Change IP address on the interface without losing the connection

I happen from time to time to configure from scratch some Checkpoint UTM/Open Server that is thousand miles away. And from experience the best way to do it is when you have out-of-band fast access to the firewall. Of course not always such well-organized beforehand set up is available. Just like today when I was asked how to change IP address on the interfcae through which you are connected to the firewall. Ok, to be more specific - client had been connected with his UTM through some ISP that included also IP addresses on the WAN (External) interface of the firewall. Time has come to change ISP and accordingly its IP addresses. All went surprisingly well, my collegue added new IP address on the External interface as the Secondary IP and from then on he could access/manage firewall through this new IP without a hitch. There is one but though - SSL VPN service was still listening on the old IP and didn't work because of that. So we had to remove the new IP as Secondary and put it as the Primary one. For this he asked my opinion , I set up some improvised lab and here is how to do it .

  1. First, for unmanned location I set up in cron to do restart in say 10-15 minutes from now so if something goes wrong restart will discard any changes done in step 2:

[Expert@R71]#crontab -l

# DO NOT EDIT THIS FILE - edit the master and reinstall.  
# (/tmp/crontab.5649 installed on Wed Jun  2 11:25:53 2010)  
# (Cron version -- $Id: crontab.c,v 2.13 1994/01/17 03:20:37 vixie Exp $)  
27 11 * * * /sbin/reboot
  1. Connected through the ssh I did the following two commands on the same line that when finished should not even disconnect you from the ssh. It brings down secondary IP (aliased interface) and assigns this IP to the External interface as the usual Primary one.

ifconfig External:0 down ; ifconfig External netmask

Follow me on not to miss what I publish on Linkedin, Github, blog, and more.