Hi All.
I'm sorry if my problem is being caused by my ingnorance.

I have a host with IP 172.18.0.43/23 which is perfectly pingable.
Now, if I add to that host an IP alias on lo then the host is unreachable.

ip addr add 172.18.0.44/23 dev lo:0

What could be the reason for this?

Thanks in advance.

Well, after such an action it decides to send all packets inside subnet (including ones to the router, including ping replies) through loopback. These attempts fail, because loopback interface doesn't allow packets to leave the box.

Excuse my ignorance, but why is this so?
And how can I avoid this while still using an alias on localhost.

Thanks!

Why do you want to have an alias assigned to loopback?

If the alias is the same as another subnet on another interface, you can bet data will not be reliably routed to that interface.

I can't see what you hope to accomplish by aliasing 'lo' - other than interfering with your routing.

I have a load balanced setup of two Apaches.
The user connects to a virtual IP (say 1.2.3.4) and gets 'balanced' to one of the Apaches.
In order for the Apache server to be able to accept connections from the virtual IP, an alias should be created, so that it is actually able to listen on that IP.
In all guides that I have read on load balancing, they specify that this virtual IP should be created as an alias on lo. Don't ask me why...

Since it is giving that many problems, I tried adding it to tunl0, which seems to be working perfectly.

I don't understand why all tutorials mention to add it to lo:0 while this causes that many problems. Any clues?

Btw, if someone could explain what exactly is going wrong during routing, I'd appreciate. Not that I still need the alias on lo, but I'm interested in understanding why it is not possible/recommended.

Thanks for your replies.

Well, adding with netmask /32 would probaly also help.

The problem with routing that I don't like is that you have 2 IP's on different devices inside the same subnet. Subnet means that sending data is simple - just send it directly over the medium used by interface that has IP in subnet. Everything is worse when you have two devices on the same subnet and on different physical layers...

That's something that I didn't know.
I thought that it would send a packet from the interface where the program is listening on, and not from another interface, although it's in the same subnet.

I mean, if I have 2 servers running on my machine, on the same subnet, I expect that responses will be send from the interface where the request arrived. I still don't really get the problem...guess my networking skills are way below zero