Otdfbt

I have Apache 2.4.18 Web server on my Ubuntu 16.04 VPS, and I configured these vHosts :

null.conf

<VirtualHost *:80>
 DocumentRoot /var/www/null

 LogLevel info
 ErrorLog $APACHE_LOG_DIR/null.error.log
 CustomLog $APACHE_LOG_DIR/null.log combined

 <Directory />
 Require all denied
 </Directory>
</VirtualHost>

dante-MyDomain.conf

<VirtualHost *:80>
 ServerName mydomain.com
 ServerAlias mydomain.com
 ServerAdmin admin@mydomain.com

 DocumentRoot /var/www/dante/MyDomain

 ErrorLog $APACHE_LOG_DIR/dante-MyDomain_Error.log
 CustomLog $APACHE_LOG_DIR/dante-MyDomain_Access.log combined

 <Directory "/var/www/dante/MyDomain">
 Require all granted
 </Directory>
</VirtualHost>

lazarus-public.conf

<VirtualHost *:80>
 ServerName lazarus
 ServerAlias lazarus
 ServerAdmin admin@lazarus.vps

 DocumentRoot /var/www/public

 ErrorLog $APACHE_LOG_DIR/public.error.log
 CustomLog $APACHE_LOG_DIR/public.access.log combined

 <Directory "/var/www/public">
 Require all granted
 Options FollowSymLinks Indexes
 </Directory>
</VirtualHost>

Where mydomain.com is a public domain available on the internet, And lazarus is a private domain name I set using hosts file in my operating system.

Also I configured apache to load null.conf before any other hosts, So if anyone tries to connect to my server using any other domain name or using IP, They'll get a 403 error.

The Problem is that the vHost on lazarus domain sometimes responds with error 503. Other vHosts usually work perfectly, unless there are some configuration or network issues, But lazarus vHost rarely works, And it works for two or three days then stops working again.

It most likely looks like a server overload because it will be available if I wait some hours. But the server is not overloaded and I'm sure of it (unless it's misconfigured), Because lazarus can only be access by those who know about it (using hosts file or any other mechanism) and my mydomain.com only has a single index.html 'Under Construction' note, So I don't think anyone will visit it !

So I tried to look in log files to see if there's anything, But there's nothing. I even tail -f the log files, there even is no access log, When I try to access the server, and even after I get my 503 response, Nothing changes in logs, as shown below :

dante@Lazarus:~$ tail -f /var/log/apache2/*.log
==> /var/log/apache2/dante-Studio313_Access.log <==

==> /var/log/apache2/dante-Studio313_Error.log <==

==> /var/log/apache2/error.log <==
[Fri Mar 29 08:29:23.438075 2019] [mpm_prefork:notice] [pid 1710] AH00163: Apache/2.4.18 (Ubuntu) configured -- resuming normal operations
[Fri Mar 29 08:29:23.438241 2019] [mpm_prefork:info] [pid 1710] AH00164: Server built: 2018-06-07T19:43:03
[Fri Mar 29 08:29:23.438276 2019] [core:notice] [pid 1710] AH00094: Command line: '/usr/sbin/apache2'

==> /var/log/apache2/null.error.log <==

==> /var/log/apache2/null.log <==

==> /var/log/apache2/other_vhosts_access.log <==

==> /var/log/apache2/public.access.log <==

==> /var/log/apache2/public.error.log <==

Nothing changes. Only some lines in error.log from when the apache2 service started.

Also, File permissions are correct. Apache has r/w access to all files in /var/www and /var/log/apache2

For the sake of verbosity, here is my apache2.conf :

DefaultRuntimeDir $APACHE_RUN_DIR
PidFile $APACHE_PID_FILE
Timeout 300
KeepAlive On
MaxKeepAliveRequests 50
KeepAliveTimeout 10
User $APACHE_RUN_USER
Group $APACHE_RUN_GROUP
HostnameLookups Off
ErrorLog $APACHE_LOG_DIR/error.log
LogLevel info
IncludeOptional mods-enabled/*.load
IncludeOptional mods-enabled/*.conf
Include ports.conf
<Directory />
 Options FollowSymLinks
 AllowOverride None
 Require all denied
</Directory>
AccessFileName .htaccess
<FilesMatch "^.ht">
 Require all denied
</FilesMatch>
LogFormat "%v:%p %h %l %u %t "%r" %>s %O "%Refereri" "%User-Agenti"" vhost_combined
LogFormat "%h %l %u %t "%r" %>s %O "%Refereri" "%User-Agenti"" combined
LogFormat "%h %l %u %t "%r" %>s %O" common
LogFormat "%Refereri -> %U" referer
LogFormat "%User-agenti" agent
IncludeOptional conf-enabled/*.conf
IncludeOptional null.conf
IncludeOptional sites-enabled/*.conf

So The Question is that how can I troubleshoot this situation ? Is there anything I'm missing ?

UPDATE : Forgot to mention
This only happens when I request it from a remote client. If I load it from localhost or using a proxy where which its server is my VPS, then lazarus works too.