Re: [gentoo-user] IP aliasing problem

[Stroller <stroller@stellar.eclipse.co.uk>]

On 6 Oct 2010, at 19:02, Grant wrote:
> ...I have 5 usable IPs from my host and I've defined 2 of them in /etc/conf.d/net like this:

> 
> config_eth0=(
>        "1.2.3.1 netmask 255.255.255.0"
>        "1.2.3.2 netmask 255.255.255.0"
> )
> 
> My apache2 config is not working as expected and I think its due to a
> networking problem because eth0:1 reports no RX or TX stats:
> 
> # ifconfig
> eth0      Link encap:Ethernet  HWaddr [removed]
>          inet addr:1.2.3.1  Bcast:[removed]  Mask:255.255.255.0
>          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>          RX packets:46047 errors:0 dropped:0 overruns:0 frame:0
>          TX packets:65703 errors:0 dropped:0 overruns:0 carrier:0
>          collisions:0 txqueuelen:1000
>          RX bytes:6131194 (5.8 MiB)  TX bytes:82568335 (78.7 MiB)
>          Interrupt:40
> 
> eth0:1    Link encap:Ethernet  HWaddr [removed]
>          inet addr:1.2.3.2  Bcast:[removed]  Mask:255.255.255.0
>          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>          Interrupt:40

It's frustrating that you must conceal the IP addresses during a question like this, because it makes it harder to know whether you've misconfigured your host or just made a typo in transcription. It's probably clearer if you alter only the second octet of your IPs, and leave everything else otherwise intact.

However... if you have 5 usable IPs then your subnet mask will not be 255.255.255.0 - it'll be 255.255.255.248. See http://krow.net/dict/subnet.html
The broadcast address will also be affected. This is also known as a /29 subnet - IIRC you actually have 8 IPs assigned to you - one of them is used by the router, one of them is the broadcast, and the other is unused (the way 192.168.0.0 or 0.0.0.0 is normally unused).

Since your router has an IP in the same subnet, you'll know this is all working when you can ping the addresses of your webserver (1.2.3.1 & 1.2.3.2) from the router. 

Alternatively, set up a second PC - e.g. your laptop - on the globally-addressable subnet, and try pinging from there.

The normal situation with a home router is that it has a globally-addressable IP on it's "WAN" interface and a private IP (192.168.x.y) on its LAN interface. When you start using your globally-addressable IPs behind the router you usually need to tell it so, and I'm guessing you haven't done that.

If your laptop is currently using 192.168.a.b with a default gateway of 192.168.0.1, then the packets will be sent to the router for routing. And if the router is in the default configuration usual for home users, it won't know there's a 1.2.3.z subnet on your LAN, and it'll be trying to route the packets out into the internet (and those poor little packets will probably get very confused when they reach your ISP and it sends them back again!).

You can triple-home by setting eth0:2 to 192.168.c.d, and can (depending on your bridge/router configuration) continue to use private addresses for additional machines if you have more hosts on the LAN than you have globally-addressable IPs. Watch out for default gateway and DNS stuff! You'll probably want to give your laptop 1 private + 1 global IP whilst you're testing.

If your router is a fully-configurable Linux box then say so, and someone will be able to point you in the right direction for this. It's been a long time since I messed with this. I'm still using an old Draytek Vigor here, and this is all done by checking a radio button and pasting IP addressy stuff into boxes in the web-based configurator. The router has to operate in bridging mode to do this - on a Linux router you need configure as a bridge and NOT as a router (or, I think, as both).
http://www.google.com/search?q=bridge+5+usable+ips

My initial reaction to your question was, "Oh, Grant, this is simple network stuff", but whilst writing this my memory has been roughly refreshed, and I realise it's really not terribly simple at all. I'm extremely rusty at all this, and I'm also very tired, so I'm not going to write any more right now. 

Stroller.