OHMS BLOG

Sunday, June 17, 2012

technology

How to switch a Cisco DPC3825 / EPC3825 to/from bridge mode at will

DISCLAIMER: This blog entry is unsupported. It outlines the steps that I successfully taken to switch this device into bridge mode. I am posting this as a courtesy to people who would like to know how to do the same. Having said that, my work here is done and my problem has been solved. This technique might not work for all firmwares. It requires knowledge of HTML and HTTP. If this isn't working for you, please do not post comments or send me emails asking me to solve your problem for you.

A few weeks ago I had to help somebody deal with an annoyance: They had a Cisco DPC3825 device that is a combination cable modem + wireless router device. They were unhappy with the device's router functionality and wanted to set it to "bridge mode," i.e. disable everything but the cable modem.

This is easier said than done; this device had its firmware customized such that no user interface was available to change such things. There were no form fields present pertaining to the device's mode. I found some limited information on other websites. Many of these users had devices with firmwares that were different from the one that I was working with. In their cases, the form fields for switching modes were present in the HTML but hidden via CSS.

On the forum website, their solution was to remove the display: none from the field's CSS, making the form field visible. I was disappointed with this solution, as it wouldn't work in my case. On the other hand, I couldn't help but notice that the forum users had posted a few images of their HTML. Indeed, one of the screenshots includes the necessary <select> tag, whose name was "working_mode"! We weren't finished yet, however: I still needed to figure out which value to use for the hidden field. I noticed that in other <select> inputs, the firmware set the field's value to the 0-based index of the option in the select. The screenshot showed that "Router Mode" was the first option, implying a value of "0", followed by "Bridged Only," implying a value of "1". This means that one can, as part of the administration form, just insert an additional field. I'm lazy and I only needed to execute the command once, so I just used Firebug to inject a hidden input into the administration form. Look for the

<form name="administration" method="POST" action="/goform/Administration">

tag and add the following:

<input name="working_mode" type="hidden" value="1">

I submitted the form and was then presented with a screen informing me that the device was rebooting. When it reboots in bridge mode, the device resets its IP address to 192.168.100.1. To switch back to router mode, do the same thing but set the input value to "0".



Of course you might want to script all of this and POST the form directly.

17 comments:

Anonymous said...

Shaw hub in Calgary catches fire taking out all their web/phone support, and you sir come to the rescue!

Anonymous said...

Shaw hub in Calgary catches fire taking out all their web/phone support, and you sir come to the rescue!

Anonymous said...

This info helped me out. Thought I'd say Thank You!

Anonymous said...

Fantastic! Using a Shaw modem in Saskatoon and your workaround worked beautifully. Cheers!

Anonymous said...

Worked great on Shaw in Vancouver! Thank you, was dreading calling in.

TMDP said...

There is another way to accomplish this that I found when I was equally frustrated.

Process:
1) Disconnect coax from modem
2) Perform factory reset by holding reset button for 30 seconds. While continuing to hold reset button, disconnect power, hold reset button for 30 seconds. While continuing to hold reset button, reconnect power, hold reset button for 30 seconds.
3) Do not plug coax in. Plug ethernet in from modem to computer.
4) Browse to router IP address. 192.168.0.1
5) You now have root access to the router until you connect coax.

How It Works:
These things are configured by DHCP option 66. This allows a config packet do be delivered when the server at your ISP is asked for an IP address to be issued. I have found that the packet activates an individual setting only and does not deactivate, this allows your settings to stay on the device, yet be rendered unchangeable when the config packet to lock out elements of the UI is delivered.

Anonymous said...

Tried it a few days ago and got 403 FORBIDDEN. I have been able to successfully enable bridge mode once before. Any one tried recently?

Anonymous said...

I've been using this solution for a year now and have been happy. Now I am trying to add some DVR Cameras through port forwarding. I have a netgear N600 that has all the port forwarding assignments. I suspect that the Cisco router is preventing the Netgear router from assigning these ports. The admin page for the reset IP address does not show the option pages I need to allow the control of forwarding. Additionally, when I check routers on the network, even though the address was reset to 192.168.100.1 it still reports as 192.168.2.1. Any advice here?

I am curious about the post by TMDP. Has anyone tried this? What success or issues?

Anonymous said...

Using a DPC3825 on Shaw in Vancouver, BC -- none of the above work.

The form manipulation returns 403 FORBIDDEN (as reported above).

The modem reset reported by TMDP did nothing for me (reset the modem to factory defaults but still no ability to switch to bridge mode).

Anonymous said...

This worked for me! Thanks for posting :)

Anonymous said...

If this process is too confusing for anyone I wrote this bookmarklet that will get the job done. Create a new bookmark and use this code as the URL:

javascript:(function(){s=document.createElement('input');s.name='working_mode';s.value='1';document.forms[0].appendChild(s);})();

Log into the modem and navigate to the page described here, click the bookmark.

Result: A white input box will appear with the number 1 inside.

Click save and the modem will reboot into "bridged" mode.

Anonymous said...

This process no longer works. I now get a 403 Forbidden error from the DPC3825's web interface -- where it used to work before. I'm guessing that there was either a firmware upgrade or that the ISP pushed a new config file that prevents this.

Anonymous said...

It worked for me on the 3827 I am curious if its normal for the admin management page to be completely blank now. maybe because it should technically only show working mode then?

Anonymous said...

IS anyone aware on how to enable the bridge mode?. I just bought a TP-LINK high end wifi router and I would like to use use instead of this cisco one the cable provider gave me.

Juan Martin Della Vecchia said...

Any update!?

Anonymous said...

any news on this topic?. I've heard to have this access you need a "advanced admin password" or something like that

Anonymous said...

For Rogers, you can just log in as administrator, and use the form window to switch to gateway mode, or to bridge mode.

As noted in the original post, you need to use 192.168.0.1 when in gateway mode, and 192.168.100.0 when in bridge mode.

Hallelujah!

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