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I'm using a dynamic DNS service provider that gives me a URL to access, and based on the IP address accessing that URL my dynamic hostname gets updated with that IP address. This is being run from a NAS device I have that is plugged in directly to the wireless router (with an Ethernet cable).

My question is whether someone who is on the network (or even not on the network) using tools like Wireshark or other methods find that URL I'm accessing? It's a plain HTTP URL not HTTPS, if that makes any difference. Because if they can, they can access it from another device and change my hostname to have an incorrect IP address. (I'm not asking so much because I'm worried about the consequences of someone doing this, I'm asking to help increase my understanding of how these things actually work.)

Does it make a difference whether they are connected to the same wireless router directly (with an Ethernet cable), or whether they're accessing its WiFi? And does it make a difference if that WiFi has a password or doesn't?

Btw, how about the case where the NAS isn't plugged in directly to the device, but is plugged in to my laptop using a crossover Ethernet cable and is accessing my laptop's internet connection to access that dynamic DNS URL, who would be able to see that URL?

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Yes and no.

If you are using a wired network and your network still uses hubs: Yes.

If you are using a wired network and your network uses switches: No.

The difference is that a hub is a layer-1 device. It will receive data on on port and echo/repeat it on all other ports. People on the other ports can hear anything you sent.

A Switch on the other hand only does this once. For future communications it remembers which port was used by which MAC and only sends the data to that port. Your data would be safe, unless it is a managed switch and someone has the username/password of the switch. (Read: Your network administrator can configure port mirroring and see all your data).

If you are using wireless then you do broadcast all your information via the air. Anyone can hear what you say. If you do not use any encryption then they literally can pluck your data out of the air. If you use WEP/WPA/WPA2 then the data is encrypted. However one might assume that others have the password for the same access point, in which case its gets interesting.

However neither of these should be a problem since your NAS is (should) be protected by a password and a secure way to enter it. If it is not an your are using security by obscurity then you are just as safe as someone who locks the door and hides the key under the door mat.

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As a general rule: if you're not accessing a url over SSL, it's always best to assume that someone (network peer, network admin, ISP) is watching and can see what URL you accessed, along with any data you sent to it, and any data it sends back. – Darth Android Jun 17 '13 at 21:30
Correct. And even with SSL or HTTPS things might run into a corporate MITM attack. So do not even try it from work or with a computer which you do not trust (e.g. no from an internet cafe). – Hennes Jun 17 '13 at 21:38

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