How to exchange a webserver maintaining the same IP address: haproxy

Scenario: An obsolete server hosting a website must be put offline and a new one must take his IP address. You don’t want to change all of yours A records on DNS to put online your new website, just exchange the internal IP address of your local network.

Your provider can exchange the addresses to assign the old IP public address to the new internal IP but it takes his time. How to cover the time between the provider action and your new website online?

Haproxy is the answer.

This is the content of the /etc/haproxy/haproxy.cfg:

# main frontend which proxys to the backends
# redirect all traffic on :80 to another server
frontend  main *:80
    default_backend             app

backend app
    balance     roundrobin
    # where 192.168.x.x is the internal address of the new server
    server  app1 192.168.x.x:80 check

And then restart haproxy (e.g. on centos 6 service haproxy restart).

Before configuring haproxy remember to stop or reconfigure the service listening to the port 80. If for example you want to reconfigure apache you have to change Listen 80 into something like Listen 8081 in /etc/httpd/conf/httpd.conf (CentOS 6 systems), if you have varnish listening to the 80 you have to change the VARNISH_LISTEN_PORT parameter in  /etc/sysconfig/varnish.

Now all the traffic arriving to the old server on port 80 will be redirected to the new server via local network. When the provider will exchange the addresses you haven’t to change anything. Meanwhile, you can test all the production settings prior to the real internal IP exchange.


Apache CentOS 6 cannot send email and Drupal get HTTP request status fails

I’m installing a Pressflow 6 on a new machine running CentOS 6. I’m using Apache MPM Worker with FastCGI. Then I get the classical e-mail error:

Unable to send e-mail. Please contact the site administrator if the problem persists.

Then I try to use sendmail:

sendmail -v < testmail

Where testmail is a file containing these lines:

Subject: test mail Ozu
Yasujiro Ozu
[blank line here]

And i get the message. PHP cannot send email through apache!

Trying a simple php script to send mail like drupal core do I got this error:

sendmail: fatal: chdir /var/spool/postfix: Permission denied

Then I check this variable following this awesome post:

# /usr/sbin/getsebool httpd_can_sendmail
httpd_can_sendmail --> off

Enable httpd_can_sendmail solve this issue:

setsebool -P httpd_can_sendmail 1

And wait. It will be a long wait using the -P option. And then PHP and Drupal can send mail.

Then check again the variable:

# /usr/sbin/getsebool httpd_can_sendmail
httpd_can_sendmail --> on

Now httpd can send mail. Try your script again.

The SMTP Authentication Support module is not working. This is another of these variables, the same that causes Drupal to show the “HTTP request status fails” message.

setsebool -P httpd_can_network_connect 1

And wait again. Both the SMTP module and the base Drupal networking are now working and Status report is all green.