[CLUG] Ping rate slow

[Ronan Kirby]

Yeah, I tend to agree with you. In fact, for the most part, your are 
indeed correct. But the rate can still be effected...

ping only does a lookup on the hostname in order to be able to send the 
ping, at the start, when the command is invoked.

However, it does a lookup on every REPLY it receives. i.e. when it gets 
the reply to a ping it has sent it, say it comes back from 194.125.2.240, 
it does a lookup on that address before it displays it to you. This is why 
you see hostname (ipaddress) in the output.

Its quite easy to demonstrate this. Below I start a ping to ns.iol.ie, 
while the ping is still running, I block[1] outbound requests on port 53 
(i.e. stop it from doing DNS lookups). Watch what happens...

64 bytes from ns.iol.ie (194.125.2.240): icmp_seq=9 ttl=247 time=21.9 ms
64 bytes from ns.iol.ie (194.125.2.240): icmp_seq=10 ttl=247 time=20.8 ms
64 bytes from ns.iol.ie (194.125.2.240): icmp_seq=11 ttl=247 time=15.7 ms
64 bytes from 194.125.2.240: icmp_seq=12 ttl=247 time=27.2 ms
64 bytes from 194.125.2.240: icmp_seq=13 ttl=247 time=15.7 ms
64 bytes from 194.125.2.240: icmp_seq=14 ttl=247 time=15.4 ms

So as we can see, it does do DNS lookups in real time on the reply 
address. But not, as you pointed out, on the destination host at each 
ping.

- Ronan

[1] Keeping the auld iptables thing going, here is the rule I used...

# iptables -A OUTPUT -o eth0 -p udp -m udp --dport 53 -j DROP

On Wed, 22 Oct 2003, David Dorgan wrote:
> 
> This is a pretty strange situation, the rate should only 
> effect it on startup, gethostbyname isn't called for
> every ping sent out. 
>