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Supratik
Asked: 2013-08-20 23:26:10 +0800 CST

How to detect a dead ISP in a multihomed Linux router and route traffic through alternate ISP?

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I am currently using a Linux box (CentOS 5.9) as my router. The router is multihomed and uses two ISP to connect to the internet. Internet traffic from the LAN is distributed through both the lines.

When a particular ISP connection goes down I had to manually route the traffic through the other ISP. What is the best way to detect a dead ISP gateway and route traffic through the alternate ISP connection?

linux-networking

2 Answers

  1. Best Answer

    I would realize this with a simple cron job

    #!/bin/sh

PING=$(ping $ETH0ISPGW -I eth0 -c1 -W5 | awk -F '[ /]*' '/rtt/ {print $8}')

if [ -z "$PING" ]; then
        echo "Line is dead"
        echo "do something"
else
        echo "everything ok"
        echo "$PING"
fi

    If the line drops delete the specific rule , and let it send you an email, or something.

    With a dedicated routing platform like pfSense, it would take care of multiple connections on its own. regardles of the configuration of the two lines (load balancing, backup ect.)

    • 4

  2. You could use route metrics.

     route add default gw $gw1 metric 1
 route add default gw $gw2 metric 2

    in this way, if link 1 goes down, system will chose the secondary route. Not a balancer but a it should do the job. If you're looking for balancing too, i'd look to something hardware.

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