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I have the current proxy modules enabled and working fine:

$ apache2ctl -M | grep proxy proxy_module (shared) proxy_fcgi_module (shared)

When I enable mod_proxy_http with a2enmod proxy_http and service apache2 restart, apache returns an error as if mod_proxy was not enabled:

oct 16 10:55:45 apachectl[15243]: apache2: Syntax error on line 140 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

ap_proxy_check_connection is a mod_proxy.h method https://ci.apache.org/projects/httpd/trunk/doxygen/group__MOD__PROXY.html#ga618c0e1bede21bdd3af5d6a802671cae

Maybe some problem on module order load?

Thanks.

UPDATE: After add LoadModule mod_proxy before IncludeOptional directive, the problem is same.

$ vi /etc/apache2/apache2.conf Include /etc/apache2/mods-available/proxy.load Include /etc/apache2/mods-available/proxy.conf # Include module configuration: IncludeOptional mods-enabled/*.load IncludeOptional mods-enabled/*.conf $ apache2ctl -M | grep proxy [Mon Oct 16 11:14:00.205439 2017] [so:warn] [pid 19944] AH01574: module proxy_module is already loaded, skipping proxy_module (shared) proxy_fcgi_module (shared) $ a2enmod proxy_http Considering dependency proxy for proxy_http: Module proxy already enabled Enabling module proxy_http. To activate the new configuration, you need to run: systemctl restart apache2 $ apache2ctl -M | grep proxy [Mon Oct 16 11:14:11.256292 2017] [so:warn] [pid 20006] AH01574: module proxy_module is already loaded, skipping apache2: Syntax error on line 142 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
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  • a2enmod proxy && a2enmod proxy_http Commented Oct 16, 2017 at 10:00
  • Same probem: $ a2enmod proxy && a2enmod proxy_http && service apache2 restart: apache2: Syntax error on line 140 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 Commented Oct 16, 2017 at 10:03
  • how did you installed your apache? Commented Oct 16, 2017 at 10:07
  • It's a Plesk installation. Enabling http proxy module from Plesk panel the error is the same. Commented Oct 16, 2017 at 10:12
  • if you are using any hosting provider, open a ticket with this error "undefined symbol: ap_proxy_check_connection Commented Oct 16, 2017 at 10:14

2 Answers 2

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This might be due to dist-upgrade. Please check Plesk support articles -

Put this after the other LoadModule directives

LoadModule proxy_module /usr/lib/apache2/modules/mod_proxy.so LoadModule proxy_http_module /usr/lib/apache2/modules/mod_proxy_http.so LoadModule rewrite_module /usr/lib/apache2/modules/mod_rewrite.so
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Sounds like the problem as described here. To fix:

apt-get purge libapache2-mod-proxy-psa a2dismod proxy a2enmod proxy proxy_fcgi systemctl restart apache2
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