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"arp who-has" doing my head in.

  • 01-07-2005 3:37pm
    #1
    Syth
    Registered Users, Registered Users 2 Posts: 1,865 ✭✭✭


    I'm having some trouble with my wireless set up at home.

    My hardware: I'm a IOL/Esat/BT broadband customer, so I use their ZyXEL Prestige 623R-T1 broadband 'modem'. I have a Linksys WRT54G wireless router. The ZyXEL is set into bridge mode and the Linksys is set to use PPPoE to connect to the internet. My only PC has a "RaLink Ralink RT2500 802.11 Cardbus Reference Card (rev 01)" (according to lspci), I'm using v1.1.0 of the rt2500 module from http://rt2x00.serialmonkey.com.

    My Problem: Sometimes my wireless card seems to be unable to connect to the linksys wireless router. I have reset the linksys wireless router, so the setting are the default, and hence correct. I can set the essid, etc of the wireless card, but whenever I try to ping the wireless router (which thanks to /etc/resolv.conf, I've named 'modem'), it fails and says "Destination host unreachable". If I run tcpdump when trying to ping, I only get loads of "arp who-has modem tell 192.168.1.10". nothing else. Resetting all the devices and rebooting doesn't change anything,

    This is not the first time this has happened. Previously, the problem would fix itself eventaully. I would get sick of trying to fix this problem and just give up. A day or two later when I went back to my PC, it would all just work again without requiring any fiddling.

    According to the man pages 'arp' stuff is to do with the hardware address of the device, is there any reason why this would not be able to find the hardware address? I think it'll 'Just Work (tm)' again in a day or two, so I'll just copy the hardware address then and save it in /etc/ethers (good old man pages). Would that stop this problem from occuring? Does/Cold the hardware address change?


Comments

  • #2
    oscarBravo
    Technology & Internet Moderators Posts: 28,830 Mod ✭✭✭✭oscarBravo


    Syth wrote:
    ...whenever I try to ping the wireless router (which thanks to /etc/resolv.conf, I've named 'modem'), it fails and says "Destination host unreachable". If I run tcpdump when trying to ping, I only get loads of "arp who-has modem tell 192.168.1.10". nothing else.
    What have you done with resolv.conf to name the router "modem"? I would have thought that was a function of "hosts".

    The arp output is strange. It should be along the lines of "arp who-has 192.168.1.1 tell 192.168.1.10" - I've never seen an arp who-has with a hostname in it.

    arp maps MAC addresses to IPs. The first time your TCP/IP stack tries to contact a local address (i.e. on your subnet) it sends an arp who-has message to the network broadcast. The host that has that address replies, giving your local stack the MAC address it needs to send the message to. The MAC gets stored in your local arp cache, where it expires after a few minutes if it's not used.


  • #3
    oscarBravo
    Technology & Internet Moderators Posts: 28,830 Mod ✭✭✭✭oscarBravo


    Looking at it some more, I suspect it's not actually an arp problem. It looks like, for whatever reason, your wireless card is not associating with the router. No association, no connection - at a level below the IP layer. It's like having an Ethernet cable unplugged - arp can send "who-has" all it likes, it's not going to get a reply.


  • #4
    Hotblack Desiato
    Registered Users, Registered Users 2 Posts: 36,537 ✭✭✭✭Hotblack Desiato


    Is it actually necessary to set your Zyxel to bridge mode? My preference would be to keep the configuration of everything as simple as possible.

    I had a simple setup where my PC was connected directly by ethernet to the Zyxel (same model but on Digiweb). Then I wanted to add a wireless laptop so I got a Dlink ap/switch and a wireless card. I thought I had to set the Zyxel to bridge mode but that didn't work. The Zyxel is now set up as before for PPPoE (and still acting as a DHCP server - assigning an IP address to the WAN side of the Dlink). The Dlink is also acting as a DHCP server assigning IPs to the LAN - no DSL settings are activated on the Dlink.

    It's far easier to hook a PC up to your access point over Ethernet to configure wireless, perhaps you can borrow a PC or laptop for this purpose?

    At the end of the day, sounds to me like a network problem not specifically a linux problem. If you're setting the wireless card to a specific ESSID then you need to set the AP accordingly - easy if you can access it by ethernet.

    As to why it works sometimes - maybe you were picking up someone else's AP? :)

    In Cavan there was a great fire / Judge McCarthy was sent to inquire / It would be a shame / If the nuns were to blame / So it had to be caused by a wire.



  • #5
    Syth
    Registered Users, Registered Users 2 Posts: 1,865 ✭✭✭Syth


    oscarBravo wrote:
    What have you done with resolv.conf to name the router "modem"? I would have thought that was a function of "hosts".
    A yes you're right. I had "192.1681.1 modem" in /etc/hosts, not /etc/resolv.conf. Must have been confused.
    ninja900 wrote:
    As to why it works sometimes - maybe you were picking up someone else's AP?
    Probably not. Many times I reset the Linksys and then when I go to http://modem, it has been reset, and I only ever get one scan result.

    As for setting the Zyxel into bridge mode, it seems to be easier. When I first got my wireless stuff, I was banging my head against the wall trying to get it to work. Getting the Zyxel to do PPPoE would involve basically two networks and make things like NAT a bit of a pain.

    After resetting the Linksys I think I can connect, I use the command "iwconfig ra0 essid linksys channel 11". That used to work before. A GUI tool came with the driver, and I can use that connect. I'll try RTFM of my linksys to see if I've left out something.


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