coredns/plugin/log/README.md

log

Name

log - enables query logging to standard output.

Description

By just using log you dump all queries (and parts for the reply) on standard output. Options exist to tweak the output a little. Note that for busy servers logging will incur a performance hit.

Enabling or disabling the log plugin only affects the query logging, any other logging from CoreDNS will show up regardless.

Syntax

log

With no arguments, a query log entry is written to stdout in the common log format for all requests. Or if you want/need slightly more control:

log [NAMES...] [FORMAT]

• NAMES is the name list to match in order to be logged
• FORMAT is the log format to use (default is Common Log Format), {common} is used as a shortcut for the Common Log Format. You can also use {combined} for a format that adds the query opcode {>opcode} to the Common Log Format.

You can further specify the classes of responses that get logged:

log [NAMES...] [FORMAT] {
    class CLASSES...
}

• CLASSES is a space-separated list of classes of responses that should be logged

The classes of responses have the following meaning:

• success: successful response
• denial: either NXDOMAIN or nodata responses (Name exists, type does not). A nodata response sets the return code to NOERROR.
• error: SERVFAIL, NOTIMP, REFUSED, etc. Anything that indicates the remote server is not willing to resolve the request.
• all: the default - nothing is specified. Using of this class means that all messages will be logged whatever we mix together with "all".

If no class is specified, it defaults to all.

Log Format

You can specify a custom log format with any placeholder values. Log supports both request and response placeholders.

The following place holders are supported:

• {type}: qtype of the request
• {name}: qname of the request
• {class}: qclass of the request
• {proto}: protocol used (tcp or udp)
• {remote}: client's IP address, for IPv6 addresses these are enclosed in brackets: [::1]
• {local}: server's IP address, for IPv6 addresses these are enclosed in brackets: [::1]
• {size}: request size in bytes
• {port}: client's port
• {duration}: response duration
• {rcode}: response RCODE
• {rsize}: raw (uncompressed), response size (a client may receive a smaller response)
• {>rflags}: response flags, each set flag will be displayed, e.g. "aa, tc". This includes the qr bit as well
• {>bufsize}: the EDNS0 buffer size advertised in the query
• {>do}: is the EDNS0 DO (DNSSEC OK) bit set in the query
• {>id}: query ID
• {>opcode}: query OPCODE
• {common}: the default Common Log Format.
• {combined}: the Common Log Format with the query opcode.
• {/LABEL}: any metadata label is accepted as a place holder if it is enclosed between {/ and }, the place holder will be replaced by the corresponding metadata value or the default value - if label is not defined. See the metadata plugin for more information.

The default Common Log Format is:

`{remote}:{port} - {>id} "{type} {class} {name} {proto} {size} {>do} {>bufsize}" {rcode} {>rflags} {rsize} {duration}`

In the default text mode, each of these logs is output with log.Info, so a typical example looks like this:

[INFO] [::1]:50759 - 29008 "A IN example.org. udp 41 false 4096" NOERROR qr,rd,ra,ad 68 0.037990251s

JSON Output

Start CoreDNS with -log-format=json to select JSON output for the entire process. This is a command-line flag, not a Corefile directive. -log-format=text is the default. The log plugin's name and response-class filters work identically in both modes.

Each query produces one JSON record with common fields time, level, msg, and plugin (always log for query records), plus these typed fields:

Field	Type	Meaning
client_ip	string	Client address, without brackets around IPv6 addresses
client_port	number	Client port
qname	string	Lowercase, fully qualified query name, in DNS presentation format
qtype, qclass	string	Query type and class, including numeric forms for unknown values
protocol	string	udp or tcp, as for {proto}
id, opcode	number	Query ID and opcode
request_size	number	Request size in bytes, as for {size}
dnssec_ok	boolean	Query's DNSSEC OK bit
bufsize	number	Effective response buffer size, as for {>bufsize}
rcode	string or null	Response RCODE, or null if no DNS response was recorded
response_size	number	Recorded response size in bytes, as for {rsize}
duration_seconds	number	Elapsed handling time in seconds

As in text mode, response sizes describe recorded, uncompressed messages, not necessarily the bytes delivered to the client. Deferred errors (such as SERVFAIL) use the response CoreDNS will generate after the plugin chain returns. A dropped request has rcode: null; it is not logged as a successful response. Raw Write calls contribute to the size but do not provide a decoded response RCODE.

FORMAT still controls msg, including custom formats and metadata placeholders. It does not replace the JSON schema or define new top-level fields. The DNS fields come directly from the request and response, not from parsing msg. For example, with log . "{name} {rcode}":

{"time":"2026-09-15T08:00:00Z","level":"INFO","msg":"example.org. NOERROR","plugin":"log","client_ip":"127.0.0.1","client_port":40212,"qname":"example.org.","qtype":"A","qclass":"IN","protocol":"udp","id":42,"opcode":0,"request_size":29,"dnssec_ok":false,"bufsize":512,"rcode":"NOERROR","response_size":29,"duration_seconds":0.001}

Additional metadata

The log plugin adds the following metadata to allow for granular differentiation of NOERROR denial vs success messages. These are mapped from plugin/pkg/response/classify.go and plugin/pkg/response/typify.go.

• {/log/class}: success, denial
• {/log/type}: NODATA, NXDOMAIN, NOERROR

. {
    log . "{proto} Request: {name} {type} {/log/class} {/log/type}"
}

Examples

Log all requests to stdout

. {
    log
    whoami
}

Custom log format, for all zones (.)

. {
    log . "{proto} Request: {name} {type} {>id}"
}

Only log denials (NXDOMAIN and nodata) for example.org (and below)

. {
    log example.org {
        class denial
    }
}

Log all queries which were not resolved successfully in the Combined Log Format.

. {
    log . {combined} {
        class denial error
    }
}

Log all queries on which we did not get errors

. {
    log . {
        class denial success
    }
}

Also the multiple statements can be OR-ed, for example, we can rewrite the above case as following:

. {
    log . {
        class denial
        class success
    }
}