Please describe the problem:
When an ill-configured DHCP server informs a gateway IP that is outside of the subnet, no route is created (but one can normally "ping" the gateway). As a result, Internet access is blocked.

An example from my ISP (data from ifconfig and syslog):

eth0's IP:      189.61.119.200
eth0's netmask: 255.255.240.0
eth0's subnet:  189.61.112.0
gateway:        189.61.96.1

"route" gives the following output (labels in portuguese):

Tabela de Roteamento IP do Kernel
Destino         Roteador        MáscaraGen.    Opções Métrica Ref   Uso Iface
189.61.112.0    *               255.255.240.0   U     1      0        0 eth0 
link-local      *               255.255.0.0     U     1000   0        0 eth0

No default route is set, so no Internet access is provided.


Steps to reproduce:
1. (Mis)configure a DHCP server that gives a gateway IP outside the subnet of the provided IP.
2. Connect the client to the server.
3. Run "ifconfig" and "route" and confirm the results.


Actual results:
No routes are established.

Expected results:
The default route should be set to the gateway's IP address.

Does this happen every time?
Yes.

Other information:
I made a workaround, but it only works when the interface is brought up:

file "/etc/network/if-up.d/fixgateway":
------
GATEWAY=189.61.96.1
if [ "$IFACE" = "eth0" ]; then
  route add -host $GATEWAY dev $IFACE
  route add default gw $GATEWAY
fi
------

When the DHCP token expires and a new IP is asked to the server, the problem reappears. I have to unplug (or shut down) the interface and plug it again for the workaround to work.