Re: Network slowing down by masquerade

[Pascal Hambourg <pascal@plouf.fr.eu.org>]

Glen Huang a écrit :
> I have a pptp client connection (ppp1) on a gateway. If I directly
> using curl --interface ppp1 to download a file, I get full download
> speed very quickly (2m/s).

Why do you have to use --interface ?

> But if I route my lan host to ppp1 and -o ppp1 -j MASQUERADE, running
> curl to download the same file on the host starts very slow (less than
> 100k/s), then the speed *slowly* increases(about 50k per second), until
> it reach about 1.8m/s. While downloading the file on the host, the
> gateway's cpu usage never reach 1 from the output of top.
> 
> If I directly establish the pptp client connection on host, I quickly
> get full speed again.
> 
> I wonder what might slow down the network when the packets are
> forwarded. I'm currently guess it's the masquerade target, but I'm not sure.

I don't think MASQUERADE is the culprit. I would suspect first MTU
issues (fragmentation, path MTU discovery).

> How do I test it?

Lower the MTU of the client host LAN interface below ~1460.