[pptp-server] GRE and traceroute - reliable?

[Justin Kreger]

On Thu, 12 Jul 2001, G. Georgiev wrote:

> 	Hello,
> 
> 	My pptp link came to live this morning, but it may be seen that my
> ISP (for whatever reason) did change the routing - the third hop is now
> 10.154.0.35, yesterday was 10.154.0.34. Quite sure I may argue to
> Videotron that .0.34 blocks the GRE traffic, but... I DID THE TRACEROUTE
> WITH -G OPTION - GRE packets yesterday, and they went beyond the router in
> question.

see below, under the two traces.

> 
> 	In reality, if I am not mistaken, tracroute should use ICMP
> packets - full imitation of GRE in traceroute it probably
> impossible? And if that's the case is there any reasonable way to check if
> a router blocks IP protocol 47 (GRE)?
> 
> 	Thanks, George.
> 
> P.S. It may be usefull to have a 'public' pptp server running somewhere
> to which everyone may try to connect - this way we may test if the problem
> is close to our side or is on the other end.
> 

A traceroute can use any type of packet, all it does is send out three
packets, and then increments the time to live on the packets up one,  It
collects the time-exceeded packets, and shows you a map, your route from
one host to another host.  The way you would know a rotuer blocks the
forwarding of GRE, is your route would end there, you would see no more
hops, you would get nothing back, it would be a black hole.

> 
>          ---- Routing from this morning - VPN works ---------
>  1  10.48.160.1 (10.48.160.1)  9.429 ms  8.989 ms  9.662 ms
>  2  modemcable090.241-200-24.mtl.mc.videotron.ca (24.200.241.90)  8.101 ms
>  3  10.154.0.35 (10.154.0.35)  8.106 ms  11.335 ms  7.884 ms
>  4  * ia-cduc-bb02-ge3-0.videotron.net (207.96.163.9)  122.174 ms  123.818
>  5  * if-7-3.core2.Montreal.Teleglobe.net (207.45.204.41)  128.289 ms *
>  ..... (other lines equal with example below)
> 
> 
> 
>         -------- Routing from yesterday - no VPN -----------
>  1  10.48.160.1 (10.48.160.1)  9.067 ms  9.352 ms  11.871 ms
>  2  modemcable090.241-200-24.mtl.mc.videotron.ca (24.200.241.90)  7.826 ms
>  3  10.154.0.34 (10.154.0.34)  9.283 ms  8.997 ms  13.797 ms
>  4  ia-cduc-bb02-ge3-0.videotron.net (207.96.163.9)  7.583 ms  8.673 ms
>  5  if-7-3.core2.Montreal.Teleglobe.net (207.45.204.41)  15.974 ms  45.709
>  

The trace you showed yesterday had traced it up to the the (i
think) alternet gateway, of where your server is most likely at.  Talk to
the netadmin in charge of that router.  

> 
> 
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