How to read an IP address backwards?

Question

If we have this string (IP address): 192.168.1.1

How can I derive the (DNS reverse record form) from this string, so it will be shown like 1.1.168.192.in-addr.arpa using a shell script?

With this amount of answers this should better be in codegolf ;) — tkausl
– tkausl, Commented Dec 6, 2014 at 12:59

Stéphane Chazelas · Accepted Answer · 2014-05-31 06:44:48Z

35

Just for curiosity value... using tac from GNU coreutils: given a variable ip in the form 192.168.1.1 then

$(printf %s "$ip." | tac -s.)in-addr.arpa

i.e.

$ ip=192.168.1.1 $ rr=$(printf %s "$ip." | tac -s.)in-addr.arpa $ echo "$rr" 1.1.168.192.in-addr.arpa

edited May 31, 2014 at 6:44

Stéphane Chazelas

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answered May 30, 2014 at 23:09

steeldriver

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9

+1 This is definitely my favorite answer here. Thanks for introducing me to tac!

Jonathon Reinhart
– Jonathon Reinhart

2014-05-31 17:41:34 +00:00
Commented May 31, 2014 at 17:41
Thanks are also due to @StéphaneChazelas for the elegant printf edit (I originally posted an ugly echo -n)

steeldriver
– steeldriver

2014-06-01 17:07:18 +00:00
Commented Jun 1, 2014 at 17:07
2

Clearly simpler: printf 'arpa.in-addr.%s.' "$ip" | tac -s.

user232326
– user232326

2018-06-20 21:27:29 +00:00
Commented Jun 20, 2018 at 21:27

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Peter Mortensen · Accepted Answer · 2014-05-31 14:06:39Z

You can do it with AWK. There are nicer ways to do it, but this is the simplest, I think.

echo '192.168.1.1' | awk 'BEGIN{FS="."}{print $4"."$3"."$2"."$1".in-addr.arpa"}'

This will reverse the order of the IP address.

Just to save a few keystrokes, as Mikel suggested, we can further shorten the upper statement:

echo '192.168.1.1' | awk -F . '{print $4"."$3"."$2"."$1".in-addr.arpa"}'

OR

echo '192.168.1.1' | awk -F. '{print $4"."$3"."$2"."$1".in-addr.arpa"}'

OR

echo '192.168.1.1' | awk -F. -vOFS=. '{print $4,$3,$2,$1,"in-addr.arpa"}'

AWK is pretty flexible. :)

If you want to save a few keystrokes, -F . should be equivalent to BEGIN{FS="."}. — Mikel
– Mikel, Commented May 30, 2014 at 22:03

Mikel · Accepted Answer · 2014-05-31 13:49:53Z

If you want to use only shell (zsh, ksh93, bash), here's another way:

IFS=. read w x y z <<<'192.168.1.1' printf '%d.%d.%d.%d.in-addr.arpa.' "$z" "$y" "$x" "$w"

Or in plain old shell:

echo '192.168.1.1' | { IFS=. read w x y z; echo "$z.$y.$w.$x.in-addr.arpa."; }

JRFerguson · Accepted Answer · 2014-05-30 22:01:51Z

13

Easily with Perl, thusly:

$ echo 192.168.1.1|perl -nle 'print join ".",reverse(split /\./,$_)' 1.1.168.192

answered May 30, 2014 at 22:01

JRFerguson

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6

Can be made even more compact: perl -F'\.' -lane '$,=".";print reverse @F'

Joseph R.
– Joseph R.

2014-05-30 22:15:12 +00:00
Commented May 30, 2014 at 22:15

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Matt · Accepted Answer · 2014-12-06 13:16:52Z

7

To round it out, Ruby:

ruby -r ipaddr -e 'puts IPAddr.new(ARGV.first).reverse' 192.168.1.1

Which also supports IPv6

2607:F8B0:4000:080A:0000:0000:0000:2000 => 0.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.a.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa

edited Dec 6, 2014 at 13:16

answered May 31, 2014 at 20:13

Matt

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you could take the "require" out of the script: ruby -r ipaddr -e 'puts ...'

glenn jackman
– glenn jackman

2014-12-05 19:22:47 +00:00
Commented Dec 5, 2014 at 19:22
I think the OP needs shell..

MaXi32
– MaXi32

2021-09-21 13:52:25 +00:00
Commented Sep 21, 2021 at 13:52

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MattBianco · Accepted Answer · 2014-06-05 10:31:07Z

Through GNU sed,

sed -r 's/^([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})$/\4.\3.\2.\1.in-addr.arpa/g' file

It reverses any IPv4-address format.

Example:

$ echo '192.168.1.1' | sed -r 's/^([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})$/\4.\3.\2.\1.in-addr.arpa/g' 1.1.168.192.in-addr.arpa $ echo '192.1.1.1' | sed -r 's/^([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})$/\4.\3.\2.\1.in-addr.arpa/g' 1.1.1.192.in-addr.arpa $ echo '1.1.1.1' | sed -r 's/^([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})$/\4.\3.\2.\1.in-addr.arpa/g' 1.1.1.1.in-addr.arpa $ sed -r 's/^([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3}).([0-9]{1,3})$/\4.\3.\2.\1.in-addr.arpa/g' <<< '192.168.189.23' 23.189.168.192.in-addr.arpa

This is the fastest solution! 45k IPs converted in just 0.794s. — Petr Javorik
– Petr Javorik, Commented Oct 4, 2019 at 13:58

Stéphane Chazelas · Accepted Answer · 2014-05-31 06:53:28Z

With zsh:

$ ip=192.168.1.1 $ echo ${(j:.:)${(s:.:Oa)ip}}.in-addr.arpa 1.1.168.192.in-addr.arpa

Those are variable expansion flags:

s:.:: split on .
Oa: reverse order the array
j:.:: join on .

Josh Lee · Accepted Answer · 2014-11-24 20:01:57Z

4

Using Python’s standard library:

>>> ipaddress.ip_address('192.168.1.1').reverse_pointer '1.1.168.192.in-addr.arpa'

answered Nov 24, 2014 at 20:01

Josh Lee

1505 bronze badges

1

This is in the 3.5 standard library and scheduled to be released September 2015. From a script you can do: python3.5 -c "import ipaddress; ipaddress.ip_address('192.168.1.1').reverse_pointer" (all on one line)

Anthon
– Anthon

2014-11-24 20:40:22 +00:00
Commented Nov 24, 2014 at 20:40

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Pablo Saratxaga · Accepted Answer · 2014-06-01 10:33:33Z

Another possibility is to use the "dig" command line tool with the "-x" switch.

It actually does a request on the PTR entry, but if you filter on "PTR" it will show you one commented line (the request) and maybe some replies.

Using "dig" can be handy for a quick writing of the PTR name, without having to write a small script. Particularly if you need it interactively (to cut and paste the result). It works on IPv6 too.

MattBianco · Accepted Answer · 2014-06-05 10:36:45Z

If you want it working with IPv6 as well, you can use dig -x.

For example:

$ dig -x 194.68.208.240 | egrep '^;.*PTR$' | cut -c 2- | awk '{print $1}' 240.208.68.194.in-addr.arpa. $ dig -x 2001:db8:dc61:2a61::1 | egrep '^;.*PTR$' | cut -c 2- | awk '{print $1}' 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.6.a.2.1.6.c.d.8.b.d.0.1.0.0.2.ip6.arpa.

c4f4t0r · Accepted Answer · 2014-06-06 09:06:29Z

In Python

 a = "192.168.1.122" import re m = re.search(r'(\d+)\.(\d+)\.(\d+)\.(\d+)',a) ip = m.group(4),m.group(3),m.group(2),m.group(1) '.'.join(ip) + ".in-addr.arpa" '122.1.168.192.in-addr.arpa'

Or more idiomatically, "{}.in-addr.arpa".format(".".join(reversed(a.split(".")))) in Python2.7 — iruvar
– iruvar, Commented Jun 8, 2014 at 11:42

mikeserv · Accepted Answer · 2014-07-27 10:09:23Z

IFS=. ; set -- ${0+192.168.1.2} printf %b. $4 $3 $2 $1 in-addr.arpa\\c IFS=. ; printf %s\\n \ in-addr.arpa ${0+192.168.1.2} | sed '1!G;$s/\n/./gp;h;d' IFS=. ; printf '[%b.] ' \ ${0+192.168.1.2.]PPPPP\\c} |dc echo in-addr.arpa

uwotm8 · Accepted Answer · 2014-05-31 01:45:29Z

$ while read ip while> do while> n=( $(echo $ip) ) && echo "${n[4]}"'.'"${n[3]}"'.'"${n[2]}"'.'"${n[1]}"'.'"in-addr.arpa" while> done 192.168.1.2 2.1.168.192.in-addr.arpa

This way you can type in an address and hit return for your result.

user232326 · Accepted Answer · 2018-06-21 00:12:58Z

Assuming a var contains the ip: ip=192.168.2.1. If the value needs to be given to a new variable, just enclose any solution inside $() and assign that to the var rr=$(...).

Some solutions are possible:

Simplest : printf 'arpa.in-addr.%s.' "$ip" | tac -s.
Most shells: IFS=. eval 'set -- $ip'; echo "$4.$3.$2.$1.in-addr.arpa"
Some shells : IFS=. read d c b a <<<"$ip"; printf %s "$a.$b.$c.$d.in-addr.arpa."
awk : echo "$ip" | awk -F. '{OFS=FS;print $4,$3,$2,$1,"in-addr.arpa"}'
sed : echo "$ip" | sed -E 's/([^.]+)\.([^.]+)\.([^.]+)\.([^.]+)$/\4.\3.\2.\1.in-addr.arpa./'
sed : echo "arpa.in-addr.$ip" | sed 'y/./\n/' | sed 'G;$s/\n/./gp;h;d'
perl : echo "$ip" | perl -F\\. -lane '$,=".";print( join(".",(reverse @F),"in-addr.arpa"))'
dig : dig -x "$ip" | awk -F '[; \t]+' '/^;.*PTR$/{print($2)}'
host: host -t ptr 192.168.2.1 | cut -d' ' -f2

Both dig and host solutions work with IPv6.

Lukas Gottschall · Accepted Answer · 2018-06-23 08:20:40Z

#!/bin/bash # script file name reverseip.sh if [ -z $1 ] || [ "help" == $1 ] then echo 'Convert a full ipv4 or ipv6 address to arpa notation' echo "usage:" echo ./reverseip.sh "help" echo ./reverseip.sh "ipv4 address format: xxxx.xxxx.xxxx.xxxx" echo ./reverseip.sh "ipv6 address format: xxxx:xxxx:xxxx:xxxx::xxxx" echo "examples:" echo ./reverseip.sh 216.58.207.35 echo ./reverseip.sh 2a00:1450:4001:824::2003 exit fi # if ip address passed containing ':' if [[ $1 = *':'* ]]; then # invert ipv6 address e.g.: 2a00:1450:4001:824::2003 to 3.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.4.2.8.0.1.0.0.4.0.5.4.1.0.0.a.2. # @see lsowen https://gist.github.com/lsowen/4447d916fd19cbb7fce4 echo "$1" | awk -F: 'BEGIN {OFS=""; }{addCount = 9 - NF; for(i=1; i<=NF;i++){if(length($i) == 0){ for(j=1;j<=addCount;j++){$i = ($i "0000");} } else { $i = substr(("0000" $i), length($i)+5-4);}}; print}' | rev | sed -e "s/./&./g" | echo "$(</dev/stdin)ip6.arpa." else # invert ipv6 address e.g.: 216.58.207.35 to 35.207.58.216.in-addr.arpa # @see Stéphane Chazelas https://unix.stackexchange.com/questions/132779/how-to-read-an-ip-address-backwards echo $(printf %s "$1." | tac -s.)in-addr.arpa fi

cuonglm · Accepted Answer · 2014-06-05 10:49:07Z

0

With host command from dnsutils:

$ host -t ptr 192.168.1.1 | cut -d' ' -f 2 1.1.168.192.in-addr.arpa.

answered Jun 5, 2014 at 10:49

cuonglm

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1

This only works for addresses you can lookup in your DNS. If you try an unknown address you will get an error message (which however includes the reverse address but need slightly different postprocessing).

Alexander Remesch
– Alexander Remesch

2017-07-11 13:40:14 +00:00
Commented Jul 11, 2017 at 13:40

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mback2k · Accepted Answer · 2018-06-16 19:53:08Z

A shorter alternative to mattbianco's answer with less tools, but using pcregrep could be:

$ dig -x 194.68.208.240 | pcregrep -o1 '^;(\S+)\s+IN\s+PTR$' 240.208.68.194.in-addr.arpa. $ dig -x 2001:db8:dc61:2a61::1 | pcregrep -o1 '^;(\S+)\s+IN\s+PTR$' 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.6.a.2.1.6.c.d.8.b.d.0.1.0.0.2.ip6.arpa.

Hongbo Liu · Accepted Answer · 2019-07-18 13:18:11Z

Use following one line shell:

echo '192.168.1.1' | tr '.' '\n' | tac | paste -s -d '.' -

Clint Pachl · Accepted Answer · 2022-03-22 11:52:29Z

Using POSIX sh:

$ ip=192.168.1.1 $ rev_ip=` IFS=. set -- $ip # set positional parameters: $1, $2, $3, ... echo $4.$3.$2.$1.in-addr.arpa ` $ echo $rev_ip 1.1.168.192.in-addr.arpa

Note: Be careful when setting IFS. In my example, I isolated the internal field separator (IFS) in a subshell using the backtick operator, otherwise you may have to reset/restore it.

Alternatively, you could avoid IFS and use tr instead:

$ set -- `echo $ip | tr . \ ` $ echo $4.$3.$2.$1.in-addr.arpa

If you're using ksh or bash, use the $( ) subshell operator instead.

$( ... ) is to be preferred over backticks

Chris Davies
– Chris Davies

2022-03-22 17:07:20 +00:00
Commented Mar 22, 2022 at 17:07 — Chris Davies
– Chris Davies, Commented Mar 22, 2022 at 17:07

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