You probably found this because you're trying to make a socket connection from Flex/Flash and getting the following error:
SecurityError: Error #2123: Security sandbox violation:
Adobe went through a number of phases making the rules for serving and checking Policyfiles stricter. There are different security sandboxes. If you publish your flex/flash application on domain.com, and the application attempts to load content from domain2.com, it will look for a Cross Domain Policy File at domain2.com/crossdomain.xml to get permission. It does this automatically, however you could specify the location of the Cross Domain Policy File in your flex application using the following method:
Security.loadPolicyFile("http://domain.com/remote_content/crossdomain.xml")
A Cross Domain Policy File only has authority to grant access to content below it in the folder hierarchy. So a policy file in /remote_content/ can't grant access to content in the root folder, and in addition a Policy File at the domain root can override any other policy file. It has maximal authority. Subdomains are considered separate domains - which as a side note most search engines see subdomains that way too.
Now that's Cross Domain Policy Files, In general Adobe Air applications operate in one of the local system sandboxes and has thus have access to content on any domain. This post is about Socket Policy Files. When you access regular web content you're generally connecting to your server on port 80 and being served content by Apache or whatever web server you happen to be running. When you do this you're using http protocols under the hood and never have to deal directly with that crazy stuff. If you want to make a raw socket connection to your server you will need to serve up a Socket Policy File on a specific port.
First I just want to stress the difference between a Cross Domain Policy File and a Socket Policy File. For some reason my dyslexia and the ton of misleading, vague, and now out of date and incorrect information I kept thinking they were the same thing. Second there is no way as far as I'm aware to serve a Socket Policy File with Apache.
The default port for flex/flash to search for a socket policy file on port 843. There are several places on the web that talk about being able to connect to higher port numbers without a Socket Policy File, well that doesn't seem to be the case anymore. Just assume that any raw socket connection from a flex/flash client requires a Socket Policy File.
You can serve the Socket Policy File from the port you're connecting to, but this is tricky considering the manner in which Flex/Flash goes about loading the Socket Policy File and rewriting the service to serve this up, especially if you're using server software built by someone else, means it's just better to keep the Socket Policy File Server as a separate always running entity on the system.
Now in the simplest implementations you need a process either written in python, perl, c++, php cli, or whatever. It needs to be listening on port 843. It has to wait for - very specifically - the following string<policy-file-request/> followed by a NULL byte. Upon receiving that it needs to serve up the policy file which needs to at least have allow-access-from domain set to *, and to-ports set to *. You should use the links at the end of this post to familiarize yourself with the differences between and all options you can specify in Policy Files. It's easiest to keep the Policy File as an actual file, instead of adding the text of the file to your custom server code. And that's it!, now you can go on with a better idea of what information out there is out of date or not.
Here are some important links to help you on your journey:
Adobe on setting up a Socket Policy File Server
Adobe on Policy File changes for flash 9 and 10
Adobe on the structure of Policy Files
An intro to Sockets
Working PHP Cli Socket Policy File Server
