[ntp:questions] NTP over redundant peer links, undetected loops

Richard B. Gilbert rgilbert88 at comcast.net
Mon Feb 16 22:34:07 UTC 2009

Dave Hart wrote:
> On Feb 16, 8:57 pm, "Richard B. Gilbert" <rgilber... at comcast.net>
> wrote:
>> Dave Hart wrote:
>>> On the nonroutability of RFC1918 addresses, have you ever seen someone
>>> try to VPN back to their home network from a hotel network and fail
>>> miserably because the hotel network and the home network have
>>> conflicting ideas of how to route RFC1918 addresses?  
>> RFC-1918 address are non-routeable by definition!  They are intended for
>> PRIVATE networks where the nodes need not be accessible from outside.
>> Generally, any link with the outside must be initiated from inside.
>> (Don't call us, we'll call you!)
> Give me a half-ounce of credit, please.  Yes, they are not routable on
> the global internet.  To suggest they are therefore never used by
> machines with internet routes or never seen except in disconnected
> islands is a vision from the past in need of a serious reality check.
>> You can reach your home system from your hotel by addressing the
>> external address of your router and selecting a port that the router
>> will map to something you can talk to and can talk to you.  This
>> requires some setup on your router before you leave home!
> The problem is on the client side of the VPN.  I am in hotel NAT
> cesspool 192.168.1.x, say gateway  Now I
> want to connect up a an IPSEC or PPTP tunnel to my home network, sure
> I target the single public IP on my router for the VPN connection, but
> when it comes up my local IP stack has a problem.  You see, my network
> at home is also in the ever-popular 192.168.1.x subnet.  Every time I
> try to send a packet to my desktop machine at, my IP
> stack tries to deliver it to some other hotel NAT cesspool customer,
> and the packet never makes it to the VPN.  There are a million
> variations possible.  Build a B2B link between two companies whose
> network architects didn't plan in advance for that scenario.

I think your problem is that you are trying to use an RFC-1918 address 
to access your desk top.  You must address the packet to your home 
router's external IP address.  Your home router must be configured to 
map some port on the router to the RFC-1918 address of your desktop. 
Pick an arbitrary port number greater than 1024 and less than 65536. 
Configure your router to send all traffic addressed to that port to the 
RFC-1918 address of your desktop.  Let's assume that your router has 
been configured to send incoming traffic addressed to port 2009 to your 
desktop.  When you connect from outside, you simply send to port 2009 at 
your external IP address and your router should hand it to your desktop.

If the manufacturer of your router offers technical support you may be 
able to call an 800 number and speak with a technician who can walk you 
through the process.  Be prepared to wait 30 to 60 minutes to talk to a 
human being who will probably speak accented English.  If you are 
extremely lucky, he will speak with a British accent.  Those who learned 
English in India are more difficult to understand.

You may have better luck at the vendor's web site if they have one. 
Linksys does.  Linksys provides a great deal of information on their web 
site.  I have a Linksys router and so have never had occasion to check 
out the competition.

More information about the questions mailing list