur at networking and this isn't my day job, so I'm open to other
> possibilities.
>
> P.S. I checked my firewall and I'm not blocking any ICMP packets.
>
>
I suspect you are correct, and tcpdump may show it clearly.
But - while we had a 'related' problem here with a broadband
supplier, initially supported by tcpdump records, and reduced,
but not fully resolved by both re-compiling Exim to use smaller
MTU AND setting the client(s) to match, we ultimately found that
a dodgy SiS 10/100 NIC & its 4.* FreeBSD driver combo was a
major contributor.
Switched the cable and IP to the Intel 10/100/Gig-E NIC in the
same server, and the problem went away and took some other
problems away with it.
So don't rule out 'hardware', or hardware+driver problems,
especially when it seems one is the 'Lone Ranger' on an issue.
JFWIW, YMMV
Bill
