4. Configuration

This document describes how to configure the VPL-Jail-System. For more details about VPL, visit the VPL home page or the VPL plugin page at Moodle.

4.1. Changing configuration

The configuration is done changing the text file /etc/vpl/vpl-jail-system.conf. To apply the new configuration value you must restart the service (as user root).

Using systemd

systemctl restart vpl-jail-system

or using system V

service vpl-jail-system restart

4.2. Configuration file format

The configuration file is in text format. Each line can be empty, be a comment, or a parameter line. A comment line starts with a ‘#’ (numeric symbol). A parameter line starts with a parameter name a ‘=’ (equals symbol) and the parameter value. Format PARAMETER=VALUE.


Beware that no space is allowed before or after the equals symbol “=”.


# This is a comment line

4.3. Configuration parameters

4.3.1. Network parameters PORT

Socket port number to listen for http and ws connections (no ciphers). The default value is 80. The value 0 indicates no use of unciphered connections.




For security reasons, current browsers do not accept ws connexions to no cipher ports. SECURE_PORT

Socket port number to listen for https and wss connections. Default value 443. The value 0 indicates no use of ciphered connections. Notice that the files with the server public certificate and secret key must exist and have proper content. Example:


If this option changes, the URL of the execution server used at Moodle must be update. URL format https://yourservername:SECURE_PORT/URLPATH. INTERFACE

Sets the network interface that the server will use. Use this parameter when your server has multiple IPs and you want to control which to use. The default value is empty, meaning the use of all network interfaces in the system.


New in version 2.6.0.

This parameter specifies ciphering options for SSL. In case of wanting to have Forward Secrecy, the value must be ECDHE. The default value is SSL_CIPHER_LIST=



New in version 2.6.0.

Indicates the path to the server’s certificate in PEM format. If your Certification Authority is not a root authority you may need to add the chain of certificates of the intermediate CAs to this file. The default value is SSL_CERT_FILE=/etc/vpl/cert.pem


SSL_CERT_FILE=/ssl/certs/mycert.pem SSL_KEY_FILE

New in version 2.6.0.

Indicates the path to the server’s private key in PEM format. The default value is SSL_KEY_FILE=/etc/vpl/key.pem.


4.3.2. Security parameters URLPATH

This parameter acts as a password to access the execution server. If the PATH of the URL request no matches URLPATH, the request is rejected. The default value is “/”. Example:


If this option changes, the URL of the execution server used at Moodle must be updated. URL format https://servername:SECURE_PORT/URLPATH. TASK_ONLY_FROM

This parameter limits the servers that can do task requests. The value must be IPs or networks (type A, B, and C) separate with spaces. The IP format is the full dot notation. Example: The network format is the incomplete dot notation ending with a dot. Example: 10.1.. The default value is empty, accepting task requests from all servers. Example:

# Accepts tasks from networt 10.10.3.X and IP

New in version 2.3.

This switch allows the execution of programs with a suid bit inside the jail. The default value is ALLOWSUID=false.


Setting true this option may be a security breach, use at your own risk.



New in version 2.5.

VPL jail service includes a logic to ban IPs with a high number of failed requests. This feature now can be controlled with a new configuration numeric parameter called FAIL2BAN. The banning and the account of failed requests take periods of 5 minutes. If one IP does more than FAIL2BAN*20 failed requests and more failed requests than succeeded, the offending network IP is banned until the next period. The FAIL2BAN set to 0 stops these checks. The default value of FAIL2BAN is 0. Examples:


Sets the system firewall using iptables. Accepted values are 0, 1, 2, 3, or 4. The preset value is 0.

0: No firewall
1: VPL service+DNS+internet access
2: VPL service+DNS+Limit Internet to port 80 (super user unlimited)
3: VPL service+No external access (super user unlimited)
4: VPL service+No external access


In level 4 stops the update/upgrade of the system.


This feature does not work in CentOS



This value goes from 0 to 8. Use 0 for the minimum log and 8 for the maximum log. Level 8 doesn’t remove the prisoners’ home directory. The default value is 3. Commonly the log is written to the file “/var/log/syslog”. Example:



Do not use a high log level in production servers; you may get a low performance and run out of disk space.

4.3.3. File system parameters JAILPATH

Sets the path to the jail directory. The system will use this directory as a fake clone of the root directory. The preset value is JAILPATH=/jail.


Path to control directory. The system saves here information on requests in progress. The preset value is CONTROLPATH=”/var/vpl-jail-system


New in version 2.3.

This switch allows the use tmpfs file system for “/home” and the “/dev/shm” directories. The preset value is USETMPFS=true. If your system memory is low you may use this switch, but changing this switch to “false” can reduce the performance of the jail system.


From version 2.3 the structure of jail file systems changed to improve the compatibility and performance of the use of overlayFS in different O.S. configurations. The upper layer of the overlaid file system is on a tmpfs file system or, if you set the USETMPFS=false, is on a loop file system located at a sibling path to the control path (by default /var/vpl-jail-system.fs).


If you set USETMPFS=false, then you can not set HOMESIZE to a system memory percent, you must set HOMESIZE to a fixed value. HOMESIZE

New in version 2.3.

This option sets the size of the “/home” directory. The value may be a percent of the system memory or a fixed value in megabyte (M) or gigabyte (G). The default value is 30% of the system memory. If USETMPFS is set to false, you must use a fixed value. Examples:




New in version 2.3.

This option sets the size of the “/dev/shm” directory. The preset value is 30% of the system memory. This option is applicable if using tmpfs file system for the “/dev/shm” directory. Example:


4.3.4. Parameters for limiting the resources used by the requested tasks

These parameters set resource limits the requested task can not exceed. MAXTIME

This parameter sets the maximum time for a request in seconds. Default value MAXTIME=1800 MAXFILESIZE

Maximum file size in bytes. No file created by a task can exceed this size. The default value is MAXFILESIZE=64000000. MAXMEMORY

Maximum memory size in bytes. No running task can exceed this size. The default value is MAXMEMORY=2000000. MAXPROCESSES

The maximum number of processes. No running task can exceed this number of processes/threads. The default value is MAXPROCESSES=500.

4.3.5. Other parameters MIN_PRISONER_UGID

This parameter sets the start value for the range of user/group ids selected randomly for prisoners. The preset value is MIN_PRISONER_UGID=10000. Example:


This parameter sets the end value for the range of user/group selected randomly for prisoners. The preset value is MAX_PRISONER_UGID=12000. Example:


This parameter sets the environment PATH variable when running tasks. IMPORTANT: If you are using RedHat or derived OSes you must set this parameter to the PATH environment variable of common users (not root). The default value is empty. Example: