dig - Unix, Linux Command

dig
(domain information groper) is a flexible tool for interrogating DNS name servers. It performs DNS lookups and displays the answers that are returned from the name server(s) that were queried. Most DNS administrators use
dig
to troubleshoot DNS problems because of its flexibility, ease of use and clarity of output. Other lookup tools tend to have less functionality than
dig.


Although
dig
is normally used with command-line arguments, it also has a batch mode of operation for reading lookup requests from a file. A brief summary of its command-line arguments and options is printed when the
-h
option is given. Unlike earlier versions, the BIND 9 implementation of
dig
allows multiple lookups to be issued from the command line.


Unless it is told to query a specific name server,
dig
will try each of the servers listed in
/etc/resolv.conf.


When no command line arguments or options are given,
dig
will perform an NS query for "." (the root).


It is possible to set per-user defaults for
dig
via
${HOME}/.digrc. This file is read and any options in it are applied before the command line arguments.


The IN and CH class names overlap with the IN and CH top level domains names. Either use the
-t
and
-c
options to specify the type and class, or use "IN." and "CH." when looking up these top level domains.


A typical invocation of
dig
looks like:



 dig @server name type


where:


server
is the name or IP address of the name server to query. This can be an IPv4 address in dotted-decimal notation or an IPv6 address in colon-delimited notation. When the supplied
server
argument is a hostname,
dig
resolves that name before querying that name server. If no
server
argument is provided,
dig
consults
/etc/resolv.conf
and queries the name servers listed there. The reply from the name server that responds is displayed.


name
is the name of the resource record that is to be looked up.


type
indicates what type of query is required — ANY, A, MX, SIG, etc.
type
can be any valid query type. If no
type
argument is supplied,
dig
will perform a lookup for an A record.


The
-b
option sets the source IP address of the query to
address. This must be a valid address on one of the host’s network interfaces or "0.0.0.0" or "::". An optional port may be specified by appending "#<port>"


The default query class (IN for internet) is overridden by the
-c
option.
class
is any valid class, such as HS for Hesiod records or CH for Chaosnet records.


The
-f
option makes
dig 
operate in batch mode by reading a list of lookup requests to process from the file
filename. The file contains a number of queries, one per line. Each entry in the file should be organized in the same way they would be presented as queries to
dig
using the command-line interface.


The
-m
option enables memory usage debugging.


If a non-standard port number is to be queried, the
-p
option is used.
port#
is the port number that
dig
will send its queries instead of the standard DNS port number 53. This option would be used to test a name server that has been configured to listen for queries on a non-standard port number.


The
-4
option forces
dig
to only use IPv4 query transport. The
-6
option forces
dig
to only use IPv6 query transport.


The
-t
option sets the query type to
type. It can be any valid query type which is supported in BIND 9. The default query type is "A", unless the
-x
option is supplied to indicate a reverse lookup. A zone transfer can be requested by specifying a type of AXFR. When an incremental zone transfer (IXFR) is required,
type
is set to
ixfr=N. The incremental zone transfer will contain the changes made to the zone since the serial number in the zone’s SOA record was
N.


Reverse lookups — mapping addresses to names — are simplified by the
-x
option.
addr
is an IPv4 address in dotted-decimal notation, or a colon-delimited IPv6 address. When this option is used, there is no need to provide the
name,
class
and
type
arguments.
dig
automatically performs a lookup for a name like
11.12.13.10.in-addr.arpa
and sets the query type and class to PTR and IN respectively. By default, IPv6 addresses are looked up using nibble format under the IP6.ARPA domain. To use the older RFC1886 method using the IP6.INT domain specify the
-i
option. Bit string labels (RFC2874) are now experimental and are not attempted.


To sign the DNS queries sent by
dig
and their responses using transaction signatures (TSIG), specify a TSIG key file using the
-k
option. You can also specify the TSIG key itself on the command line using the
-y
option;
name
is the name of the TSIG key and
key
is the actual key. The key is a base-64 encoded string, typically generated by
dnssec-keygen(8). Caution should be taken when using the
-y
option on multi-user systems as the key can be visible in the output from
ps(1 )
or in the shell’s history file. When using TSIG authentication with
dig, the name server that is queried needs to know the key and algorithm that is being used. In BIND, this is done by providing appropriate
key
and
server
statements in
named.conf.


dig
provides a number of query options which affect the way in which lookups are made and the results displayed. Some of these set or reset flag bits in the query header, some determine which sections of the answer get printed, and others determine the timeout and retry strategies.


Each query option is identified by a keyword preceded by a plus sign (+). Some keywords set or reset an option. These may be preceded by the string
no
to negate the meaning of that keyword. Other keywords assign values to options like the timeout interval. They have the form
+keyword=value. The query options are:


+[no]tcp
Use [do not use] TCP when querying name servers. The default behavior is to use UDP unless an AXFR or IXFR query is requested, in which case a TCP connection is used.


+[no]vc
Use [do not use] TCP when querying name servers. This alternate syntax to
+[no]tcp
is provided for backwards compatibility. The "vc" stands for "virtual circuit".


+[no]ignore
Ignore truncation in UDP responses instead of retrying with TCP. By default, TCP retries are performed.


+domain=somename
Set the search list to contain the single domain
somename, as if specified in a
domain
directive in
/etc/resolv.conf, and enable search list processing as if the
+search
option were given.


+[no]search
Use [do not use] the search list defined by the searchlist or domain directive in
resolv.conf
(if any). The search list is not used by default.


+[no]defname
Deprecated, treated as a synonym for
+[no]search


+[no]aaonly
Sets the "aa" flag in the query.


+[no]aaflag
A synonym for
+[no]aaonly.


+[no]adflag
Set [do not set] the AD (authentic data) bit in the query. The AD bit currently has a standard meaning only in responses, not in queries, but the ability to set the bit in the query is provided for completeness.


+[no]cdflag
Set [do not set] the CD (checking disabled) bit in the query. This requests the server to not perform DNSSEC validation of responses.


+[no]cl
Display [do not display] the CLASS when printing the record.


+[no]ttlid
Display [do not display] the TTL when printing the record.


+[no]recurse
Toggle the setting of the RD (recursion desired) bit in the query. This bit is set by default, which means
dig
normally sends recursive queries. Recursion is automatically disabled when the
+nssearch
or
+trace
query options are used.


+[no]nssearch
When this option is set,
dig
attempts to find the authoritative name servers for the zone containing the name being looked up and display the SOA record that each name server has for the zone.


+[no]trace
Toggle tracing of the delegation path from the root name servers for the name being looked up. Tracing is disabled by default. When tracing is enabled,
dig
makes iterative queries to resolve the name being looked up. It will follow referrals from the root servers, showing the answer from each server that was used to resolve the lookup.


+[no]cmd
Toggles the printing of the initial comment in the output identifying the version of
dig
and the query options that have been applied. This comment is printed by default.


+[no]short
Provide a terse answer. The default is to print the answer in a verbose form.


+[no]identify
Show [or do not show] the IP address and port number that supplied the answer when the
+short
option is enabled. If short form answers are requested, the default is not to show the source address and port number of the server that provided the answer.


+[no]comments
Toggle the display of comment lines in the output. The default is to print comments.


+[no]stats
This query option toggles the printing of statistics: when the query was made, the size of the reply and so on. The default behavior is to print the query statistics.


+[no]qr
Print [do not print] the query as it is sent. By default, the query is not printed.


+[no]question
Print [do not print] the question section of a query when an answer is returned. The default is to print the question section as a comment.


+[no]answer
Display [do not display] the answer section of a reply. The default is to display it.


+[no]authority
Display [do not display] the authority section of a reply. The default is to display it.


+[no]additional
Display [do not display] the additional section of a reply. The default is to display it.


+[no]all
Set or clear all display flags.


+time=T
Sets the timeout for a query to
T
seconds. The default timeout is 5 seconds. An attempt to set
T
to less than 1 will result in a query timeout of 1 second being applied.


+tries=T
Sets the number of times to try UDP queries to server to
T
instead of the default, 3. If
T
is less than or equal to zero, the number of tries is silently rounded up to 1.


+retry=T
Sets the number of times to retry UDP queries to server to
T
instead of the default, 2. Unlike
+tries, this does not include the initial query.


+ndots=D
Set the number of dots that have to appear in
name
to
D
for it to be considered absolute. The default value is that defined using the ndots statement in
/etc/resolv.conf, or 1 if no ndots statement is present. Names with fewer dots are interpreted as relative names and will be searched for in the domains listed in the
search
or
domain
directive in
/etc/resolv.conf.


+bufsize=B
Set the UDP message buffer size advertised using EDNS0 to
B
bytes. The maximum and minimum sizes of this buffer are 65535 and 0 respectively. Values outside this range are rounded up or down appropriately.


+[no]multiline
Print records like the SOA records in a verbose multi-line format with human-readable comments. The default is to print each record on a single line, to facilitate machine parsing of the
dig
output.


+[no]fail
Do not try the next server if you receive a SERVFAIL. The default is to not try the next server which is the reverse of normal stub resolver behavior.


+[no]besteffort
Attempt to display the contents of messages which are malformed. The default is to not display malformed answers.


+[no]dnssec
Requests DNSSEC records be sent by setting the DNSSEC OK bit (DO) in the OPT record in the additional section of the query.


+[no]sigchase
Chase DNSSEC signature chains. Requires dig be compiled with -DDIG_SIGCHASE.


+trusted-key=####
Specifies a file containing trusted keys to be used with
+sigchase. Each DNSKEY record must be on its own line.


If not specified
dig
will look for
/etc/trusted-key.key
then
trusted-key.key
in the current directory.


Requires dig be compiled with -DDIG_SIGCHASE.


+[no]topdown
When chasing DNSSEC signature chains perform a top-down validation. Requires dig be compiled with -DDIG_SIGCHASE.


The BIND 9 implementation of
dig 
supports specifying multiple queries on the command line (in addition to supporting the
-f
batch file option). Each of those queries can be supplied with its own set of flags, options and query options.


In this case, each
query
argument represent an individual query in the command-line syntax described above. Each consists of any of the standard options and flags, the name to be looked up, an optional query type and class and any query options that should be applied to that query.


A global set of query options, which should be applied to all queries, can also be supplied. These global query options must precede the first tuple of name, class, type, options, flags, and query options supplied on the command line. Any global query options (except the
+[no]cmd
option) can be overridden by a query-specific set of query options. For example:



dig +qr www.isc.org any -x 127.0.0.1 isc.org ns +noqr


shows how
dig
could be used from the command line to make three lookups: an ANY query for
www.isc.org, a reverse lookup of 127.0.0.1 and a query for the NS records of
isc.org. A global query option of
+qr
is applied, so that
dig
shows the initial query it made for each lookup. The final query has a local query option of
+noqr
which means that
dig
will not print the initial query when it looks up the NS records for
isc.org.


/etc/resolv.conf


${HOME}/.digrc


There are probably too many query options.


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