Writing a UDP server behind a firewall

Writing a UDP server behind a firewall

Post by brend » Wed, 05 Jun 2002 13:14:50

hi all,

I am writing a little peer2peer chat application which I have running
on my computer, and I'm having some trouble with my firewall software.
 It's actually on Win, but I think some Linux UDP gurus are gonna be
my greatest hope :)  (And the backend runs on Linux, anyhow.)

You can read about my program at www.brendanland.com if you're
interested; there are quite a few explanations on how it works.

I have ZoneAlarm running here.  When someone wants to connect to my
computer, they send a UDP message to a known port on my box.  I have a
socket bind()ed to the port, but I never called listen() because I'm
only accepting UDP, not TCP.

However, the UDP message never arrives.

I thought I could get around this by having the server on my box first
send a UDP message out to the client who's about to connect.  That way
ZoneAlarm will see their incoming packet as a "reply" to the one I
sent, and will let the packet through.  Should this work?  Because it
didn't seem to.

Should I perhaps be calling listen() on my server port?  Would that

The basic question is this: how can my program tell ZoneAlarm that it
should let UDP packets through to a given port?

thanks very much!

- Brendan


1. FTP server behind linux firewall communicating w/ FTP behind linux firewall

I have a Windows-based FTP server (G6) behind a linux firewall box
running ipchain and ipmasqadm portfw rules to enable communication
with the out side world. I can connect to this server from the
outside, but PASV doesn't work. I have rules that allow ports above
1023 for the PASV traffic and I also had put the FTP server on a
haigher port other than 21.  I portfw'd the same port through to the
internal Windows machine running the ftp server as well as forwarding
the ftp-data. I have the ip_masq_ftp module loaded. I'm not sure why
PASV doesn't work.

Also, the other thing I'm trying to get working is communicating with
this same FTP server from a client within another linux-firewalled
(also using ipchains and portfw rules) LAN. I can connect, but can't
get any data transfers going, including directory listings, using
either PASV or regular FTP. I'm not sure if I should be forwarding
ftp-data to the internal machine running the ftp client.

What I ultimately want to do is be able to connect from a client
within on linux firewalled LAN to an ftp server inside another linux
firewalled LAN on a non-standard port and using PASV if possible. Any
help would be appreciated.

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