This has been a problem since the 2.0 days, with anyone using IP Masq.
There seems to be an interaction problem when the masq box has a smaller
MTU than the firewalled system, and Path MTU Discovery breaks down. At
some point, the ICMP messages necessary to support it are not being
passed along correctly. Unfortunately, no one seems to have a clear
handle no the nature of the problem. Setting the PPP MTU to be the same
as the ethernet MTU (1500) cures the problem in all cases, but many
people dislike the performance ramifications this has.
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