Question

14

Not able to get the reverse DNS working

rated 0 times [ 14] [ 0] / answers: 1 / hits: 40592 / 3 Years ago, sat, october 16, 2021, 6:34:10

I had been struggling to get reverse DNS working using nslookup on Ubuntu 12.04. Here is the output of the nslookup command and the content of the different network related configuration files.

What could be the possible reason for the nslookup not working? Also, why do I need an internet connection to get a reverse lookup work for an entry in the /etc/hosts file?

training@guest1:/etc/network$ nslookup 192.168.1.101

Server:     8.8.8.8

Address:    8.8.8.8#53

server can't find 101.1.168.192.in-addr.arpa.: NXDOMAIN



training@guest1:/etc/network$ cat /etc/hosts

127.0.0.1   localhost

#127.0.1.1  Slave1

192.168.1.101 MyUbuntuLaptop



# The following lines are desirable for IPv6 capable hosts

::1     ip6-localhost ip6-loopback

fe00::0 ip6-localnet

ff00::0 ip6-mcastprefix

ff02::1 ip6-allnodes

ff02::2 ip6-allrouters



training@guest1:/etc/network$ cat /etc/network/interfaces

# This file describes the network interfaces available on your system

# and how to activate them. For more information, see interfaces(5).

# The loopback network interface

auto lo

iface lo inet loopback

auto eth0

iface eth0 inet static

address 192.168.1.201

netmask 255.255.255.0

network 192.168.1.0

broadcast 192.168.1.255

gateway 192.168.1.1

dns-nameservers 8.8.8.8 8.8.4.4



training@guest1:/etc/network$ ping MyUbuntuLaptop

ping MyUbuntuLaptop (192.168.1.101) 56(84) bytes of data.

64 bytes from MyUbuntuLaptop (192.168.1.101): icmp_req=1 ttl=64 time=0.113 ms

64 bytes from MyUbuntuLaptop (192.168.1.101): icmp_req=2 ttl=64 time=0.222 ms

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answered 3 Years ago utonmbo · Accepted Answer

About private addresses

The IP addresses you're trying to resolve to names (the reverse lookups) are so-called private addresses, as defined in RFC 1918. In short, the following IPv4 range of addresses are defined in there:

10.0.0.0/8

172.16.0.0/12

192.168.0.0/16

Opposed to public addresses, these aren't routed across the Internet and are solely to be used behind NAT in a private network. Many people are using this and therefore absolutely not unique. For this reason it is of no interest to have public DNS servers respond to reverse lookup requests for private ones.

Why it isn't working

Google and all other public DNS servers on the Internet aren't interested in private addresses as they're not routable over the Internet. So, basically, you're doing it wrong by asking a server totally not responsible for your network.

In case you were looking to override this with a local configuration like in /etc/hosts, this won't work:

Can /etc/hosts config reverse resolution? No.

Do entries in local 'hosts' files override both forward and reverse name lookups? Yes, but no, actually

Local DNS server

If you want to be able to do reverse lookups within your private LAN, then you'll have to set up your own DNS server with the appropriate records and have all your local clients using this DNS server. You may already be running one, but you didn't specify anything of it in your question and if you have, you appear not to be using it.

A quick all-in-one solution I can recommend is Dnsmasq . It's a DHCP and DNS server in one¹ with the advantage of using the hostname for DHCP also in DNS. From the manpage of dnsmasq is the following excerpt:

Dnsmasq accepts DNS queries and either answers them from a small,
local, cache or forwards them to a real, recursive, DNS server. It
loads the contents of /etc/hosts so that local hostnames which do not
appear in the global DNS can be resolved and also answers DNS queries
for DHCP configured hosts.

Setting up a complete Dnsmasq is a bit out of the scope here as it will replace your current DHCP server, which I don't know is an accepted solution. Anyway, for this part of the configuration this setting will be important:

domain=mydomain.lan,192.168.1.0/24,local

In which you tell it to serve the 192.168.1.0/24 network with short hostnames to use the mydomain.lan domain name and will not be forwarded to the upstream recursive DNS server. Result will be that a machine with the hostname myhost requesting a dynamic address will be automatically used in the local resolver and listens to queries myhost.mydomain.lan and the reverse pointer will resolve likewise.

^{I'm not aware of a "How do I set up Dnsmasq for a home network the good way?" question on this site. If you or others are interested, I'll be happy to Q&A it.}

¹ _{Actually even more, like TFTP server as well.}