[CentOS] OT: Setting a CentOS to gateway a private IP address

Ross Walker rswwalker at gmail.com
Tue Oct 21 19:20:38 UTC 2008

On Tue, Oct 21, 2008 at 3:02 PM, James B. Byrne <byrnejb at harte-lyne.ca> wrote:
> This is mildly off topic since it has more to do with TCP/IP networking
> than with CentOS per se.  However, at the risk of ruffling a few feathers
> I would like some advice on how to resolve the following routing problem:
> Given:
> dual homed host running CentOS-5.2 with
> eth0 = 2xx.yyy.zzz.23
> eth1 =
> and a Cisco gateway at 2xx.1xx.y7y.1 with the following networks:
> interface FastEthernet0/1
>  description connected to EthernetLAN
>  ip address secondary
>  ip address secondary
>  ip address secondary
>  ip address secondary
>  ip address 2xx.yyy.zzz.1
>  ip access-group 101 in
>  ip inspect FastEthernet_0_1 in
>  speed 100
>  full-duplex
> !
> router rip
>  version 2
>  passive-interface FastEthernet0/0
>  network
>  network
>  network
>  network 2xx.yyy.zzz.0
>  no auto-summary

Assuming that this Cisco box is for routing your Internet
traffic, I would NOT put any private network routing, or
any dynamic routing at all on it and use NAT on the
CentOS box.

> !
> ip classless
> ip route FastEthernet0/0
> and with the following routes on the CentOS host:
> # route
> Kernel IP routing table
> Destination     Gateway       Genmask         Flags Metric Ref    Use Iface
>   *      U     0      0        0 eth1
> 2xx.yyy.zzz.0   *      U     0      0        0 eth0
>     *        U     0      0        0 eth1
> default         gw-fibrewire         UG    0      0        0 eth0
> # ip route
> dev eth1  proto kernel  scope link  src
> 2xx.yyy.zzz.0/24 dev eth0  proto kernel  scope link  src 2xx.yyy.zzz.23
> dev eth1  scope link
> default via 2xx.yyy.zzz.1 dev eth0
> The situation is this.
> >From the host itself I can ping on

Was that a typo

> >From any other host on 2xx.1xx.y7y.0/24 I can ping

Yes, cause it a local interface on router 2xx.yyy.zzz.23, discoverable
by ICMP.

> >From any other host on 2xx.1xx.y7y.0/24 I cannot ping

You need to have a route in the Cisco's table for or you
need to get RIP working between 2xx.yyy.zzz.23 and 2xx.yyy.zzz.1

> What setup steps on the CentOS host have I overlooked or what
> configuration errors have I committed?

Probably getting routed/gated running, setting active/passive interfaces,
broadcast or multicast RIP, host routes/default routes, etc.

> As I am a digest subscriber please use "reply all" so that I get an
> immediate copy of any replies.

List always strips off the members email :-(


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