Understanding Basic Networking

carlosinfl · 05-05-2006, 02:04 PM

OK - So my motherboard has 2 onboard NIC's. I am a little confused since it is not very easy to know what NIC eth0 or eth1 is. I also can't determine which NIC it using. Obviously I know what NIC has the CAT5 cable however I don't know how to associate this on my Linux box. Here is what I see what I do an "ifconfig" on my system...

Code:

nucleus:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:01:29:FC:B8:05
          inet addr:192.168.1.102  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST MULTICAST  MTU:1500  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:1000
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:66

eth1      Link encap:Ethernet  HWaddr 00:01:29:FC:77:DB
          inet addr:192.168.1.102  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::201:29ff:fefc:77db/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:63523 errors:0 dropped:0 overruns:0 frame:0
          TX packets:47247 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:82594714 (78.7 MiB)  TX bytes:5558977 (5.3 MiB)
          Interrupt:217 Base address:0xc000

eth2      Link encap:UNSPEC  HWaddr 00-01-29-20-00-02-64-C0-00-00-00-00-00-00-00-00
          inet addr:192.168.1.102  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:239 errors:45 dropped:45 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b)  TX bytes:8604 (8.4 KiB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:190 errors:0 dropped:0 overruns:0 frame:0
          TX packets:190 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:26097 (25.4 KiB)  TX bytes:26097 (25.4 KiB)

sit0      Link encap:IPv6-in-IPv4
          inet6 addr: ::127.0.0.1/96 Scope:Unknown
          inet6 addr: ::192.168.1.100/96 Scope:Compat
          UP RUNNING NOARP  MTU:1480  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:34 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

Here I see 3 NICS...I don't know why I have 3 but as you can see they have all been dynamicly set and issued the same IP. I guess this does not matter since the cable is only in one NIC but which I don't know.

Can someone please clearify this mess for me and also let me know why a machine with 2 NICS shows so many...?

pljvaldez · 05-05-2006, 03:27 PM

Not sure why there are three, but I would guess that eth1 has the network connected right now because it has the largest number of TX (transmit) and RX (receive) bytes, not to mention that there are no errors.

carlosinfl · 05-05-2006, 04:20 PM

I think you're right about ETH1 but I think that is a very confusing way to guage that answer. I would think there would be a more presise way of knowing for sure.

uselpa · 05-05-2006, 04:52 PM

Perhaps check the routing table, mine looks like this:

Code:

root@slackw:/etc/rc.d$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
loopback        *               255.0.0.0       U     0      0        0 lo
default         router          0.0.0.0         UG    1      0        0 eth0

In the last column you see it uses eth0.