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Kernel Routing Table - Interface specific routes


I have a question on kernel routing table. It will be great if any of you can throw some light on this topic

In general, whenever an inteface is brought to 'up' state, an interface route comes up. for ex. If eth0 with Ip address is brought up, the following route is created by default

" U 0 0 0 eth0"

After this route is created, if i try to add the same route manually, with the following command, the same route gets added again.

"route add -net netmask dev eth0"

I do not see any difference in both of these routes when i verified with 'route -n' or 'netstat -r' or 'cat /proc/net/route'. How is this possible?

In general, at any point of time, you cannot have two same entries in the kernel routing table. Can any of the linux expert help me with this question??




  • Set_Killer
    Set_Killer Posts: 31
    with route you can and should add gateway.
  • joetech20
    joetech20 Posts: 5
    In the case of network specifc routes, gateway address will be try route -n on any linux Box/PC and verify.

    My question is how can there be two identical routes in the routing table at a point of time.

  • Set_Killer
    Set_Killer Posts: 31
    == newbiesh is your IP of the interface. if your IP is, then is equal to this. is something like localhost, but uses the ethX interface and its IP.

    this is my route -n
    Kernel IP routing table
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface   U     202    0        0 eth0         UG    202    0        0 eth0

    as you see they are not identical. the first line is for incoming trafic and the second for the outgoing (or vice versa).

    in my case to add gateway i should use:
    route add -host
    i hope you know the diffrence betwen -host and -net.
    == end-newbiesh (LoL)

    So maybe is wrong with the init script. but be sure that if you still have net then everythink is okey. it does not matter wich one of the 2 equal lines the kernel is using, becaouse they are pointing at same thing. the solution is to write a patch wich makes (unnesessery) checking for existance of the route before adding it.

    so dont worry about it. i hope i've helped you in some or other way.
  • joetech20
    joetech20 Posts: 5
    The situation which you have explained is the difference between default route (route to destination) and the interface specifc route (destination
    My question is different. Probably, i did not explain it clearly enough. Let me try to explain the situation with an example.

    Consider the scenario where you have eth1 interface with IP address as In such a case when the interface comes up (assuming that there is no other route configured) , the following is the entry in the kernel route table

    Destination Gateway Genmask Flags Metric Ref Use Iface U 0 0 0 eth1

    Now, if i try to add the same entry again into the kernel routing table using the following command, the entry gets added and route entry shows two similar entries
    route add -net dev eth1
    Destination Gateway Genmask Flags Metric Ref Use Iface U 0 0 0 eth1 U 0 0 0 eth1

    I understand that there will not be any problem because of this duplicate rule. But, Can someone clarify the following questions based on the scenario which i explained above?

    1. Are these two entries (the entry which got added by default and the entry which i added manually) different by any means??
    2. why should there be two similar routes in the kernel routing table?
    3. Is there a patch available to solve this?
    4. In general, there should not be identical entries in the kernel routing table. 5. Is this not applicable to all routing entries (be it host specific, default route or interface specific route)?



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