weblog d’un abbe

20060722

Day with a more secure modem or a buggy modem or some advanced bridging

Filed under: Experiences — abbe @ 2141

Today, I’ve visited Dad’s Surajmal Vihar office. They recently got an internet connection from Airtel with a static IPv4 address. So, I’ve to just check that internet connection for their Linux server. It was installed on a box with a proprietary OS. I figured out that it is installed in bridge mode, as the computer has a non-private IPv4 address manually set. The Airtel Broadband has provided them with a Zyxel Prestige 600 series modem.

I just plugged out the internet cable (actually a CAT5 cable with a RJ45 connector at both ends) from the box with proprietary OS and inserted in the eth1 of the Linux server, and configured eth1 with the static IPv4 address and appropriate gateway , but to my surprise I’m unable to ping the gateway :-(. Then I rechecked settings, and found them correct, and re-initialized eth1. But still I’m unable to ping the gateway. I re-inserted cable into previous box ( with proprietary OS), and there I’m able to ping the gateway. Well I suspected that modem gets bound to a MAC address, as Gautam thrice told me that in his university, they can’t connect any other network equipment into their university’s network, as MAC addresses of all network devices have to be registered, as each port in the network switch is specifically bound to that MAC address. Anyways, I’m unable to figure out how to change settings in my modem, i.e. I don’t know what is my modem’s IPv4 address. So I just called the Airtel guy, who has installed the connection. He told me that modem is accessible at 192.168.1.1, and he is also surprised why modem is not responding when plugged into a Linux box. So, I replugged my internet cable into my Linux box. And modified eth1 to be in 192.168.1.0/24 network. Then I pinged my modem. :-). And then I telnetted to the modem. And I found the modem interface quite complex as I’m a n00b in networking. So I simply exited from the modem interface without making any changes. And I restored eth1 to the static IPv4 address and appropriate gateway. To my surprise it now worked :)

After some more disconnecting and reconnecting, I finally figured out that this modem gets binded to a MAC address and hence routes packet only for that source MAC address. And to change that binding, one has to just telnet (or probably just ping) the modem (i.e. 192.168.1.1 IPv4 address ) and modem will then record the new MAC address.

This is what I observed today. I don’t know if they did this for the security purpose, or it is just a bug. Chances are more that it is due to security purpose. Or is that some special bridging technique ? I’m confused :-S

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