iptables trouble: nat with ip alias (virtual interface) not working

Question

I am calculating the day difference between two day using JavaScript. For some cases I am getting right result but some cases I cannot get the right result. My code is below:

function caldate(){
       var difference=Math.abs((parseInt(document.getElementById('dayto').value)%7)-(parseInt(document.getElementById('dayfrom').value)%7))+1;
           alert(difference);
}

Here when day to is Sunday I am getting the wrong result. My plunkr code is here. I need to calculate the difference between two day including both selected days.

Solution

When you use virtual interfaces just use the option -d (destination), ignoring the option -i (interface)

Instead of:
iptables -t nat -A PREROUTING -i eth0 -d 25.25.25.26 -j DNAT --to-destination 172.16.2.1

use:
iptables -t nat -A PREROUTING -d 25.25.25.26 -j DNAT --to-destination

Work with iptables v1.4.14 in Debian

Comments

[adeneo]

Why are you changing the values with .abs() and %7, just compare the values

[satya]

@adeneo : suppose i selected from day=wedensday and to day=sunday the difference should 5 .

[charlietfl]

it is 5 if you remove %7

[adeneo]

The values are numeric strings, just subtract them, as in value1 - value2, and if you want to avoid negative numbers wrap the whole thing in a Math.abs call

[Emil S. Jørgensen]

Question: If dayfrom is greater than dayto, are we to assume another that day, but in next week?

[Larssend]

The FORWARD chain is not part of the nat table. Your proposed 'correction' will not be accepted by iptables -- it will give you an error message instead.

[Larssend]

Also, DNAT is only valid for PREROUTING and OUTPUT chains. Please avoid recommending anything that you don't really know.