数据包允许分片

node2:/root/test#cat t22.py 
import socket
import IN
s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
hostName = '192.168.137.2'
Port = 8080
s.connect((hostName, Port))
#s.setsockopt(socket.IPPROTO_IP, IN.IP_MTU_DISCOVER, IN.IP_PMTUDISC_DO)
s.setsockopt(socket.IPPROTO_IP, IN.IP_MTU_DISCOVER, IN.IP_PMTUDISC_DONT)
try:
    aa='#' * 147
    print len(aa)
    s.send(aa)
except socket.error:
    print 'The message did not make it'
    option = getattr(IN, 'IP_MTU', 14)
    print 'MTU:', s.getsockopt(socket.IPPROTO_IP, option)
else:
    print 'The big message was sent! Your network supports really big packets!'
	
	


IP_PMTUDISC_DONT策略表示从不设置DF位，即不进行PMTU发现（参见函数ip_dont_fragment）。


IP_PMTUDISC_DO策略总是设置DF位，除非内核设置了忽略df（ignore_df）