I use libvirt to run a Windows virtual machine, which is unfortunately required for certain services that my company provides (and requires that I use).

I am connected to the company via a VPN, with an MTU of 1266.

There are pMTU black holes in our network, and HR are quite insistent that I am not permitted to visit the office location of those responsible, and apply percussive education until the problem is fixed.

Thus, I must work around it at my end. My options include MSS clamping on the NAT, or asking dnsmasq to advertise the smaller MTU in its DHCP responses. Both of those would need to be set up by libvirt.

If there is an existing facility in the network configuration where I can add verbatim options to the dnsmasq config such as 'dhcp-option=virbr0,26,1266', that would probably suffice...