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Apache Logs in AWS

I recently started using CloudWatch, Amazon's host monitoring service. It has a number of features but I just wanted a way to view all my logs in one place. Configuring hosts to use CloudWatch was easy, and I described the process in an article on setting up Anonymous FTP.

Timeout Errors

With monitoring in place, I noticed a lot of 408 errors coming from the Apache logs on my WordPress instance. The errors appeared in:


This was caused by the way an Elastic Load Balancer (ELB) was conducting its health check. I resurrected a thread in the AWS Forums to talk about it. The solution was to increase Apache's timeout values using the mod_reqtimeout module. I created the file /etc/httpd/conf.d/mod_reqtimeout.conf and added the following lines:

<IfModule reqtimeout_module>
    RequestReadTimeout header=62,MinRate=500 body=62,MinRate=500

Then I saved it and restarted Apache. Those header and body values correspond to the interval at which my ELB conducts its health check, which is set at 60 seconds. A slightly larger timeout value here prevents the errors from occurring.

Correcting Logs Entries

Because all incoming requests pass through the ELB, no external IP addresses are logged by default. The LogFormat directive must be modified to capture the original IP address using a special header that the ELB creates.

Edit the /etc/httpd/conf/httpd.conf file and find where LogFormat is defined. Comment out the defaults and add the following lines:

LogFormat "%{X-Forwarded-For}i %h %l %u %t \"%r\" %>s %b"
LogFormat "%{X-Forwarded-For}i %h %l %u %t \"%r\" %>s %b" common
LogFormat "%{X-Forwarded-For}i %h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined

Also find and replace the ErrorLogFormat directive with the following (just add this if no such directive exists):

ErrorLogFormat "[%{u}t] [%-m:%l] [pid %P:tid %T] %7F: %E: [client\ %a] [%{X-Forwarded-For}i] %M% ,\ referer\ %{Referer}i"

The X-Forwarded-For header field should contain the original IP address for any incoming request. I also wanted to ignore the health check and any request coming from the host itself. The SetEnvIf directive allows you to filter out requests based on their characteristics. I added these lines right above the new LogFormat directives:

<IfModule mod_setenvif>
    SetEnvIf Request_URI "^/wp-content/health_check\.txt$" dontlog
    SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog
    SetEnvIf Remote_Addr "::1" dontlog

This sets the "dontlog" variable if the request meets any of the above conditions. The last step is to attach it to the CustomLog directive. Comment out the existing CustomLog lines and add the following:

CustomLog "logs/access_log" combined env=!dontlog

The /etc/httpd/conf.d/ssl.conf file also need to be updated. Find the TransferLog and CustomLog directives and comment them out, then add the following:

CustomLog logs/ssl_access_log combined env=!dontlog
CustomLog logs/ssl_request_log "%t %{X-Forwarded-For}i %h %{SSL_PROTOCOL}x %{SSL_CIPHER}x \"%r\" %b" env=!dontlog

Also find and replace the ErrorLogFormat directive with the following (just add this if no such directive exists):

ErrorLogFormat "[%{u}t] [%-m:%l] [pid %P:tid %T] %7F: %E: [client\ %a] [%{X-Forwarded-For}i] %M% ,\ referer\ %{Referer}i"

Note: TransferLog differs from CustomLog in that it does not allow custom log formats or conditional logging.


Lastly I noticed an error in /var/log/ssl_error_log that said "server certificate does NOT include an ID which matches the server name". I just needed to update /etc/httpd/conf/httpd.conf with the following line:


Hopefully that helps anyone tuning their Apache logs in AWS!

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