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Increase file descriptors

Linux manages open files as well as open network connections via a file descriptor table (network connections are also files in Linux). And sometimes the default size of that table is not sufficient.

I'll explain how to modify that table size in this tutorial. Also I tested it using Ubuntu Linux 18, this should work for pretty much every Linux distribution and version.

The current limits can be obtained via the following commands:

  1. root@machine:~# ulimit -n ; ulimit -Hn ; ulimit -Sn
  2. 1024
  3. 1048576
  4. 1024

First is the soft limit (which is the default limit ulimit shows, the second is the hard limit and the last one is the soft limit (this time explicitly).

About the difference between soft limits and hard limits I found the following explanation (Actually from a manual for a UNIX version, but also applies for Linux):

The soft limits are the ones that actually affect processes; hard limits are the
maximum values for soft limits. Any user or process can raise the soft limits up
to the value of the hard limits. Only processes with superuser authority can
raise the hard limits.

Now we edit the configuration:

  1. root@machine:~# nano /etc/security/limits.conf

The default file shipped with Ubuntu 18 should look like this:

  1. # /etc/security/limits.conf
  2. #
  3. #Each line describes a limit for a user in the form:
  4. #
  5. #<domain>        <type>  <item>  <value>
  6. #
  7. #Where:
  8. #<domain> can be:
  9. #        - a user name
  10. #        - a group name, with @group syntax
  11. #        - the wildcard *, for default entry
  12. #        - the wildcard %, can be also used with %group syntax,
  13. #                 for maxlogin limit
  14. #        - NOTE: group and wildcard limits are not applied to root.
  15. #          To apply a limit to the root user, <domain> must be
  16. #          the literal username root.
  17. #
  18. #<type> can have the two values:
  19. #        - "soft" for enforcing the soft limits
  20. #        - "hard" for enforcing hard limits
  21. #
  22. #<item> can be one of the following:
  23. #        - core - limits the core file size (KB)
  24. #        - data - max data size (KB)
  25. #        - fsize - maximum filesize (KB)
  26. #        - memlock - max locked-in-memory address space (KB)
  27. #        - nofile - max number of open files
  28. #        - rss - max resident set size (KB)
  29. #        - stack - max stack size (KB)
  30. #        - cpu - max CPU time (MIN)
  31. #        - nproc - max number of processes
  32. #        - as - address space limit (KB)
  33. #        - maxlogins - max number of logins for this user
  34. #        - maxsyslogins - max number of logins on the system
  35. #        - priority - the priority to run user process with
  36. #        - locks - max number of file locks the user can hold
  37. #        - sigpending - max number of pending signals
  38. #        - msgqueue - max memory used by POSIX message queues (bytes)
  39. #        - nice - max nice priority allowed to raise to values: [-20, 19]
  40. #        - rtprio - max realtime priority
  41. #        - chroot - change root to directory (Debian-specific)
  42. #
  43. #<domain>      <type>  <item>         <value>
  44. #
  45.  
  46. #*               soft    core            0
  47. #root            hard    core            100000
  48. #*               hard    rss             10000
  49. #@student        hard    nproc           20
  50. #@faculty        soft    nproc           20
  51. #@faculty        hard    nproc           50
  52. #ftp             hard    nproc           0
  53. #ftp             -       chroot          /ftp
  54. #@student        -       maxlogins       4
  55.  
  56. # End of file

Between the example and the # End of file comment we add the following lines:

  1. *                hard    nofile          8192
  2. *                soft    nofile          8192
  3. root             hard    nofile          8192
  4. root             soft    nofile          8192

This will change the soft and hard limit for all users as well as the root (which also includes the processes / services running as root) to 8192.

In our case (Ubuntu 18) this will increase the soft limit from 1024 to 8192, but will decrease the hard limit from 1048576 to 8192 - at least for the root user.