Find the answer to your Linux question:
Page 1 of 2 1 2 LastLast
Results 1 to 10 of 20
Hi, I'm having problems with an embedded linux platform and a wireless access point (I've tried the wireless forum but to no avail). I want my network to be as ...
Enjoy an ad free experience by logging in. Not a member yet? Register.
  1. #1
    Just Joined!
    Join Date
    Dec 2009
    Posts
    19

    DHCP problem with embedded Linux platform


    Hi,

    I'm having problems with an embedded linux platform and a wireless access point (I've tried the wireless forum but to no avail). I want my network to be as follows:
    ADSL wireless BB router (and DHCP server) -> WAP (wireless access point acting as client, DHCP server not enabled) -> switch -> embedded platform

    However the embedded platform sends out its DHCP discover request (noted by using wireshark on a linux PC) but nothing responds.

    However
    ADSL BB router -> WAP -> switch -> Linux PC and Windows PC get IP addresses as expected
    ADSL BB router -> switch -> swtich -> embedded platform gets IP address
    but
    ADSL BB router -> WAP -> embedded platform doesn't get IP addreess

    So the Linux PC and a Windows PC all successfully can use all combinations of switches and WAP to get IP addresses using DHCP but the embedded device and the WAP don't get on.

    I've also tried a hub instead of a switch incase that makes a difference but it doesn't.

    I've looked through the DHCP discover packets using Wireshark from all machines and I can't spot anything obvious (the only 3 differences I could see I bodged in ipconfig.c (length of packet, whether the packet allows fragmentation, stay alive timeout) but still no joy).

    I've looked for updates to the ethernet driver for my embedded platform but nothing. The embedded platform is using kernel 2.6.21.3 so quite old however I am not able to upgrade the kernel to the latest as there are too many driver patches for the platform that I know nothing about and have been warned that the platform experts expect to take a week to do a kernel upgrade so it would take me much longer. I have tried applying various IP patches I've found around but nothing has worked so far.

    Any ideas gratefully received (assume a certain level of ignorance about IP as you can probably tell by my comments above) as well as telling me to try a more appropriate forum.

    Rachel

    PS The WAP is a Edimax EW-7206APg, the ADSL BB router is a NETGEAR DG834GT if that makes a difference. The ethernet driver the embedded platform is using is the 8139too driver.

  2. #2
    Linux User
    Join Date
    Dec 2009
    Posts
    264
    I've had a similar problem ...
    Some WAP just start the connection to the WLAN when they see a device at their ethernet which is turned on.

    That would explain why it works with a switch but not with a client.

    I assume
    ADSL BB router -> WAP -> switch -> embedded platform
    is working?!

  3. #3
    Just Joined!
    Join Date
    Dec 2009
    Posts
    19
    Unfortunately BB router -> WAP -> switch -> embedded platform doesn't work either. The presence of or lack of the switch seems to make no difference to the behaviour - ie BB router -> WAP -> Linux PC works as does BB router -> WAP -> switch -> Linux PC, but not for the embedded platform.

  4. $spacer_open
    $spacer_close
  5. #4
    Linux Guru Rubberman's Avatar
    Join Date
    Apr 2009
    Location
    I can be found either 40 miles west of Chicago, in Chicago, or in a galaxy far, far away.
    Posts
    11,754
    What address for the gateway do you have your embedded platform set to? Also, what about its netmask, etc? In any case, the default gateway address has to be set to the router address. If it is incorrectly set, then I don't think the dhcp request will be handled properly.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  6. #5
    Just Joined!
    Join Date
    Dec 2009
    Posts
    19
    Netmask, gateway all set as expected.

    If I use a wire to the switch ie.
    BB router -> switch -> embedded platform then the embedded device gets its IP address as expected from DHCP however it is when I add in the WAP i.e.
    BB router -> WAP -> switch -> embedded platform that DHCP discover message doesn't get a response.

  7. #6
    Linux Guru Rubberman's Avatar
    Join Date
    Apr 2009
    Location
    I can be found either 40 miles west of Chicago, in Chicago, or in a galaxy far, far away.
    Posts
    11,754
    A few possible issues. One, are you sure you the WAP key/passphrase is correct? Two, which version of WAP are you using? Three, is the computer version of WAP the same as what the router is set to? Four, if all the above are correct, are you really getting a connection? If you are getting a connection, then you should be able to get a dhcp address. To verify that, give your computer's wifi port a local static IP address and see if you can ping the router.

    Honestly, I don't think the router is the problem, unless for some reason you've selected the enterprise WAP setting that requires a Radius server for key management.

    Finally, you can try to temporarily reset the router to use WEP instead of WAP and try connecting with that instead. If you can, then you know that the problem is with the WAP settings and/or passphrase.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  8. #7
    Just Joined!
    Join Date
    Dec 2009
    Posts
    19
    Hi rubberman - thanks for the reply. I know the WAP WPA key is correct as the linux PC and windows PC also connected to the same switch via the WAP i.e.
    BB router -> WAP -> switch -> linux pc etc can get IP addresses using DHCP from the router, as well as browser the internet etc.
    The wireless access point is a Edimax EW 7206APg with the latest firmware I could find for it.

    If I connect wired (i.e. BB router -> switch -> platform), boot up my embedded platform, get an IP address from the router, then replace the ethernet cable from the router with the WAP (i.e. BB router -> WAP -> switch -> platform) the platform continues to perform as expected - so it is just the DHCP step in the boot up of the embedded platform that something is going amiss.

    Could the WAP be throwing away packets it doesn't like? Perhaps the formatting of the dhcp discover packet is slightly wrong from the embedded platform and the WAP bins it? Would a wireless access point do that?

  9. #8
    Linux Guru Rubberman's Avatar
    Join Date
    Apr 2009
    Location
    I can be found either 40 miles west of Chicago, in Chicago, or in a galaxy far, far away.
    Posts
    11,754
    The wireless access point should not modify the packets after decrypting them. However, if the encrypted packet had been corrupted, then it would not have been able to decrypt it. Like I asked, did you try a static address for the wireless port with WAP enabled? If you do that, can you get on the network? If so, then you know a couple of things for sure. One is that the WAP key/passphrase is correct. Two is that your network is healthy (mostly) and that as you are thinking, the interaction of DHCP and WAP is causing a problem. If that appears after all other considerations to be the problem, then get something like wireshark and look at the dhcp packets getting sent and the replies coming back from/to the client system from the router.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  10. #9
    Just Joined!
    Join Date
    Dec 2009
    Posts
    19
    Hi thanks for the reply.

    I'm not sure what you mean by "try a static address for the wireless port with WAP enabled" - the WAP has a static IP address (192.168.0.2 on my network as compared to the router with 192.168.0.1), I've tried a static IP address for the embedded platform, all work as expected. As I said I know the WAP WPA key is correct as the other machines are using the same setup at the same time and if they can get IP addresses using DHCP over the wireless link, then its can't be the wireless link. Also the WPA key part manages the link between the wireless points (router and WAP acting as client) so how would that have any different effect on the machines that are down stream from the switch?

    I have looked at wireshark as I state in my original message - I see the DHCP discover packet get sent out but nothing comes back. I can compare this with a Linux PC and a Windows PC which get a request packet back from the router. So either my embedded platforms discover packet is binned by the wireless access point (WAP) or by the router. Being as when I'm connected using wire dhcp discover and request packets are seen, I'm guessing someone doesn't like the discover packet and it must be the WAP.

    But why would a WAP throw away a packet that the router understands? Is the router being more flexible? Can I configure wireshark to show where in the network a packet gets to? To see if the packet makes it passed the wireless access point?

  11. #10
    Linux Guru Rubberman's Avatar
    Join Date
    Apr 2009
    Location
    I can be found either 40 miles west of Chicago, in Chicago, or in a galaxy far, far away.
    Posts
    11,754
    I mean that the computer you are trying to get a dhcp address for. Use a static address for the wifi connection instead of dhcp to see if the problem is specifically associated with the dhcp services.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

Page 1 of 2 1 2 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •