Question for Jeff Liebermann

Hi Jeff,

I found an old post of yours where you said "DD-WRT and OpenWRT both have a bridging mode. However, it's not exactly transparent bridging and may cause problems if you expect transparent bridging."

I've got an old Win98 computer with PCI 2.1 slots, and I wanted to give it wireless access using WPA encryption. Seems like most/all wireless adapters today don't support WPA on Win98 and also require PCI 2.2 slots. So I was thinking of getting an Asus WL-520gU router and using DD-WRT to set it up as a wireless bridge for the Win98 machine to talk to my Linksys WRT54GL wireless access point. I was wondering what you meant by "transparent bridging". Would my scenario require transparent bridging? How would using the DD-WRT bridging be different than if I had a wireless PCI card in the computer? What could or couldn't I do?

Thanks for any help.

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Not high enough.


Read the first 3 messages. The thread rapidly degenerates into a muddle. Basically, it says that DD-WRT does *NOT* return a unique MAC address and IP address pair, as transparent bridging would require. Subsequent messages indicate that with later versions of DD-WRT, one does get a unique pair. I don't know what is the current status and I'm too busy|lazy|bored to test it myself.

More on bridging (all flavors):

Client mode:

Bridge mode:

(Not specific to client bridge mode).

Note: You can have more than one IP address for a given MAC address but that's not transparent bridging. Transparent means that you can't tell that there's an intermediate device on the LAN.

Replace computer and operating system. No need to thank me.


Yep. No problem with only one machine connected to the WL-520GU. It's when you stuff an ethernet switch behind the WL-520GU and connect multiple machines that the fun starts. It can be done, with a workgroup bridge, transparent bridge, or client bridge).

See previous explanation.

No. One machine behind a wireless bridge requires exactly one IP address and MAC address pair.

I don't understand what you're asking. Functionally, with one computah, they're identical.

I'm not sure exactly what will fail if you build a network behind a client mode radio (the one with multiple IP's on a single MAC). If I think of something, I'll post it.

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Jeff Liebermann

So the only difference would be that machines in front of the bridge would think that the Win98 machine has a MAC address equivalent to the bridge's MAC address? And there would be no way that they could tell there's a bridge? I did read that you could set up the bridge to clone the MAC of the machine behind it, so that the Win98 machine wouldn't even have its MAC address changed.

I was thinking of attaching 2 computers to the WL-520gU, although now that I know there could be issues with that, I probably won't do it. Do you have any recommendations for routers that support transparent bridging? Thanks.

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That sounds correct, but I'm not 100.0% sure as I haven't bothered to try it. The usual failure is DHCP. If you place more than one computah behind a client mode radio, only one machine gets a DHCP assigned IP address. The other's just sit there and do nothing until the initial machine releases it's IP address.

It's not that difficult to just try it. I normally would try DD-WRT but don't have the time. You might ask the same question in the DD-WRT forums:

Sure. See:

The one's labeled "multi" can handle multiple IP's behind the wireless ethernet bridge. There's an issue with one version of the DWL-2100AP firmware so avoid that one.

I've been using the Buffalo WLI-TX4-G54HP for wireless ethernet bridging. The problem is finding one. Buffalo lost the suit by CSIRO, who claims to own the patents to wi-fi, and is not allowed to sell in the USA.

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Jeff Liebermann

I got the WL-520gU, and I flashed it with DD-WRT and set it up as a client bridge. It talks to my WAP, which sits behind a router. The bridge seems to be working, as the computer attached to it can get on the internet, but there seems to be one problem I noticed. While testing the bridge, I would plug the computer directly into the bridge, and then plug it directly into the router, and back and forth. I noticed that whenever I plugged the computer into the bridge, the computer could not get on the internet, and it couldn't ping the router either. I would have to power-cycle the router before the computer could get on the internet. Any idea what that's about?

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Nothing wrong. You computah is retaining the MAC address of the router in the ARP table. Run: arp -a to dump the current MAC -> IP address values. When you move the connection to wireless, the IP address (of the router) doesn't change, but the MAC address does. Same problem at the router, which previously had the MAC address of your ethernet card for your PC, but now has the MAC address of the wireless card for the same IP address.

Most PC's are fairly smart about recognizing the change, but it's possible to configure your unspecified operating system to fail to recognize the change. Power cycling the router is one way to clear the ARP table. You can run: arp -d ip_address_of_the_router or possibly: ipconfig /release ipconfig /renew to kick start the boxes.

Look for a check box on the operating systems ethernet setup with something like "detect media changes". It should be checked.

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Jeff Liebermann

When you juggle connections, it's also possible you're running into a "route metric" (priority) problem:

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Jeff Liebermann

So then shouldn't my computer have the same problem when I connect it directly to the router, after it has been working behind the wireless bridge? But it has no problem moving from bridge to router.

Here is what I have seen tonight:

With my computer plugged into the router and able to access the internet, doing an "arp -a" on the computer shows with the router's MAC, and the router's web site shows my computer's IP with the ethernet card MAC.

Then after plugging the computer into the wireless bridge, I can't ping the router. Trying a "arp -d" doesn't help the computer ping the router. I have to do "ipconfig /release" and "ipconfig /renew" and then my computer is able to ping the router and go on the internet. But then "arp -a" still shows with the router's MAC, and the router's web site still shows my computer's IP with the ethernet card MAC. I don't see the MAC of the wireless bridge anywhere. Does that sound right? (Note that I am not doing any MAC cloning on the bridge.)

I am running Windows 2000, and I don't see any sort of "detect media changes" option anywhere. I also looked at that route metric problem link you provided, and it seems to be a new feature in XP, so that wouldn't affect my computer.

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Yes, that would seem logical. However, I note from your original posting that you're using Windoze 98. I have no clue how it would be expected to respond. I'm going to do myself a big favor and not excavate an old W98SE laptop and see. Incidentally, if it's Win98SE (second edition), you have a chance of getting things working. If it's the original Win98, or Win98SE without any updates, give up while you're still sane.

That's because the web page(s) are in the web browsers cache. Can you change pages in the web browser? (especially one's that you have seen before)?

You're right (I'm wrong). The computer should show the router MAC address in arp -a, not that of the wireless bridge. If you ping the IP address of the Asus WL-520gU running DD-WRT in client mode, you should see both the WRT54GL router and the client adapter with arp -a.

At this point, I'm not sure where the problem is hiding. Something is having a problem deciding whether packets should go via ethernet or wireless, but I can't tell from here.

That's good to know. Please re-read your original posting and explain where you switched from Windoze 98 to Windoze 2000. "I've got an old Win98 computer with PCI 2.1 slots, and I wanted to give it wireless access using WPA encryption."


In Win98, it was up to the ethernet driver manufacture how this was set. Some, particularly laptops, had some fairly creative default setups. In 2000 and XP, it's on by default. Some drivers have check boxes or settings to change it.

The route metric is in every version of Windoze since the stone age. The problem is that each version and some updates behave differently.

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Jeff Liebermann

Since I don't have 98 or 2000 I decided to have a check using Ubuntu

7.10 and the following items: Zyxel commbo P-660HW-T1 V2 Buffalo WHR-G125 running DD-WRT V24 RC-5 as the Client Bridge.

Having just set up the bridge and giving it an IP and doing arp -a from the connected laptop I get: ? ( at 00:02:CF:xx:xx:xx [ether] on eth0 ? ( at 00:16:01:xx:xx:xx [ether] on eth0 These are the Zyxel and Buffalo MAC's. Having set the bridge up if I now reboot the laptop and do an arp -a I only get the Zyxel MAC, however should I log onto the Buffalo I will then again get both MAC's. I have no problem moving the laptop between the Buffalo and the Zyxel. I also have no problems accessing the buffalo from a Vista laptop which is on the wireless network and not physically connected.

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This is where I say damn. Now that I have put everything away I have just realised that I had a DHCP server running on both routers although not in the same range. I may try again tomorrow.

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Ubuntu 8.04 arrives tomorrow or the day after. update-manager -d I've been running the beta version with current updates. Seems to work ok, but I'm not digging very deep.

dd-wrt V24 SP1 final release has been out for several months.

I don't think the DHCP server matters as it looks like your IP addresses both came from a single DHCP server. Whether the Zyxel can deliver a DHCP assigned IP through the wireless bridge is an open question. If not, it might offer yet another explanation as to what's happening.

The problem (if it's there), is that when you plug in the ethernet cable, you're still connected via wireless. Windoze, Ubuntu, and the router both have to figure out which way to send packets, via wired or wireless. W2K and current versions of XP do it right. I vaguely recall that some early version of XP got confused.

I've played with switching back and forth and found that Windoze and Ubuntu usually get it right. The problems start when some manner of overly user-friendly "wireless manager" crams itself into the picture and takes over control of the switching. For example, Toshiba has the ConfigFree wireless manager which has some nifty radar like displays for finding SSID's, but can't seem to figure out how to switch between ethernet and wireless.

The clue with XP and W2K is that when you unplug the ethernet or disconnect wireless, and run "ipconfig" you should see something like: Windows 2000 IP Configuration Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : IP Address. . . . . . . . . . . . : Subnet Mask . . . . . . . . . . . : Default Gateway . . . . . . . . . : If you see IP addresses when it's disconnected, you're either running static IP's on the PC, or your wireless/ethernet/whatever interface is not recognizing that it's disconnected and should try some other interface.

Sorry, but I don't have time to setup a similar network on the bench and try all the various combination and permutations. Maybe next week.

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Jeff Liebermann

I installed 8.04 when it first came out and had a few hiccups with a few programs I was running so I reverted this one machine back to 7.10. I do run 8.04 on a couple of other machines but it doesn't reliably identify one inbuilt wireless card and I have tried several different things to get it to work but it is unreliable. 8.04 works ok with a usb wireless card so I use that at home. Nvidia graphics cards are also a pain.

This is a known reliable router that I loan out and I have just been too lazy to upgrade.

No, I gave the bridge an IP Address of and when I actually did an ifconfig on the laptop it did actually say the interface was which was out of the DHCP range of the Zyxel so must have come from the Buffalo though I just ignored it until I had put everthing away. Whether the Zyxel can
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I should really engage my brain. The Client Bridge setup was still in my browser cache, DHCP Server was disabled so the laptop was issued with an IP address prior to the router being converted to a bridge and the IP was within the same subnet of both routers so I really never tested whether the laptop got an IP address from the Zyxel.

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Yeah I do have a computer with Win98SE and updates.

After doing the "ipconfig /release" and "ipconfig /renew", I can visit pages that I never visited before in the browser.

OK, after pinging the WL-520gU, I see them both in the "arp -a" output.

At least I know of a couple workarounds now.

Yes, I will eventually move the bridge to be connected with my Win98 machine, although right now it is a little more convenient for me to test it out with my Win2000 machine. And the Win2000 machine may need wireless access at some point in the future, so it doesn't hurt to see if it works there too.

Looks like mine is on, according to the lack of that registry entry mentioned in the link you provided.

Thanks for the help, Jeff.

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