1

The iputils ping program specifies the following for the -Woption:

-W timeout
       Time to wait for a response, in seconds. The option affects only
       timeout in absence of any responses, otherwise  ping  waits  for
       two RTTs.

In which cases would the -W value not take effect, and the "wait for two RTTs" apply?

1

In the case where there has been any reply from the pinged machine during the ping sequence. In that case, ping times out after a time equal to the max value of all RTTs in the ping sequence, multiplied by two.

In this example, timeout is set at 55.392 × 2 = 110.784 ms:

PING www.google.com (173.194.44.80) 56(84) bytes of data.
64 bytes from 173.194.44.80: icmp_seq=1 ttl=47 time=55.3 ms
64 bytes from 173.194.44.80: icmp_seq=2 ttl=47 time=52.6 ms
64 bytes from 173.194.44.80: icmp_seq=3 ttl=47 time=52.7 ms
64 bytes from 173.194.44.80: icmp_seq=4 ttl=47 time=53.9 ms
64 bytes from 173.194.44.80: icmp_seq=5 ttl=47 time=52.5 ms
64 bytes from 173.194.44.80: icmp_seq=6 ttl=47 time=52.7 ms
64 bytes from 173.194.44.80: icmp_seq=7 ttl=47 time=52.8 ms
64 bytes from 173.194.44.80: icmp_seq=8 ttl=47 time=53.2 ms
^C
--- www.google.com ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 7010ms
rtt min/avg/max/mdev = 52.535/53.273/55.392/0.939 ms

If the machine you're trying to ping does not reply at all:

  • if you do not use the -W flag, ping timeouts after a time that depends on the implementation, usually 4 or 10 seconds
  • if you use the -W flag, ping timeouts after the specified time

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