Unable to start Apache after dist-upgrade on a Plesk server: undefined symbol:

[Suhitha]

Question: Unable to start Apache after dist-upgrade on a Plesk server: undefined symbol: ap_proxy_check_connection


Applicable to:

• Plesk for Linux

Symptoms

• After a dist-upgrade (from Debian 8 or Ubuntu 16), Apache fails to start with th the error message:

# systemctl start apache2.service
...
apachectl[14512]: apache2: Syntax error on line 146 of /etc/apache2/apache2.conf: Syntax error on line 2 of /etc/apache2/mods-enabled/proxy_http.load: Cannot load /usr/lib/apache2/modules/mod_proxy_http.so into server: /usr/lib/apache2/modules/mod_proxy_http.so: undefined symbol: ap_proxy_check_connection

• The same error message may appear in the Home page in Plesk.

Cause

The libapache2-mod-proxy-psa package exists on the server which is a leftover from Debian 8/Ubuntu 16:

# dpkg -l libapache2-mod-proxy-psa
...
ii libapache2-mod-proxy-psa 2.4.10-debian8.0.17100418 amd64 Apache Module mod_proxy


Resolution

1.Connect to the Plesk server via SSH.

2.Remove the package libapache2-mod-proxy-psa:

# dpkg -r --force-depends libapache2-mod-proxy-psa

3.Disable proxy modules:

# a2dismod proxy*

4.Start the Apache service:

# systemctl restart apache2

5.Enable the default proxy modules.

# a2enmod proxy proxy_fcgi proxy_http

6.Restart the Apache service:

# systemctl restart apache2