Bash: Restart program when IP change for domain.com

haliban · 10-04-2012, 01:51 AM

[Task description]

We have a dns record domain.com it can resolve to IP1 or IP2. We don't know which one exactly (GTM load balancing)

The script will check every 5 minutes if the IP address for domain.com has changed. If it has than it will execute restartprogram1.sh if it hasn't do nothing.

[End of task description/]

I was thinking maybe to set both IPs as variable but it will work only in one direction

Than I was thinking to store the IP in file and use this file to check if the IP has changed every 5 mins.

Code:

#!/bin/bash
set -x

# domain.com can resolve to 100.24.34.4 or 100.24.35.4

now=`/usr/sbin/nslookup domain.com | /usr/bin/grep 100 | cut -d" " -f2`

before=`cat /home/scripts/lookup`
    
    if [ now =! before ] || [ -z `$before`] then
         `/usr/sbin/restartprogram.sh`
          echo $now > lookup`
         
        else
                `/usr/sbin/restartprogram.sh`
                 echo $now > lookup
fi

#END

I'm learning and will appreciate every helpful feedback and idea

Cheeerss

acid_kewpie · 10-04-2012, 02:46 AM

Hmm, well I'd be more interested to know what the program being restarted was, maybe there's a way to improve that at source rather than hacking around it.

Obviously there are plenty of ways to resolve this. Presumably you are running through a cron job. If this program being restarted maintains a persistent network connection then I would look for connections in the output of netstat to see what destination is currently being used, and compare that to the output from host (not nslookup), although your system is probably fine too, especially in a more general purpose solution.

You're a bit backtick happy there. Note that backticks were superseded by $(this) notation a long time ago in bash, it's much clearer to use. But your restart line in the else block doesn't make sense. You're going to try to run the output of the script, which, unless the script outputs "/usr/bin/somecommand" makes no sense.

haliban · 10-06-2012, 07:54 AM

Cheers Chris,

To add more meat on the bone of the story. The restartprogram.sh is actually Apache working as reverse proxy and pointing to the destination hosting server domain.com. The domain.com can change its IP without notification at any time (no control on this process). In our case Apache is cashing the DNS until restart. I'm not sure if Apache can check for IP change every few minutes and went to the hacky way

acid_kewpie · 10-08-2012, 07:05 AM

check this issue report, seems similar to what you're looking at https://issues.apache.org/bugzilla/s...g.cgi?id=50551

fundamentally, or at least, as *I* read it, there's no fault per se, and you could do well to just tune down the mod_proxy config to kill off connections pretty quickly.

haliban · 10-10-2012, 01:34 AM

This looks pretty much the same to what i'm trying to do.
We are using Apache/2.2.9
I'll give it a go and will let you know how it goes.

haliban · 10-14-2012, 07:25 PM

My config as bellow

NameVirtualHost 192.168.100.10:443
<VirtualHost 192.168.100.10:443>
SSLEngine ON
SSLProxyEngine ON
ProxyPass / https://backend.mydomain.com:8080/ disablereuse=On smax=0 ttl=30
ProxyPassReverse / https://backend.mydomain.com:8080/
</VirtualHost>I have changed the DNS for backend.mydomain.com to different IPs. Apache picked up the new IP without restart or additional interventions, pretty cool!

I haven't done packet captures at this time as it seems to look may do later on if needed.

Thanks hips for the support! Much appreciated!