Measure the time difference between two Linux hosts
Timestamp data can be placed in the headers of IP and ICMP packets. The clockdiff program uses these timestamps to measure the system time difference between the destination host and the local host.
-o: Use the IP timestamp option to measure the system time difference. Uses 3 timestamps.
-o1: Use the IP timestamp option to measure the system time difference. Uses 4 timestamps. If neither -o nor -o1 is set, ICMP timestamps are used.
lixi@lixi-desktop:~$ ping -T tsandaddr www.ustc.edu.cn -c 1
PING www.ustc.edu.cn (202.38.64.9) 56(124) bytes of data.
64 bytes from 202.38.64.9: icmp_seq=1 ttl=62 time=0.823 ms
TS: lixi-desktop.local (210.45.74.25) 12522473 absolute
210.45.74.1 -251
local-gw.ustc.edu.cn (202.38.64.126) 248
202.38.64.9 -857514
Unrecorded hops: 3
--- www.ustc.edu.cn ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.823/0.823/0.823/0.000 ms
From the above, when the RTT is small, the relationship between ICMP timestamps can be determined. The time difference between the local host and 202.38.64.9 is approximately: -857514 + 248 - 251 = -857517. Test the system time of the route using both the -o option (IP timestamp) and without any option (ICMP timestamp). Results:
lixi@lixi-desktop:~# ./clockdiff -o 202.38.64.9
..................................................
host=202.38.64.9 rtt=1(0)ms/1ms delta=-857517ms/-857517ms Wed Dec 17 11:28:30 2008
lixi@lixi-desktop:~# ./clockdiff 202.38.64.9
.
host=202.38.64.9 rtt=750(187)ms/0ms delta=-857517ms/-857517ms Wed Dec 17 11:28:35 2008
Both methods are relatively accurate.
lixi@lixi-desktop:~# ./clockdiff gigagate1.Princeton.EDU
..................................................
host=gigagate1.Princeton.EDU rtt=307(21)ms/271ms delta=-5ms/-5ms Wed Dec 17 11:50:16 2008
The above example measures the time difference with a destination host that has a larger RTT. Note that using clockdiff requires some luck, as many routers ignore ICMP or IP timestamps.