Without port forwarding there's no way a NAT-enabled router can know
what class C IP to forward the packet or connection to so the  only
valid option is to drop the packet or connection.

If a router I owned forwarded any such packet or connection I'd
return it whence it came accompanied by a complaint.

How about using e-mail?

That way firewalls and NAT routers are not an issue because the
destination(s) will fetch your message from their ISP's mail servers.

JavaMail is pretty easy to use, especially to send messages, and its
easy to send the file as a MIME attachment. Get JavaMail from
http://java.sun.com/products/javamail/index.jsp and make sure you grab
both the ADI Design Specification (which includes useful example code)
and the API documentation. You'll need the JavaBeans Activation
Framework as well: http://java.sun.com/products/javabeans/jaf/index.jsp

aca04pds@shef.ac.uk wrote in news:1177268794.163530.313070
@e65g2000hsc.googlegroups.com:

Use something like the FTP portion of Apache's commons-net component
(http://jakarta.apache.org/commons/net/), which contains the underlying code
necessary to construct both the client (sender) and the server (receiver)
code.

Cheers!

Java will require that you set up a program, some program, also.
There's nothing to receive the transfer request if you don't.  Even the
other comments about using Apache code will require some kind of server.

There is an FTP server built into most version of Windows.  Have you
looked into just enabling that?

How about Windows file sharing?  Just turn that on and use Explorer.
Any already working solution, whether it's FTP, HTTP or Windows, will be
much easier to implement than trying to roll your own.

Doesn't matter.  Everything above should work still, unless your router
is borked.  And even then everything will still work if your router
isn't involved (ie, all PCs are connected via a switch).

FTP pretty easy, and supported by most web browsers.  Windows file
sharing harder on you, but probably easier on the users once it's going.

On Apr 22, 3:06 pm, aca04...@shef.ac.uk wrote:

Set up a VPN or use an SSH tunnel.  This requires an initiation from
the host behind the NAT to some publicly accessible place.  I don't
deal with VPNs, but SSH-tunnels are trivial and work on all OS's.

While you may already be aware of how to do this.. You're basically
taking a host accessible to the client and defining a host:port
combination that is the proxy.  All TCP/IP traffic then gets forwarded
through the NAT to your target machine where the real server port
resides.

The only problem is that your SSH connections need proper keep-alives
- or at least detection which will restart the SSH connection
(assuming a public/private keypair).

So in the ideal case, you have a publicly accessible host "pubHost"
and a NAT host "natHost".  natHost needs to SSH to pubHost (possibly
via pub/priv keys) and establish a port only accessible to pubHost
(e.g. not exposed to the world - which is the default in SSH) say port
5555 which forwards to natHost port 12345 (the server port).  Now the
client on pubHost connects to localhost:5555 and is really connecting
to natHost:12345, but in an encrypted manner and passing through all
firewalls.

If pubHost's proxied port was exposed to the world (instead of just
localhost), then it had best be behind it's own firewall to restrict
accessibility to an extranet.  Otherwise it would be easier to just
port-forward on the NAT router as others' have suggested.

And FYI, an FTP server is no different than running the java process
listening on port 21 and utilizing the FTP protocol (which is rather
trivial - I believe there are open-source java libraries that can do
this).  The only difference is that by default the FTP server likes
using a pair of sockets which doesn't work at all with NAT boxes.

On Apr 22, 3:06 pm, aca04...@shef.ac.uk wrote:

Are you talking about arbitrary servers?  Meaning some anonymous
person downloads your software, runs it, and it's supposed to act as a
server?  Good lord that would be horrible.. Consider all the personal
firewalls which don't even allow arbitrary sockets to open - you'd
have tech support nightmares.

Be that as it may, the only way to have a dumb-user get this to work,
is to switch the direction of operation for server behind the NAT to
client behind the NAT.  Meaning the natHost connects to a central
server and all connections are thereafter initiated by the server
through the existing channel.

If you're trying to do a napster or bittorrent type deal, then what
happens is that natHost1 connects to central server - there's nothing
to do.. So it sends keep-alive or ping messages down the TCP channel.
Later natHost2 connects to the central server, which says there's
another host connected - i.e. central-server sends a hostID generated
for natHost1 to natHost2.. Now natHost2 wants to send a file to
natHost1 (or rather client-id-1), it sends a forward request
containing raw file-data.  The server forwards the request to natHost1
who then allows the download.

Now either the download can be sequential and the server has to block
natHost2 until natHost1 is available (e.g. blocking natHost3 who wants
to upload at the same time) or the upload request would require that
natHost1 establish a new TCP connection to the server for the express
purpose of receiving the file-upload (which presumably will take a
while).

The parallel method is more complicated because the new connections
have to be associated with the master command-channel.  Also more
things can go wrong.  Plus you can DOS the poor natHost1 by giving it
thousands of connections (or even 2).

Obviously the server node needs to be on a publicly available
address.  But you can even do this through a NAT, so long as you port-
forward on your router and use dynDNS or some other such DNS remapper.

Obviously this is a bandwidth hog for the poor server as it has to
both upload and download all data for all clients.  But there is no
other good way to reliably allow arbitrary NAT-hosted machines to talk
to one another.

Mark Space schrieb:

something like that:

http://redwing.hutman.net/%7Emreed/warriorshtm/necromancer.htm