Screen Sharing over VPN inconsistency

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Level 1

5 points

Screen Sharing over VPN inconsistency

Hi,

In short: I can connect to one Mac at work via Screen Sharing over VPN from my home network, but can only connect to another Mac on the same work network when using 3G.

At work:

A Mac Mini with Snow Leopard Server, behind a NAT (static IP), VPN on, Screen Sharing on, and DNS seemingly setup properly.

Also, a client Mac Mini 10.6.8 on the same LAN behind the NAT, a LAN static IP, DNS entry on the Server, and Screen Sharing is on.

At home:

A mac mini with 10.6.8 that can connect to the work Mac Mini Server over VPN (L2TP) over Comcast broadband, and subsequently control it via Screen Sharing.

An iPhone 4 with iOS 5 that can connect to the work Mac Mini Server over VPN (same L2TP account) using home Comcast wifi, and subsequently control via Remoter VNC app (port 5900).

What I cannot do is connect to the non-Server Mac Mini located at work via Screen Sharing from my home network. There is no authentication attempt from either my home Mac Mini, or my home iPhone over wifi. However, I can connect to that same work non-Server Mac Mini with my iPhone when -- and only when -- using 3G over VPN.

Why can I connect to my Server computer at work from home using the Mac Mini or iPhone over broadband and 3G, but the non-Server Mac on the same work network is only accessible via 3G?

Please help. It doesn't make any sense to me. 😕

Mac OS X (10.6.8)

Posted on Dec 29, 2011 4:06 AM

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Answer 1

Best reply

MrHoffman

Community+ 2024

Level 10

116,103 points

Dec 29, 2011 6:40 AM in response to smallIT

You can't use a VPN with the same IP subnet present on both ends of the connection. IP routing doesn't appreciate that, as it can't determine the adjacencies; what packets are routed directly, versus what packets are routed over the VPN, versus what packets are sent to the designated router.

This is also why you'll see me posting the "get out of 192.168.0.0/24 and 192.168.0.0/24 subnet" message with some regularity, too. (Those two subnets are commonly used, thus mess up IP routing, thus moving to subnets in the 10.0.0.0/8 or 172.16.0.0/16 can avoid derailing the routing.)

Are your two networks using the same subnet? (Your success with 3G implies this is the case.)

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Answer 2

smallIT Author

Level 1

5 points

Dec 29, 2011 12:31 PM in response to MrHoffman

Hi,

Are your two networks using the same subnet? (Your success with 3G implies this is the case.)

Yes. 😊

Ugh. Thank you for the quick reply.

The amusing thing is that I used a Hoffman Labs article as a guide when I originally set up the 10.6 Server over a year ago at work. I apparently used similar principles when I later revamped my home network, because I used the same uncommon subnet in both networks.

A rookie mistake, made by a rookie.

I thank you again. You were very helpful.

Cheers!

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