[Gllug] Re: hello, and a NAT problem

[lists]

Thanks Richard, that was it. Luckily iptables can clamp it : 

iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS 
 --clamp-mss-to-pmtu 

so I don't have to set the MTU to 1492 on all the clients to match the 8-bit 
short DSL. 

Nice one! 

Julian 

Richard Cohen writes: 

> On Sun, 7 Mar 2004, lists wrote: 
> 
>> Hello, Im Julian, Ive just upgraded to London & Linux. I hope that this is
>> an appropriate place to air my dumb questions...
> 
> Yes, it is.  Welcome :-) 
> 
>> PROBLEM with NAT 
>>
>> This is what I have done to try get this mandrake 9.2 (2.4.22) to share a
>> dsl connection across the wireless lan:
>> # echo "1" > /proc/sys/net/ipv4/ip_forward
>> # iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE 
>>
>> Now, most services work: I can do dns lookups on tiscali's dns from the
>> clients, I can fetch an email from a pop3 server from the clients, I can
>> ping, traceroute etc. But /most/ of the web times out (I can do google
>> searches and open some other near, fast sites that I have definitely not
>> been to before). On some sites, I can open some pages, but not others. And
>> if I a client has more than one pop3 message waiting, the mail client times
>> out. 
>>
>> I don't know how to start with this one -- can anyone point me in the right
>> direction?
> 
> This sounds to me a little like an MTU problem - like you have two networks
> connected back-to-back, a different MTU on each, and when one tries to send
> a packet which is too big for the other, things stop working.  That would
> mean that small web pages - like google, say, and single mails which don't
> generate a large stream of data, all work fine, but larger things don't. 
> 
> I could be talking nonsense, but I have seen this sort of thing before... 
> 
> ifconfig should tell you what the MTU is on the relevent interfaces. 
> 
>> thanks for your time, 
>>
>> Julian
> 
> Cheers
> Richard
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