Web

HOW TO : Apache and SELinux

Quick note for future reference..

If you ever run into errors like this

[code]
<pre>Starting httpd: Warning: DocumentRoot [/var/www/html/static] does not exist
Warning: DocumentRoot [/var/www/html/static] does not exist
Warning: DocumentRoot [/var/www/html/static] does not exist
Warning: DocumentRoot [/var/www/html/static] does not exist
(13)Permission denied: httpd: could not open error log file /etc/httpd/logs/error_log.
Unable to open logs
[FAILED]
[/code]
And you are scratching your head why Apache is throwing these errors, even when the said directory and files exist. And you have the right permissions!! Check if you have SELinux running and being enforced.
On RHEL, you can check if SELinux is running by
[code]cat /selinux/enforce [/code]
The two values are 0 and 1. 0 means, SELinux is not being enforced and 1 means it is.
You can quickly disable SELinux temporarily by
[code]echo 0 >/selinux/enforce [/code]
If you want to disable it permanently (i.e. survive reboots), you have to edit the file /etc/selinux/config and change the SELINUX line from enabled to disabled.

August 22, 2011 by HOWTO Linux Networking Technology Uncategorized Web 4

Overheard : Random comments about technology

Here are some interesting titbits from a executive summary event hosted by Redhat/Intel that I attended yesterday.

We decreased the execution times for our orders from 1.5 seconds to 5 milliseconds

This from an executive managing the technology organization for a large trading company. Imagine the geekiness in accomplishing this :).

For every 450 smartphones that get activated a server is added to support them

This from an Intel executive. So if there are 500000 android phones being activated every day.. that’s around 1111 servers being added just to serve the android fans :).

1 in 4 servers currently runs Linux

This from a Redhat executive. If anyone doubts that Linux is mainstream.. they are living under a rock šŸ™‚

August 19, 2011 by Linux OverHeard Technology Uncategorized Web 0

HOW TO : Enable global reverse proxy with certain exclusions in Apache

Say you want to enable reverse proxy on a site powered by Apache Web Server where all traffic to the web site it reverse proxied to a different server, but you want to exclude certain paths from being reverse proxies. I don’t know why you would want to do that :).. but we ran into that scenario at work and I wanted to document the config for future reference. The picture below shows a high level view of the traffic

  • Ensure the following modules are being loaded in Apache.

[bash]

LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
LoadModule proxy_connect_module modules/mod_proxy_connect.so
LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule rewrite_module modules/mod_rewrite.so

[/bash]

  • In the virtual host configuration for kudithipudi.org add the following lines

[bash]

ProxyRequests Off

<Proxy *>
Order deny,allow
Allow from all
</Proxy>

ProxyPass /static !
ProxyPass /media !
ProxyPass / http://INTERNAL_SERVER:8888
ProxyPassReverse / http://INTERNAL_SERVER:8888

[/bash]

March 8, 2011 by HOWTO Technology Uncategorized Web 0

HOW TO : Trick to find out your IP address from a web server farm

This is a quick trick I came up with to find out the IP address of a client that is trying to access a farm of web servers that you have access to. The diagram below shows the network path for a typical web server.

You have a client that might be sitting behind a (or multiple) proxy server. And there is a load balancer involved because you have multiple web servers for redundancy.

We were recently working on some rewrite rules for our web servers at work and we needed to find out what IP address the web servers were seeing the client traffic come from. Couple of challenges

  • Which web server do you check? The load balancer can send you traffic to any server.
  • What IP address are you going to look for? Wait that is the original problem right :).

The web servers usually write an entry to the error log when they serve a 404 error. So we can use that to figure out which web server you are hitting and what IP address the web server is seeing you as. Here’s the trick

  • On the client side go to http://WEBSITE_ADDRESS/Get_Me_My_IP (or some other URL, which you know doesn’t exist on the web site)
  • On the server side, grep for “Get_Me_My_IP” in the web server error logs

Here is an example, I ran on this website (https://kudithipudi.org)

[bash]
root@samurai:/var/log/apache2# grep -i what_is_my_ip access_kudithipudi.log
199.27.130.105 – – [04/Mar/2011:16:07:18 +0000] "GET /what_is_my_ip HTTP/1.0" 40 4 5495 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.14) Gecko/2 0110218 Firefox/3.6.14 ( .NET CLR 3.5.30729; .NET4.0E)"
[/bash]

  • From this entry I can figure out that my client is appearing as “199.27.130.105” to the web server.
March 4, 2011 by HOWTO Technology Uncategorized Web 0

HOW TO : Setup Global Redirect in Lighttpd

If you have ever managed a web application, you know you have to take it down at times :). And you usually want to show an simple page stating that you are down for maintenance. Here is a simple way to setup a “maintenance” splash page. The assumption is that you have a Linux server to host the maintenance page.

  • Configure lighttpd (HTTP Server) on the server using instructions from this article on Cyberciti.
  • Edit the lighttpd.conf file and add the following line in your site configuration

[bash]Ā server.error-handler-404Ā Ā  = "index.html" [/bash]

  • Name your maintenance page as index.html and upload it to the document root (in this example, it is /var/www/html)

You are essentially telling the web server to display index.html whenever the user is trying to access content that is not present on the server. And since there is no content on the server other than the index.html, the web browser will always display the index.html page..

February 11, 2011 by HOWTO Linux Networking Technology Uncategorized Web 0

HOW TO : Capture HTTP Headers using tcpdump

Quick how to on capturing HTTP headers using tcpdump on a web server (running Linux).

    • On the web server, issue the following command

      [bash] tcpdump -s 1024 -C 1024000 -w /tmp/httpcapture dst port 80 [/bash]

        • Stop the capture by issuing the break command (ctrl + c)
        • Open the capture file (httpcapture in this example) in wireshark and check out the headers under theĀ  the HTTP protocol
        February 7, 2011 by HOWTO Linux Networking Technology Uncategorized Web 2

        HOW TO : Configure Cache Expiration in Apache

        Cache servers depend on cache control headers provided by the web server. Essentially, the web server (based on the configuration) specify’s what content isĀ cache-ableĀ and for how long. (Note: Some of the cache servers might ignore this and have a default cache period for specific content. But that is not for another post šŸ™‚ )

        Here is a quick and dirty way to configure Apache 2.x server to enable cache control settings on all content in a directory

        [bash]
        ExpiresActive On
        <Directory "/var/www/html/static">
        Options FollowSymLinks MultiViews
        Order allow,deny
        Allow from all
        ExpiresDefault "modification plus 1 hour"
        </Directory>
        [/bash]

        This configuration tells apache to enable cache headers for all content in the /var/www/html/static folder. The cache expiration is set to expire 1 hour from the modification time of the content.

        January 31, 2011 by HOWTO Technology Uncategorized Web 0