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# 7 Custom LLD rules

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### Overview

It is also possible to create a completely custom LLD rule, discovering
any type of entities - for example, databases on a database server.

To do so, a custom item should be created that returns JSON, specifying
found objects and optionally - some properties of them. The amount of
macros per entity is not limited - while the built-in discovery rules
return either one or two macros (for example, two for filesystem
discovery), it is possible to return more.

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### Example

The required JSON format is best illustrated with an example. Suppose we
are running an old Zabbix 1.8 agent (one that does not support
"vfs.fs.discovery"), but we still need to discover file systems. Here is
a simple Perl script for Linux that discovers mounted file systems and
outputs JSON, which includes both file system name and type. One way to
use it would be as a UserParameter with key "vfs.fs.discovery\_perl":

``` {.perl}
#!/usr/bin/perl

$first = 1;

print "[\n";

for (`cat /proc/mounts`)
{
    ($fsname, $fstype) = m/\S+ (\S+) (\S+)/;

    print "\t,\n" if not $first;
    $first = 0;

    print "\t{\n";
    print "\t\t\"{#FSNAME}\":\"$fsname\",\n";
    print "\t\t\"{#FSTYPE}\":\"$fstype\"\n";
    print "\t}\n";
}

print "]\n";
```

::: noteimportant
Allowed symbols for LLD macro names are **0-9** , **A-Z** , **\_** , **.** 
Lowercase letters are not supported in the names.
:::

An example of its output (reformatted for clarity) is shown below. JSON
for custom discovery checks has to follow the same format.

``` {.javascript}
[
    { "{#FSNAME}":"/",                           "{#FSTYPE}":"rootfs"   },
    { "{#FSNAME}":"/sys",                        "{#FSTYPE}":"sysfs"    },
    { "{#FSNAME}":"/proc",                       "{#FSTYPE}":"proc"     },
    { "{#FSNAME}":"/dev",                        "{#FSTYPE}":"devtmpfs" },
    { "{#FSNAME}":"/dev/pts",                    "{#FSTYPE}":"devpts"   },
    { "{#FSNAME}":"/lib/init/rw",                "{#FSTYPE}":"tmpfs"    },
    { "{#FSNAME}":"/dev/shm",                    "{#FSTYPE}":"tmpfs"    },
    { "{#FSNAME}":"/home",                       "{#FSTYPE}":"ext3"     },
    { "{#FSNAME}":"/tmp",                        "{#FSTYPE}":"ext3"     },
    { "{#FSNAME}":"/usr",                        "{#FSTYPE}":"ext3"     },
    { "{#FSNAME}":"/var",                        "{#FSTYPE}":"ext3"     },
    { "{#FSNAME}":"/sys/fs/fuse/connections",    "{#FSTYPE}":"fusectl"  }
]
```

In the previous example it is required that the keys match the LLD macro
names used in prototypes, the alternative is to extract LLD macro values
using JSONPath `{#FSNAME}` → `$.fsname` and `{#FSTYPE}` → `$.fstype`,
thus making such script possible:

``` {.perl}
#!/usr/bin/perl
 
$first = 1;
 
print "[\n";
 
for (`cat /proc/mounts`)
{
    ($fsname, $fstype) = m/\S+ (\S+) (\S+)/;
 
    print "\t,\n" if not $first;
    $first = 0;
 
    print "\t{\n";
    print "\t\t\"fsname\":\"$fsname\",\n";
    print "\t\t\"fstype\":\"$fstype\"\n";
    print "\t}\n";
}
 
print "]\n";
```

An example of its output (reformatted for clarity) is shown below. JSON
for custom discovery checks has to follow the same format.

``` {.javascript}
[
    { "fsname":"/",                           "fstype":"rootfs"   },
    { "fsname":"/sys",                        "fstype":"sysfs"    },
    { "fsname":"/proc",                       "fstype":"proc"     },
    { "fsname":"/dev",                        "fstype":"devtmpfs" },
    { "fsname":"/dev/pts",                    "fstype":"devpts"   },
    { "fsname":"/lib/init/rw",                "fstype":"tmpfs"    },
    { "fsname":"/dev/shm",                    "fstype":"tmpfs"    },
    { "fsname":"/home",                       "fstype":"ext3"     },
    { "fsname":"/tmp",                        "fstype":"ext3"     },
    { "fsname":"/usr",                        "fstype":"ext3"     },
    { "fsname":"/var",                        "fstype":"ext3"     },
    { "fsname":"/sys/fs/fuse/connections",    "fstype":"fusectl"  }
]
```

Then, in the discovery rule's "Filter" field, we could specify
"{\#FSTYPE}" as a macro and "rootfs|ext3" as a regular expression.

::: noteclassic
You don't have to use macro names FSNAME/FSTYPE with custom
LLD rules, you are free to use whatever names you like. In case JSONPath
is used then LLD row will be an array element that can be an object, but
it can be also another array or a value. 
:::

Note that, if using a user parameter, the return value is limited to 512
KB. For more details, see [data limits for LLD return
values](/manual/discovery/low_level_discovery/notes#data_limits_for_return_values).

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