ColdFire/68K Microcontrollers and ProcessorsのトピックHow to access a ethernet device behind an ADSL modem

How to access a ethernet device behind an ADSL modem

Ricardo_RauppV — Sat, 25 Oct 2008 22:30:58 GMT

Hi Folks

I developped a board with ethernet and some relays to home automation.

Till now, I know that the only one way to connect with this board is:

- know the modem IP

- set a Port Forwarding (NAT?) feature in the modem web browser.

Well, ok...

But, imagine the modem has been replaced by a new one....the NAT settings disappeared !!therefore the board is now unreachable!

I read a protocol (?) / feature(?) called uPnP : Universal Plug and Play and I understood the

own device connected to a switch could ask for Port Fowarfing.

It is a very important feature for people who wants to connect devices wich are behind switchers / modensa etc..

Has anyone done it?

Will it be what MSN and others do to be able to receive external connections?

Ricardo Raupp

Re: How to access a ethernet device behind an ADSL modem

RichTestardi — Sun, 26 Oct 2008 02:27:26 GMT

You might also consider a form of http tunneling, where your board behind the firewall just initiates an *outbound* http connection on port 80 (assuming that is permitted by the firewall) to connect to a public Internet server which in turn behaves as a proxy/tunnel to allow it to connect to (or accept connections from) any other system on the Internet, on any port.

http://en.wikipedia.org/wiki/HTTP_tunnel

A number of commercial and open source http tunnels exist:

http://www.google.com/search?q=http+tunnel

Re: How to access a ethernet device behind an ADSL modem

JimDon — Sun, 26 Oct 2008 02:50:59 GMT

Well, upnp is considered a security risk. ddwrt has it disabled by default. Don't know about other routers.

It's one thing on your internal network, but allowing a programs or devices to open ports thru the router without permission does not sound like something I would want or recommend.

On the other hand, I can't image outbound port 80 connections being blocked.

Re: How to access a ethernet device behind an ADSL modem

Ricardo_RauppV — Tue, 28 Oct 2008 20:24:21 GMT

Hi guys

Thanks for your help ...

A friend of mine had success in implement uPNP command in Java.

I will try it in my board also..However I´m sitll evaluiating this situation.

I received a interesting help from a guy from another forum ...

Follow below ...

...

uPnP is certainly a way for devices to ask a router to enable port
forwarding. However, not every NAT router supports it, and many have it
disabled (either by default, or intentionally by the user). It is an
insanely insecure concept which allows any software or hardware to
bypass your firewall. It's a typical MS idea - in order to make it as
easy as possible for legitimate software or hardware to open incoming
connections (a laudable aim in itself), you allow *any* software or
hardware to open incoming connections.

Even if you use uPnP to open connections (or ask users to manually
configure a port forward), you also have to consider how you are going
to find the modem's IP - try a dynamic dns service.

An alternative is for the board to initiate contact - it could regularly
contact a server somewhere looking for new orders. That way you have no
issues regarding connectivity or addressing. Remember also that there
is no need for this communication to follow web-style request then
immediate answer - your board could open a connection and leave it open.
The server can then send commands as and when it wants to (such as in
response to the user connecting to a web interface on the server). The
only two disadvantages here are that open connections require some
resources on your server, which could be an issue if you have many
thousands of boards installed (consider regular polling, and only hold
the connection open if it is needed), and that the user's Internet
connection must be open all the time (not a problem for most people).
...