Lost Denon on network with linux firewall.

We installed a remote backup server running Debian Linux in a client’s home. The backup server is also a firewall to protect the home’s Personal Computers. The home network also had a Twonky MediaServer to manage the components of a home theater.

Due to the way the home is wired (and other reasons) the Denon receiver sits outside the firewall in a DMZ behind the internet modem on a separate LAN. This caused the Twonky MediaServer to not be able to connect to the Denon receiver without opening some ports on the firewall. We typically run a closed outbound policy and open ports as needed.

We spent some time trying to research what ports the Denon was using. Unfortunately documentation was lacking so we had to resort to the old fashioned method. Here was the steps we took and the results.

The DMZ network where the Denon resides is on the network.

First step was to port scan the network with nmap to find any responding IP’s. The Denon as you can see was kind enough to identify itself.

firewall:/$ nmap -sP
Host appears to be up.
MAC Address: 00:00:00:00:00:00 (Denon)

We found the Denon so let’s see what ports are open.

firewall:/$ nmap
Starting Nmap ( ) at 2009-01-19 16:19 PST
Interesting ports on
Not shown: 1673 closed ports

23/tcp   open  telnet
80/tcp   open  http
443/tcp  open  https
5000/tcp open  UPnP
5001/tcp open  commplex-link
6666/tcp open  irc-serv
8080/tcp open  http-proxy
MAC Address: 00:00:00:00:00:00 (Denon)

Ahh, looks like UPnP, so open it up and test connectivity.

firewall:/# telnet 5000
Connected to
Escape character is ‘^]’.

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