13 Restricting agent checks

Overview

It is possible to restrict checks on the agent side by creating an item blacklist, a whitelist, or a combination of whitelist/blacklist.

To do that use a combination of two agent configuration parameters:

• AllowKey=<pattern> - which checks are allowed; <pattern> is specified using a wildcard (*) expression
• DenyKey=<pattern> - which checks are denied; <pattern> is specified using a wildcard (*) expression

Note that:

• All system.run[*] items (remote commands, scripts) are disabled by default, even when no deny keys are specified;
• Since Zabbix 5.0.2 the EnableRemoteCommands agent parameter is:

   * deprecated by Zabbix agent
          * unsupported by Zabbix agent2

Therefore, to allow all remote commands, specify an AllowKey=system.run[*] parameter. To allow only some remote commands, create a whitelist of specific system.run[] commands. To disallow specific remote commands, add DenyKey parameters with system.run[] commands before the AllowKey=system.run[*] parameter.

Important rules

• A whitelist without a deny rule is only allowed for system.run[*] items. For all other items, AllowKey parameters are not allowed without a DenyKey parameter; in this case Zabbix agent will not start with only AllowKey parameters.
• The order matters. The specified parameters are checked one by one according to their appearance order in the configuration file:
  • As soon as an item key matches an allow/deny rule, the item is either allowed or denied; and rule checking stops. So if an item matches both an allow rule and a deny rule, the result will depend on which rule comes first.
  • The order affects also EnableRemoteCommands parameter (if used).
• Unlimited numbers of AllowKey/DenyKey parameters is supported.
• AllowKey, DenyKey rules do not affect HostnameItem, HostMetadataItem, HostInterfaceItem configuration parameters.
• Key pattern is a wildcard expression where the wildcard (*) character matches any number of any characters in certain position. It might be used in both the key name and parameters.
• If a specific item key is disallowed in the agent configuration, the item will be reported as unsupported (no hint is given as to the reason);
• Zabbix agent with --print (-p) command line option will not show keys that are not allowed by configuration;
• Zabbix agent with --test (-t) command line option will return "Unsupported item key." status for keys that are not allowed by configuration;
• Denied remote commands will not be logged in the agent log (if LogRemoteCommands=1).

Use cases

Deny specific check

• Blacklist a specific check with DenyKey parameter. Matching keys will be disallowed. All non-matching keys will be allowed, except system.run[] items.

For example:

# Deny secure data access
       DenyKey=vfs.file.contents[/etc/passwd,*]

Deny specific command, allow others

• Blacklist a specific command with DenyKey parameter. Whitelist all other commands, with the AllowKey parameter.

# Disallow specific command
       DenyKey=system.run[ls -l /]
        
       # Allow other scripts
       AllowKey=system.run[*]

Allow specific check, deny others

• Whitelist specific checks with AllowKey parameters, deny others with DenyKey=*

For example:

# Allow reading logs:
       AllowKey=vfs.file.*[/var/log/*]
       
       # Allow localtime checks
       AllowKey=system.localtime[*]
       
       # Deny all other keys
       DenyKey=*

Pattern examples

system.run and AllowKey

A hypothetical script like 'myscript.sh' may be executed on a host via Zabbix agent in several ways:

1. As an item key in a passive or active check, for example:

• system.run[myscript.sh]
• system.run[myscript.sh,wait]
• system.run[myscript.sh.nowait]

Here the user may add "wait", "nowait" or omit the 2nd argument to use its default value in system.run[].

2. As a global script (initiated by user in frontend or API).

A user configures this script in Administration → Scripts, sets "Execute on: Zabbix agent" and puts "myscript.sh" into the script's "Commands" input field. When invoked from frontend or API the Zabbix server sends to agent:

• system.run[myscript.sh,wait] - up to Zabbix 5.0.4
• system.run[myscript.sh] - since 5.0.5

Here the user does not control the "wait"/"nowait" parameters.

3. As a remote command from an action. The Zabbix server sends to agent:

• system.run[myscript.sh,nowait]

Here again the user does not control the "wait"/"nowait" parameters.

What that means is if we set AllowKey like:

AllowKey=system.run[myscript.sh]

then

• system.run[myscript.sh] - will be allowed
• system.run[myscript.sh,wait], system.run[myscript.sh,nowait] will not be allowed - the script will not be run if invoked as a step of action

To allow all described variants you may add:

AllowKey=system.run[myscript.sh,*] 
       DenyKey=system.run[*]

to the agent/agent2 parameters.