Amending netmask in RedHat ES4

[columb]

The network configuration that one of my servers sits on is changing from a 24 bit netmask to a 23 bit netmask and I need to change the NIC configuration to match. The only problem is I'm on a remote site and have no GUI.

What I propose to do is run

ifconfig eth0 netmask 255.255.254.0
ifconfig eth0 down; ifconfig eth0 up

As I'm very aware that any mistakes will mean that I can't connect to the system I'd appreciate any comments before I do something dumb.

Thanks

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Columb Healy

 

[thedaver]

I'll toss a couple of ideas out to you...

First, the service "network" on RH boxes is a better choice to refresh your networking. However, it is file driven for its configuration. Which brings me to my next point.

If you ONLY make the changes through ifconfig, these are RAM-only changes that will be lost when you next reboot the box or restart the network service. At such point, you will revert to the settings in your config files...

Sooooooooooooooooooooooooooooo...

Spend some time thinking about the contents of these two files:

/etc/sysconfig/network
/etc/sysconfig/network-scripts/ifcfg-eth0

Between these two files you will need to make the relevant changes to your IP, mask, broadcast, gateway, and network settings (a couple of which you might not have thought of in your first scenario)

You can make changes to these files and they won't impact the system until you either reboot or issue
"service network restart"

You WILL (99%) lose your connection to the server upon either method of restarting...

ALSO!!!! Make VERY sure that you understand your firewall rules. RH has a habit of there being some default firewall rules that are ENABLED when you build the machine. Generally these rules are written agnostically to your IP addressing scheme, but I recommend you be sure by checking iptables before you restart.

Beyond these points you would minimally want to ensure that any IP-specific assignments in your major service configuration files are managed before rebooting.

Generally 'sshd' is NOT bound to listen on a specific IP/mask, so unless you did it specifically you probably are ok.

Often 'httpd'/apache is bound to listen on a specific PORT not IP, but depending upon your environment, it might have been configured for specific IPs.

You may want to let this thread run for a day to see if anyone else spots ways to control your risks before you make your changes.... good luck!








D.E.R. Management - IT Project Management Consulting

http://www.dermanagement.com/

 

[columb]

Thanks thedaver.

I've had a poke around and as
[ul]
[li]The only thing that is changing is the netmask[/li]
[li]I do not use the built in firewall[/li]
[/ul]
Taking your suggestions I've now formulted plan B
[ol]
[li]Make a backup copy of /etc/sysconfig/network-scripts/ifcfg-eth0[/li]
[li]Edit /etc/sysconfig/network-scripts/ifcfg-eth0 to change the line NETMASK=255.255.255.0 to NETMASK=255.255.254.0[/li]
[li]run 'service network restart'[/li]
[/ol]
This change is planned for 18:00 GMT on Tuesday 5th so there's a little time if anyone would like to comment.

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Columb Healy

 

[thedaver]

You sure your gateway and broadcast are not changing on a netmask change?! I would think broadcast HAS to change.

D.E.R. Management - IT Project Management Consulting

http://www.dermanagement.com/

 

[columb]

thedaver

The gateway is not changing.

My copy of /etc/sysconfig/network-scripts/ifcfg-eth0 does not have a broadcast setting and the RedHat manual says that BROADCAST is depreciated so I guess it will work it out for itself.

Thanks again for your help.

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Columb Healy

 

[piti]

i would recomend first run some shell script that changes the network setup (ifconfig, eth down and up), while having scheduled the network restart in about 5 minutes or so - this way you can check if the setup is working - you can reach the network in- and outbound - and if not, everything is reverted to the working current setup (providing that you do not change the network init scripts) after those 5 mins ;-)

 

[columb]

piti
Hmm... Good idea. Something along the lines of

cd /etc/sysconfig/network-scripts
mv ifcfg-eth0 backup_ifcfg-eth0
mv amended_ifcfg-eth0 ifcfg-eth0
service network restart
sleep 600
[[ -f all_ok ]] && exit
mv ifcfg-eth0 amended_ifcfg-eth0
mv backup_ifcfg-eth0 ifcfg-eth0
service network restart

So I've got 10 minutes to login and touch /etc/sysconfig/network-scripts/all_ok to confirm all OK otherwise it will revert the changes.

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Columb Healy

 

[piti]

well ... looks nice, but i personaly would use this approach
1. "at" to schedule the network restart
2. "ifconfig" to change the settings and bring interface down/up

i think this is less failure jeopardized ...