Changing Servers - Redirect to new IP = No Downtime?

Solution 1:

Here's the method that worked for me:

1. Sync the files and databases with the new server.
2. Perform a re-sync just before cut-off.
3. Change the DNS to point to the new server.
4. Forward the request coming to the old ip to the new server until DNS propagation completes.

Here's how I would do the step 4:

We will configure IPTables on a Linux server to redirect all the traffic coming on port 80, (which is the default web server port), to a server with the IP 122.164.34.240. The first step is to set your Linux box to allow this kind of forwarding to take place. Open a terminal window, log in as root user and run the following command:

# echo 1 >/proc/sys/net/ipv4/ip_forward

The next step is to tell IPTables to redirect the traffic to the new server:

# iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 122.164.34.240

Here’s where the IPTables magic happens. With the third and final step, we tell IPTables to rewrite the origin of connections to the new server’s port 80 to appear to come from the old server.

# iptables -t nat -A POSTROUTING -p tcp -d 122.164.34.240 --dport 80 -j MASQUERADE

The final step is required because if we don’t tell the web server of the new server that the connections are coming from the client machines, it would think that they are originating from the old server.

You may want to repeat this for the databases and email server port as well.

Solution 2:

You can also look into adding multiple A-records. For example, Google uses this, check their nslookup output:

Name:       google.com
Addresses:  209.85.148.101
            209.85.148.102
            209.85.148.113
            209.85.148.138
            209.85.148.100
            209.85.148.139

If you add multiple A-records to a domain, visitors will receive multiple IP's and try them in that order. If one fails, the client moves to the next one to try.

Setup the new IP as an extra A-record 24 hours up front, start new server, shutdown the old, remove IP.