On 07/18/2015 11:13 PM, Frank Cox wrote:
[root@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 204.83.15.254 tell 204.83.15.1
00:11:01.412135 arp who-has 204.83.15.254 tell 204.83.15.1
...
Is the gateway's MAC address listed in the output of "arp"?
[root@audio ~]# arp
Address HWtype HWaddress Flags Mask Iface
204.83.15.254 (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 204.83.15.254, 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, 204.83.15.1 is on a /24 network with its gateway,
204.83.15.254. 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.
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