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    Posted Dec 23, 2009
    9:23:03 AM
    IP Conflict with DHCP

    I've noticed a strange DHCP problem since migrating to Uverse and the 2Wire 3800HGV-B RG. I use VMWare Workstation and often have an XP VM running on my Vista laptop. Lately, the RG's DHCP server seems to be assigning the same IP address to the VM as it has already assigned to the host and of course both system lose connectivity until I manually do a release/renew on the VM. Note that I always get a unique address on the VM as I should when running "ipconfig /release" then "ipconfig  /renew" but it seems the lease is very short and the address gets changed in the background while I'm working....somtimes. I've never seen this behavior in any other DHCP environment. I haven't investigated it deeply yet (I'm always in the middle of something when it happens) but I wanted to see if anyone else has seen any similar behavior with the DHCP server in the RG.

    IP Conflict with DHCP

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    Dec 23, 2009 1:32:20 PM
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    The U-Verse RG does not support multi-homed hosts.  To work properly, your VM must have a different MAC address than the host.  Adjust VMware workstation to either use NAT for the VM, put the VM on its own Physical NIC, or make sure the VM can make a DHCP request with a different MAC address than the host.

     

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    Dec 23, 2009 3:17:48 PM
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    SomeJoe7777 wrote:

    The U-Verse RG does not support multi-homed hosts

     


     

    Some Joe,

     

    Awesome... finally, I have an more concrete answer why when I went to UVerse, I had to reconfigure my Buffalo Wireless ethernet bridges (WLI-TX4-G54HP) to only use one computer... instead of plugging in multiple devices into it's built in swtich and bridging everything.

     

    I did some research at the time, and figured it's probably because the RG doesn't support proxy-arp / MAC-NAT.  Is multi-homed hosts just another way of saying the same thing, or is there some fine-grain technical difference, and if so, could you provide a brief summary, if you don't mind.  I'm highly interested in this.

     

    I wonder if the x.135.x firmware fixed this, and if not, maybe the next RG interface upgrade will.

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    Dec 23, 2009 6:32:15 PM
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    nishiko70 wrote:

     

    I did some research at the time, and figured it's probably because the RG doesn't support proxy-arp / MAC-NAT.  Is multi-homed hosts just another way of saying the same thing, or is there some fine-grain technical difference, and if so, could you provide a brief summary, if you don't mind.  I'm highly interested in this.

     

    I wonder if the x.135.x firmware fixed this, and if not, maybe the next RG interface upgrade will.


     

    Proxy ARP, MAC-NAT, and multi-homed hosts are different things, but they're all essentially the same thing to the U-Verse RG -- multiple IP addresses sharing 1 MAC address.  This is what the OP is running into (the VM and the XP host have different IPs but are sharing the NIC, so they're sharing a MAC), and what you were running into (wireless bridge is the same thing -- layer 2 sharing device, all stations behind the wireless bridge have their own IP, but appear to the RG as sharing the MAC address of the bridge).  The RG does not support anything other than the idea that 1 MAC = 1 IP.

     

    I doubt the firmware did anything to address this -- as far as I know, all 2Wire routing devices are like this, not just the 3800HG-V.

     

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    Dec 23, 2009 8:03:23 PM
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    SameJoe7777,

     

    Thank you for the reply.  Highly interesting to me, as I mentioned.  Thanks so much for your time in replying!

     

    Interesting background you also provide as in your doubts about the firmware upgrades based on 2Wire's long history w/regard to this issue on their gateways.

     

    Do you know of a good reference (link) that summarizes the differerence between multihoming, proxy arp / MAC-NAT? If not, not worries.  I did a little google searching, and didn't turn up much good stuff in 10 minutes or so.

     

    Thanks again SameJoe!

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    Dec 23, 2009 9:18:11 PM
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    Multihoming is when a single computer has 2 or more IP addresses.  This is common on servers -- there are certain conditions, especially with web servers, where multiple IP addresses are required.  You can't use DHCP to multihome a host, the multiple IP addresses are usually assigned statically.

     

    Proxy-ARP is a very old mechanism that routers used to use to allow hosts with only partial implementations of TCP/IP to communicate.  Some very old hosts had networking stacks that did not support a default gateway entry.  The host knew how to communicate via IP, but had no knowledge of the concepts of subnets or default gateways.  In a modern host, if the host wants to communicate with a machine not on its own subnet, it would send the packet to the default gateway.  On these old hosts, the host would believe that the target machine was on its own network, and would proceed to send out an ARP packet for it, attempting to discover its MAC address.  The ARP packet wouldn't get responded to (of course, because the target machine isn't on this network), and the host would fail to communicate.  Proxy-ARP is a mechanism where the router on the network snoops all broadcast traffic (including ARP packets), looking for ARP requests for hosts not on the network, and if it sees one, the router will respond to the ARP packet itself (this is the Proxy ARP).  Now, the host with partial networking stack believes it has found the MAC address of the target machine, and will send the IP packet to it.  The host doesn't know that what its really doing is sending the IP packet to the router, which the router will forward, and the host is none the wiser.

     

    The effect of Proxy ARP is that the router that issued the proxy ARP now looks like it has multiple IP addresses on the same MAC address.  The 2Wire equipment would now be unable to communicate with that router.

     

    MAC-NAT is not a true term, but the effect can be seen with certain bridges, mostly wireless ones.  On a wired bridge (network switch), the network switch itself has a MAC address, but this MAC address doesn't appear in packets traversing the switch.  Ethernet frames entering the switch are addressed to a target station from the source station, and the frame leaving the switch is exactly the same.

     

    With a wireless bridge that's designed to connect wirelessly to a standard 802.11 access point, this can't be done.  Each session on the access point has to be seen as a client, which by definition has only one MAC address.  Thus, the wireless access point, even though it's a layer 2 device just like the switch, can't transparently pass the MAC addresses of the stations on the other side of it because that would mean one session connected to the access point would have multiple MAC addresses.  So the wireless bridge is performing "MAC-NAT".  The U-Verse RG sees this as one MAC address with multiple IP addresses, which it can't handle.

     

    Some wireless bridges use a proprietary protocol to encapsulate the Ethernet frames, and work as a true transparent bridge.  But the wireless bridges on both ends of the wireless link have to be dedicated to this task for this to work -- you can't use a normal access point on one end.  Some of the custom firmwares available for the Linksys WRT series may be able do this, including DD-WRT and Tomato, but I'm not positive of that, and you would need 2 Linksys WRT routers dedicated to the task.

     

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    Dec 23, 2009 10:02:04 PM
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    Excellent reply, SomeJoe7777!  What an excellent response.  Makes sense to me.  My so far most respected commenters now on this and DSLReports are you, Dave006, and gdm.  There are other regulars that are growing on me as well.... but so far, you three are the ones I respect most.

     

    BTW, on that Buffalo bridge I used before, I had no problems using it with other routers, and I know for sure, I didn't have a problem with Tomato or DD-WRT.  The first time I ran into this issue, is with the UVerse 3800 HGV-B GW... like you said/implied, it's a relativley unique issue with these gateways / 2 Wire gateways in general.  Though, now that I come to think of it, I think I had the Buffalo bridges when I had standard AT&T ADSL on an 1800 HGV and I think it worked OK (multiple computers/devices on the same bridge's switch), but I could easily be wrong.  That was several years ago... like 4 or 5.

     

    As for some wireless bridges using a proprietary protocol for transparent bridging, yep, gotcha on that, and good point.  Even though transparent bridging wasn't the main aim of WDS, many people use the transparent wireless bridging aspect of it to accomplish single hop bridging as we've been discussing.  I believe that's how it's often accomplished using Tomato or DD-WRT, though they may also have custom ways of doing it as well.  I've heard my favorite wireless bridge, the Netgear WG602v3 (v3 is important) also uses WDS when setting two of them up in bridge mode... and I've verified with those that bridging is transparent with UVerse.  They just don't tell you its using WDS... think they call it point-to-point bridging or something to that effect.

     

    Because of all this, for most people, a powerline or COAX briding solution would probably be best (Powerline AV or MOCA) where able to.  I would have included HPNA but that's what's included with the UVerse RG and may interfere... but I'm not sure.  Can you purchase third party HPNA devices and have them connect via the RG's HPNA "radio".  I would think not, since I've never heard anyone mention it.  It's definitley technically possible, but I imagine AT&T has that locked down for use just with their STB's and DVR's.

     

    Any thoughts on these last few points, SomeJoe, especially on my comments on WDS and also add-on HPNA adapter interfacing to the RG?

     

    Much appreciation for your generosity in information sharing!

     

     

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    Dec 23, 2009 10:16:17 PM
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    SomeJoe7777,

     

    I also neglected to fully thank you for even adding the historical perspective and everything your your explanation of the differences.  I really helps to remember what they are and adds some richness to it all.

     

    Also, I said for most, it's probably best to use powerline/coax bridging... I should have been more precise to say "easier to use".  Not necessarily best, though often also best.  For me, I don't mind a little learning curve most of the time, especially with networking subject matter I'm so interested in.

     

    When I first ran into the 1:1 issue with the UVerse GW in June when I had it installed, it blew me away for a little while, but I soon tracked it down to a good enough extent that I realized the gw wasn't happy with ProxyArp/MAC-NAT (which I didn't know existed, until I ran into this problem and started poking around on Google).  Not finding such a good explanation that you provided so easily, I finally just satisfied myself that I understood the problem well enough to fix it, and left it at that.  You improved my knowledge on this subject, like I said, complete with historical perspective/reasonings, which really helps remember the differences too.  The historical interleaving typically (hey a UVerse term too!) also makes reading such subject matter so much more interesting, and your post was no exception.

     

    Thanks again!   - Alan

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    Dec 24, 2009 8:21:47 AM
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    SomeJoe7777 wrote:

    The U-Verse RG does not support multi-homed hosts.  To work properly, your VM must have a different MAC address than the host.  Adjust VMware workstation to either use NAT for the VM, put the VM on its own Physical NIC, or make sure the VM can make a DHCP request with a different MAC address than the host.

     


    I've used VMWare in the past (although I'm not using it now) and put my VM's network in bridged mode, not NAT.  In other words it's the mode where the VM appears as a host on the network with its own IP address, doled out by the RG's DHCP support.  It always worked flawlessly for me.  (I only have one physical network adapter in the PC and I don't run a separate router at all, just the RG).  So I wonder how to reconcile this with your statement that it won't work.   

     

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    Dec 24, 2009 4:23:47 PM
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    ignore
    Message Edited by Wardawg1001 on 12-24-2009 04:26 PM

    Re: IP Conflict with DHCP

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    Dec 24, 2009 4:26:22 PM
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    nishiko70 wrote:

     

    Because of all this, for most people, a powerline or COAX briding solution would probably be best (Powerline AV or MOCA) where able to.  I would have included HPNA but that's what's included with the UVerse RG and may interfere... but I'm not sure.  Can you purchase third party HPNA devices and have them connect via the RG's HPNA "radio".  I would think not, since I've never heard anyone mention it.  It's definitley technically possible, but I imagine AT&T has that locked down for use just with their STB's and DVR's.

     

     


     

    Other devices work just fine connected with coax to the RG, at least in my (albeit limited) experience with it. I've never seen another router connected with it, but I don't imagine it would work any differently than connecting it with ethernet.

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    Dec 26, 2009 1:20:26 PM
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    MyDogHasFleas wrote:
    I've used VMWare in the past (although I'm not using it now) and put my VM's network in bridged mode, not NAT.  In other words it's the mode where the VM appears as a host on the network with its own IP address, doled out by the RG's DHCP support.  It always worked flawlessly for me.  (I only have one physical network adapter in the PC and I don't run a separate router at all, just the RG).  So I wonder how to reconcile this with your statement that it won't work.

     

    On page 308 of the VMware Workstation 7.0 user manual, it is implied that VMware Workstation assigns a unique MAC address to each virtual machine.  If this is the case, the VMs should have no problem operating with the RG using bridged networking.

     

    However, I'm not sure if this was always the case with previous versions of VMware Workstation or other VMware products, such as the old VMware Server.

     

    In most versions of VMware products that I've seen, the VM can always be manually assigned a MAC address for it's network adapter in the VM config file if there is a problem.

     

    Re: IP Conflict with DHCP

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    Dec 27, 2009 8:17:09 PM
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    OK so your advice to the OP should be to ensure that VMware is assigning unique virtual MAC addresses, not to tell him it won't work. 

    Re: IP Conflict with DHCP

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    Dec 27, 2009 9:56:42 PM
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    MyDogHasFleas wrote:
    OK so your advice to the OP should be to ensure that VMware is assigning unique virtual MAC addresses, not to tell him it won't work. 

     

     Isn't that exactly what this says:

     


    SomeJoe7777 wrote:

    The U-Verse RG does not support multi-homed hosts.  To work properly, your VM must have a different MAC address than the host.  Adjust VMware workstation to either use NAT for the VM, put the VM on its own Physical NIC, or make sure the VM can make a DHCP request with a different MAC address than the host.


     

    Did you somehow read something completely different when you read my answer to the OP (the second post in this thread if you missed it)?

     

    Re: IP Conflict with DHCP

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    Dec 28, 2009 7:20:11 AM
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    You are right, I was not correct. You had the correct information but you kind of "buried the lead" as they say in the journalism biz.  

     

    The change I would make to your advice is to say:

     

    If you want to run your VM NIC in bridged mode, make sure that it has a unique virtual MAC address on the network.  VMware attempts to make it unique but it may not succeed, and you may need to manually assign unique MAC addresses to your virtual NICs.  Another solution would be to use NAT mode for your virtual network, or put the VM on its own physical NIC.  

     

    And, hey, thank you for your participation and expert advice!  I learn a lot from you (including this particular fact).  If I sounded negative I didn't mean to.   

     

     

     

     

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    Dec 28, 2009 7:29:26 AM
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    Not a problem -- I don't know how he was getting multiple IPs on the same MAC with VMware workstation, but evidently he somehow was.

     

    I know that I have seen the multihomed thing not work with my own eyes.  I tried to set up a Routing and Remote Access server on Windows Server 2003.  To do it, RRAS is supposed to require 2 NICs in the computer.  I thought I could do it with just 1 NIC, and put 2 IP addresses on it, and then just bind the RRAS ports to the proper interfaces.

     

    This didn't work.  I was looking at the RG's network devices list from another computer, and the server kept changing it's IP address in the list. (!)

     

    What was happening was that the RG was sending out ARP packets for the server NIC's MAC address, and sometimes getting a reply on one IP and sometimes getting the other.  Each time it would receive an ARP reply for 1 IP address, the RG would "forget" about the other one, and packets from that IP wouldn't route through the RG.

     

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    Dec 28, 2009 8:53:14 AM
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    SomeJoe7777 wrote:

    Not a problem -- I don't know how he was getting multiple IPs on the same MAC with VMware workstation, but evidently he somehow was.


    When I was still on DSL, I used a Westell 327w modem.  My laptop is dual boot Windows/linux.  I always got a different IP in Windows than in linux.

     

    I'm not sure what the DHCP server was doing, but obviously it was using information other than the MAC address for assigning IPs.

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