DNS Records
Common DNS records
- Name Server (NS)
Defines the nameservers for a domain.
- IPv4 Address (A)
Returns the IPv4 address for a given name
- IPv6 Address (AAAA)
Returns the IPv6 address for a given name
- Canonical Name (CNAME)
Alias one name to another
- Pointer (PTR)
Points to a canonical name. Used for reverse DNS lookup.
Using the host command
The host command is part of the bind-utils package on CentOS. It
enables a simple way to make DNS queries on your system.
Forward Lookup with host
If you make a DNS query for a modern website, such as www.google.com you will likely get
both the IPv4 and IPv6 address.
[root@master ~]# host www.google.com
www.google.com has address 172.217.12.164
www.google.com has IPv6 address 2607:f8b0:4006:81a::2004
During this course we will set up our own DNS server to host the .hpc
domain. It will know the IP addresses of our systems.
[root@master ~]# host master.hpc
master.hpc has address 192.168.16.1
Reverse Lookup with host
In a similar way, if properly configured, a DNS server will be able to determine the name of a system based on its IP.
[root@master ~]# host 192.168.16.1
1.16.168.192.in-addr.arpa domain name pointer master.hpc.
Note that for the IP we got for www.google.com the returned reverse lookup
doesn’t actually return www.google.com. Instead it returns the hostname of
that particular IP address. What is going on?
[root@master ~]# host 172.217.12.164
164.12.217.172.in-addr.arpa domain name pointer lga25s62-in-f4.1e100.net.
As mentioned each DNS record can have one or more A records. Which IP
address you get is then determined by a round-robin algorithm. During forward
lookup of the the domain, the DNS server will iterate through the A records and
return them in a different order for each query. That means there can be
multiple IPs hosting a single domain, which is useful for load balancing. Each
host has its own unique name, which is returned in the PTR record of that address.
Note
Advanced configurations allow nameservers to return different results based on on the geographic location of a request. This way data centers in Europe can respond to requests from Europe, while US data centers data centers can respond to requests from the US.
Using the dig command
Another utility part of bind-utils is the dig command. It lets you
inspect the actual DNS response you get during a query. The text output you see
is in the Zone Files Format, which is described in the next section.
Forward Lookup with dig
The following is an example for resolving www.google.com. This will ask for
the A record, the IPv4 address. As you can see the response returns the A
record with an IP address attached.
Additional DNS records are returned containing the nameservers and their IP addresses used to resolve this query.
[root@master ~]# dig www.google.com
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.2 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60676
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 9
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.google.com. IN A
;; ANSWER SECTION:
www.google.com. 300 IN A 172.217.12.164
;; AUTHORITY SECTION:
google.com. 154535 IN NS ns1.google.com.
google.com. 154535 IN NS ns4.google.com.
google.com. 154535 IN NS ns3.google.com.
google.com. 154535 IN NS ns2.google.com.
;; ADDITIONAL SECTION:
ns1.google.com. 239233 IN A 216.239.32.10
ns2.google.com. 239233 IN A 216.239.34.10
ns3.google.com. 239233 IN A 216.239.36.10
ns4.google.com. 50135 IN A 216.239.38.10
ns1.google.com. 239233 IN AAAA 2001:4860:4802:32::a
ns2.google.com. 239233 IN AAAA 2001:4860:4802:34::a
ns3.google.com. 239233 IN AAAA 2001:4860:4802:36::a
ns4.google.com. 50135 IN AAAA 2001:4860:4802:38::a
;; Query time: 23 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Jan 14 15:15:51 EST 2021
;; MSG SIZE rcvd: 307
Once we’ve configured DNS in our cluster, these responses will be much shorter for hostnames within our own domain:
[root@master ~]# dig master.hpc
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.2 <<>> master.hpc
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 25546
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;master.hpc. IN A
;; ANSWER SECTION:
master.hpc. 300 IN A 192.168.16.1
;; AUTHORITY SECTION:
hpc. 300 IN NS 192.168.16.1.
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Jan 14 15:13:06 EST 2021
;; MSG SIZE rcvd: 81
Reverse Lookup with dig
For a reverse lookup with dig you need to add the -x flag. The
following shows the reverse lookup of a system in our own .hpc domain that we
will configure during this course.
[root@master ~]# dig -x 192.168.16.1
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.2 <<>> -x 192.168.16.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7346
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;1.16.168.192.in-addr.arpa. IN PTR
;; ANSWER SECTION:
1.16.168.192.in-addr.arpa. 300 IN PTR master.hpc.
;; AUTHORITY SECTION:
16.168.192.in-addr.arpa. 300 IN NS 192.168.16.1.
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Jan 14 15:12:01 EST 2021
;; MSG SIZE rcvd: 104
Note again that when doing a reverse lookup for the IP we got from the Google
Nameservers, we don’t get www.google.com but the hostname of their server
for that IP.
[root@master ~]# dig -x 172.217.12.164
; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.el7_9.2 <<>> -x 172.217.12.164
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31115
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 9
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;164.12.217.172.in-addr.arpa. IN PTR
;; ANSWER SECTION:
164.12.217.172.in-addr.arpa. 86400 IN PTR lga25s62-in-f4.1e100.net.
;; AUTHORITY SECTION:
217.172.in-addr.arpa. 86400 IN NS ns4.google.com.
217.172.in-addr.arpa. 86400 IN NS ns1.google.com.
217.172.in-addr.arpa. 86400 IN NS ns2.google.com.
217.172.in-addr.arpa. 86400 IN NS ns3.google.com.
;; ADDITIONAL SECTION:
ns4.google.com. 50412 IN A 216.239.38.10
ns2.google.com. 239510 IN A 216.239.34.10
ns1.google.com. 239510 IN A 216.239.32.10
ns3.google.com. 239510 IN A 216.239.36.10
ns4.google.com. 50412 IN AAAA 2001:4860:4802:38::a
ns2.google.com. 239510 IN AAAA 2001:4860:4802:34::a
ns1.google.com. 239510 IN AAAA 2001:4860:4802:32::a
ns3.google.com. 239510 IN AAAA 2001:4860:4802:36::a
;; Query time: 29 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Jan 14 15:11:14 EST 2021
;; MSG SIZE rcvd: 352