[pptp-server] Poptop and port 47

Cowles, Steve Steve.Cowles at gte.net
Fri Sep 1 09:10:35 CDT 2000

> -----Original Message-----
> From: S.Ecker [mailto:emmet___ at yahoo.com]
> Sent: Friday, September 01, 2000 2:17 AM
> To: pptp-server at lists.schulte.org
> Subject: RE: [pptp-server] Poptop and port 47
> If you look a little closer it says 'proto 47', not
> port 47.  If you want to know what port 47 is, check
> out http://www.normos.org/en/lists/iana/port-numbers-0.html,
> but to save you the trip it's NI-FTP.  If you have a
> flowpoint router for instance you need to issue the
> following command:

Hmmm... Since you replied to my post, I'm curious - Why you are asking me to
take a closer look? I may not be an expert on PPTP, but I do understand the
difference between ports/protos and (i believe) also correctly stated that
in my reply to this list. Please correct me if I'm wrong.

FWIW: From www.cisco.com (Terms and Acronyms)
GRE: generic routing encapsulation. Tunneling protocol developed by Cisco
that can encapsulate a wide variety of protocol packet types inside IP
tunnels, creating a virtual point-to-point link to Cisco routers at remote
points over an IP internetwork. By connecting multiprotocol subnetworks in a
single-protocol backbone environment, IP tunneling using GRE allows network
expansion across a single-protocol backbone environment.

Whether or not you choose to believe Cisco's term (definition) for GRE, all
of us on this list are obviously not creating a tunnel into a cisco router.
Given my limited understanding of PPTP, I believe PPTP (the tunnel part)
along with PPP are being ecapsulated and de-enacapsulated using GRE. I base
my belief on viewing some of the source code from PPTP client.

/* pptp_gre.c  -- encapsulate PPP in PPTP-GRE.
 *                Handle the IP Protocol 47 portion of PPTP.
And some of the functions defined within...

int decaps_hdlc(...)
int encaps_hdlc(...)
int decaps_gre (...)
int encaps_gre (...)

Getting back to the original intent of my post, both PROTO 47 and TCP PORT
1723 must be ACCEPTed on the external interface of your firewall in order to
establish a PPTP/PPP tunnel. Based on your particular network architecture,
some additional configuration might need to be done. i.e. ipmasqadm and
ipfwd along with the possibility of adding routes to your route tables.

Steve Cowles

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