ifdown. is it really down?

[Bruno Wolff III]

On Tue, Jul 01, 2008 at 06:27:16 +0000,
  g <geleem at bellsouth.net> wrote:
>
> my understanding was that 'ifdown' _stopped_ traffic thru port interface.

I tried downing interfaces and the route tables looked to be properly
updated. Maybe there is an issue when you have downed all of the external
interfaces? (I didn't want to down my ppp link at this moment, so I didn't
test that case.)

> with 'ifdown eth0'
> [geo at argosyiayia ~]$ ip link
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
>     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
> 2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000
>     link/ether 00:07:95:2b:26:23 brd ff:ff:ff:ff:ff:ff
> [geo at argosyiayia ~]$ ip addr
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
>     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
>     inet 127.0.0.1/8 scope host lo
>     inet6 ::1/128 scope host
>        valid_lft forever preferred_lft forever
> 2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000
>     link/ether 00:07:95:2b:26:23 brd ff:ff:ff:ff:ff:ff

Well that didn't really down the device, but it did remove the net block
associated with it, so I wouldn't expect any IP traffic to be sent from it.
It might still respond to external layer 2 packets sent to it. But that
wouldn't explain outbound packets matched up with when you try to run
firefox.

>
> with 'ifup eth0'
> [geo at argosyiayia ~]$ ip link
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
>     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
>     link/ether 00:07:95:2b:26:23 brd ff:ff:ff:ff:ff:ff
> [geo at argosyiayia ~]$ ip addr
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
>     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
>     inet 127.0.0.1/8 scope host lo
>     inet6 ::1/128 scope host
>        valid_lft forever preferred_lft forever
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
>     link/ether 00:07:95:2b:26:23 brd ff:ff:ff:ff:ff:ff
>     inet 192.168.1.97/24 brd 192.168.1.255 scope global eth0
>     inet6 fe80::207:95ff:fe2b:2623/64 scope link
>        valid_lft forever preferred_lft forever