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

By default we don’t trust protocol path MTUs while forwarding because they could be easily forged and can lead to unwanted fragmentation by the router. You only need to enable this if you have user-space software which tries to discover path mtus by itself and depends on the kernel honoring this information. This is normally not the case. Default: 0 (disabled) Possible values: 0 - disabled 1 - enabled