dhclient fails setting default gateway to unreachable host

[HonoredMule]

When receiving an IP from a DHCP server and a router IP that is not the same IP, dhclient-script silently fails to add the router as a default route for the interface (RTNETLINK answers: Network is unreachable). Error occurs on line 320:

    # set default route
    ip -4 route add default via ${router} dev ${interface} \
        ${if_metric:+metric $if_metric} >/dev/null 2>&1

  With no control over the DHCP server (an ISP-provided appliance configured to propagate its external IP to this DHCP client), the apparent solution (other than quick hard-coded hacks like `route add default MYINTERFACE`) is to first create a single-ip route, adding the preceding line:

    ip -4 route add ${router} dev ${interface}

(Alternatively, collapsing the resulting two rules by replacing original line's use of ${router} with ${new_ip_address} also works, and also preserves the route table's resolution of the gateway ip to a hostname - unlike the hack.)

I could probably write a more complex custom enter hook to correctly handle the scenario, but this does not seem so terribly obscure to justify it (standard appliance from a major Canadian ISP). With sufficient information present to infer the correct routing, I believe it should work out of the box.

I do not rule out consequences of doing so beyond my current grasp, and it does seem odd to me that my ISP provides a gateway outside the subnet. Nevertheless I hope some attention on the issue will yeild a more robust stock configuration or at least some deeper insight.

(Sanitized) variable dump for dhclient-script:
    Tue Dec 12 00:59:23 AST 2017: entering /etc/dhcp/dhclient-enter-hooks.d, dumping variables.
    reason='BOUND'
    interface='eth0'
    new_ip_address='123.45.114.112'
    new_network_number='123.45.114.0'
    new_subnet_mask='255.255.255.0'
    new_broadcast_address='123.45.114.255'
    new_routers='123.45.112.1'
    new_domain_name='redacted.com'
    new_domain_name_servers='127.0.0.1'
    old_ip_address='123.45.114.112'
    old_network_number='123.45.114.0'
    old_subnet_mask='255.255.255.0'
    old_broadcast_address='123.45.114.255'
    old_routers='123.45.112.1'
    old_domain_name='redacted.com'
    old_domain_name_servers='127.0.0.1'

Routing rules with stock configuration (the only one that doesn't route internet traffic):
    Destination Gateway Genmask Flags Metric Ref Use Iface
    123.45.114.0 * 255.255.255.0 U 0 0 0 eth0

With my added line:
    Destination Gateway Genmask Flags Metric Ref Use Iface
    default fctnnbsc38w-123 0.0.0.0 UG 0 0 0 eth0
    fctnnbsc38w-123 * 255.255.255.255 UH 0 0 0 eth0
    123.45.114.0 * 255.255.255.0 U 0 0 0 eth0

With interface ip as router ip:
    Destination Gateway Genmask Flags Metric Ref Use Iface
    default fctnnbsc38w-123 0.0.0.0 UG 0 0 0 eth0
    123.45.114.0 * 255.255.255.0 U 0 0 0 eth0

With hardcoded hack:
    Destination Gateway Genmask Flags Metric Ref Use Iface
    default * 0.0.0.0 U 0 0 0 eth0
    123.45.114.0 * 255.255.255.0 U 0 0 0 eth0