file: /proc/sys/net/ipv4/ip_no_pmtu_disc
variable: net.ipv4.ip_no_pmtu_disc
Official reference

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 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 hardend 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 they verify e.g. the sequence number or the association. This mode should not be enabled globally but is only intended to secure e.g. name servers in namespaces where TCP path mtu must still work but path MTU information of other protocols should be discarded. If enabled globally this mode could break other protocols.

Possible values: 0-3 Default: FALSE