[Mimedefang] Strange activity

[David F. Skoll]

Kevin A. McGrail wrote:

> However, I've also read similar recommendations with exim and VPN
> especially so it's not really the craziest thing I've ever heard.  I'm
> interested in hearing more.  I mean 8 bytes every 1300 bytes rather than
> every 1500 bytes just doesn't seem like that big a deal if it fixes
> real-world problems.

I've never had anyone complain about our mail server, which uses the standard
Ethernet MTU of 1500.  (And it's typically 40 bytes of overhead, not 8.
You're adding an IP header and a TCP header to each packet.)

I remember when PPPoE was introduced (with an MTU of 1492), there were
lots of problems, but I haven't heard of those problems lately.  Are
they really still happening?

My PPPoE client (rp-pppoe, the most popular PPPoE client for Linux
systems) attempts to get around the problem by tweaking the MSS option
in the TCP header, so that even hosts that don't do PMTU discovery
still work.  In other words, it intercepts Maximum Segment Size
negotation and lowers it if it would cause problems.  Neither side is
any the wiser.  This is a Horrible Hack (tm), but it works.  I believe
the 2.6 Linux kernel has an iptables hack that does something similar.

Regards,

David.