19. Logging

19.1. Logging Configuration

During its operation Kea may produce many messages. They differ in severity (some are more important than others) and source (different components, like hooks, produce different messages). It is useful to understand which log messages are critical and which are not, and to configure logging appropriately. For example, debug-level messages can be safely ignored in a typical deployment. They are, however, very useful when debugging a problem.

The logging system in Kea is configured through the loggers entry in the server section of your configuration file. In previous Kea releases this entry was in an independent Logging section; this was still supported for backward compatibility until Kea 1.7.9 included.

19.1.1. Loggers

Within Kea, a message is logged through an entity called a “logger.” Different components log messages through different loggers, and each logger can be configured independently of the others. Some components, in particular the DHCP server processes, may use multiple loggers to log messages pertaining to different logical functions of the component. For example, the DHCPv4 server uses one logger for messages about packet reception and transmission, another logger for messages related to lease allocation, and so on. Some of the libraries used by the Kea server, such as libdhcpsrv, use their own loggers.

Users implementing hooks libraries (code attached to the server at runtime) are responsible for creating the loggers used by those libraries. Such loggers should have unique names, different from the logger names used by Kea. In this way the messages produced by the hooks library can be distinguished from messages issued by the core Kea code. Unique names also allow the loggers to be configured independently of loggers used by Kea. Whenever it makes sense, a hooks library can use multiple loggers to log messages pertaining to different logical parts of the library.

In the server section of a configuration file the configuration for zero or more loggers (including loggers used by the proprietary hooks libraries) can be specified. If there are no loggers specified, the code will use default values; these cause Kea to log messages of INFO severity or greater to standard output. There is a small time window after Kea has been started but before it has read its configuration; logging in this short period can be controlled using environment variables. For details, see Logging During Kea Startup.

The three main elements of a logger configuration are: name (the component that is generating the messages), severity (what to log), and output_commands (where to log). There is also a debuglevel element, which is only relevant if debug-level logging has been selected. The name (string) Logger

Each logger in the system has a name: that of the component binary file using it to log messages. For instance, to configure logging for the DHCPv4 server, add an entry for a logger named “kea-dhcp4”. This configuration will then be used by the loggers in the DHCPv4 server and all the libraries used by it, unless a library defines its own logger and there is a specific logger configuration that applies to that logger.

When tracking down an issue with the server’s operation, use of DEBUG logging is required to obtain the verbose output needed for problem diagnosis. However, the high verbosity is likely to overwhelm the logging system in cases where the server is processing high-volume traffic. To mitigate this problem, Kea can use multiple loggers, for different functional parts of the server, that can each be configured independently. If the user is reasonably confident that a problem originates in a specific function of the server, or that the problem is related to a specific type of operation, they may enable high verbosity only for the relevant logger, thereby limiting the debug messages to the required minimum.

The loggers are associated with a particular library or binary of Kea. However, each library or binary may (and usually does) include multiple loggers. For example, the DHCPv4 server binary contains separate loggers for packet parsing, dropped packets, callouts, etc.

The loggers form a hierarchy. For each program in Kea, there is a “root” logger, named after the program (e.g. the root logger for kea-dhcp, the DHCPv4 server) is named kea-dhcp4. All other loggers are children of this logger and are named accordingly, e.g. the allocation engine in the DHCPv4 server logs messages using a logger called kea-dhcp4.alloc-engine.

This relationship is important, as each child logger derives its default configuration from its parent root logger. In the typical case, the root logger configuration is the only logging configuration specified in the configuration file and so applies to all loggers. If an entry is made for a given logger, any attributes specified override those of the root logger, whereas any not specified are inherited from it.

To illustrate this, suppose we are using the DHCPv4 server with the root logger “kea-dhcp4” logging at the INFO level. In order to enable DEBUG verbosity for DHCPv4 packet drops, we must create a configuration entry for the logger called “kea-dhcp4.bad-packets” and specify severity DEBUG for this logger. All other configuration parameters may be omitted for this logger if the logger should use the default values specified in the root logger’s configuration.

If there are multiple logger specifications in the configuration that might match a particular logger, the specification with the more specific logger name takes precedence. For example, if there are entries for both “kea-dhcp4” and “kea-dhcp4.dhcpsrv”, the main DHCPv4 server program — and all libraries it uses other than the dhcpsrv library (libdhcpsrv) — will log messages according to the configuration in the first entry (“kea-dhcp4”). Messages generated by the dhcpsrv library will be logged according to the configuration set by the second entry.

Currently defined loggers are defined in the following table. The “Software Package” column of this table specifies whether the particular loggers belong to the core Kea code (open source Kea binaries and libraries), or hooks libraries (open source or premium).

List of Loggers Supported by Kea Servers and Hooks Libraries Shipped With Kea and Premium Packages
Logger Name Software Package Description
kea-ctrl-agent core The root logger for the Control Agent exposing the RESTful control API. All components used by the Control Agent inherit the settings from this logger.
kea-ctrl-agent.auth core A logger which covers access control details, such as a result of the basic HTTP authentication.
kea-ctrl-agent.http core A logger which outputs log messages related to receiving, parsing, and sending HTTP messages.
kea-dhcp4 core The root logger for the DHCPv4 server. All components used by the DHCPv4 server inherit the settings from this logger.
kea-dhcp6 core The root logger for the DHCPv6 server. All components used by the DHCPv6 server inherit the settings from this logger.
kea-dhcp4.alloc-engine, kea-dhcp6.alloc-engine core Used by the lease allocation engine, which is responsible for managing leases in the lease database, i.e. creating, modifying, and removing DHCP leases as a result of processing messages from clients.
kea-dhcp4.bad-packets, kea-dhcp6.bad-packets core Used by the DHCP servers for logging inbound client packets that were dropped or to which the server responded with a DHCPNAK. It allows administrators to configure a separate log output that contains only packet drop and reject entries.
kea-dhcp4.bootp-hooks libdhcp_bootp hook library This logger is used to log messages related to the operation of the BOOTP hook library.
kea-dhcp4.callouts, kea-dhcp6.callouts core Used to log messages pertaining to the callouts registration and execution for the particular hook point.
kea-dhcp4.commands, kea-dhcp6.commands core Used to log messages relating to the handling of commands received by the DHCP server over the command channel.
kea-dhcp4.database, kea-dhcp6.database core Used to log messages relating to general operations on the relational databases and Cassandra.
kea-dhcp4.ddns, kea-dhcp6.ddns core Used by the DHCP server to log messages related to Client FQDN and Hostname option processing. It also includes log messages related to the relevant DNS updates.
kea-dhcp4.dhcp4 core Used by the DHCPv4 server daemon to log basic operations.
kea-dhcp4.dhcpsrv, kea-dhcp6.dhcpsrv core The base loggers for the libkea-dhcpsrv library.
kea-dhcp4.eval, kea-dhcp6.eval core Used to log messages relating to the client classification expression evaluation code.
kea-dhcp4.host-cache-hooks, kea-dhcp6.host-cache-hooks libdhcp_host_cache premium hook library This logger is used to log messages related to the operation of the Host Cache hooks library.
kea-dhcp4.flex-id-hooks, kea-dhcp6.flex-id-hooks libdhcp_flex_id premium hook library This logger is used to log messages related to the operation of the Flexible Identifiers hooks library.
kea-dhcp4.ha-hooks, kea-dhcp6.ha-hooks libdhcp_ha hook library This logger is used to log messages related to the operation of the High Availability hooks library.
kea-dhcp4.hooks, kea-dhcp6.hooks core Used to log messages related to the management of hooks libraries, e.g. registration and deregistration of the libraries, and to the initialization of the callouts execution for various hook points within the DHCP server.
kea-dhcp4.host-cmds-hooks, kea-dhcp6.host-cmds-hooks libdhcp_host_cmds premium hook library This logger is used to log messages related to the operation of the Host Commands hooks library. In general, these will pertain to the loading and unloading of the library and the execution of commands by the library.
kea-dhcp4.hosts, kea-dhcp6.hosts core Used within the libdhcpsrv, it logs messages related to the management of DHCP host reservations, i.e. retrieving reservations and adding new reservations.
kea-dhcp4.lease-cmds-hooks, kea-dhcp6.lease-cmds-hooks libdhcp_lease_cmds hook library This logger is used to log messages related to the operation of the Lease Commands hooks library. In general, these will pertain to the loading and unloading of the library and the execution of commands by the library.
kea-dhcp4.leases, kea-dhcp6.leases core Used by the DHCP server to log messages related to lease allocation. The messages include detailed information about the allocated or offered leases, errors during the lease allocation, etc.
kea-dhcp4.legal-log-hooks, kea-dhcp6.legal-log-hooks libdhcp_legal_log premium hook library This logger is used to log messages related to the operation of the Forensic Logging hooks library.
kea-dhcp4.options, kea-dhcp6.options core Used by the DHCP server to log messages related to the processing of options in the DHCP messages, i.e. parsing options, encoding options into on-wire format, and packet classification using options contained in the received packets.
kea-dhcp4.packets, kea-dhcp6.packets core This logger is mostly used to log messages related to transmission of the DHCP packets, i.e. packet reception and the sending of a response. Such messages include information about the source and destination IP addresses and interfaces used to transmit packets. The logger is also used to log messages related to subnet selection, as this selection is usually based on the IP addresses, relay addresses, and/or interface names, which can be retrieved from the received packet even before the DHCP message carried in the packet is parsed.
kea-dhcp4.radius-hooks, kea-dhcp6.radius-hooks libdhcp_radius premium hook library This logger is used to log messages related to the operation of the RADIUS hooks library.
kea-dhcp4.stat-cmds-hooks, kea-dhcp6.stat-cmds-hooks libdhcp_stat_cmds hook library This logger is used to log messages related to the operation of the Statistics Commands hooks library. In general, these will pertain to loading and unloading the library and the execution of commands by the library.
kea-dhcp4.subnet-cmds-hooks, kea-dhcp6.subnet-cmds-hooks libdhcp_subnet_cmds hook library This logger is used to log messages related to the operation of the Subnet Commands hooks library. In general, these will pertain to loading and unloading the library and the execution of commands by the library.
kea-dhcp4.mysql-cb-hooks, kea-dhcp6.mysql-cb-hooks libdhcp_mysql_cb_hooks hook library This logger is used to log messages related to the operation of the MySQL Configuration Backend hooks library.
kea-dhcp-ddns core The root logger for the kea-dhcp-ddns daemon. All components used by this daemon inherit the settings from this logger unless there are configurations for more specialized loggers.
kea-dhcp-ddns.dctl core The logger used by the kea-dhcp-ddns daemon for logging basic information about the process, received signals, and triggered reconfigurations.
kea-dhcp-ddns.dhcpddns core The logger used by the kea-dhcp-ddns daemon for logging events related to DDNS operations.
kea-dhcp-ddns.dhcp-to-d2 core Used by the kea-dhcp-ddns daemon for logging information about events dealing with receiving messages from the DHCP servers and adding them to the queue for processing.
kea-dhcp-ddns.d2-to-dns core Used by the kea-dhcp-ddns daemon for logging information about events dealing with sending and receiving messages to and from the DNS servers.
kea-netconf core The root logger for the NETCONF agent. All components used by NETCONF inherit the settings from this logger if there is no specialized logger provided.
kea-dhcp4.lease-query-hooks, kea-dhcp6.lease-query-hooks libdhcp_lease_query hook library This logger is used to log messages related to the operation of the Leasequery hooks library

Note that user-defined hook libraries should not use any of the loggers mentioned above, but should instead define new loggers with names that correspond to the libraries using them. Suppose that a user created a library called “libdhcp-packet-capture” to dump packets received and transmitted by the server to a file. An appropriate name for the logger could be kea-dhcp4.packet-capture-hooks. (Note that the hook library implementer only specifies the second part of this name, i.e. “packet-capture”. The first part is a root-logger name and is prepended by the Kea logging system.) It is also important to note that since this new logger is a child of a root logger, it inherits the configuration from the root logger, something that can be overridden by an entry in the configuration file.

The easiest way to find a logger name is to configure all logging to go to a single destination and look there for specific logger names. See Logging Message Format for details. The severity (string) Logger

This specifies the category of messages logged. Each message is logged with an associated severity, which may be one of the following (in descending order of severity):

  • FATAL - associated with messages generated by a condition that is so serious that the server cannot continue executing.
  • ERROR - associated with messages generated by an error condition. The server will continue executing, but the results may not be as expected.
  • WARN - indicates an out-of-the-ordinary condition. However, the server will continue executing normally.
  • INFO - an informational message marking some event.
  • DEBUG - messages produced for debugging purposes.

When the severity of a logger is set to one of these values, it will only log messages of that severity and above (e.g. setting the logging severity to INFO will log INFO, WARN, ERROR, and FATAL messages). The severity may also be set to NONE, in which case all messages from that logger are inhibited.


The keactrl tool, described in Managing Kea with keactrl, can be configured to start the servers in verbose mode. If this is the case, the settings of the logging severity in the configuration file will have no effect; the servers will use a logging severity of DEBUG regardless of the logging settings specified in the configuration file. To control severity via the configuration file, please make sure that the kea_verbose value is set to “no” within the keactrl configuration. The debuglevel (integer) Logger

When a logger’s severity is set to DEBUG, this value specifies what level of debug messages should be printed. It ranges from 0 (least verbose) to 99 (most verbose). If severity for the logger is not DEBUG, this value is ignored. The output_options (list) Logger

Each logger can have zero or more output_options. These specify where log messages are sent and are explained in detail below. The output (string) Option

This value determines the type of output. There are several special values allowed here: stdout (messages are printed on standard output), stderr (messages are printed on stderr), syslog (messages are logged to syslog using the default name), syslog:name (messages are logged to syslog using a specified name). Any other value is interpreted as a filename to which messages should be written. The flush (true or false) Option

Flush buffers after each log message. Doing this will reduce performance but will ensure that if the program terminates abnormally, all messages up to the point of termination are output. The default is “true”. The maxsize (integer) Option

This option is only relevant when the destination is a file; this is the maximum size in bytes that a log file may reach. When the maximum size is reached, the file is renamed and a new file opened. For example, a “.1” is appended to the name; if a “.1” file exists, it is renamed “.2”, etc. This is referred to as rotation.

The default value is 10240000 (10MB). The smallest value that can be specified without disabling rotation is 204800. Any value less than this, including 0, disables rotation.


Due to a limitation of the underlying logging library (log4cplus), rolling over the log files (from “.1” to “.2”, etc) may show odd results; there can be multiple small files at the timing of rollover. This can happen when multiple processes try to roll over the files simultaneously. Version 1.1.0 of log4cplus solved this problem, so if this version or later of log4cplus is used to build Kea, the issue should not occur. Even for older versions, it is normally expected to happen rarely unless the log messages are produced very frequently by multiple different processes. The maxver (integer) Option

This option is only relevant when the destination is a file and rotation is enabled (i.e. maxsize is large enough). This is the maximum number of rotated versions that will be kept. Once that number of files has been reached, the oldest file, “log-name.maxver”, will be discarded each time the log rotates. In other words, at most there will be the active log file plus maxver rotated files. The minimum and default value is 1. The pattern (string) Option

This option can be used to specify the layout pattern of log messages for a logger. Kea logging is implemented using the Log4Cplus library and whose output formatting is based, conceptually, on the printf formatting from C and is discussed in detail in the next section Logging Message Format.

Each output type (stdout, file, or syslog) has a default pattern which describes the content of its log messages. This parameter can be used to specify a desired pattern. The pattern for each logger is governed individually so each configured logger can have its own pattern. Omitting the pattern parameter or setting it to an empty string, “”, causes Kea to use the default pattern for that logger’s output type.

In addition to the log text itself, the default patterns used for stdout and files contain information such as date and time, logger level, and process information. The default pattern for syslog is limited primarily to log level, source, and the log text. This avoids duplicating information which is usually supplied by syslog.


You are strongly encouraged to test your pattern(s) on a local, non-production instance of Kea, running in the foreground and logging to stdout.

19.1.2. Logging Message Format

As mentioned above, Kea log message content is controlled via a scheme similar to the C language’s printf formatting. The “pattern” used for each message is described by a string containing one or more format components as part of a text string. In addition to the components the string may contain any other arbitrary text you find useful.

The Log4Cplus documentation provides a concise discussion of the supported components and formatting behavior and can be seen here:

It is probably easiest to understand this by examining the default pattern for stdout and files (currently they are the same). That pattern is shown below:

"%D{%Y-%m-%d %H:%M:%S.%q} %-5p [%c/%i.%t] %m\n";

and a typical log produced by this pattern looks something like this:

2019-08-05 14:27:45.871 DEBUG [kea-dhcp4.dhcpsrv/8475.12345] DHCPSRV_TIMERMGR_START_TIMER starting timer: reclaim-expired-leases

That breaks down as like so:

  • %D{%Y-%m-%d %H:%M:%S.%q} ‘%D’ is the date and time in local time that the log message is generated, while everything between the curly braces, ‘{}’ are date and time components. From the example log above this produces: 2019-08-05 14:27:45.871
  • %-5p The severity of message, output as a minimum of five characters, using right-padding with spaces. In our example log: DEBUG
  • %c The log source. This includes two elements: the Kea process generating the message, in this case, kea-dhcp4; and the component within the program from which the message originated, dhcpsrv (e.g. the name of the library used by DHCP server implementations).
  • %i The process ID. From the example log: 8475
  • %t The thread ID. From the example log: 12345. Note the format of the thread ID is OS dependent: e.g. on some systems it is an address so it is displayed in hexadecimal.
  • %m The log message itself. Kea log messages all begin with a message identifier followed by arbitrary log text. Every message in Kea has a unique identifier, which can be used as an index to the Kea Messages Manual, where more information can be obtained. In our example log above, the identifier is DHCPSRV_TIMERMGR_START_TIMER. The log text is typically a brief description detailing the condition that caused the message to be logged. In our example, the information logged, starting timer: reclaim-expired-leases, explains that the timer for the expired lease reclamation cycle has been started.


Omitting %m will omit the log message text from your output making it rather useless. You should consider %m mandatory.

Finally, note that spacing between components, the square brackets around the log source and PID, and the final carriage return ‘n’ are all literal text specified as part of the pattern.


In order to ensure each log entry is a separate line, your patterns must end with an \n. There may be use cases where it is not desired so we do not enforce its inclusion. Be aware that if you omit it from your pattern that to common text tools or displays, the log entries will run together in one long, endless “line”.

The default for pattern for syslog output is as follows:

"%-5p [%c.%t] %m\n";

You can see that it omits the date and time as well the process ID as this information is typically output by syslog. Note that Kea uses the pattern to construct the text it sends to syslog (or any other destination). It has no influence on the content syslog may add or formatting it may do.

Consult your OS documentation for “syslog” behavior as there are multiple implementations. Example Logger Configurations

In this example we want to set the server logging to write to the console using standard output.

"Server": {
    "loggers": [
            "name": "kea-dhcp4",
            "output_options": [
                    "output": "stdout"
            "severity": "WARN"

In this second example, we want to store debug log messages in a file that is at most 2MB and keep up to eight copies of old log files. Once the logfile grows to 2MB, it will be renamed and a new file will be created.

"Server": {
    "loggers": [
            "name": "kea-dhcp6",
            "output_options": [
                    "output": "/var/log/kea-debug.log",
                    "maxver": 8,
                    "maxsize": 204800,
                    "flush": true
                    "pattern": "%d{%j %H:%M:%S.%q} %c %m\n"
            "severity": "DEBUG",
            "debuglevel": 99

Notice that the above configuration uses a custom pattern which produces output like this:

220 13:50:31.783 kea-dhcp4.dhcp4 DHCP4_STARTED Kea DHCPv4 server version 1.6.0-beta2-git started

19.1.3. Logging During Kea Startup

The logging configuration is specified in the configuration file. However, when Kea starts, the configuration file is not read until partway into the initialization process. Prior to that, the logging settings are set to default values, although it is possible to modify some aspects of the settings by means of environment variables. Note that in the absence of any logging configuration in the configuration file, the settings of the (possibly modified) default configuration will persist while the program is running.

The following environment variables can be used to control the behavior of logging during startup:


Specifies a directory where the logging system should create its lock file. If not specified, it is prefix/var/run/kea, where “prefix” defaults to /usr/local. This variable must not end with a slash. There is one special value: “none”, which instructs Kea not to create a lock file at all. This may cause issues if several processes log to the same file.


Specifies logging output. There are several special values:

stdout Log to standard output.

stderr Log to standard error.

syslog[:fac] Log via syslog. The optional fac (which is separated from the word “syslog” by a colon) specifies the facility to be used for the log messages. Unless specified, messages will be logged using the facility “local0”.

Any other value is treated as a name of the output file. If not specified otherwise, Kea will log to standard output.

19.2. Logging levels

All Kea servers follow the overall intention to strike a balance between letting the user know what is going on and not overloading the logging system with too much information as that could easily be used as a Denial Of Service attack.

A wealth of information is available on debug level. Opposed to FATAL, ERROR, WARN and INFO levels, DEBUG has additional debuglevel parameters. The following table offers a rough idea of what kind of information is logged on which level. Sadly, that information is not very consistent. Future Kea versions may attempt to improve consistency in this regard. Also, keep in mind that sometimes the circumstances determine if an information is logged on higher or lower level. For example, if packet is being dropped due to configured classification, that is an execution of the configured policy and would be logged on debuglevel 15. However, if the packet is dropped due to an exception being thrown, it is much more important, as it may indicate software bug, serious problems with memory, database connectivity and similar. As such it may be logged on much higher levels, such as WARN or even ERROR.

  • 0 - singular messages printed during start or shutdown of the server.
  • 10 - logs information about received API commands.
  • 15 - information about reasons why a packet was dropped.
  • 40 - a lot of tracing information, including processing decisions, results of expression evaluations and more.
  • 45 - similar to level 40, but with more details, e.g. the subnet being selected for incoming packet.
  • 50 - evaluations of expressions, status received from hook points, lease processing, packet processing details, including unpacking, packing, sending etc.
  • 55 - includes all details available, including full packet contents with all options printed.

The debug levels apply only to messages logged on DEBUG. The debug levels are configured using the debuglevel option. See Section The debuglevel (integer) Logger for details.