Linux Routing table
We can check current Linux route table with netstat command
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 12.96.176.1 0.0.0.0 UG 0 0 0 eth2
5.72.185.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
12.96.154.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
12.96.176.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
13.122.288.72 5.72.185.1 255.255.255.240 UG 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
135.188.0.0 5.72.185.1 255.255.0.0 UG 0 0 0 eth0
135.188.45.153 5.72.185.1 255.255.255.255 UGH 0 0 0 eth0
174.65.146.0 5.72.185.1 255.255.255.0 UG 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
Q: How to add a new route?
To add a single host destination:
# route add $Host_IP gw $Gateway
e.g:
# route add 12.153.4.22 gw 6.64.215.1
To add a network destination:
# route add –net $Network_IP netmask $MASK gw $Gateway
e.g:
# route add -net 12.153.4.0 netmask 255.255.255.248 gw 6.64.215.1
Q: How to delete route?
# route del –net $Network_IP netmask $MASK gw $Gateway
Q: How to check the default gateway?
In routing table, the row start with 0.0.0.0 has gateway IP which in this example is 12.96.176.1
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 12.96.176.1 0.0.0.0 UG 0 0 0 eth2
5.72.185.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
12.96.154.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
12.96.176.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
13.122.288.72 5.72.185.1 255.255.255.240 UG 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
135.188.0.0 5.72.185.1 255.255.0.0 UG 0 0 0 eth0
135.188.45.153 5.72.185.1 255.255.255.255 UGH 0 0
0 eth0
174.65.146.0 5.72.185.1 255.255.255.0 UG 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
and finally to set default gateway we can use following command:
# route add default gw 192.168.1.254 eth0
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 12.96.176.1 0.0.0.0 UG 0 0 0 eth2
5.72.185.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
12.96.154.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
12.96.176.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
13.122.288.72 5.72.185.1 255.255.255.240 UG 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
135.188.0.0 5.72.185.1 255.255.0.0 UG 0 0 0 eth0
135.188.45.153 5.72.185.1 255.255.255.255 UGH 0 0 0 eth0
174.65.146.0 5.72.185.1 255.255.255.0 UG 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
Q: How to add a new route?
To add a single host destination:
# route add $Host_IP gw $Gateway
e.g:
# route add 12.153.4.22 gw 6.64.215.1
To add a network destination:
# route add –net $Network_IP netmask $MASK gw $Gateway
e.g:
# route add -net 12.153.4.0 netmask 255.255.255.248 gw 6.64.215.1
Q: How to delete route?
# route del –net $Network_IP netmask $MASK gw $Gateway
Q: How to check the default gateway?
In routing table, the row start with 0.0.0.0 has gateway IP which in this example is 12.96.176.1
# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 12.96.176.1 0.0.0.0 UG 0 0 0 eth2
5.72.185.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
12.96.154.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
12.96.176.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
13.122.288.72 5.72.185.1 255.255.255.240 UG 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
135.188.0.0 5.72.185.1 255.255.0.0 UG 0 0 0 eth0
135.188.45.153 5.72.185.1 255.255.255.255 UGH 0 0
0 eth0
174.65.146.0 5.72.185.1 255.255.255.0 UG 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
and finally to set default gateway we can use following command:
# route add default gw 192.168.1.254 eth0
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