Re: ping to local interface v4 address works, even though it's admin/line down

From	Tauno Voipio <tauno.voipio@notused.fi.invalid>
Newsgroups	comp.os.linux.development.system
Subject	Re: ping to local interface v4 address works, even though it's admin/line down
Date	2012-09-16 10:06 +0300
Organization	A noiseless patient Spider
Message-ID	<k33tp2$4o2$1@dont-email.me> (permalink)
References	<26eec478-85d3-4ba9-9830-d8312eb894bc@googlegroups.com>

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On 16.9.12 5:15 , anjanishp@gmail.com wrote:
> Can somebody help me understand why ping works to a local interface, even if it's admin and line protocol down in linux.
>
> I have a eth1 interface, i assign v4 address to it.
> I make the interface down.
> try ping to the v4 address, and ping is success.
>
> Please find the logs for the bahaviour.
>
> root@avi:/home/avinash# uname -a
> Linux avi 3.2.0-23-generic-pae #36-Ubuntu SMP Tue Apr 10 22:19:09 UTC 2012 i686 i686 i386 GNU/Linux
> root@avi:/home/avinash#
>
> root@avi:/home/avinash# ifconfig eth1 22.0.0.1
> root@avi:/home/avinash# ifconfig eth1
> eth1      Link encap:Ethernet  HWaddr 00:30:48:b1:ee:33
>            inet addr:22.0.0.1  Bcast:22.255.255.255  Mask:255.0.0.0
>            UP BROADCAST MULTICAST  MTU:1500  Metric:1
>            RX packets:65 errors:0 dropped:0 overruns:0 frame:0
>            TX packets:17082 errors:0 dropped:0 overruns:0 carrier:0
>            collisions:0 txqueuelen:1000
>            RX bytes:5926 (5.9 KB)  TX bytes:3841723 (3.8 MB)
>            Interrupt:44 Base address:0xe000
>
> root@avi:/home/avinash# route -n
> Kernel IP routing table
> Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
> 0.0.0.0         10.16.48.1      0.0.0.0         UG    100    0        0 eth0
> 10.16.48.0      0.0.0.0         255.255.255.0   U     0      0        0 eth0
> 22.0.0.0        0.0.0.0         255.0.0.0       U     0      0        0 eth1
> 169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
> root@avi:/home/avinash# ifconfig eth1 down
> root@avi:/home/avinash# route -n
> Kernel IP routing table
> Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
> 0.0.0.0         10.16.48.1      0.0.0.0         UG    100    0        0 eth0
> 10.16.48.0      0.0.0.0         255.255.255.0   U     0      0        0 eth0
> 169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
> root@avi:/home/avinash# ifconfig eth1
> eth1      Link encap:Ethernet  HWaddr 00:30:48:b1:ee:33
>            inet addr:22.0.0.1  Bcast:22.255.255.255  Mask:255.0.0.0
>            BROADCAST MULTICAST  MTU:1500  Metric:1
>            RX packets:65 errors:0 dropped:0 overruns:0 frame:0
>            TX packets:17082 errors:0 dropped:0 overruns:0 carrier:0
>            collisions:0 txqueuelen:1000
>            RX bytes:5926 (5.9 KB)  TX bytes:3841723 (3.8 MB)
>            Interrupt:44 Base address:0xe000
>
> root@avi:/home/avinash# ping 22.0.0.1
> PING 22.0.0.1 (22.0.0.1) 56(84) bytes of data.
> 64 bytes from 22.0.0.1: icmp_req=1 ttl=64 time=0.027 ms
> 64 bytes from 22.0.0.1: icmp_req=2 ttl=64 time=0.019 ms
> ^C
> --- 22.0.0.1 ping statistics ---
> 2 packets transmitted, 2 received, 0% packet loss, time 999ms
> rtt min/avg/max/mdev = 0.019/0.023/0.027/0.004 ms
> root@avi:/home/avina

This has already been answered to you in another newsgroup.
Are you not happy with the answers?

The IP stack is smart enough to route packets destined to
*any* of the local addresses via the loopback interface lo.
The local ping never gets so far that it could detect that
the interface is downed.

-- 

Tauno Voipio

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ping to local interface v4 address works, even though it's admin/line down anjanishp@gmail.com - 2012-09-15 19:15 -0700
  Re: ping to local interface v4 address works, even though it's admin/line down Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2012-09-16 10:06 +0300

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