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
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