A few things that could cause that are:
Possible Causes:
1) MaxClients set to a value too low
2) If you have over about 800 sites, the ErrorLog files open too many file descriptors and apache won't be able to log the errors and may stop responding.
Solutions:
1) edit /etc/httpd/conf/httpd.conf and increase the MaxClients setting to something like 200 or 300.
2) cd /usr/local/directadmin/data/templates
cp virtual_host*.conf custom
cd custom
# remove all the ErrorLog lines (or comment them out) from the 4 virtual_host*.conf files that are in the custom directory.
echo "action=rewrite&value=httpd" >> /usr/local/directadmin/data/task.queue
Apache should be restarted automatically after a few minutes later (rewrite might take a while with over 800 sites).
3) Other possible information:
Edit /usr/include/bits/typesizes.h and set
#define __FD_SETSIZE 32768
and then recompile with customapache.
On FreeBSD, it's /usr/include/sys/select.h
Change:
#define FD_SETSIZE 1024U
to
#define FD_SETSIZE 32768U
then recompile apache/php
4) edit /etc/sysctl.conf and add:
fs.file-max = 32768
and run:
/sbin/sysctl -w fs.file-max=32768
then recompile apache/php
5) Another way to free up FileDescriptors (FDs) is to disable ssl on any domain that does not require it.
A quck way to check is to type:
ls -la /home/*/domains/*/private_html/index.html
quickly scan the list for any index.html that isn't betwen 200-300 bytes in size. Any that are not in that range will have been edited and the user is probably using ssl, so take note of those usernames and domains. Now, the quick way to do a mass SSL shutoff for domains is to type:
perl -pi -e 's/ssl=ON/ssl=OFF/' /usr/local/directadmin/data/users/*/domains/*.conf
Then turn ssl=ON back on for any users who need it. Note that this is an end user level setting, so they have the ability to turn it back on themselves via Domain Setup. Then type the action=rewrite&value=httpd command as mentioned in step 2 above.
What this does is reduced the number of FD's by 50%. Since many people rarely use SSL, disabling it reduceds half of all virtualhosts, since all domains, subdomains, etc.. have 2 virtualhosts each with ssl, and only 1 each without ssl.
6) Openssl bug. Either update openssl and recompile apache, or patch apache 2:
http://issues.apache.org/bugzilla/show_bug.cgi?id=43717
Related error messages:
[error] System: Too many open files in system (errno: 23)
host: isc_socket_create: not enough free resources socket.c:2117: REQUIRE(maxfd <= (int)1024) failed.
host: isc_socket_create: not enough free resources
Referance from
http://help.directadmin.com/item.php?id=9
