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gre tunnel MTU adjustment

Dear List,

I have a GRE tunnel setup between a debian linux/zebra router at my
co-lo and my home office.  This allows me to have a /27 without coughing
up $7/IP to the local cable monopoly.  There are no other broadband IP
options available.

My problem is I can't raise the MTU on the intermediate links over which
the tunneled packets must travel, thus the MTU of my GRE tunnel is less
than 1500.  Many popular Internet sites, including paypal, hotmail,
portions of Yahoo, and my beloved friendster, have utterly broken Path
MTU Detection.  The problem is wide-spread, and I don't think these
sites are going to correct their problem or disable PMTUd on their
servers, load balancers, and whatnot.

Cisco routers have the ability to fragment and reassemble IP packets
traversing GRE tunnels in order to effectively increase the tunnel MTU. 
The command syntax is e.g. `ip mtu 1500` in interface configuration.

Is similar functionality available on linux?  If not, can someone with
iptables clue give me an example of how to disable the IP Don't-Fragment
bit on ip packets that are being routed to my tunnel, allowing them to
be fragmented even though the transmitting TCP stack has set DF?

Kind thanks,

Jeff S Wheeler <jsw@five-elements.com>

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