Re: [vserver] Static Route on Loopback

From: Gordan Bobic <>
Date: Fri 15 Oct 2010 - 13:08:22 BST
Message-ID: <>

The problem is that loopback reacts differently. It responds locally to
all addresses in the subnet.

e.g. if you set your loopback, the localhost will listen to
ALL of the addresses in that subnet. If you set it for,
then ALL of the IPs in that subnet will get answered by localhost,
rather than passed out.

Either way, the solution using a dummy NIC works. It turns out the lo IP
bindings were a hangover from before, after I removed them they didn't
come back, so it's all good.

I think this is a point that's worth pointing in the dummy vs lo
argument. With lo, the routing and filtering can be quite unintuitive,
while dummy works exactly as you'd expect.


Rik Bobbaers wrote:
> might be a stupid remark, but if you want machines to talk to each other
> inside the same network, you have to put them in the same network
> so, if you configure:
> and, then these 2 machines are on a
> different network, so outgoing traffic will go via it's network with the
> default route, which is your 192.168.1.x/24 network.
> So... i suggest you configure the interface on lo on at least
> (or maybe more clear: 24) and go with that....
> What's the reason you put /32 on those lo interfaces? what will you use
> them for?
> kr,
> Rik Bobbaers
> --
> linux/unix/system/network/security/hardware admin
> infrastructure architect
>> I just confirmed that using dummy interfaces with a real netmask does
>> actually make things work the way I want. I notice the lo /32 interface
>> still gets created with the same IP. Are there any drawbacks in doing it
>> this way?
>> Gordan
>> Gordan Bobic wrote:
>>> Adrian Reyer wrote:
>>>> Hi Gordon,
>>>> On Thu, Oct 14, 2010 at 10:22:15PM +0100, Gordan Bobic wrote:
>>>>> Is there a way to add a static route to a VM on loopback?
>>>> You don't need to. As the guest has no network, it doens't need to
>>>> route, either. The kernel has the network and does the routing.
>>>> Do you experience any problems with this setup?
>>> Yes I am seeing a problem with this setup.
>>> Host A:
>>> lo:
>>> eth0:
>>> Host B:
>>> lo:
>>> eth0:
>>> When host A connects to, the connection looks like it came
>>> from, rather than I don't want my app on host B
>>> binding on listening on interface. I want the connection
>>> to be going via the internal loopback only. I also want to keep the
>>> iptables rules relatively sane and intuitive.
>>> Normally, this would be implicit by the network scope, but since lo is
>>> different and setting on it would make the local host
>>> respond on the entire range, I need an alternative solution that would
>>> work more sensibly. Would using a dummy network device work for this? Or
>>> is there a better way?
>>> Gordan
Received on Fri Oct 15 20:56:12 2010

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