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Sanya Snex
Asked: 2016-04-29 10:21:45 +0800 CST

How do I find out linux gateway IP address?

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I have CentOS 6.7 with two Internet connections

eth0 - dynamic IP address and dynamic gateway via DHCP (controlled by NetworkManager)

ppp0 - dynamic IP address and dynamic gateway (configured with rp_pppoe)

I want write script which will change the default gateway on in some conditions something like this:

ip route replace default scope global via $IP1 dev eth0
# or
ip route replace default scope global via $IP2 dev ppp0
# or
ip route replace default scope global nexthop via $IP1 dev eth0 weight 8 nexthop via $IP2 dev ppp0 weight 10

How to find out dynamic gateway $IP1 and $IP2?

update:

from this answer https://unix.stackexchange.com/a/124341/157086

in file /var/lib/dhclient/dhclient.leases I can find gateway to eth0

option routers 12.34.59.28;

How to find out dynamic gateway $IP2 for ppp0?

gateway route dynamic-ip linux-networking ifconfig

4 Answers

  1. As @Alex mention netstat -r, if that's not available there is also another way:

    $ ip route list dev eno1
default via X.X.X.X  proto static  metric 100 
X.X.X.X/24  proto kernel  scope link  src X.X.X.X  metric 100 
$

    replace eno1 with your device.

    dhclient -R routers - I can't test it, as I don't have dhcp set-up anywhere.

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  2. For the ppp0 device you can query the IP address of the peer with ip addr show ppp0.

    However, you don’t even need one, as for a point-to-point device like ppp0 you can simply set the route only to the interface, omitting the router IP.

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  3. This command will give you all the routes set in your Linux machine, including gateway:

    route -n
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  4. Best Answer

    For eth0:

    #!/bin/bash    
gwip=$(nmcli dev list iface eth0 | grep IP4-SETTINGS.GATEWAY: | awk '{ print $2}')
ip route replace default scope global via $gwip dev eth0

    for ppp0:

    #!/bin/bash 
gwip2=$(/sbin/ip addr show ppp0 | grep peer | awk ' { print $4 } ' | sed 's/\/32//')
ip route replace default via $gwip2 dev ppp0
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