My computer has two ethernet interfaces eth0 and eth1.
The setup are:

# ifconfig
eth0 Link encap:Ethernet HWaddr 00:00:50:98:38:AE
inet addr:172.20.11.200 Bcast:172.20.255.255 Mask:255.255.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:171 errors:0 dropped:0 overruns:0 frame:0
TX packets:9 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:24983 (24.3 Kb) TX bytes:458 (458.0 b)
Interrupt:11 Base address:0x7000

eth1 Link encap:Ethernet HWaddr 00:00:50:98:38:AF
inet addr:172.20.11.199 Bcast:172.20.255.255 Mask:255.255.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:229 errors:0 dropped:0 overruns:0 frame:0
TX packets:62 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:31971 (31.2 Kb) TX bytes:8351 (8.1 Kb)
Interrupt:9 Base address:0x9000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:8 errors:0 dropped:0 overruns:0 frame:0
TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:560 (560.0 b) TX bytes:560 (560.0 b)

# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
172.20.0.0 * 255.255.0.0 U 0 0 0 eth1
172.20.0.0 * 255.255.0.0 U 0 0 0 eth1
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default 172.20.11.10 0.0.0.0 UG 0 0 0 eth0

# uname -a
Linux rose 2.4.18 #22 Mon Nov 20 13:34:04 EST 2006 i686 unknown

I can ping both eth0 and eth1 from another computer.
If I do "ifdown eth1", then eth0 can be pinged and only eth1 cannot be pinged - which is expected.
If both eth0 and eth1 are up (enabled), and if one ethernet cable is unplugged, both eth0 and eth1 cannot be pinged, why and how to fix it?

I am guessing you set a gateway on both interfaces. Even though the same it is in conflict as to which is right. You should have one set with an IP, netmask, gateway and the other as IP, natmask, and no gateway.

Brian

Hmm, could have something to do with ARP and MAC address's.

If you pull the plug on your interfaces does the interface go down ?

You may need an app to drop the interface if the physical link goes down ... such as ifplugd.

or look into using ifenslave to manage both interfaces ?

Are you using bonding or acting as a gateway between two physically disconnected network segments? Both NICs are on the same subnet.

As far as the network going down on disconnect then it may depend on if NetwaorkManager is running. Also NetworkManager does not work well sometimes and in constiant confusion as to what is real and not. That has been my experience with it.

Brian

Only eth0 has "GATEWAY" defined.
# cd /etc/sysconfig/network-scripts/
# cat ifcfg-eth0
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=static
IPADDR=172.20.11.200
NETMASK=255.255.0.0
GATEWAY=172.20.11.10

# cat ifcfg-eth1
DEVICE=eth1
ONBOOT=yes
BOOTPROTO=static
IPADDR=172.20.11.199
NETMASK=255.255.0.0

How to find out whether I am using bonding or acting as a gateway between two physically disconnected network segments?

Connecting to my computer through the serial port, after pulling the ethernet cable from eth1, "ifconfig" show that both eth0 and eth1 are still up.
However, I cannot use ssh, i.e. no character can be typed, but ssh is not disconnected. If the ethernet cable is reinserted, then ssh can be used right away, no need to login again.

You will have problems with both interfaces being on the same subnet with no additional configuration.

There are solutions for this, but it depends on what you are trying to accomplish by having them on the same subnet.

We have no specific reason having them on the same subnet other than that it can be done. We want to allow both ethernet interface addresses to be any addresses (if possible).
What is the easiest solution?

Easiest solution is to put them on separate subnets.


When multiple interfaces share the same subnet it is usually for a reason.

Like bridging the interfaces.

http://www.linux.org/docs/ldp/howto/...irewall-3.html



You can use iproute2, and configure the interfaces to send data out on the same interface it is received on.

http://www.policyrouting.org/iproute2-toc.html
http://www.linuxjournal.com/article/7291

No idea as to what it will take to get it to work. Bonding is not going on here. If it was there would be a bond0 and the ifcfg-eth* would look different.

Brian

After reading the linuxjournal article, I want to set up source based routing.
Why there is "Network is unreachable" error?

# ifconfig
eth0 Link encap:Ethernet HWaddr 00:00:50:1A:CE:8D
inet addr:172.20.11.242 Bcast:172.20.11.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1916 errors:0 dropped:0 overruns:0 frame:1
TX packets:381 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:220079 (214.9 Kb) TX bytes:52506 (51.2 Kb)
Interrupt:16 Base address:0x6000

eth1 Link encap:Ethernet HWaddr 00:00:50:1A:CE:8E
inet addr:172.20.11.199 Bcast:172.20.11.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1066 errors:0 dropped:0 overruns:0 frame:1
TX packets:504 errors:0 dropped:0 overruns:0 carrier:0
collisions:2 txqueuelen:100
RX bytes:117390 (114.6 Kb) TX bytes:66184 (64.6 Kb)
Interrupt:17 Base address:0x8000

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)

# ip route show
172.20.11.0/24 dev eth1 scope link
127.0.0.0/8 dev lo scope link
default via 172.20.11.10 dev eth0

# ip route add default via 172.20.11.10 dev eth0 tab 1
RTNETLINK answers: Network is unreachable

# ping 172.20.11.10
PING 172.20.11.10 (172.20.11.10) from 172.20.11.199 : 56(84) bytes of data.
64 bytes from 172.20.11.10: icmp_seq=0 ttl=255 time=1.005 msec
64 bytes from 172.20.11.10: icmp_seq=1 ttl=255 time=466 usec