MTU Problem

[mossywell]

OPNsense 18.7.5_1-amd64
FreeBSD 11.1-RELEASE-p14
OpenSSL 1.0.2p 14 Aug 2018

I might be misunderstanding how it is supposed to work, but if I set MTU to, say, 1300 and then ping that interface with the "do not fragment" option from a machine on the same LAN as the interface, I'd expect Opnsense to tell me that the packet needs to be fragmented. But it doesn't  -I just get no reply.

How do I configure Opnsense to tell me that the packet needs to be fragments, assuming that MTU isn't the option I need? (I don't really want to have to set a small MTU on all my servers.)

Thanks
Mark