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Milkfish and AVM Fritzbox - Introduction

The FritzBox by AVM, Berlin, is a widely used Router Hardware in Germany. This is not only due to its very well engineered embedded hardware but also to its software capabilities which we will both look at briefly in this chapter.

Below, we will sketch and describe how to connect and configure the Fritzbox to work behind a Milkfish NAT Router. In other words: Fritzbox inside a LAN using the Milkfish Internet Router in the same LAN as its SIP Outbound Proxy and RTP Proxy. The Fritzbox therefore acts exclusively as an ISDN-/Analog-Terminal-Adapter for SIP IP Telephony.
Please note: The Fritzbox itself is not hooked up directly to the internet. It uses the Milkfish Router for Internet traffic.

More information about the Fritzbox you may find at the manufacturers website or at

Milkfish and AVM Fritzbox - Overview

Various Fritzbox Hardwares are available, having different interfaces and properties. Relevant here are the following:

  • Up to three analogue FXS ports for connecting POTS phone sets or PBXes
  • One dual POTS-FXO/ISDN-BRI landline trunk connection
  • One internal ISDN S0 bus for interfacing existing ISDN equipment (PBX, DECT, Phones...)
  • One or more Ethernet LAN ports

The FXS ports, as well as the internal ISDN port, can be mapped through an configurable dialplan to SIP User Accounts for Dialout, effectively turning the Fritzbox into an ISDN-/Analogue-Terminal-Adapter for Internet Telephony.

Please note that this setup is not found working for Milkfish Boozy Releases 3.0.0 and older!
The Fritzbox we use sucessfully for this setup:
AVM FRITZ!Box Fon 5050 (UI) 12.04.31 (Feb 5 2007)

 .         POTS/ISDN Cabling         |          Intranet          ||     Internet       |      PSTN
 .                                   |                            ||                    |
 .                                   |                            ||                    |
 .                                                                ||                    |
 .  Old Phone                                                                           |
 .   ________                  AVM Fritzbox        Milkfish Router      VoIP-Provider   |     ________
 .  |_Phone1_|               ________________       ______________       ____________        |_Phone3_|
 .    / .. \   twin wire    |                | eth |              |     |            |         / .. \  
 .   [______]---------------| Fritzbox IATA  |--+--|  SIP Proxy   |-----|  Gateway   |--------[______]
 .                          |________________|  |  |______________| DSL |____________|
 .                         /        ||          |                                            Landline Phone
 .   ________             /         /           | Ethernet        ||                    |
 .  |        |  int.     /         /ext.        |    ________     ||                    |  
 .  |ISDN PBX|  S0-Bus  /         / S0-Bus      |   |_Phone2_|    ||                    |
 .  |________[---------+         / (optional)   |     / .. \      ||                    |
 .                              /               +----[______]     ||                    |
 .                   BRI-NTBA--+                     SIP Phone

Milkfish and AVM Fritzbox 5050 - Example 1

For the network setup as sketched above you need to enable the Fritzbox to work behind the Milkfish Router. This means you need to configure the Fritzbox to use the Milkfish Router as its Internet Gateway and connect it to the Milkfish Router using an Ethernet cable. With the factory settings, the Fritzbox may try to dial up the internet through its integrated DSL Modem, which needs to be deactivated for the setup above.

For the signalling (SIP) to work, you need to set the Milkfish Routers LAN IP address as the SIP-Proxy address in the SIP Account settings, as shown in the screenshot below:

Milkfish and AVM Fritzbox 7270 - Example 2

Based on the howto given here, find below the procedure to make the box get the current time set at startup automatically, which is particularly interesting if you use the fax-to-email or push service feature of the box when the box is only connected to the LAN and cannot get the current time by itself:

First you need to activate the telnetd using a analogue phone by dialing:


Open a shell/DOS prompt on your PC and execute


Then, on the box enter the following commands...

 # cd /var
 # echo > /var/flash/debug.cfg
 # nvi /var/flash/debug.cfg

...and edit the file debug.cfg so it looks like this:

 # cat /var/flash/debug.cfg 
 wget -O /var/gettime.tmp -q
 date $(cat /var/gettime.tmp)
 rm /var/gettime.tmp

Below you find a local subscriber configuration for a Fritzbox 7270 (v54.04.58) at using a Milkfish-dd 1.0 router at as its registrar.

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Page last modified on October 19, 2008, at 05:20 PM