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  1. #1

    Loop back (lo) .. maybe ? HELP im a newbie

    Can someone please help.

    I am very new to Linux .. but I took the plunge - I got a nice new pc and installed Ubuntu 10.10.

    I’ve learnt some basics and everything runs nice but I need to do something with the network to emulate my old set-up, so first let me tell you my old set-up.

    Firstly I have a netgear DG834 v3 router without NAT loop back and it doesn't support the newer firmware supporting this, so on my old pc set-up I have various server set-ups one of which is Apache which I use to build websites and test flash content etc.., obviously without NAT loop back others can access my server but I cannot because of NAT conflicts - so I utilised the Microsoft loop back utility - and ill say right now .. I didn’t really configure it myself, I simply followed some instructions I found online and it all worked fine .. I can type in my dns address - and voilà I connect no problems.

    OK here's what the Microsoft loop back adapter looks like:

    client for Microsoft networks unchecked

    file and print sharing for Microsoft networks unchecked

    internet protocol (TCP/IP) enabled (checked)


    properties on internet protocol (TCP/IP):

    use following IP is checked and is:

    IP address

    Subnet mask

    Default gateway left blank

    use the following DNS is checked but -

    Preferred DNS Server and Alternative DNS Server are both blank

    clicking the advanced button on this tab shows the IP settings as set i.e and subnet mask

    DNS tab is blank

    WINS tab is blank

    Options tab shows TCP/IP filtering with no properties.

    under the advanced tab:

    allow other networks users to connect to connect through this computers internet services is unchecked

    this connection is fire walled with no exceptions.

    that’s it, its un bridged .. and just works, I can launch my browser or client of choice and it just seems to do what its supposed to.


    Now ... my old PC is still up and running .. and works as is .. providing I try to access my server from the same PC... if I try to access this pc from my new Ubuntu set-up .. I cant connect to it .. I get a conflict and it just hangs.. in other words .. I type in the the address this connects to my dns account which then routes over the Internet to my router.. and that’s as far as it gets (I presume).

    My old pc with my old set-up does have a network card .. and I have tried connecting my new pc (Ubuntu) via this card also with shared Internet services .. in the hope it would just utilise the loop back adapter ... but it doesn’t and if I try to force the loop back adapter to share Internet services .. it defaults to IP - which is my router IP .. and so it conflicts and messes everything up.

    OK.. at the risk of babbling on endlessly - hopefully I’ve painted the picture. How can I, either over the Internet or through a shared connection via my old PC .. loop back to my server ?
    (I don't want to edit the HOST file I want to do this over the Internet)

    I have tried to set-up the loop back utility on my Ubuntu system .. but its very confusing for a novice .. I see its there .. and working, its IP and subnet which is the default, but I don’t see it in the Network Manager Applet and not editable in network tools (config button grey'd out) I haven’t the slightest idea how to configure it.. and all the information on this I read just confuses me. Also .. just as a test .. I booted my Ubuntu PC in XP .. same system as my old pc.. and also set-up a loop back adaptor on that.. just as a test .. but when I try to access my server I cant connect either .. this was with duplicated IP etc. .. identical set-up .. so please, if anyone has some simple instructions .. or obvious ideas how I can set this up I would be very grateful - but remember im an idiot .. don't get all sudo on me - or if you can tell me what I need to achieve then I can focus on that .. at the moment I'm just wandering about in the ether .. or is that the eth0 )
    ... oh .. p.p.s,, my Internet service provider wont allow use of NAT loop back router - so I need to do it this way )

    p.p.p.p.s .. I am more than happy to throw my old pc in the trash .. my new p.c is more than fast enough to handle the servers and my general use .. so thats an alternative .. but I would prefer to keep the servers separate.

  2. #2
    Linux Engineer Kloschüssel's Avatar
    Join Date
    Oct 2005
    Hi Caren,

    your confusion confused me too. Would you please summarize the problem into a paragraph or two including a small and nice question? I've got lost after the second paragraph.

    First of all we need to know what you want. After that we may have a possible solution and as last step we may have to clear up some details.


  3. #3
    Hi Kloschüssel,

    ty for your response .. and yes .. ill try to keep it simple .. I just thought some background wd help .. im trying to emulate the Microsoft Loopback adapter setup I have on my xp server pc .. (as described in my post) – in other words .. how would I configure the loopback facility (lo) in Ubuntu 10.10 to do this exact same job, namely .. to ba able to access my network via my dyndns address .. without loopback problems as I am unable to use a NAT loopback router on my service.

    Many thanks )

  4. $spacer_open
  5. #4
    Linux Engineer Kloschüssel's Avatar
    Join Date
    Oct 2005
    I do not know what the loopback adapter is or how the loopback adapter works.

    NAT forwarding with masquerading works well as long as you don't want to forward one public port to two different hosts.

    What are those loopback problems you are talking about?

  6. #5
    Hi Kloschüssel,

    A server on a home connection with a broadband router needs, NAT Loop back functionality.
    Many DSL routers/modems prevent loop back connections as a security feature. This means that a machine on your local network (e.g. behind your DSL router/modem) cannot connect to a forward facing IP address (such as of a machine that it also on your local network. Connecting to the local IP address (such as of that same machine works fine.
    This is an issue since a server has to specify an IP address for the client to connect. This is the ExternalHostName. In the absence of NAT loop back, if a forward facing IP address is specified (such as then external clients will be able to connect to the server but clients on your local network will not. If the internal address were put in ExternalHostName instead (e.g. then clients on the local network will be able to connect but clients from an external network would not.

    Microsoft have a solution known as the Microsoft Loop back adapter and to the best of my knowledge .. it creates a psudo or virtual network card (software not hardware) which u can configure and use for testing servers in a virtual environment .. it also has many other uses such as circumventing the said NAT loopback issue - your network .. and DNS etc sees it as a seperate network as if it were not on the host .. or something like that .. please forgive my lack of total knowlage on this .. but there is a knowlage base on Microsoft (sorry to swear) website regarding this if your interested..
    I knew this wouldn’t be simple, but .... I suppose im looking for advice from someone who has experience of this on windows systems who has successfully translated it into Linux speak – Many thanks again Kloschüssel )

  7. #6
    Linux Engineer Kloschüssel's Avatar
    Join Date
    Oct 2005
    Ok. I will try to rewrite it how i understood the problem:

    public ip
    | \
    | v
    v lan_client

    you have set up nat forwarding on the router to lan_server such that the lan_server is reachable on the public ip. am i right?

    But you cannot access that public ip from another lan client as that device does not forward the packets properly because the router is unable to forward the packets properly.

    In this case I always suggest to configure the local (lan) dns with a alias (cname) for the domain.

    when a lan client requests the IP from the public domain from your local dns server it will no longer receive the public ip, but the lan ip. thus it will communicate with the server directly through the lan.

    see here:

    please correct me if I'm wrong.

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