Re: [Fwbuilder-discussion] Clamp MSS to MTU

[Nicole H. <nic...@gm...>]

Hi,

how can I add this rule in fwbuilder?
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS 
--clamp-mss-to-pmtu
Do I have to make a custom service?

Thanks!

Nicole


Nicolás López schrieb:
>
> Hello Vadim
>
> I think I’ve found a little mistake in the order rules are placed when 
> using the options “Accept ESTABLISHED and RELATED packets before the 
> first rule” _/and/_ “Clamp MSS to MTU”.
>
> When using those options, the following rules are placed in this order:
>
> $IPTABLES -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>
> $IPTABLES -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
>
> $IPTABLES -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
>
> $IPTABLES -A FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS 
> --clamp-mss-to-pmtu
>
> In 99% of the cases this rules order will not be of any trouble but, 
> for packets going through the _/forward/_ chain (ie. nated and/or 
> routed) which has a _/RELATED/_ connection (ie. an ftp file transfer 
> session) the 4th^ rule will never apply so the tcp SYN or SYN/ACK 
> packet will go out with the wrong MSS value causing the connection to 
> freeze.
>
> To resolve this there are a couple of workarounds or corrections to be 
> made:
>
> 1 – When the option “Clamp MSS to MTU” is enabled, then place the 4th 
> rule _/always/_ before the 3th one.
>
> 2 – If you don’t want to change the order rules are placed, then you 
> can write it “$IPTABLES –t mangle -A POSTROUTING -p tcp -m tcp 
> --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu”
>
> Hope I’ve been clear enough.
>
> Regards,
>
> Nicolas.
>
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