PING
NAME
ping -- send ICMP ECHO_REQUEST packets to network hosts
DESCRIPTION
-c count
Stop after sending (and receiving) count ECHO_RESPONSE packets. If this option is not specified, ping will operate until interrupted. If this option is specified in conjunction with ping sweeps, each sweep will consist of count packets.
-D
Set the Do not Fragment bit.
-s packetsize
Specify the number of data bytes to be sent. The default is 56, which translates into 64 ICMP data bytes when combined with the 8 bytes of ICMP header data. This option cannot be used with ping sweeps.
MTU = max icmp data + icmp header(8 bytes) + ip header(20 bytes)
（题外话：MTU = MSS + tcp header(20 bytes) + ip header(20 bytes)）
因为 ping -s 选项指定的是 icmp data size，所以：
ping -s (MTU-28) -D -c 1 ip_address
不断试探 -s 选项的参数最后可以查找到 MTU。
➜ ~ ping -s 1473 -D -c 1 192.168.1.105
PING 192.168.1.105 (192.168.1.105): 1473 data bytes
ping: sendto: Message too long
--- 192.168.1.105 ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss
➜ ~
➜ ~ ping -s 1472 -D -c 1 192.168.1.105
PING 192.168.1.105 (192.168.1.105): 1472 data bytes
1480 bytes from 192.168.1.105: icmp_seq=0 ttl=64 time=103.999 ms
--- 192.168.1.105 ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 103.999/103.999/103.999/0.000 ms
上面的例子可以看出测试机的 MTU 是 1472 + 28 = 1500（bytes）
