Windows machines causing ISDN-dialouts



When having a network with NT workstations/servers connected to the

internet over a Brick, the Brick unreasonably establishes connections

over ISDN to the internet without any user interaction.

This behaviour is not a Brick's bug or problem, because the Brick simply

connects to the internet when any PC from the LAN tries to reach an ip-

adress outside the LAN.

Especially Windows NT PCs are sending udp-packets at the broadcast-

address of the LAN and therefore an internet connection will be established.

The corresponding service is called Netbios over tcp/ip and it periodically

transmits udp-packets at port 137 to search for other workstations and 


Like described below, it's possible to suppress this behaviour, filtering

the udp-broadcast-packets sent to the Brick.

But there's still another NT service running over port 137, also udp, 

which has the purpose to synchronize the explorers content. If you

display a remote networkdrives' (mounted over ISDN) content, the ISDN

connection will remain established as long as the window is opened.

You can't filter this too, you can't do anything but close the window!



PC--------------LAN-----------Brick--------ISDN-----------Internet                  Interface 'ISP'


1. Connect to Brick with telnet or serial connection and log in

2. enter 'setup'

3. choose IP->Access Lists

4. add a 'deny'-filter for interface 'en':

BIANCA/BRICK-XM Setup Tool                           BinTec Communications GmbH

[IP][ACCESS][EDIT]: Configure Access List Entry                          brick3




  Mode                        deny                                            


  Protocol                    udp

  Source Interface/Partner    en


  Source Address               

  Source Mask                  

  Source Port                 any



  Destination Address

  Destination Mask  

  Destination Port            specify

  Specify Port                137


                    SAVE                               CANCEL




This filter will cause all broadcasts using udp-port 137 on the ethernet,
not to be transported over ISDN, eg using interface 'ISP'.


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If you have comments or suggestions, send email  at support@BinTec.DE