Re: Problem about host addresses to names

[Guy Harris <guy () alum mit edu>]

On Jun 26, 2017, at 5:57 PM, Zheng, Ruoqin <zhengrq.fnst () cn fujitsu com> wrote:

> Hi Guy
>    Thank you for your answer, and here is the output with -d:
>
>    root # ping -q -c 50 -I eth0.100 192.168.255.2&
>       [2] 1208
>       root # tcpdump -d -i eth0 ether broadcastPING 192.168.255.2 (192.168.255.2) from 192.168.255.1 eth0.1
>
>       (000) ld       [2]
>       (001) jeq      #0xffffffff      jt 2    jf 5
>       (002) ldh      [0]
>       (003) jeq      #0xffff          jt 4    jf 5
>       (004) ret      #262144
>       (005) ret      #0
>
>       root # tcpdump -n -d -i eth0 ether broadcast
>       (000) ld       [2]
>       (001) jeq      #0xffffffff      jt 2    jf 5
>       (002) ldh      [0]
>       (003) jeq      #0xffff          jt 4    jf 5
>       (004) ret      #262144
>       (005) ret      #0
>       root #
>
>    You can see they are same.

As I expected.

So what happens if you run tcpdump, without -n, do the ping, and wait a long time to see whether tcpdump eventually 
prints something?  It might be taking a long time to try to resolve a host name, for some reason or another, so that it 
can't print anything until that attempt either succeeds or times out.
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