We have an application that negotiates a different port over time. We want to lock down our firewall to particular port(s), but this seems impossible to do with this apps. I know we can use inspection (fixup) on ftp, dns, sip, ... However, the application we're using is just some not so popular/standard software.
Basically, this is how the client/server talk:
- Client always initiates the converstation with Server on UDP port 19777 (I made this up)-> Server will tell Client "talk to me using UDP/3331"->Client then starts talking to server UDP/3331 until end of conversation.
If there is no inspection for this application then you are limited in what you can do. You could
1) Open up all UDP ports and try to be as specific as you can regarding the source and destination IP addresses.
2) Isolate that server onto it's own DMZ so at least if it is compromised you still have a firewall between it and the rest of your estate.
3) Depends on where the connections are initiated from. You could VPN the connection which would allow you to just open the IPSEC ports but if this is a general application open to all users this probably isn't practical.
4) If you can, talk to the developers of the software. Many times these type of apps can be locked down to a range of ports at the very least.
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