The Directory Server allows for an unlimited number of connections by default, but the file descriptor limit on the operating system restrict the number of connections. Many Linux distributions have a default file descriptor limit of 1024 per process, which might be too low for the server if it needs to handle a large number of concurrent connections.

If the operating system relies on systemd, see the Linux operating system documentation for instructions on setting the file descriptor limit.

After you set the operating system limit, you can configure the number of file descriptors that the server will use either by using a NUM_FILE_DESCRIPTORS environment variable, or by creating a config/num-file-descriptors file with a single line such as NUM_FILE_DESCRIPTORS=12345. If these are not set, the operating system uses the default of 65535 descriptors. This is an optional change you can make if you want to ensure that the server shuts down safely prior to reaching the file descriptor limit.

  1. Display the current fs.file-max limit of the system. 
    sysctl fs.file-max

    The fs.file-max limit is the maximum server-wide file limit you can set without tuning the kernel parameters in the proc file system.

  2. Edit the /etc/sysctl.conf file.

    If there is a line that sets the value of the fs.file-max property, make sure that its value is set to at least 1.5 times the per-process limit.

    If there is no line that sets a value for this property, add the following to the end of the file:

    fs.file-max = 100000
    Note:

    100000 is just an example here. Specify a value of at least 1.5 times the per-process limit.

  3. Display the current hard limit of the system. 
    ulimit -aH

    The open files (-n) value is the maximum number of open files per process limit.

    The value should be set to at least 65535.

  4. Edit the /etc/security/limits.conf file.

    If the file has lines that set the soft and hard limits for the number of file descriptors, make sure the values are set to 65535. If the lines are not present, add the following lines before #End of file, making certain to insert a tab between the columns.

    *   soft   nofile   65535
*   hard   nofile   65535
    Note:

    The number of open file descriptors is limited by the physical memory available to the host. You can determine this limit with the following command.

    cat /proc/sys/fs/file-max

    If the file-max value is significantly higher than the 65535 limit, consider increasing the file descriptor limit to between 10% and 15% of the system-wide file descriptor limit. For example, if the file-max value is 810752, you could set the file descriptor limit to 100000. If the file-max value is lower than 65535, the host is likely not sized appropriately.

  5. Reboot your system, and then use the ulimit command to verify that the file descriptor limit is set to 65535. 
    # ulimit -n
Note:

For RedHat 7 or later, modify the 20-nproc.conf file to set both the open files and max user processes limits.

/etc/security/limits.d/20-nproc.conf

 Add or edit the following lines if they do not already exist:

 *          soft    nproc     65536
 *          soft    nofile    65536
 *          hard    nproc     65536
 *          hard    nofile    65536
 root       soft    nproc     unlimited