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At work we're a bunch of .NET developers who don't know much about server setups. We've run into a problem that we 'believe' might be an iis6 issue. The problem is consistently reproduceable from some client machines and consistently not reproduceable on others.

But the reason for I think it might have something to do with the server is that I have nailed down consistent reproduction to the following steps from a linux console (i.e. no browsers involved). - wget

Always the same result. The download starts and hangs at some different point each time. The file is about 1.5MiB. The problem reproduces regardless of the MIME type related to swf files (i.e octet stream or x-shockwave)

So my question is in 2 parts: - Where on the server can I look for what information to prove whether its a server issue or not - Is this any sort of known issue fixed by an update or service pack that we may not have.


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up vote 1 down vote accepted

May be a problem with the TCP Clamp MSS to PMTU feature of Netfilter and is usually required when your internet connection is through PPPoE or PPTP.

If you are reaching the file through such a kind of connectiopn try to add the following iptables rules:

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -m tcpmss
       --mss 1400:1536 -j TCPMSS --clamp-mss-to-pmtu
       -o ppp0

Of course you can try debugging in various ways:

  • try to see if a gif or jpeg or doc file of about 1.5 Mb is correctly downloaded
  • ask someone you know in a different geographic place to download the swf too, and see if he has the same problem
  • try with a smaller swf, then a bigger one. It really hangs, or slows down its speed to a very poor level?
share|improve this answer
No, I don't think so. I didn't know what that "TCP Clamp..." bit you mentioned was, but googling seems to show its related to Apache, or at least *nix. And there's no VPN tunnelling going on in our scenario where we can recreate the problem – David Archer Oct 13 '09 at 15:06
never mentioned a VPN :) I can't argue what Apache matters, you have tagged your question iis6. MSS clamp has nothing to do with apache, it is a network related issue. – drAlberT Oct 13 '09 at 16:36
ok cool, thanks. This however is over my head. As a test we set up the same site on an iis7 server and now there is no problem. So because we have no clue... we'll just... very quietly... ignore it. But thanks for trying – David Archer Oct 14 '09 at 9:21
IIS idiosyncrasies :) nope, you are welcome. – drAlberT Oct 14 '09 at 11:21
Accepted because its the best answer I got! :-) – David Archer Jan 29 '10 at 10:43

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