Re: NATed computer can't access certain websites

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On Wed, 2006-07-05 at 14:17 +0200, Roberto Ragusa wrote:
Chris Linton-Ford wrote:
> Hi,
> 
> I've had a recurring problem on our office network where some computers
> suddenly stop being able to access certain websites. We're currently
> using a mixture of FC3-5 behind an OpenBSD gateway, which performs
> firewalling and NAT; however we had the same problem when using an FC5
> gateway.
> 
> The computer in question at the moment is running FC4, has no problems
> resolving the IP address of the website, and its routing tables are
> fine. It doesn't have a firewall, and can access all other websites no
> problems, as far as we can tell. Doing a tcpdump shows that the http
> packets are going out and ack packets are coming back, but no http data
> comes back from the website.
> 
> I've tried accessing the website using Firefox, elinks and using telnet
> to do the basic GET /index.htm etc. They all hang after the sending the
> request.

Can you sniff the packets on the external interface of the gateway to
understand if something is blocked by the firewall on the gateway?

Some things I'd check:

1) "ifconfig eth0 mtu 1000" on the FC4 box to use smaller packets

2) "cat /proc/sys/net/ipv4/tcp_window_scaling"; if it is enabled,
try disabling it

3) "cat /proc/sys/net/ipv4/tcp_ecn" to see if ECN is enabled (better
if disabled, but it should be 0 by default)

Best regards.

-- 
   Roberto Ragusa    mail at robertoragusa.it

Hi Roberto,

Changing the MTU size didn't help, but disabling tcp_window_scaling did the trick. Having looked around various groups, it sounds like this is a problem with the 2.6.17-1.2139 kernel; is it a better idea for me to permanently disable window scaling on our machines or wait until a fix comes out for the kernel? Is window scaling a Good Thing?

Thanks for your help,

Chris


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