[pptp-server] how to detect GRE filtering?
Neale Banks
neale at lowendale.com.au
Tue May 29 23:23:11 CDT 2001
On Wed, 30 May 2001, kukulkan wrote:
> Hi there again,
> I am trying to check that whether my internet connection(our network and
> ISP) is implementing some form of filtering. Help me to make a decision. I have
> downloaded the patch for GRE traceroute , compile and run successfully. Again,
> please, please correct me if I am wrong.
>
> I also have tried to use the traceroute with -G option:
>
> test # 1 - traceroute -G 192.168.216.98
>
> and it gave me several error just before the pptp server, the router in front
> of my PPTP server(192.168.216.98)
>
> test # 2 - traceroute -G -p 1723 192.168.216.98
>
> and both gave me the same error.
>
> 1 e0.kch8.xxx.my (xxx.142.221.9) 116.096 ms 117.333 ms 120.209 ms
> 2 fe0-0-0.kch.xxx.my (xxx.142.221.1) 119.418 ms 129.945 ms 119.755 ms
> 3 fe0.kch15.xxx.my (xxx.142.221.16) 119.861 ms fe1.kch15.xxx.my
> (xxx.142.0.74) 119.767 ms fe0.kch15.xxx.my (xxx.142.221.16) 119.798 ms
> 4 xxx.142.33.210 (xxx.142.33.210) 339.815 ms !X 321.584 ms !X *
Acording to "man traceroute" on my Debian system, that "X" means
"communication administratively prohibited".
I suspect that you've just identified the point that's blocking GRE ;-)
> My question is,
>
> i) is there any difference with the traceroute with -G(use GRE packets instead
> of UDP packets) test # 1.
> ii)or the -G with port specification 1723?
>
> in other words, is it *neccessary* for me to include the port specification -
> port 1723 to check whether my GRE is being filtered?
No, GRE does not use port numbers.
PPTP uses two separate channels:
- a contol/setup channel over TCP which uses TCP port 1723.
- a data channel for the PPP which uses GRE (no port# applicable).
HTH,
Neale.
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