[SOLVED] Router/Arch is creating a new MAC address in a dual boot

j33205 · 2015-08-06 05:46:58

Hello. I'm dual booting Win7 and Arch in the same tower connected directly via ethernet cable to my router and therefore the LAN. It's a fairly new system and I haven't really configured anything on it in the way of networking (i.e. no network manager). I haven't needed to so I've been letting Arch's default configuration handle my networking. On the router end, though, I have been configuring. I need remote desktop access to the machine for both OSes, so I need RDP, VNC, and SSH ports all forwarded to my machine. I've noticed, however, that my router is identifying two separate machines when there is only one. My router's DHCP settings shows all active leases, which include my Windows system as 192.168.0.100 under MAC address say 12-34-56-78-9A-BC and my Arch machine as 192.168.0.129 under MAC address 56-78-9A-BC-00-01. Note that Arch's MAC is different but contains part of the correct MAC plus some extra zeros and a one. This seems odd to me (under the definition of a what a MAC address refers to), is this normal? Does it affect normal behavior? Can it be changed, and whose fault is it, Arch or the router? I kind of expected my router to assign the two OSes the same IP and just change the host name and not the MAC (which I believe it did when I had a Debian based system).

The main reason I ask is because in order to successfully port forward consistently I would like to utilize my router's DHCP server to assign my machine a static local IP address. The IP addresses do tend to stay the same, as per their DHCP leases, so I could blindly forward to 192.168.0.129 but when that suddenly changes without my knowing, then I'm screwed, so preventing that would be nice. I don't really mind having to forward two IPs but I tried assigning the two different MACs two different static IPs but the router ignored the static IP listing and just assigned a new different IP address to Arch even though the MAC matched one in the static list.

Any suggestions as to how to properly configure this, if I can? All ideas welcome and appreciated.

Last edited by j33205 (2015-08-07 17:39:46)

ewaller · 2015-08-06 14:07:26

Well, the 12-34-56-78-9A-BC MAC is clearly bogus. And, although it has to do with the Windows mode, I suspect it is not Windows fault. My guess would have been that the router made that up. What would have happened is you connected as a Linux machine and passed in your MAC address and host name, and the router issued you an IPv4 address and added all that information to a database. You rebooted to windows, and passed in the (correct) MAC and a different host name. The router noted there had been a MAC with multiple host names and decides it is going to create a new, unique MAC.

I used the past imperfect tense because I don't really believe that is what is happening either. I say that because now I don't believe the other address MAC address is valid either. Both MAC addresses have way to many sequential hex digits to be a coincidence. What make and model router are you using, and what is the make and model of the NIC card in your computer?

Last edited by ewaller (2015-08-06 14:08:04)

j33205 · 2015-08-06 18:04:08

I'm sorry I think you misunderstood. 12-34-56-78-9A-BC is bogus because I made it up, that's not actually what it is, although it does Represent the true MAC of my NIC and the one associated with the Windows host. I just wanted to avoid posting my real MAC. Now, following that idea, the MAC that is associated with Arch is 56-78-9A-BC-00-01 I also made up. But the real scenario follows the same pattern in that the Arch MAC shares the same last 8 digits as the real one and then appends three zeros and a one to the end of it.

Apologies for not making that clear.

ewaller · 2015-08-06 18:12:41

Okay. I would chase down the router as the cause. I think it is creating MAC out of whole cloth to allow it to cope with the different host names.
Perhaps if you were to go into the router and delete both entries. Then reconnect from Linux. I am guessing that the router will suddenly get the MAC correct. Then, I would lock that down in the routers DHCP configuration to allow you to have a predicable address to which you can forward ports. Some (good) routers will let you forward ports based on a host name rather than a static IPv4 address.

I really cannot wait until we move on to IPv6

rsmarples · 2015-08-07 09:39:57

DHCP leases are stored by ClientID, not MAC address.
If no ClientID is present in the message then the DHCP server will generate one from the HW address presented in the DHCP message.

Newer DHCP clients found in open source tend to use a RFC4361 style ClientID to more readily allow matching DHCP and DHCPv6 clients.
This is where you probably see the diifference as Windows, OSX don't use this.
If you use dhcpcd and don't care for this, you can change this in /etc/dhcpcd.conf - there's a big comment about it at the top.

j33205 · 2015-08-07 17:39:28

rsmarbles: Thank you, that's exactly what I was looking for. dhcpcd was sending my DUID to the router, which is quite old, and it was getting very confused, though handling it surprisingly well I guess. Anyway, now dhcpcd.conf is set to sent clientid instead of DUID. My router kept the static IPv4 lease entry that I made for the Windows machine and just changed the Host Name, so now I have the same IP for both machines. Cool.

ewaller wrote:

I really cannot wait until we move on to IPv6

Amen.

Arch Linux

#1 2015-08-06 05:46:58

[SOLVED] Router/Arch is creating a new MAC address in a dual boot

#2 2015-08-06 14:07:26

Re: [SOLVED] Router/Arch is creating a new MAC address in a dual boot

#3 2015-08-06 18:04:08

Re: [SOLVED] Router/Arch is creating a new MAC address in a dual boot

#4 2015-08-06 18:12:41

Re: [SOLVED] Router/Arch is creating a new MAC address in a dual boot

#5 2015-08-07 09:39:57

Re: [SOLVED] Router/Arch is creating a new MAC address in a dual boot

#6 2015-08-07 17:39:28

Re: [SOLVED] Router/Arch is creating a new MAC address in a dual boot

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