Why do packets to cause dial-up?

Nik Clayton nik at freebsd.org
Thu Mar 30 10:58:58 BST 2000

Hi guys,

This is a bit of a noddy question, but I'm damned if I'm exposing my
ignorance to the entirety of the world via -questions.  The whole of 
the UK (and certain small European countries, hi Brad) should be more
than sufficient.

I've got a bog standard PPP set up at home, dialling in to Demon.  No 
PPP filters yet, though there will be soon.  I'm running Samba on the
FreeBSD box.

Periodically, as shown by "tcpdump -i tun0" Samba is sending packets like

    09:51:21.680366 localhost.nothing-going-on.org.netbios-dgm \ 
        > udp 220
    09:52:31.678547 nothing-going-on.demon.co.uk.netbios-ns \
        > udp 50
    09:52:33.698393 nothing-going-on.demon.co.uk.netbios-ns \
        > udp 50

This will bring the modem line up.

I'm going to track down the whys and wheres of Samba doing this in a minute,
and block it.  That should be fairly easy with some dial filters.

What I don't understand is how packets for the 127/8 broadcast address are
getting on to tun0 in the first place.  Why aren't they staying on lo0 like 
they should be?

"ifconfig -a" shows

        inet netmask 0xffffff00 broadcast
        inet netmask 0xffffffff broadcast
        inet netmask 0xffffffff broadcast
        inet netmask 0xffffffff broadcast
        inet netmask 0xffffffff broadcast
        inet netmask 0xffffffff broadcast
        ether 00:90:27:30:9c:88 
        media: autoselect (100baseTX) status: active
        supported media: autoselect 100baseTX <full-duplex> 100baseTX 10baseT/UTP <full-duplex> 10baseT/UTP
lp0: flags=8810<POINTOPOINT,SIMPLEX,MULTICAST> mtu 1500
        inet --> netmask 0xffffff00 
sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 552
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
        inet netmask 0xff000000 

which is expected (hmm, must get around to disabling lp0 and sl0 some time)

And I've got the following entries in smb.conf:

    interfaces = lo0
    bind interfaces only = true

As you can see, tun0, or it's IP address, aren't mentioned anywhere.

There's obviously a gap in my understanding of how IP works.  Any pointers
appreciated (and, if you're going to be at the LUUG meeting tonight, 
they'll get a beer bought for you as well).

Internet connection, $19.95 a month.  Computer, $799.95.  Modem, $149.95.
Telephone line, $24.95 a month.  Software, free.  USENET transmission,
hundreds if not thousands of dollars.  Thinking before posting, priceless.
Somethings in life you can't buy.  For everything else, there's MasterCard.
  -- Graham Reed, in the Scary Devil Monastery

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