这个参数的迷惑性在于双重否定，字面意思是关闭PMTU发现的功能。如果设置为1，代表关闭；如果是0，代表不关闭pmtu发现的功能。所以说明里，有disable/enable，就容易搞混。所以要甄别网上的某些博客的说明，不要被误导。
ip_no_pmtu_disc - INTEGER
Disable Path MTU Discovery. If enabled in mode 1 and a fragmentation-required ICMP is received, the PMTU to this destination will be set to the smallest of the old MTU to this destination and min_pmtu (see below). You will need to raise min_pmtu to the smallest interface MTU on your system manually if you want to avoid locally generated fragments.
In mode 2 incoming Path MTU Discovery messages will be discarded. Outgoing frames are handled the same as in mode 1, implicitly setting IP_PMTUDISC_DONT on every created socket.
Mode 3 is a hardened pmtu discover mode. The kernel will only accept fragmentation-needed errors if the underlying protocol can verify them besides a plain socket lookup. Current protocols for which pmtu events will be honored are TCP, SCTP and DCCP as th
