Knowledge Base

Pages of a website hosted in Plesk fail to load: Size of a request header field exceeds server limit

Symptoms

Website pages fail to open with the following error message in a web-browser:

Bad request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.

Cause

Size of a request header field has exceeded the server limit defined in the Apache web server (most often) configuration. The default Apache header size that is allowed is 8 KB.

Resolution

For Apache on Debian-based distributions (all domains)

For Apache on RHEL-based distributions (all domains)

For one website served by nginx only (single domain)

For one website served by Apache (single domain)

Connect to the Plesk server via SSH
Create a backup of the Apache configuration file (just in case):

# cp /etc/apache2/apache2.conf /etc/apache2/apache2.conf_`date +%F`
Open the /etc/apache2/apache2.conffile with your favorite command-line text editor
Add the following line right after the # Global configuration line (near the top of the file) and specify your value (16000 is an example):

LimitRequestFieldSize 16000
Save the changes and close the file
Restart the Apache service to apply the changes:

# systemctl restart apache2

Note: In case the value of 16000 (16KB) is not enough, increase it to the greater value, for example, 32000 (32KB).

Connect to the Plesk server via SSH
Create a backup of the Apache configuration file (just in case):

# cp -a /etc/httpd/conf/httpd.conf /root/httpd.conf_`date +%F`
Open the /etc/httpd/conf/httpd.conf file with your favorite command-line text editor
Add the following line right after the # Global configuration line (near the top of the file) and specify your value (16000 is an example):

LimitRequestFieldSize 16000
Save the changes and close the file
Restart the Apache service to apply the changes:

# systemctl restart httpd

Note: In case the value of 16000 (16KB) is not enough, increase it to the greater value, for example, 32000 (32KB).

If the website is using nginx-only hosting, apply the following steps:

Log into Plesk
Go to Domains > example.com > Hosting & DNS > Apache & nginx Settings.
Add the following line to Additional nginx directives:

large_client_header_buffers 4 16k;

Note: Edit 16k to match whatever request buffer size you need (8k is default), and keep in mind that the larger the buffer size, the greater the negative impact would be in case of a DDOS attack.
Press Apply

If the website is served by Apache, apply the following steps:

Log into Plesk
Go to Domains > example.com > Hosting & DNS > Apache & nginx Settings.
Add the following line to Additional directives for HTTP and Additional directives for HTTPS:

LimitRequestFieldSize 16000
Press Apply