topic Re: networking issue in eth0 and eth1 , arp in Operating System - Linux

networking issue in eth0 and eth1 , arp

guna81_m — Thu, 06 Aug 2009 09:12:39 GMT

hi friends ,

I have two network cards eth0 and eth1 in rhel5 .

eth0 - 192.168.168.23 - macaddress1
eth1 - 192.168.168.26 - macaddress2

Iam able to ping both ip address from windows xp machine

if iam typing arp -a command ,iam getting same mac address for both 192.168.168.23 and 192.168.168.26

arp -a in win xp gives

192.168.168.23 macaddress1
192.168.168.26 macaddress1

iam getting the mac address of eth0 for both the ip .

How to trouble shoot ????

thanks and regards,
M.Gunasekaran.

Re: networking issue in eth0 and eth1 , arp

Goran Koruga — Thu, 06 Aug 2009 09:16:25 GMT

Hello.

Are they in the same subnet?

Goran

Re: networking issue in eth0 and eth1 , arp

guna81_m — Thu, 06 Aug 2009 09:20:32 GMT

yes they are in the same subnet

Re: networking issue in eth0 and eth1 , arp

Goran Koruga — Thu, 06 Aug 2009 09:34:02 GMT

There you go - that explains it.

Re: networking issue in eth0 and eth1 , arp

Matti_Kurkela — Thu, 06 Aug 2009 09:35:30 GMT

Too bad you did not specify the name and version of your Linux distribution.

Connecting one machine with two separate NICs and IP addresses to one subnet is not a recommended configuration and will usually lead to exactly the same symptoms you saw.

If you only need two IP addresses, configure one of the addresses as an IP alias:

192.168.168.23 = eth0 just as before,
192.168.168.26 = eth0:1 IP alias.

Please read the documentation of your Linux distribution to find the recommended way to configure IP aliases.

If you need more network bandwidth than one NIC can provide, you should bond the interfaces together into a single bond0 interface. When the bond0 interface is configured, you can then use IP aliases just like with a single NIC.

See this URL for general information about NIC bonding:
http://www.mjmwired.net/kernel/Documentation/networking/bonding.txt

Please read the documentation of your Linux distribution to find how to configure NIC bonding.

MK

Re: networking issue in eth0 and eth1 , arp

rick jones — Fri, 07 Aug 2009 15:43:48 GMT

The Linux networking stack takes a very hard interpretation of the so called weak end system model. It interprets IP addresses as being assigned to the *system* and not just to an interface. As such, any interface will be perfectly willing to respond to any ARP request it sees for any IP address assigned to the system. Regardless of IP subnet.

One way to deal with this is to use sysctl to set the "arp_ignore" parameters to a value of 1. You can put the setting for the "def" one you see in:

sysctl -a | grep ignore

into /etc/sysctl.conf and it will be applied on subsequent reboots.

Now, backing up from your specific question, why are you configuring multiple interfaces (from the point of view of the transport) into the same IP subnet? What end do you wish to achieve?