Two elementary questions on LANs

[Ed Greshko]

On 06/17/2011 03:06 PM, Joe Zeff wrote:
> On 06/16/2011 09:56 AM, Tim wrote:
>> Try some other addresses.  Perhaps google.co.uk doesn't respond to
>> pings.  Or, what happens when you try pinging it while on the gateway
>> computer?
> Until recently, mit.edu was a good choice.  Checking, it no longer 
> replies.  However, using traceroute, I found that this works:
>
> ping OC11-RTR-1-BACKBONE.MIT.EDU

One thing that people sometimes forget is that it is possible for an
outbound packet to reach its destination but for the return inbound
packet to get "lost" due to bad routes or other things on the return path.

Sometimes, it is necessary to use sites such as
http://network-tools.com/ to initiate a test from external to your
system and using something like tcpdump or wireshark determine if the
packets are arriving were they should be.  Yes, this is complicated by
local firewalls and/or NAT, but that can be "fixed" during testing using
port forwarding or other means.

I would also caution against doing something like "ping google.co.uk"
not only do you have to contend with the DNS resolution, you also have
to realize that it may return many IP address (at least 6 in this case)
and one of those may be configured differently than the others and/or
one of them may be down at any one time.  If you use an IP address, also
use -n to inhibit reverse lookups.

In addition, at times a better tool than a standard ping is to use a
tool that uses SYN/ACK packets to confirm end to end connectivity.