I configured a DHCP server to listen on a dummy interface:

interfaces {
    dummy dum0 {
        address 10.255.255.60/32
    }
}
service {
    dhcp-server {
        listen-address 10.255.255.60
    }
}

Whenever it received a request, it failed to respond with (the request came in via eth9):

DHCP4_PACKET_SEND_FAIL [hwtype=1 <mac>], cid=[no info], tid=xxx: failed to send DHCPv4 packet: Interface eth9/11 does not have any suitable IPv4 sockets open.

Setting the "outbound-interface": "use-routing" interfaces-config option resolves the problem, but it's not possible to make this happen using vyos configuration.

https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#interface-configuration

Possible solutions:

• Add the option whenever we currently set "dhcp-socket-type": "udp" (i.e. listen_address is defined)
• Add the option whenever listen_address contains a dummy interface.
• Add an explicit configuration option to allow the administrator to enable the option.

I'm happy to send a patch, but I'm not sure which option would be preferable.