ping waits 5 seconds between packets if RDNS is not present

If you use ping from the command line and the host you are pinging does not have a rDNS entry, then ping pauses for 5 seconds or more between packets. Since there's no output on the terminal it looks as if the host is down, but when you press Ctrl-C, you get a line about the first packet.

Here's the output from ping as well as a packet capture. b.b.b.b doesn't have an rDNS entry, but c.c.c.c does.

a.a.a.a = my ip address
b.b.b.b = pinged.machine.com
c.c.c.c = 2nd.pinged.machine.com
d.d.d.d = first dns server in resolv.conf

21:56:10.348040 IP a.a.a.a.35806 > d.d.d.d.domain: 19049+ A? pinged.machine.com. (39)
21:56:10.359921 IP d.d.d.d.domain > a.a.a.a.35806: 19049* 1/0/0 A[|domain]
21:56:10.401162 IP a.a.a.a.35806 > d.d.d.d.domain: 10575+ PTR? b.b.b.b.in-addr.arpa. (44)
21:56:10.413424 IP d.d.d.d.domain > a.a.a.a.35806: 10575 NXDomain 0/1/0 (121)
21:56:15.497545 IP a.a.a.a.35806 > d.d.d.d.domain: 60024+ PTR? b.b.b.b.in-addr.arpa. (44)
21:56:15.509826 IP d.d.d.d.domain > a.a.a.a.35806: 60024 NXDomain 0/1/0 (121)
21:56:20.588606 IP a.a.a.a.35806 > d.d.d.d.domain: 28660+ PTR? b.b.b.b.in-addr.arpa. (44)
21:56:20.624358 IP d.d.d.d.domain > a.a.a.a.35806: 28660 NXDomain 0/1/0 (121)
21:56:25.671853 IP a.a.a.a.35806 > d.d.d.d.domain: 63426+ PTR? b.b.b.b.in-addr.arpa. (44)
21:56:25.684198 IP d.d.d.d.domain > a.a.a.a.35806: 63426 NXDomain 0/1/0 (121)

me@me-desktop:~/Desktop$ ping pinged.machine.com
PING pinged.machine.com (b.b.b.b) 56(84) bytes of data.
64 bytes from b.b.b.b: icmp_seq=1 ttl=124 time=41.0 ms
64 bytes from b.b.b.b: icmp_seq=2 ttl=124 time=81.0 ms
64 bytes from b.b.b.b: icmp_seq=3 ttl=124 time=76.1 ms
64 bytes from b.b.b.b: icmp_seq=4 ttl=124 time=43.3 ms

--- pinged.machine.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 15268ms
rtt min/avg/max/mdev = 41.009/60.377/81.041/18.299 ms
me@me-desktop:~/Desktop$

22:15:38.966121 IP a.a.a.a.35806 > d.d.d.d.domain: 61313+ A? 2nd.machine.pinged.com. (42)
22:15:38.966447 IP a.a.a.a.35807 > d.d.d.d.domain: 44215+ PTR? b.b.b.b.in-addr.arpa. (43)
22:15:38.978502 IP d.d.d.d.domain > a.a.a.a.35806: 61313* 1/0/0 (58)
22:15:38.980133 IP d.d.d.d.domain > a.a.a.a.35807: 44215 NXDomain* 0/1/0 (127)
22:15:38.995406 IP a.a.a.a.35808 > d.d.d.d.domain: 45092+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:39.017572 IP d.d.d.d.domain > a.a.a.a.35808: 45092* 1/0/0 (81)
22:15:39.991054 IP a.a.a.a.35808 > d.d.d.d.domain: 48973+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:40.002881 IP d.d.d.d.domain > a.a.a.a.35808: 48973* 1/0/0 (81)
22:15:40.998392 IP a.a.a.a.35808 > d.d.d.d.domain: 78+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:41.010430 IP d.d.d.d.domain > a.a.a.a.35808: 78* 1/0/0 (81)
22:15:41.988781 IP a.a.a.a.35808 > d.d.d.d.domain: 21736+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:42.000789 IP d.d.d.d.domain > a.a.a.a.35808: 21736* 1/0/0 (81)
22:15:42.996923 IP a.a.a.a.35808 > d.d.d.d.domain: 43427+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:43.009297 IP d.d.d.d.domain > a.a.a.a.35808: 43427* 1/0/0 (81)
22:15:43.984441 IP a.a.a.a.35808 > d.d.d.d.domain: 59199+ PTR? a.a.a.a.in-addr.arpa. (41)
22:15:43.997827 IP a.a.a.a.35809 > d.d.d.d.domain: 23386+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:44.009554 IP d.d.d.d.domain > a.a.a.a.35809: 23386* 1/0/0 (81)
22:15:44.036071 IP d.d.d.d.domain > a.a.a.a.35808: 59199 NXDomain*- 0/1/0 (118)
22:15:44.990557 IP a.a.a.a.35809 > d.d.d.d.domain: 51955+ PTR? c.c.c.c.in-addr.arpa. (43)
22:15:45.002374 IP d.d.d.d.domain > a.a.a.a.35809: 51955* 1/0/0 (81)

me@me-desktop:~/Desktop$ ping 2nd.machine.pinged.com
PING 2nd.machine.pinged.com (c.c.c.c) 56(84) bytes of data.
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=1 ttl=127 time=12.1 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=2 ttl=127 time=18.7 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=3 ttl=127 time=18.3 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=4 ttl=127 time=21.9 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=5 ttl=127 time=12.3 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=6 ttl=127 time=11.8 ms
64 bytes from 2nd.machine.pinged.com (c.c.c.c): icmp_seq=7 ttl=127 time=16.2 ms

--- 2nd.machine.pinged.com ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 7018ms
rtt min/avg/max/mdev = 11.894/16.067/21.995/3.431 ms
me@me-desktop:~/Desktop$

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: iputils-ping 3:20071127-2ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-27.49-generic-pae 2.6.32.26+drm33.12
Uname: Linux 2.6.32-27-generic-pae i686
NonfreeKernelModules: nvidia
Architecture: i386
Date: Fri Mar 18 21:50:08 2011
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.utf8
 SHELL=/bin/bash
SourcePackage: iputils