I‘ve Squid Proxy server with over 100 Windows workstation. I see following error message in my cache.log file:

WARNING! Your cache is running out of filedescriptors

Do I need to filter proxy server requests to avoid this problem? How do I fix this problem under CentOS / Fedora / RHEL / Debian Linux?

If you see this message in your /var/log/squid/cache.log file, it means Squid proxy server is running out of file descriptors and cannot handle the extra requests sent by your client computers.

Task: Find out current file descriptors limits

Login to your Squid proxy server and type the following command at shell prompt:
# squidclient mgr:info | grep 'file descri'
# squidclient -p {port} -u {proxy-user} -w '{proxy-password}' mgr:info | grep 'file descri'
# squidclient -p 8080 -u admin -w 'secretePassword' mgr:info | grep 'file descri'
Sample output:

        Maximum number of file descriptors:   1024
        Available number of file descriptors: 1008
        Reserved number of file descriptors:   100

Squid increase the maximum file descriptors

First, update /etc/security/limits.conf file:
# vi /etc/security/limits.conf
Append following line to increase current limit from 1024 to 4096:
* - nofile 4096
Save and close the file. You must re-login to check new limits:
# ulimit -a | grep ‘open files’
Sample output:

open files                      (-n) 4096

Next you need to set the maximum number of open file descriptors by modifying squid proxy server configuration as per your Linux distribution.

Set max_filedesc under RHEL / Red Hat / CentOS / Fedora Linux

Stop the Squid as changes of this value isn’t respected by reconfigure command. This value should be changed only if there isn’t any active squid process.
# service squid stop
Open /etc/squid/squid.conf file
# vi /etc/squid/squid.conf
Append / add / modify max_filedesc directive:
max_filedesc 4096
Save and close the file. Start the squid proxy server:
# service squid start
Now again run squidclient command or check log files for messages:
# squidclient mgr:info

Set max_filedesc under Ubuntu / Debian Linux

First, stop squid proxy server, enter:
# /etc/init.d/squid stop
OR
$ sudo /etc/init.d/squid stop
Open /etc/default/squid to set max. number of filedescriptors to use. You can increase this on a busy
cache to a maximum of (currently) 4096 filedescriptors. Default is 1024.
$ sudo vi /etc/default/squid
OR
# vi /etc/default/squid
Now set SQUID_MAXFD:
SQUID_MAXFD=4096
Save and close the file. Start the squid proxy server:
$ sudo /etc/init.d/squid start
Verify that it is working with new limits:
# squidclient mgr:info | grep 'file descri'

See also:

1. FreeBSD Squid Proxy Caching Server Increase File Descriptors Limits