Logging

Cloud-init supports both local and remote logging configurable through python’s built-in logging configuration and through the cloud-init rsyslog module.

Command Output

Cloud-init can redirect its stdout and stderr based on config given under the output config key. The output of any commands run by cloud-init and any user or vendor scripts provided will also be included here. The output key accepts a dictionary for configuration. Output files may be specified individually for each stage (init, config, and final), or a single key all may be used to specify output for all stages.

The output for each stage may be specified as a dictionary of output and error keys, for stdout and stderr respectively, as a tuple with stdout first and stderr second, or as a single string to use for both. The strings passed to all of these keys are handled by the system shell, so any form of redirection that can be used in bash is valid, including piping cloud-init’s output to tee, or logger. If only a filename is provided, cloud-init will append its output to the file as though >> was specified.

By default, cloud-init loads its output configuration from /etc/cloud/cloud.cfg.d/05_logging.cfg. The default config directs both stdout and stderr from all cloud-init stages to /var/log/cloud-init-output.log. The default config is given as

output: { all: "| tee -a /var/log/cloud-init-output.log" }

For a more complex example, the following configuration would output the init stage to /var/log/cloud-init.out and /var/log/cloud-init.err, for stdout and stderr respectively, replacing anything that was previously there. For the config stage, it would pipe both stdout and stderr through tee -a /var/log/cloud-config.log. For the final stage it would append the output of stdout and stderr to /var/log/cloud-final.out and /var/log/cloud-final.err respectively.

output:
    init:
        output: "> /var/log/cloud-init.out"
        error: "> /var/log/cloud-init.err"
    config: "tee -a /var/log/cloud-config.log"
    final:
        - ">> /var/log/cloud-final.out"
        - "/var/log/cloud-final.err"

Python Logging

Cloud-init uses the python logging module, and can accept config for this module using the standard python fileConfig format. Cloud-init looks for config for the logging module under the logcfg key.

Note

the logging configuration is not YAML, it is python fileConfig format, and is passed through directly to the python logging module. please use the correct syntax for a multi-line string in YAML.

By default, cloud-init uses the logging configuration provided in /etc/cloud/cloud.cfg.d/05_logging.cfg. The default python logging configuration writes all cloud-init events with a priority of WARNING or higher to console, and writes all events with a level of DEBUG or higher to /var/log/cloud-init.log and via syslog.

Python’s fileConfig format consists of sections with headings in the format [title] and key value pairs in each section. Configuration for python logging must contain the sections [loggers], [handlers], and [formatters], which name the entities of their respective types that will be defined. The section name for each defined logger, handler and formatter will start with its type, followed by an underscore (_) and the name of the entity. For example, if a logger was specified with the name log01, config for the logger would be in the section [logger_log01].

Logger config entries contain basic logging set up. They may specify a list of handlers to send logging events to as well as the lowest priority level of events to handle. A logger named root must be specified and its configuration (under [logger_root]) must contain a level and a list of handlers. A level entry can be any of the following: DEBUG, INFO, WARNING, ERROR, CRITICAL, or NOTSET. For the root logger the NOTSET option will allow all logging events to be recorded.

Each configured handler must specify a class under the python’s logging package namespace. A handler may specify a message formatter to use, a priority level, and arguments for the handler class. Common handlers are StreamHandler, which handles stream redirects (i.e. logging to stderr), and FileHandler which outputs to a log file. The logging module also supports logging over net sockets, over http, via smtp, and additional complex configurations. For full details about the handlers available for python logging, please see the documentation for python logging handlers.

Log messages are formatted using the logging.Formatter class, which is configured using formatter config entities. A default format of %(message)s is given if no formatter configs are specified. Formatter config entities accept a format string which supports variable replacements. These may also accept a datefmt string which may be used to configure the timestamp used in the log messages. The format variables %(asctime)s, %(levelname)s and %(message)s are commonly used and represent the timestamp, the priority level of the event and the event message. For additional information on logging formatters see python logging formatters.

Note

by default the format string used in the logging formatter are in python’s old style %s form. the str.format() and string.Template styles can also be used by using { or $ in place of % by setting the style parameter in formatter config.

A simple, but functional python logging configuration for cloud-init is below. It will log all messages of priority DEBUG or higher both stderr and /tmp/my.log using a StreamHandler and a FileHandler, using the default format string %(message)s:

logcfg: |
 [loggers]
 keys=root,cloudinit
 [handlers]
 keys=ch,cf
 [formatters]
 keys=
 [logger_root]
 level=DEBUG
 handlers=
 [logger_cloudinit]
 level=DEBUG
 qualname=cloudinit
 handlers=ch,cf
 [handler_ch]
 class=StreamHandler
 level=DEBUG
 args=(sys.stderr,)
 [handler_cf]
 class=FileHandler
 level=DEBUG
 args=('/tmp/my.log',)

For additional information about configuring python’s logging module, please see the documentation for python logging config.

Rsyslog Module

Cloud-init’s cc_rsyslog module allows for fully customizable rsyslog configuration under the rsyslog config key. The simplest way to use the rsyslog module is by specifying remote servers under the remotes key in rsyslog config. The remotes key takes a dictionary where each key represents the name of an rsyslog server and each value is the configuration for that server. The format for server config is:

optional filter for log messages (defaults to *.*)

optional leading @ or @@, indicating udp and tcp respectively (defaults to @, for udp)

ipv4 or ipv6 hostname or address. ipv6 addresses must be in [::1] format, (e.g. @[fd00::1]:514)

optional port number (defaults to 514)

For example, to send logging to an rsyslog server named log_serv with address 10.0.4.1, using port number 514, over udp, with all log messages enabled one could use either of the following.

With all options specified:

rsyslog:
    remotes:
        log_serv: "*.* @10.0.4.1:514"

With defaults used:

rsyslog:
    remotes:
        log_serv: "10.0.4.1"

For more information on rsyslog configuration, see Rsyslog.