Multiple default gateways on the same interface

Question

Is it valid to have multiple default gateways associated with the same interface. I thought that didn't make sense, but I'm seeing this on my laptop

$ route
Kernel IP routing table                                                                                                              
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.1.0.0        *               255.255.248.0   U     1      0        0 eth0
default         10.1.1.1        0.0.0.0         UG    0      0        0 eth0
default         10.1.1.1        0.0.0.0         UG    202    0        0 eth0

The reason I ask is that in an embedded system we're developing for, we need to get the default gateway associated with a particular interface. Is this just a display issue where it's showing the same default gateway twice? Or does having multiple default gateways on 1 interface mean something that I should have to take into account.

Answer 1

It's not really valid to have multiple default gateways on the same machine let alone on a single interface. Sure there's some tiny edge cases where it might help but I've never come across any that jump to mind. You generally have a default then statics to anything outside that.

Answer 2

When you look carefully, you see different values in the "metric" column. Everything else is equal so I suspect some mis(-auto-)configuration.

With a "real" routing protocol you can have different routes with the same destination (although one wouldn't speak of multiple "default" gateways) which would be selected according their routing "cost" e.g. the higher the "metric" on the route, the lower the chance of the route actually being used. When there's network contention, the metrics will be dynamically adapted to reflect this, so that in this case it would be possible that another route to the same destination could be selected.

Answer 3

The default route, even as shown above in 2 lines, is still the same node. (10.1.1.1) The only difference between the two is the Metric column. This is typically counted in hops. Has anything changed in your network topology that would impact this field? It's likely only displaying twice because of this. Reboot or restart the network services and this should clear.

What distribution are you using? In Red Hat, you can check the default gateway in /etc/sysconfig/network. By using the route command you will be subject to any unexepected output like you see here. Stick to getting the default route from the OS documented configuration, not another binary.

Answer 4

It really doesn't make sense to have more then one default gateway in a single route table. Usually on Linux only the destination address of the packet is used to select which route will be used. Routes will be check from the most specific to the least specific to determine if that route applies to the destination. If no specific route is found the default route with the lowest metric will be used.

If you want or need to select the route based on other criteria like the source address you will need to use the advanced routing features. This will mean creating multiple route tables and using ip rule to select the proper table based on whatever criteria you like.

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It sounds like you are trying to get the route to display in some kind of GUI you are developing? Hopeful in your embedded setup you don't have a situation where multiple default gateways get set. In any case you probably should not be managing the default gateway per-interface. The default and most routes should be managed separately from the IP configuration for a specific interface. Routes really are system wide, not per-interface.