[CentOS] Multiple network cards - routing issue?

Sun Jul 19 19:25:04 UTC 2015
Gordon Messmer <gordon.messmer at gmail.com>

On 07/18/2015 11:13 PM, Frank Cox wrote:
> [root at audio ~]# tcpdump -nn -i eth1
> tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
> listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
> 00:11:00.412188 arp who-has tell
> 00:11:01.412135 arp who-has tell
>>   Is the gateway's MAC  address listed in the output of "arp"?
> [root at audio ~]# arp
> Address                  HWtype  HWaddress           Flags Mask            Iface
>                    (incomplete)                              eth1
> I don't know what that means; this is the first time I ever typed the arp command.

When you're using Ethernet, packets are transmitted between cards using 
the MAC address of the recipient's interface.  IPv4 resolves hardware 
addresses (MAC address) using the ARP protocol.  In order to send a 
packet to, a host on the same network segment sends a 
broadcast request (arp who-has) request for the hardware address 
associated with that IPv4 address.  The host with that IPv4 address 
should send a unicast reply to the host that sent the request.  
Understanding arp is essential to troubleshooting IPv4 and Ethernet.  
(IPv6 does not use ARP to resolve MAC addresses)

Your host, is on a /24 network with its gateway,  I would expect that a /24 network would probably have 
more than two hosts.  If that's the case, it would be extremely unusual 
to see no broadcast traffic when you run "tcpdump" on that interface.  
Normally you'd see arp broadcast requests every few seconds, even if you 
didn't see any other traffic.

It's hard to say specifically what the problem might be without knowing 
more about the physical topology of your network, but the most likely 
problems are that you're connected to a network segment with no other 
hosts, or that you're on a segment with only one host (the gateway) 
which has no need to broadcast anything and is on a different address 
than you expect, or that your cable is defective (even with link), or 
that the device your host is physically attached to is defective.