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Thread: Bridging two of three Ethernet connections

  1. #1
    Join Date
    Mar 2013
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    Angry Bridging two of three Ethernet connections

    I have a fresh install of Ubuntu Server 12.04, I've added a password for the root user so I can type commands a little faster, installed bridge utilities with "sudo apt-get install bridge-utils", I made "eth0" dhcp (which is working correctly), and created a bridge "br0" with a static IP using "eth1" and "eth2". PC #1 and #3 are able to communicate. This is literally all I have done. Now from my understanding since I haven't set up any sort of forwarding to my knowledge these two networks are completely separate, and shouldn't be able to access each other, right?

    Apparently not, because PC #2 can SSH into the "br0" IP (192.168.0.10), and PC #1 and #3 can SSH into "eth0" (192.168.0.122). I want "br0", and "eth0" networks separate, and don't understand why, or how this is even happening since I haven't even un-commented the line to forward ipv4 addresses..

    *edit* PC #2 cannot access PC #1, or PC #3. Which is fine, and how I'd like it. PC #1, and #3 can't reach PC #2 as well.


    Last edited by TheHammer101; March 17th, 2013 at 09:18 PM.

  2. #2
    Join Date
    Feb 2011
    Location
    Coquitlam, B.C. Canada
    Beans
    1,414
    Distro
    Ubuntu 12.04 Precise Pangolin

    Re: Bridging two of three Ethernet connections

    Everything is on the same sub-net so there is nothing to "forward".
    Try changing br0 and pc1 and pc3 to 192.168.234.0 sub-net (mask 255.255.255.0), and where the "234" can be any integer from 1 to 254.

    Edit: assuming the 192.168.0.0 subnet mask is 255.255.255.0. If not then tell us what it is.
    Any follow-up information on your issue would be appreciated. Please have the courtesy to report back.

  3. #3
    Join Date
    Mar 2013
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    31

    Re: Bridging two of three Ethernet connections

    Why would it being on the same sub-net matter though? They are on different interfaces, so wouldn't that separate them? If I change the subnet of PC #1, PC #3, and "br0" so that they are 192.168.1.1 192.168.1.2 192.168.1.3 would they become inaccessible even if I tried to SSH into that "br0" IP (192.168.1.3) from PC #2 (192.168.0.122)? For instance if I am running a DHCP server on "br0" (192.168.1.3) won't that spill over onto "eth0" (192.168.0.122). I tried running a setup like that before, and it was creating problems for PC #2 being able to connect to the router. I think it was getting DHCP from "br0" (192.168.1.3) while switching between that and the DHCP from the router (192.168.0.1)...

  4. #4
    Join Date
    Feb 2011
    Location
    Coquitlam, B.C. Canada
    Beans
    1,414
    Distro
    Ubuntu 12.04 Precise Pangolin

    Re: Bridging two of three Ethernet connections

    Why would it being on the same sub-net matter though? They are on different interfaces, so wouldn't that separate them?
    I'm not sure, but based on what you have said i'm guessing that somehow your Ubuntu server is figuring it out.
    For instance if I am running a DHCP server on "br0" (192.168.1.3) won't that spill over onto "eth0" (192.168.0.122).
    I do not see how. Just make sure that your DHCP server is only listening to br0 and not all interfaces.
    If I change the subnet of PC #1, PC #3, and "br0" so that they are 192.168.1.1 192.168.1.2 192.168.1.3 would they become inaccessible even if I tried to SSH into that "br0" IP (192.168.1.3) from PC #2 (192.168.0.122)?
    PC #2 would not have any idea how to get to br0. It wouldn't know the route.

    Edit: see the file /etc/default/isc-dhcp-server for dhcp server listening interfaces list, which defaults to all.

    Edit 2: By the way, It is not clear to me why you would run a DHCP server on your Ubuntu server. Your diagram seems to indicate that everthing on br0 is setup manually.
    Last edited by Doug S; March 17th, 2013 at 11:37 PM. Reason: add a file location
    Any follow-up information on your issue would be appreciated. Please have the courtesy to report back.

  5. #5
    Join Date
    Mar 2013
    Beans
    31

    Re: Bridging two of three Ethernet connections

    So, I have no idea why being on the same subnet mattered. When I set up my server with a "br0" IP (1.1.1.1) manually, made PC #1 (1.1.1.100) manually, and made PC #2 IP (1.1.1.10) manually I could not reach the "br0" IP (1.1.1.1), or PC #1 IP (1.1.1.100) from PC #2 IP (1.1.1.10) unless I had set up iptables to forward correctly. Then I set up DHCP and DNS servers on "br0". Thanks for your help, but I'm still confused as to why it makes a difference that they are on the same subnet.


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