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 From:  Tiresias <tiresias at gmail dot com>
 To:  m0n0wall at lists dot m0n0 dot ch
 Subject:  [m0n0wall] VPN only works for small files / packets
 Date:  Fri, 17 Dec 2004 16:15:09 -0500
Hello,
I have established my IPSec VPN between two m0n0walls (both v1.11) and
I have a machine on the LAN on each end. I can ping between the
machines but when I attempted to transfer data (ftp/http/etc) it
failed. I have been using ftp for testing to narrow down the problem,
and the ftp client seems to hang while waiting to receive the data.
Sometimes I can get the first part of a file right away (~1KB), then
no more; other times I got nothing at all. I end up having to abort
the ftp transaction. I have been able to narrow this down to a
specific file size. I can ftp a file of 2179 bytes over the VPN and
the transfer completes instantly, but 2180 bytes fails every time.

I also tried pinging the box on the other end and established that I
can ping with up to 1410 bytes; any more than that gets no reply.
After this test I tried changing  the MTU settings of the WAN
interface on both m0n0walls. I tried setting them both to
1410,1400,1200 and 900. Each time I changed the MTU of the WAN
interface, I retried my ftp transfer as above. Changing the MTU of 
the WAN interface seemed to have NO effect on the size of the file
that I could transfer. 2179b or less works; 2180b or more fails to
transfer.

Is there any other MTU setting that can/should be changed? Any other
config option that I need to look at? Would it help to look at my
configs? Any help/advice I can get would be most appreciated.