DiG HOW-TO
How to use dig to query DNS name servers.
Table of Contents
dig is a command-line tool for querying DNS name servers for information about host addresses, mail exchanges, name servers, and related information. The dig(1) man page is somewhat lacking when it comes to examples, a shortcoming this article tries to remedy.
The source code for dig is part of the larger ISC BIND distribution. Compiling and installing BIND are topics outside the scope of this document, but on UNIX™ (*BSD), and UNIX-like (Linux) systems dig is usually part of a common package: bind-tools (Gentoo), bind-utils (Red Hat, Fedora), or dnsutils (Debian). On UNIX™, (*BSD — FreeBSD, NetBSD, & OpenBSD), it is installed as part of the system.
If you’re looking for information on configuring the BIND name server, you might find the article BIND for a Small LAN more to your liking.
The most typical, simplest query is for a single host. By default, however, dig is pretty verbose. You probably don’t need all the information in the default output, but it’s probably worth knowing what it is. Below is an annotated query.
$ dig www.isc.org
That’s the command-line invocation of dig I used.
; <<>> DiG 9.6.-ESV-R5 <<>> www.isc.org ;; global options: printcmd
The opening section of dig’s output tells us a little about itself
(version 9.6.-ESV-R5) and the global options that are set (in this case,
printcmd). This part of the output can be
quelled by using the +nocmd option, but only
if it’s the very first argument on the command line (even preceeding the
host you’re querying).
;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43071 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 3
Here, dig tells us some technical details about the answer received from
the DNS server. This section of the output can be toggled using the
+[no]comments option—but beware that
disabling the comments also turns off many section headers.
;; QUESTION SECTION: ;www.isc.org. IN A
In the question section, dig reminds us of our query. The default query
is for an Internet address
(A). You can turn this output on or
off using the +[no]question option.
;; ANSWER SECTION: www.isc.org. 600 IN A 204.152.184.88
Finally, we get our answer: the address of
www.isc.org is
204.152.184.88. I don’t know why you’d
ever want to turn off the answer, but you can toggle this section of the
output using the +[no]answer option.
;; AUTHORITY SECTION: isc.org. 2351 IN NS ns-int.isc.org. isc.org. 2351 IN NS ns1.gnac.com. isc.org. 2351 IN NS ns-ext.isc.org.
The authority section tells us what DNS servers can provide an
authoritative answer to our query. In this example,
isc.org has three name servers. You can
toggle this section of the output using the
+[no]authority option.
;; ADDITIONAL SECTION: ns1.gnac.com. 171551 IN A 209.182.216.75 ns-int.isc.org. 2351 IN A 204.152.184.65 ns-int.isc.org. 2351 IN AAAA 2001:4f8:0:2::15
The additional section typically includes the IP addresses of the DNS
servers listed in the authority section. This section of the output can
be toggled with the +[no]additional option.
;; Query time: 2046 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Fri Aug 27 08:22:26 2004 ;; MSG SIZE rcvd: 173
The final section of the default output contains statistics about the
query; it can be toggled with the
+[no]stats option.
dig
will let you perform any valid DNS query, the most common of which are
A (the IP address),
TXT (text annotations),
MX (mail exchanges),
NS name servers, or the omnibus
ANY.
# get the address(es) for yahoo.com dig yahoo.com A +noall +answer # get a list of yahoo's mail servers dig yahoo.com MX +noall +answer # get a list of DNS servers authoritative for yahoo.com dig yahoo.com NS +noall +answer # get all of the above dig yahoo.com ANY +noall +answer
More obscurely, for the present anyway, you can also poll for a host’s
IPv6 address using the AAAA option.
dig www.isc.org AAAA +short
If the domain you want to query allows DNS transfers, you can get those, too. The reality of life on the Internet, however, is that very few domains allow unrestricted transfers these days.
dig yourdomain.com AXFR
When all you want is a quick answer, the
+short option is your friend:
$ dig www.isc.org +short
204.152.184.88
Note that a short answer is different from only an answer. The way to
get a detailed answer, but without any auxiliary information, is to turn
off all the results (+noall) and then turn
on only those sections you want.
Here’s a short answer followed by only an answer; the latter includes all the configuration information, including time-to-live (TTL) data, displayed in a format compatible with BIND configuration files.
$dig fsf.org mx +short20 mx20.gnu.org. 30 mx30.gnu.org. 10 mx10.gnu.org. $dig +nocmd fsf.org mx +noall +answerfsf.org. 3583 IN MX 30 mx30.gnu.org. fsf.org. 3583 IN MX 10 mx10.gnu.org. fsf.org. 3583 IN MX 20 mx20.gnu.org.
According to its man page, the
+multiline option will give you an answer
with “the SOA records in a
verbose multi-line format with human-readable comments.”
In general, the answers retrieved using the
+multiline option will appear more like BIND
config files than they will without it.
$ dig +nocmd ogi.edu any +multiline +noall +answer
ogi.edu. 14267 IN A 129.95.59.31
ogi.edu. 14267 IN MX 5 cse.ogi.edu.
ogi.edu. 14267 IN MX 15 hermes.admin.ogi.edu.
ogi.edu. 14267 IN SOA zeal.admin.ogi.edu. hostmaster.admin.ogi.edu. (
200408230 ; serial
14400 ; refresh (4 hours)
900 ; retry (15 minutes)
3600000 ; expire (5 weeks 6 days 16 hours)
14400 ; minimum (4 hours)
)
ogi.edu. 14267 IN NS zeal.admin.ogi.edu.
ogi.edu. 14267 IN NS cse.ogi.edu.
ogi.edu. 14267 IN NS fork.admin.ogi.edu.
Use the -x option to lookup the main
hostname associated with an IP address.
$ dig -x 204.152.184.167 +short
mx-1.isc.org.
In a loop, this is a slick way to map the names in a given subnet:
#!/bin/bash
NET=18.7.22
for n in $(seq 1 254); do
ADDR=${NET}.${n}
echo -e "${ADDR}\t$(dig -x ${ADDR} +short)"
done
Just specify it on the command line:
dig @ns1.google.com www.google.com
search list in
/etc/resolv.conf?
The
host
utility will automatically use the search
list in your /etc/resolv.conf file.
$ host www
www.dnswatch.com has address 24.113.41.84
By default, however,
dig
doesn’t—which may produce some unexpected results. If you want to
use local hostnames instead of fully qualified domain names, use the
+search option.
dig www +search
If you want to look up a large number of
hostnames, you can put them in a file (one
name per line) and use the -f option to
query each one in turn.
# do full lookups for a number of hostnames dig -f /path/to/host-list.txt # the same, with more focused output dig -f /path/to/host-list.txt +noall +answer
As far as I can tell,
dig
versions up to and including 9.6 are unable to do reverse lookups using
the -f option.
An improperly configured DNS setup can be really annoying. You want to make sure that your mappings work both ways:
Each hostname should resolve to an address, and that address ought to resolve back to the proper hostname.
If an address on your subnet(s) has been assigned a reverse pointer to a hostname, that hostname ought to point back to the original address.
There are exceptions to those two rules, of course. A CNAME will resolve to another hostname first, and only then to an address. Sometimes multiple hostnames will point to the same address, but that address will have only one reverse pointer.
Still, it’s good to know that your basic mappings work as expected.
You can script such a test if you build a file containing your known
hostnames. The example script below is
pretty simple; it will break if fed a CNAME, and it’ll report a failure
somewhere if multiple hostnames point to
the same address. Let’s assume the file containing your
hostnames is named
named-hosts.
#!/bin/bash
#
# test DNS forward- and reverse-mapping
#
# edit this variable to reflect local class C subnet(s)
NETS="192.168.1 192.168.2"
# Test name to address to name validity
echo
echo -e "\tname -> address -> name"
echo '----------------------------------'
while read H; do
ADDR=$(dig $H +short)
if test -n "$ADDR"; then
HOST=$(dig -x $ADDR +short)
if test "$H" = "$HOST"; then
echo -e "ok\t$H -> $ADDR -> $HOST"
elif test -n "$HOST"; then
echo -e "fail\t$H -> $ADDR -> $HOST"
else
echo -e "fail\t$H -> $ADDR -> [unassigned]"
fi
else
echo -e "fail\t$H -> [unassigned]"
fi
done < named-hosts
# Test address to name to address validity
echo
echo -e "\taddress -> name -> address"
echo '-------------------------------------'
for NET in $NETS; do
for n in $(seq 1 254); do
A=${NET}.${n}
HOST=$(dig -x $A +short)
if test -n "$HOST"; then
ADDR=$(dig $HOST +short)
if test "$A" = "$ADDR"; then
echo -e "ok\t$A -> $HOST -> $ADDR"
elif test -n "$ADDR"; then
echo -e "fail\t$A -> $HOST -> $ADDR"
else
echo -e "fail\t$A -> $HOST -> [unassigned]"
fi
fi
done
done
named.root file
Any DNS server connected to the Internet is likely to have a copy of the
InterNIC’s named.root file that lists the
root name servers for the entire Internet. You can always download that
file in the boring way from
the
InterNIC’s ftp server. Or, in a true build-it-yourself fashion, you
can build it with
dig.
# compare with ftp://ftp.internic.net/domain/named.root dig +nocmd . NS +noall +answer +additional
Your TTL values might be a little on the short side, but otherwise, it’s as up-to-date a list as you’ll find!
Perhaps you’re a devotee of
traceroute
and like to watch how to get from point A to point B. You can do a
similar thing with dig’s +trace option.
dig freebsd.org +trace
You’ll see dig head to the root name servers, then to the servers
responsible for all the *.org domains, and finally to the name servers
responsible for freebsd.org.
As a DNS administrator, I sometimes make changes and want to see if any
of my name servers are still pushing the old data. The
+nssearch provides a clear accounting of
your public servers.
# the unvarnished truth dig cse.ogi.edu +nssearch # the same, displaying only serial number and hostname dig cse.ogi.edu +nssearch | cut -d' ' -f4,11
I love Google for many reasons, one of which is that it provides referrer strings in our web logs that make it easy to figure out what sort of queries lead people to pages on this site.
Somewhat unexpectedly, we’ve seen a lot of queries asking for information about TTL (time-to-live) numbers. We would have never guessed that TTL would be a topic of interest, but you learn something new every day. So, by popular demand, here’s a brief intro to TTL.
If you ask your local DNS server for an Internet address, the server figures out where to find an authoritative answer and then asks for it. Once the server receives an answer, it will keep the answer in a local cache so that if you ask for the same address again a short time later, it can give you the answer quickly rather than searching the Internet for it all over again.
When domain administrators configure their DNS records, they decide how long the records should remain in remote caches. This is the TTL number (usually expressed in number of seconds).
Typically, a remote server will only cache those records for the length of time specified by the TTL. After that, the remote server will flush it's local cache and ask again for an authoritative answer.
When you use dig to query a DNS server concerning a certain record, the server will tell dig the time (in seconds) that record will remain in cache.
For example, as of this writing, the TTL for the MX records for the
gmail.com domain is 300 seconds. The
gmail.com admins are asking that remote
servers cache their MX records for no more than five minutes. So when
you first ask for that record set, dig will report a TTL of 300.
$ dig +nocmd gmail.com MX +noall +answer
gmail.com. 300 IN MX 20 gsmtp57.google.com.
gmail.com. 300 IN MX 10 gsmtp171.google.com.
If you ask a few seconds later, you’ll see the TTL number reduced by approximately the number of seconds you waited to ask again.
$ dig +nocmd gmail.com MX +noall +answer
gmail.com. 280 IN MX 10 gsmtp171.google.com.
gmail.com. 280 IN MX 20 gsmtp57.google.com.
If your timing is good, you can catch the record at the very end of its life.
$ dig +nocmd gmail.com MX +noall +answer
gmail.com. 1 IN MX 10 gsmtp171.google.com.
gmail.com. 1 IN MX 20 gsmtp57.google.com.
After that, the DNS server you’re querying will “forget” the answer to that question, so the whole cycle will start over again (in this example, at 300 seconds) the next time you perform that query.