Why request.getRemoteAddr() returns ipv4 or ipv6 depending on context (post query or ajax query)

Question

I've donne a web app with Spring/GWT that uses Flash to upload files.

When I send an ajax request with GWT and try to get the user ip address, I get an ipv4 address like: 127.0.0.1

but when I upload my files with flash (and so a post request on the same webapp) I get an ipv6 address 0:0:0:0:0:0:0:1

I use the same code to get the user ip which is:

ServletRequestAttributes att = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
att.getRequest().getRemoteAddr();

The problem is that I use a signature which uses the ip address to be sure that user can upload files (due to bug with session lost when uploading with flash) and the ip address changes when I use ajax or Post via flash.

What's the problem and how can I get the same ip format ?

thanks

Answer 1

You can fix it by starting your server with this flag

-Djava.net.preferIPv4Stack=true

Answer 2

The reason is that the name you type in your browser has both ipv4 and ipv6 addresses. Browser chooses to use ipv4, flash chooses to use ipv6. There is nothing you can do about that on the server side (there is no way to convert).
However, you can change your DNS and make your name have only one address, either ipv4 or ipv6. You can also connect with a literal IP address (127.0.0.1), not a name (localhost).

Answer 3

you get an IPv6 address then IPv6 is being used. IPv4 and IPv6 are different protocols, and the client chooses which one to use when both are available.

If you want the client's IPv4 address then you can force them to use it by not advertising the IPv6 address in DNS. That would be a bad idea though with the increasing deployment of IPv6. Supporting both is good, so it is better to deal with clients using IPv6.