Re: clearing dont-fragment bit

[Ramin Dousti <ramin@cannon.eng.us.uu.net>]

On Thu, Oct 09, 2003 at 04:08:19PM +0200, Abraham van der Merwe wrote:

> Hi Maciej                                        >@2003.10.09_16:03:26_+0200
> 
> > > Are there any iptables extensions out there that allow you to clear the DF
> > > (Dont Fragment) bit in ip headers?
> > AFAIK no. Why would you want to do that?
> > I think I might write a module that would do that.
> 
> I need it for tunnels. In a perfect world that wouldn't be necessary at all,
> but reality is that there's many brain dead admins that filter icmp, so if
> you build a tunnel over the big bad internet, you're screwed.
> 
> You can use the TCPMSS target which solves it for tcp, but you still have
> the same problem with udp packets, so imho the only way to solve this
> properly is to fragment packets even if DF=1.

The applications that set the DF bit, do so for a reason not just for the
fun. Sometimes (well, actually most of the time) it's for the performance
reasons in which case turning it off and having a poor performance is
preferable than it not working at all. On the other hand, the DF bit would be
set by the application probes to figure the PMTU. Setting that off on the
firewall would harm the purpose.

Can you come up with a list of the non-TCP-based application protocols that
would use the PMTU (DF bit)?

Ramin

> 
> 
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> 
> Regards
>  Abraham
> 
> Ask not for whom the telephone bell tolls...
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