Lookup Tables

NOTE: this is proposed functionality, which is NOT YET IMPLEMENTED!

Lookup tables are a powerful construct to obtain “class” information based on message content (e.g. to build log file names for different server types, departments or remote offices).

The base idea is to use a message variable as an index into a table which then returns another value. For example, $fromhost-ip could be used as an index, with the table value representing the type of server or the department or remote office it is located in. A main point with lookup tables is that the lookup is very fast. So while lookup tables can be emulated with if-elseif constructs, they are generally much faster. Also, it is possible to reload lookup tables during rsyslog runtime without the need for a full restart.

The lookup tables itself exists in a separate configuration file (one per table). This file is loaded on rsyslog startup and when a reload is requested.

There are different types of lookup tables:

  • string - the value to be looked up is an arbitrary string. Only exact some strings match.

  • array - the value to be looked up is an integer number from a consecutive set. The set does not need to start at zero or one, but there must be no number missing. So, for example 5,6,7,8,9 would be a valid set of index values, while 1,2,4,5 would not be (due to missing 2). A match happens if the requested number is present.

  • sparseArray - the value to be looked up is an integer value, but there may be gaps inside the set of values (usually there are large gaps). A typical use case would be the matching of IPv4 address information. A match happens on the first value that is less than or equal to the requested value.

Note that index integer numbers are represented by unsigned 32 bits.

Lookup tables can be access via the lookup() built-in function. The core idea is to set a local variable to the lookup result and later on use that local variable in templates.

More details on usage now follow.

Lookup Table File Format

Lookup table files contain a single JSON object. This object contains of a header and a table part.


This must be an array of elements, even if only a single value exists (for obvious reasons, we do not expect this to occur often). Each array element must contain two fields “index” and “value”.


This is a sample of how an ip-to-office mapping may look like:

{ "version":1, "nomatch":"unk", "type":"string",
  "table":[ {"index":"", "value":"A" },
          {"index":"", "value":"A" },
          {"index":"", "value":"A" },
          {"index":"", "value":"B" },
          {"index":"", "value":"B" },
          {"index":"", "value":"B" }

Note: if a different IP comes in, the value “unk” is returned thanks to the nomatch parameter in the first line.

RainerScript Statements

lookup_table() Object

This statement defines and initially loads a lookup table. Its format is as follows:

lookup_table(name="name" file="/path/to/file" reloadOnHUP="on|off")


  • name (mandatory)

    Defines the name of lookup table for further reference inside the configuration. Names must be unique. Note that it is possible, though not advisible, to have different names for the same file.

  • file (mandatory)

    Specifies the full path for the lookup table file. This file must be readable for the user rsyslog is run under (important when dropping privileges). It must point to a valid lookup table file as described above.

  • reloadOnHUP (optional, default “on”)

    Specifies if the table shall automatically be reloaded as part of HUP processing. For static tables, the default is “off” and specifying “on” triggers an error message. Note that the default of “on” may be somewhat suboptimal performance-wise, but probably is what the user intuitively expects. Turn it off if you know that you do not need the automatic reload capability.

lookup() Function

This function is used to actually do the table lookup. Format:

lookup("name", indexvalue)


  • return value

    The function returns the string that is associated with the given indexvalue. If the indexvalue is not present inside the lookup table, the “nomatch” string is returned (or an empty string if it is not defined).

  • name (constant string)

    The lookup table to be used. Note that this must be specified as a constant. In theory, variable table names could be made possible, but their runtime behaviour is not as good as for static names, and we do not (yet) see good use cases where dynamic table names could be useful.

  • indexvalue (expression)

    The value to be looked up. While this is an arbitrary RainerScript expression, it’s final value is always converted to a string in order to conduct the lookup. For example, “lookup(table, 3+4)” would be exactly the same as “lookup(table, “7”)”. In most cases, indexvalue will probably be a single variable, but it could also be the result of all RainerScript-supported expression types (like string concatenation or substring extraction). Valid samples are “lookup(name, $fromhost-ip & $hostname)” or “lookup(name, substr($fromhost-ip, 0, 5))” as well as of course the usual “lookup(table, $fromhost-ip)”.

load_lookup_table Statement

Note: in the final implementation, this MAY be implemented as an action. This is a low-level decesion that must be made during the detail development process. Parameters and semantics will remain the same of this happens.

This statement is used to reload a lookup table. It will fail if the table is static. While this statement is executed, lookups to this table are temporarily blocked. So for large tables, there may be a slight performance hit during the load phase. It is assume that always a triggering condition is used to load the table.

load_lookup_table(name="name" errOnFail="on|off" valueOnFail="value")


  • name (string)

    The lookup table to be used.

  • errOnFail (boolean, default “on”)

    Specifies whether or not an error message is to be emitted if there are any problems reloading the lookup table.

  • valueOnFail (optional, string)

    This parameter affects processing if the lookup table cannot be loaded for some reason: If the parameter is not present, the previous table will be kept in use. If the parameter is given, the previous table will no longer be used, and instead an empty table be with nomath=valueOnFail be generated. In short, that means when the parameter is set and the reload fails, all matches will always return what is specified in valueOnFail.

Usage example

For clarity, we show only those parts of rsyslog.conf that affect lookup tables. We use the remote office example that an example lookup table file is given above for.

lookup_table(name="ip2office" file="/path/to/ipoffice.lu"

template(name="depfile" type="string"

set $usr.dep = lookup("ip2office", $fromhost-ip);
action(type="omfile" dynfile="depfile")

# support for reload "commands"
if $fromhost-ip == ""
   and $msg contains "reload office lookup table"
   load_lookup_table(name="ip2office" errOnFail="on")

Note: for performance reasons, it makes sense to put the reload command into a dedicated ruleset, bound to a specific listener - which than should also be sufficiently secured, e.g. via TLS mutual auth.

Implementation Details

The lookup table functionality is implemented via highly efficient algorithms. The string lookup has O(log n) time complexity. The array lookup is O(1). In case of sparseArray, we have O(log n).

To preserve space and, more important, increase cache hit performance, equal data values are only stored once, no matter how often a lookup index points to them.

See also

Help with configuring/using Rsyslog:

See also

Contributing to Rsyslog:

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.