ARP shows client is given wrong MAC Address for system with 2 NICs

ARP shows client is given wrong MAC Address for system with 2 NICs

Post by Q A » Thu, 06 Dec 2001 08:30:21



Hello,

I am not on the linux-kernel mailing list so please
reply all.

My problem:

Note that in the following description _(A) refers to
the interface that was brought on line first in the
system after boot up.   That is if eth0 came up first,
_(A) refers to eth0.   If eth1 came up first, _(A)
refers to eth1.  Read on and you will see the problem.

I have a system with 2 NICs in one box.  When I telent
to IP(A) and I am allowed to connect with no problems
and are given MAC(A) which I check with the arp
command.  I then telnet to IP(B) and are given MAC(A).
 Yes, MAC(A).  I then ifdown NIC(B) device and telent
to IP(B) and connect with no problems.

Now looking at ARP I see I am connected to NIC(A)
because I have the MAC(A) according to ARP.  I then
bring up NIC(B) and shutdown NIC(A) while still
connected as it looks to NIC(A).  I look at ARP and I
see I still have the MAC(A). I press enter in the
telnet window connected to what I ARP tells me is
NIC(A) and everything works.   I check ARP and see
that the I now have MAC(B) for the connection.

I really need to fix this soon so that I can bring
this box online.

The system:
uname -a...
Linux system-name 2.4.9-13SGI_XFS_PR1enterprise #1 SMP
Fri Nov 2 23:44:39 CST 2001 i686 unknown

lsmod...
Module                  Size  Used by
nfs                    94706   0  (autoclean)
lockd                  60515   0  (autoclean) [nfs]
sunrpc                 84519   0  (autoclean) [nfs
lockd]
autofs                 13596   6  (autoclean)
e1000                  47194   0  (autoclean)
eepro100               19799   1  (autoclean)
lvm-mod                64482  16
usb-ohci               23358   0  (unused)
usbcore                66308   1  [usb-ohci]
raid1                  16069   4
raid0                   4314   1
cciss                  21124   4
aic7xxx               119883   8
sd_mod                 12859   8
scsi_mod              110515   2  [aic7xxx sd_mod]

This is eth1: e1000                  
This is eth0: eepro100              

Based on RH Linux 7.1 and as you can see SGI XFS.

I found two notes on this subject on the net.

Both are from this list.  
One has the subject:  RE: ARP out the wrong interface
From Date:  Feb, 09 2001

This one notes that 2.2.18 and 2.4 have a patch to
allow you to:

# sysctl -w net.ipv4.conf.all.arpfilter=1

Yet I only found net.ipv4.conf.all.arp_filter
setting this to 1(on) has no effect on the problem.

The second one has subject:  RE: [2.2] Network
Interface aliasing
From Date:  Nov, 9 1999

This on talks about IP in the 2.2 kernel.  It talks
about an "optimization to reduce work on hosts
monitoring in promiscuous mode."  "The decision is
made on IP level and by IP address, MAC level details
are inessential."  So I gather what I am seeing is to
optimise network trafic.  But I think there is a bug.

I am not a kernel programmer.  Please have a look and
let me know what more info you need.

Thanks,

Q

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ARP shows client is given wrong MAC Address for system with 2 NICs

Post by Richard B. Johnso » Fri, 07 Dec 2001 04:10:15


[SNIPPED...]
There is an ARP cache, always has been, always will be. This is so
an ARP (Address Resolution Protocol) probe doesn't have to occur for
every data transmission. It is presumed that an IP address, including
your own, won't jump around from device-to-device.

You are moving your IP address to another device (MAC address). What
do you expect?

You can delete the old entries from your ARP cache, but it has to
be done for every system that would be affected or you can just wait
for the ARP cache entry to expire.

    /sbin/arp -d ipaddress

Cheers,
* Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

    I was going to compile a list of innovations that could be
    attributed to Microsoft. Once I realized that Ctrl-Alt-Del
    was handled in the BIOS, I found that there aren't any.

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ARP shows client is given wrong MAC Address for system with 2 NICs

Post by Christopher Friese » Fri, 07 Dec 2001 05:00:11



> You are moving your IP address to another device (MAC address). What
> do you expect?

> You can delete the old entries from your ARP cache, but it has to
> be done for every system that would be affected or you can just wait
> for the ARP cache entry to expire.

>     /sbin/arp -d ipaddress

It is also possible to send out a gratuitous arp packet that will force everyone
else on the subnet to update their arp caches.  This is the normal technique for
moving an IP address from one NIC to another.

Chris

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ARP shows client is given wrong MAC Address for system with 2 NICs

Post by Q A » Fri, 07 Dec 2001 11:20:09


Thanks, but I am not moving the IP from one to the
other.  I am just saying it doesn't matter _(A) does
not have to be eth0.  Try setting up a system with 2
NICs and follow my notes.  I have checked another
system with a normal 2.4.3 kernel.

Thanks for yours and everyone elses help.

Q



Quote:

> [SNIPPED...]
> There is an ARP cache, always has been, always will
> be. This is so
> an ARP (Address Resolution Protocol) probe doesn't
> have to occur for
> every data transmission. It is presumed that an IP
> address, including
> your own, won't jump around from device-to-device.

> You are moving your IP address to another device
> (MAC address). What
> do you expect?

> You can delete the old entries from your ARP cache,
> but it has to
> be done for every system that would be affected or
> you can just wait
> for the ARP cache entry to expire.

>     /sbin/arp -d ipaddress

> Cheers,
>* Johnson

> Penguin : Linux version 2.4.1 on an i686 machine
> (799.53 BogoMips).

>     I was going to compile a list of innovations
> that could be
>     attributed to Microsoft. Once I realized that
> Ctrl-Alt-Del
>     was handled in the BIOS, I found that there
> aren't any.

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1. MAC address does not show up on the telco's ARP table caches

// __ 20050915 14:15
 Hi,

 I am using as ISP that uses the telephone/DSL network from the local
telco to provide their clients with broadband/DSL access (they do
provide a better/marginally cheaper service).

 The thing is that I am repeatedly (and way too often) kicked off the
net in ways that even my ISP tech suppost considers to be "very
strange"

 I know I am connected to the Net because if I run ethereal setting my
card on promiscuos mode (I am connecting to the Net directly), I can
see the whole traffic of what is going on on my domain. I must be a
heck of a guesser to make up an ethereal ethernet traffic log.

 They have told me things like:

 ._ your MAC address does not show on the telco's cache.

 ._ your MAC does temporarily show on their ARP table cache
sometimes(?!), but as an imcomplete address

 ._ their ARP tables also show lots of different MAC addresses assigned
to you that are actually not yours

 Also my ISP has a procedure to refresh your NIC's MAC addresses that I
have used to no avail. I have even tried using dirrent NICs on X86
boxes and even a Mac.

 What could be the 'technical' reasons why this could be happening?

 Thanks
 dennishartman

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