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The instructions for setting up a DD-WRT router in client bridge mode say:

There are errors in this video especially as related to security settings and the use of the 'join' button. It should not be used in substitution of the better instructions above.

But it doesn't explain why "join" is bad, and I'm not understanding what the use case of the "join" button under site survey is if not for one of the client modes. The docs for client mode don't use "join" either. Rather, both versions tell you to do something like:

  1. Set the DD-WRT router up in AP mode
  2. Make sure all of the settings are the same as the router you want it to be a bridge for
  3. Change it to "client mode," after which all of the settings you changed in (2) disappear

And I don't follow. What good is the "join" button, if not to automate these steps?

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A major reason to not use "Join" is simply to prevent accidentally joining someone else's network that has the same name as yours. This could be because multiple people in your area have chosen the same name, or it could be because a malicious neighbor is trying to trick you into connecting to his router instead of to your other one, thus allowing him to spy on your traffic. If you enter all of the settings on both routers manually, it's much less likely that you will connect to the wrong one.

This is very similar to the reason that WPS "Push to Connect" buttons are bad security. If you and your neighbor both push the "Connect" buttons on your respective routers, then it's a tossup for which of the routers your device will actually connect to. (Additionally, in the WPS case, the reverse is also true - if you push the "Connect" button on your router and then your neighbor pushes "Connect" on his device before you push "Connect" on yours, your neighbor will be connected to your network without needing to enter the key.)

Basically, the reason that these types of "Join/Connect" buttons are insecure is that any system that allows you to bypass its security for your convenience also allows an attacker to bypass the security too.

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    I don't think the comparison goes through: WPS is the source of a major vulnerability in WPA, exploited by tools like Reaver. It doesn't appear that "join" lets you connect without a key --- you still need to set the key appropriately (you're trying to join a foreign network as a client, you don't get to break the rules just because you're a router). It looks like "join" just fills in stuff like SSID/channel/etc for you automatically. Mar 31, 2015 at 4:50
  • @PatrickCollins First of all, the basic premise of both buttons I mentioned is still the same - putting information in manually is safer than attempting to automatically infer it from the environment. This should be true regardless of the fact that you need to set the key; deauthentication attacks mean that the key will never be checked. Second, you and I are talking about two different issues with WPS. You are talking about the PIN code version, for which there exists an attack to discover the choose. I am referring to the push-button version, for which the security issue is as I described.
    – Moshe Katz
    Mar 31, 2015 at 5:16
  • I was talking about a different WPS issue, my mistake. What do you mean by "deauthentication attacks mean that the key will never be checked"? Also, I don't think it's fair to describe it as "inferring information from the environment" --- the "join" button lets me see the BSSID of the router whose network I want to become a client of. I can't --- as far as I can tell -- specify anything other than the SSID of the router whose network I want to join without use of "join." The "join"-less method seems to have much more implicit work, to me. Mar 31, 2015 at 15:49
  • @PatrickCollins I don't have the source in front of me, but I read something a while ago about bad wireless implementations seeing an insecure network with the same name as an already-known secure network and connecting to the insecure one automatically. As far as the Join button goes, unless someone else can provide another reason that it's bad, the only thing I know of is that it lets you click "Join" on a router that isn't yours.
    – Moshe Katz
    Mar 31, 2015 at 16:13
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    I would hope that DD-WRT doesn't fall under the umbrella of "bad wireless implementations." I can't find anywhere that explains what DD-WRT is actually doing when you attempt to join a network, however, so it's tough to say what's going on in either case. Mar 31, 2015 at 16:15

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