dnsmasq

Static routes may be issued to clients using the dns option 121 (classless-static-route) RFC3442.

eg: This configures the route "10.10.10.0/24 via 192.168.1.100"

Note: there is also the older option 33 (static-route) which allows specifying classful addresses. It doesn't allow classless CIDR notation.

dhcp-option=option:classless-static-route,0.0.0.0/0,192.168.1.1,10.10.10.0/24,192.168.1.100

Stating the 0.0.0.0/0 route is also requires as clients ignore the default route if a classless route is issued.

From RFC3442 Sec: DHCP Client Behavior

If the DHCP server returns both a Classless Static Routes option and
a Router option, the DHCP client MUST ignore the Router option.