Java Firewall

Question

Hi all,
I have very recently begun using java as a development package, and I love the network capability that it has. It makes it quite a bit easier to make a network application, and implement it.
Something I would like to try would be to make a fairly simple firewall. For instance, when a user inside the network wants to make a connection, it would forward it through to the outside world, and keep the thread open to allow communication back to the host. If the outside world tried getting in, it would reject the host unless is met specific requirements (right port, etc).
Three questions I have:
1. Is this possible? I mean, it seems possible, but would there be a problem with the data, even if I used very basic I/O streams and such? Or would there be problems because of the vast number of protocols and such?
2. How can I differentiate between 2 NICs? On my existing firewall, I'd have eth0 and eth1 for my internet and local net Ethernet cards. How would java know which is which? Creating a server socket is simple, but how do I tell it which interface to listen on?
3. Obviously a firewall would need to listen to all incoming and outgoing ports. Do I seriously need to make 65535 threads to have a server socket listen on every port? I would hope there is a more simple way, but I'm just not finding it anywhere.

Answer 1

It's possible to write a proxy. Taht said, this is
not really a very good first java network assignment
as java network access is quite high level, and this
is a complex project.As far as having the knowledge behind it, that is not
a problem. This isn't really a school project per-say.
I am persuing a degree as a networking specialist, so
network addressing and protocols is not an issue. It
is simply taking that understanding and putting it
into (java) words.
In combination to that, I have a decent programming
background, just not much in java. I hope to learn
quickly, and this is more like a final goal. Java
seems like the perfect language to compliment an
understanding of a network infrastructure.Your OP didn't specify a level of knowledge other than that you were new to java. I assumed you had quite a bit of network experience to even consider just a project.
The 2 NIC have different IP addresses, when creatinga
ServerSocket you can specify the ip address as wellas
the port,Makes sense, I guess, but I was more or less hoping to
have it dynamic. If I ever want to change the address
or move this to another system, I wouldn't want to
have to recompile with the new address. This could
easily be solved with a textbox to input the addresses
though, so it's not a big deal.You could use a Runtime.exec() call to run a command such as ipconfig on windows or ifconfig -a on unix to get that info.
I believe there is no way to do this, a port of 0
means to use an ephemeral port.If this is not possible, then I would think a java
firewall is not possible. Yes, when I said it requests an ephemeral port I meant that you can't do what you want.
Granted, maybe I am just not
understanding the software end of it, but a packet
sniffer, like Ethereal, is able to see all data
passing through, and I doubt it has 65k threads per
protocol to listen. Is there a way to have the program
see all data hitting it's NIC?Java definitely cannot listen to packets in promiscuous mode. There is a package called JPCap that provides packet capture using JNI.
I am not sure where an ephemeral port would solve
this. Granted, it would help with the internals of the
program, passing data from eth0 to eth1, but how would
this solve the problem of knowing with port to recieve
data on eth0?
Thanks again!You're welcome, and good luck