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Thread: Computer unreachable through shared connection

  1. #1
    Join Date
    Apr 2015
    Beans
    3

    Computer unreachable through shared connection

    Hi,
    I have the following setup:
    Computer with Ubuntu 14.04 LTS with two ethernet ports. One of them is connected to our office LAN, the other is connected to a desktop PC running Windows 8.1. I enabled 'Shared to other computers' on this connection.

    From the W8.1 computer I am able to connect to the Ubuntu computer and, through it, to the office LAN.
    From the Ubuntu computer I am able to ping the W8.1 computer.
    From the Ubuntu computer I am not able to connect to the VNC server that is running on the W8.1 computer.

    When I connect the W8.1 computer to the office LAN directly, I can reach the VNC server just fine.

    Below is the output of the iptables /L command:

    Chain INPUT (policy ACCEPT)
    target prot opt source destination
    ACCEPT udp -- anywhere anywhere udp dpt:bootps
    ACCEPT tcp -- anywhere anywhere tcp dpt:bootps
    ACCEPT udp -- anywhere anywhere udp dpt:domain
    ACCEPT tcp -- anywhere anywhere tcp dpt:domain

    Chain FORWARD (policy ACCEPT)
    target prot opt source destination
    ACCEPT all -- anywhere 10.42.0.0/24 state RELATED,ESTABLISHED
    ACCEPT all -- 10.42.0.0/24 anywhere
    ACCEPT all -- anywhere anywhere
    REJECT all -- anywhere anywhere reject-with icmp-port-unreachable
    REJECT all -- anywhere anywhere reject-with icmp-port-unreachable

    Chain OUTPUT (policy ACCEPT)
    target prot opt source destination



    Can someone tell me how to get the vnc connection working?

    Thanks,

    Gilbert

  2. #2
    Join Date
    Nov 2007
    Location
    London, England
    Beans
    7,751

    Re: Computer unreachable through shared connection

    I don't think your post contains the full information about the rules. Specifically, any interface-specific info is missing, and I suspect there may be some.
    I don't see any reason the VNC connection should fail from the above post. Perhaps the missing info provides a clue.
    Please can you post the output of
    Code:
    sudo iptables-save -c
    which will give all the details of the rules.

  3. #3
    Join Date
    Apr 2015
    Beans
    3

    Re: Computer unreachable through shared connection

    Here it is:

    # Generated by iptables-save v1.4.21 on Wed Apr 29 13:46:28 2015
    *filter
    :INPUT ACCEPT [58370:7386105]
    :FORWARD ACCEPT [0:0]
    :OUTPUT ACCEPT [86353:74624668]
    [4:1392] -A INPUT -i eth4 -p udp -m udp --dport 67 -j ACCEPT
    [0:0] -A INPUT -i eth4 -p tcp -m tcp --dport 67 -j ACCEPT
    [1180:76357] -A INPUT -i eth4 -p udp -m udp --dport 53 -j ACCEPT
    [0:0] -A INPUT -i eth4 -p tcp -m tcp --dport 53 -j ACCEPT
    [65449:35289684] -A FORWARD -d 10.42.0.0/24 -o eth4 -m state --state RELATED,ESTABLISHED -j ACCEPT
    [44009:15653243] -A FORWARD -s 10.42.0.0/24 -i eth4 -j ACCEPT
    [0:0] -A FORWARD -i eth4 -o eth4 -j ACCEPT
    [0:0] -A FORWARD -o eth4 -j REJECT --reject-with icmp-port-unreachable
    [0:0] -A FORWARD -i eth4 -j REJECT --reject-with icmp-port-unreachable
    COMMIT
    # Completed on Wed Apr 29 13:46:28 2015
    # Generated by iptables-save v1.4.21 on Wed Apr 29 13:46:28 2015
    *nat
    :PREROUTING ACCEPT [6876:633319]
    :INPUT ACCEPT [5010:441074]
    :OUTPUT ACCEPT [4107:269478]
    :POSTROUTING ACCEPT [4097:268763]
    [1711:181365] -A POSTROUTING -s 10.42.0.0/24 ! -d 10.42.0.0/24 -j MASQUERADE
    COMMIT
    # Completed on Wed Apr 29 13:46:28 2015

  4. #4
    Join Date
    Nov 2007
    Location
    London, England
    Beans
    7,751

    Re: Computer unreachable through shared connection

    I can't see anything in those rules that would prevent the RDP from working.

    My next thing would be to try to trace the packets as you try to make the RDP connection. Use tcpdump to print the packets. You need to trace on the interface that connects to the W8.1 PC - which I think is not eth4 but I don't know which one it is. The command below assumes eth0, but adjust it to trace the correct interface:
    s
    Code:
    udo tcpdump -np -i eth0
    then try the RDP connection and then stop the trace (with Ctrl-C). We might see why the connection fails.

  5. #5
    Join Date
    Apr 2015
    Beans
    3

    Re: Computer unreachable through shared connection

    Hi,
    I got it to work but I don't know how. The only thing I (can remember I) did was unplug the network cable between the two PCs and plug it back in.
    I am trying to retrace my steps and figure out what happened.
    Will let you know when I do.

    Thank you for your support so far.

    Gilbert

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