why port forwarding is not working in this setup?

I'm trying to setup a docker virtualization environment. This is a follow up of this question.

I have a virtual eth0:0 interface, and I would like to forward it using iptables.

The public, main IP is 93.93.93.93

The failover IP is 5.6.7.8

I've a server where IP aliasing is configured:

/etc/network/interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 93.93.93.93
        netmask 255.255.255.0
        network 93.93.93.0
        broadcast 93.93.93.255
        gateway 93.93.93.254

# IPFO 1
    post-up /sbin/ifconfig eth0:0 5.6.7.8 netmask 255.255.255.255 broadcast 5.6.7.8
    pre-down /sbin/ifconfig eth0:0 down

This way, when I ping eth0:0 from outside it works. It seems like the problem start from routing maybe?

root@aldebaran:~# ip route
default via 93.93.93.254 dev eth0 
93.93.93.0/24 dev eth0  proto kernel  scope link  src 94.23.55.226 
172.17.0.0/16 dev docker0  proto kernel  scope link  src 172.17.42.1 

No sign of eth0.0

Then I add iptables rules:

iptables -A PREROUTING -t nat -i eth0:0 -p tcp --dport 80 -j DNAT --to 172.17.0.2:80
iptables -A FORWARD -p tcp -d 172.17.0.2 --dport 80 -j ACCEPT

Then I try

root@aldebaran:~# curl 172.17.0.2:80
WORKS!
root@aldebaran:~# curl IP_FAILOVER
curl: (7) Failed to connect to IP_FAILOVER port 80: Connection refused

According to this tutorial, I haven't setup routing correctly. Is this right? How do I fix this?

The problem was that I was using IP alias, which creates an interface that Iptables cant use. Eth0:0 is not a valid target. I should use instead:

iptables -t nat -I PREROUTING --dst 5.6.7.8 -p tcp --dport 80 -j DNAT --to 172.17.0.2:80
iptables -A FORWARD -p tcp -d 172.17.0.2 --dport 22 -j ACCEPT
iptables -t nat -A POSTROUTING -s 172.17.0.2 -j MASQUERADE