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Remote Access - can ping between but not outside.

Posted on 1998-10-29
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Last Modified: 2013-12-23
PPP connection over TCP/IP established over modem between home (Windows95) and work (Digital AlphaStation 200 4/100 running Digital Unix 3.2G).

I can ping/telnet from home to my work PC and vice-versa.

I CANNOT ping/telnet from home through my office to another workstation on the same subnet...so I also cannot ping the gateway router to to rest of the network...from home.

I CANNOT ping/telnet from any other workstation on the same subnet at work through my workstaion in my office in which my modem is connected to home.

I am always using the IP address.

I have researched this problem for many months and consulted the MAN pages as well as networking/TCP/IP books.

I need my workstation in my office to recognize that it needs to listed for my home PC's IP address in addition to its own and pass that information on to my home PC and vice-versa.

I though I could do it with ARP...but I am unable to determine the MAC address for my home PC since I am not using a network card...but am using Dialup Networking.

I have read that perhaps I need to add an additional IP address for the PPP to the host table in addition to my home PC IP address and somehow use that additional address to "bind" the two together ...in some sort of routing table or something like that.

Any time I need to get a file or something, I have to log onto the office PC from home first and then ftp from there.  Then I can ftp from there to home or vice-versa.

Thanks in advance for your help,

Greg Green
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Question by:Greg99
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mart010897 earned 100 total points
ID: 1582536
You're correct that this is a routing issue.  Your UNIX workstation needs to have the ability to forward packets from your home machine out over your office network.  Unfortunately, this means (for conventional IP routing) that you need not just an IP address for your home PC when it dials in, but you need an entire IP *network*.

Read your tcp/ip book for the definition of an IP network.  If you have a spare Class C IP network, you can subnet it further using a 30-bit subnet mask (instead of the standard class c 24 bit subnet mask) to give you an IP network with only 2 hosts, one for your UNIX machine's PPP address and one for your home PC.  Then you will be able to route traffic between the two IP networks that are defined on your work machine.  If you are interested in this, read your tcp/ip book for a description os "variable-length subnetting".

I am not going in to a great amount of detail above because it's tricky stuff and not too many people have spare class c's lying around.  But you do have an alternative.  Try to see if your UNIX can support "IP Masquerading" (your TCP/IP book probably talks about this too).  IP Masq. will allow you to configure your PPP connection to reside on an imaginary IP network (10.0.0.0/255.0.0.0 or "reserved class 10" is the most common).  Then your UNIX host PPP interface can be 10.0.0.1, your home machine can dial in on 10.0.0.2, and your routing software will change the IP address of all packets going to and from your home machine so that they are legal addresses for your network.  In fact what it does is change your home machine's address to its own, then when it gets the response, it changes it back to the imaginary address.

IP Masquerading is more often a viable approach because there's no shortage of addresses, and you don't have to go through your network admin.  Also you don't have to spend time mucking around with non-byte-aligned subnet masks.

So... read up on IP Masquerading.  Most likely your OS will support it.  The commands you'll probably end up using are "ipfw" and/or "ipfwadm".

Let me know how it works out...

Mart

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