Arnold is a port-blocker and vlan changer, and was originally made to be able to easier remove mischievers from the campus-internet.

This document will give you information about how Arnold works and how to use and configure it. A FAQ-section will be added as questions are received.

What does Arnold do?

Arnold is a system that blocks or changes vlan on (we use the label detains) switch-ports by using SNMP-set commands. Arnold uses an IP- or MAC-address to locate the switch-port the address is operating from using the NAV-database, and then attempts to detain it.


You must assign a write-enabled SNMP profile to your devices, otherwise Arnold will not be able to detain or enable ports on them. You can select profiles for devices individually in the Seed Database tool.

Using Arnold

Arnold consists of a couple of scripts, configuration files and a web-interface. For basic use you will use the web-interface to disable and enable ports. Automatic use of Arnold requires some setup and use of cron to execute jobs periodically.

The web-interface

The web-interface is accessible from the Toolbox. When using Arnold you have several views available. They are as follows:

  • History lists the detentions for the last x days. This will list both active detentions and detentions no longer in effect.

  • Detained ports lists all active detentions.

  • Detention reasons lists all existing reasons for a detention. A reason is directly connected to a detention, and makes it easy to group detentions by reasons.

  • Quarantine vlans lists all existing quarantine vlans. Instead of shutting down the port, you can switch the access vlan to that of a quarantine vlan. This vlan could be configured for instance to rate limit traffic or be restricted in some other way.

  • Detention profiles lists all detention profiles. A detention profile is used when executing automatic detentions.

In addition you have two actions you can use - Search and Manual detention.

  • Search lets you search for detentions given some search parameters.

  • Manual detention lets you manually detain a port given an IP- or MAC-address.

Predefined detentions

The only way to use a predefined detention is by using the start_arnold shell script. After creating a predefined detention you usually want to create a cron-job for running the script with some input parameters. See section about start_arnold.

Using the scripts

Arnold consists of three scripts, which all are located in the nav/bin directory.

  • autoenable enables ports based on the autoenable variable available for both manual and predefined detentions.

  • start_arnold is used in combination with predefined detentions to invoke a series of detentions.

  • t1000 verifies that the MAC-addresses that should be offline are not active on other ports. If a detained MAC-address is online on another port, it will try to detain it there aswell.

More details about the different scripts can be seen below.


autoenable fetches all detained ports with an autoenable-value and enables each of those detentions if the time is due. It can be run manually or as a periodic cron job.

The simplest way of running automatic enabling periodically is to create a file containing cron configuration that calls the autoenable program as often as you would like:

0 * * * * some_prefix/nav/bin/autoenable  # Run every hour on the hour

Save this snippet in a file called autoenable in NAV’s etc/cron.d/ directory. That way, you can add it to the navcron user’s crontab by calling nav start autoenable.


When a predefined detention is created you can use start_arnold to invoke a series of detentions based on the input to the script.

If the file or list of addresses exist locally then you can pipe it in using for instance cat:

# cat scanresult.txt | nav/bin/start_arnold -i

or you can do it from a remote server using ssh commands:

# cat scanresult.txt | ssh -i

To avoid having to type passwords you want to create public keys, like described for instance here.

File format

Each line in this file is assumed to consist of an IP- or MAC-address and optionally a comment (separated by a space). For each valid address a detention will be made. Lines starting with # will be skipped.


This script needs to be set up to run in the same way as autoenable.

t1000 fetches all detained ports and checks if the MAC-address which was behind the detained port is active on another port. If it is, it enforces the detention on that port aswell. Depending on options given at detention-time it will either remove the detention on the old port or just leave it.


This does not detain the new port immediately after a detained computer has moved to it, because it takes some time before NAV discovers the new location of the MAC-address. This combined with the interval t1000 runs in could give the user quite some time with access before being detained again. This on-and-off behavior of internet access has been known to cause confusion and annoyance among the users - use this script knowing that.

Configuring Arnold

Config files

The following configuration files are used by Arnold.


nav/etc/arnold/arnold.conf is divided into three sections.

  • arnold is the section that contains information about what database to use and on what networking equipment Arnold should be able to detain ports. You also define email-addresses here.

  • loglevel is deprecated. See the section about Logging.

  • arnoldweb has just one config option, which sets the default detention method when loading the web interface.


nav/etc/arnold/nonblock.conf is not really a config-file but an exception list. Some addresses should, for various reasons, not be detained. They can be added to this file. The format is defined in the file, and supports single addresses, lists and subnets.

On reading this file you will maybe notice options for defining netbox types that are to be ignored. This is a deprecated option that existed because Arnold had trouble communicating with some types of equipment. These kind of problems are now handled automatically.



When creating a predefined detention there is an option for “Path to mailfile”.

Arnold is able to send mail to those listed as responsible for the address it tries to detain. The mail-address is the contact address defined for an organisation derived for this IP- or MAC-address. You have to create the mail template yourself. The default template directory contains a README-file that has more information about how to create a template.


The arnold scripts logs to individual files stored in nav/var/log/arnold/. The webinterface logs to STDERR, which Apache most probably puts in it’s error.log. The loglevel used for each script must be set in logging.conf.

The loggers (with default loglevels) are:

start_arnold = INFO
t1000 = INFO
autoenable = INFO


Missing interfaces

When an interface that is a part of a detention is removed from NAV, commonly by removing the switch, Arnold will display a message regarding this. The last known interface and switch will be displayed.

To close this detention just enable it manually. This will not send any commands to any network equipment, only close the detention as seen from the web interface.