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I have a linux server that needs to get some routing. I'm fairly new at this and i don't find any clear source on google.

The setup should be simple:
All traffic to a server on ip 192.168.72.20 should be sent over interface 3. All other interfaces don't matter at the moment.

Here you have the output for route -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.72.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0
192.168.72.0    0.0.0.0         255.255.255.0   U     0      0        0 eth1
192.168.72.0    0.0.0.0         255.255.255.0   U     0      0        0 eth2
192.168.72.0    0.0.0.0         255.255.255.0   U     0      0        0 eth3
192.168.72.0    0.0.0.0         255.255.255.0   U     0      0        0 eth4
169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth0
0.0.0.0         192.168.72.12   0.0.0.0         UG    0      0        0 eth0
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  • 1
    Are you using the old fashioned ifconfig and route command or the modern ip commands? (old ones would be something like route add -host 192.168.72.20 GW eth3_ip)
    – Hennes
    Mar 29 '13 at 13:48
  • i don't know, i guess the old ones, how can i check that?
    – Frederiek
    Mar 29 '13 at 13:58
31

Try:

ip route add 192.168.72.20/32 dev eth3

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  • 4
    Just a note: to permanently keep this configuration make sure you add it to /etc/rc.local or your distro's equivalent. It won't persist between reboots.
    – LawrenceC
    Mar 29 '13 at 16:32
  • 2
    I tried this same format for mapping an IP to the wlan0 on Ubuntu 14.04 and got RTNETLINK answers: Invalid argument I had to remove the /** from the IP so I ended up with: sudo ip route add 192.168.50.15 dev wlan0 -- thought I'd mention that in case anyone else ran into this issue. Thank you for your answer ultrasawblade, it was helpful.
    – mason81
    Jul 10 '15 at 14:54
  • 5
    Do not forget via <gateway_ip> if the target IP is not directly reachable!
    – calandoa
    Dec 4 '17 at 15:00
  • How would I add another entry saying "use this interface for all other destination IPs"? Jul 29 '20 at 12:51
  • 1
    Thanks! What would be the $ROUTER_IP? Jul 29 '20 at 18:27
0

you might need "via" in the command:

ip route add 192.168.72.20/32 via dev eth3

See this similar issue CentOS 7 custom routes with 2 NIC's

1
  • No, via specifically indicates that a gateway address follows. The output interface is a separate parameter from a gateway.
    – user1686
    Aug 12 at 13:25

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