[CentOS] routing multiple network cards on a single subnet

Clint Dilks clintd at scms.waikato.ac.nz
Wed Jan 20 21:27:17 UTC 2010


Frank Cox wrote:
> I have dealt with machines that have multiple network cards in them
> before, but never when they were on the same subnet so this issue has
> never come up before.
>
> My problem is that I can only access one IP address at a time.  I
> started out using dhcp and found that if I went through the dhcp
> song-and-dance then that address became active and the other one was
> disabled, and vice versa.
>
> On our local tech mailing list, a couple of the guys advised me that
> this is due to a routing issue and, after a bit of googling around I now
> understand why that is.
>
> However, I have so far been unable to fix it.
>
> I got rid of dhcp and set up static addresses using
> system-config-network.
>
> This machine has three network cards in it, eth0 is 192.168.1.5 and I
> use that one to ssh into the box from this computer.  (I really don't
> want to lose my ability to connect to eth0; this machine runs headless
> on a shelf about 7 feet above the floor and it would be quite an
> undertaking to dismantle it and bring it down to hook a monitor and
> keyboard to it again.)
>
> The solution to this problem appears to be easier to describe than to
> implement, at least for me.  I need to have each network card reply back
> on the same interface that it received a request from.
>
> eth1 is 24.89.92.178
> eth2 is 24.89.92.180
>
> The gateway for both of these is 24.89.92.1
>
> The suggestion that I got was to add two entries to the end
> of /etc/iproute2/rt_tables (which I did with a text editor) and run a
> series of ip route commands which set up a custom routing table but I'm
> missing something because while the custom routing tables appear to be
> getting set up, it's still not working.
>
> I have studied the suggested routing commands and I think I understand
> what they are doing and what is supposed to be happening.  But something
> is still missing because it's not working.
>
> Here is what I did and what the results are.  If I have missed anything
> let me know; this covers what I think is the relevant information  as I
> currently understand it.  I would sincerely appreciate any further
> advice regarding this situation.  I really would like to know what I am
> doing wrong and also why (in the interest of learning something from
> this situation).  It's new territory for me.
>
> [root at audio ~]# cat /etc/iproute2/rt_tables 
> #
> # reserved values
> #
> 255     local
> 254     main
> 253     default
> 0       unspec
> #
> # local
> #
> #1      inr.ruhep
> 50      access1
> 60      access2
> [root at audio ~]# ip route add 24.89.92.0/24 dev eth1 table access1
> [root at audio ~]# ip route add default via 24.89.92.1 table access1
> [root at audio ~]# ip rule add from 24.89.92.178/32 lookup access1
> [root at audio ~]# 
> [root at audio ~]# ip route add 24.89.92.0/24 dev eth2 table access2
> [root at audio ~]# ip route add default via 24.89.92.1 table access2
> [root at audio ~]# ip rule add from 24.89.92.180/32 lookup access2
> [root at audio ~]# ip route show table access2
> 24.89.92.0/24 dev eth2  scope link 
> default via 24.89.92.1 dev eth1 
> [root at audio ~]# ip route show table access1
> 24.89.92.0/24 dev eth1  scope link 
> default via 24.89.92.1 dev eth1 
> [root at audio ~]# ip route
> 24.89.92.0/24 dev eth1  proto kernel  scope link  src 24.89.92.178 
> 24.89.92.0/24 dev eth2  proto kernel  scope link  src 24.89.92.180 
> 192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.5 
> 169.254.0.0/16 dev eth2  scope link 
> default via 24.89.92.1 dev eth1 
>
> [frankcox at mutt ~]$ ping 24.89.92.178
> PING 24.89.92.178 (24.89.92.178) 56(84) bytes of data.
> 64 bytes from 24.89.92.178: icmp_seq=1 ttl=50 time=92.2 ms
> 64 bytes from 24.89.92.178: icmp_seq=2 ttl=50 time=96.2 ms
> 64 bytes from 24.89.92.178: icmp_seq=3 ttl=50 time=91.0 ms
>
> --- 24.89.92.178 ping statistics ---
> 3 packets transmitted, 3 received, 0% packet loss, time 2001ms
> rtt min/avg/max/mdev = 91.023/93.193/96.263/2.245 ms
> [frankcox at mutt ~]$ ping 24.89.92.180
> PING 24.89.92.180 (24.89.92.180) 56(84) bytes of data.
>
> --- 24.89.92.180 ping statistics ---
> 6 packets transmitted, 0 received, 100% packet loss, time 5000ms
>
> Incidentally, it is my current understanding that anything that I do
> with an "ip route" command will go away on a reboot, therefore if I
> somehow screw up the routing on this box completely all I have to do is
> reboot it and I'll be back to what I had before.  Which is not a bad
> thing at the moment.  Once I have this nailed down should I put the "ip
> route" commands into /etc/rc.local?  Or is there a better place?
>
>
>   
Hi
This Article should be exactly what you need

http://www.linuxjournal.com/node/7291/print


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