Grip linux networking commands short

1. NETWORKING IN LINUX
DNS Related Commands
Sayed Ahmed
Computer Engineering, BUET, Bangladesh (Graduated on
2001 )
MSc, Computer Science, U of Manitoba, Canada
http://www.justetc.net
http://sayed.justetc.net
sayed@justetc.net

2. INTRODUCTION
 Will list mostly DNS related commands and
concepts
 At one point, will try to discuss DNS system
 At another point, will try to explain how to
configure DNS system
 Related
 Zones (Forward Zones, Reverse Zones)
 And related

3. BUT SOME RESOURCES
 How to configure DNS in Linux
 http://www.tldp.org/LDP/lame/LAME/linux-admin-
made-easy/domain-name-server.html
 http://computernetworkingnotes.com/network-
administrations/dns-server.html
 How does the DNS system work
 http://en.wikipedia.org/wiki/Domain_Name_Syste
m
 http://wiki.bravenet.com/How_the_domain_name
_system_works

4. DNS COMMAND ANSWER TYPES
 Authoritative Answer vs Non-Authoritative
Answer
 For a DNS related Linux command
 Any answer that originates from the DNS Server
which has the complete zone file information
available for the domain is said to be authoritative
answer.
 In many cases, DNS servers will not have the
complete zone file information available for a given
domain. Instead, it maintains a cache file which has
the results of all queries performed in the past for
which it has gotten authoritative response. When a
DNS query is given, it searches the cache file, and
return the information available as “Non-Authoritative
Answer”.

5. GET DOMAIN/HOST IP ADDRESS
 Get domain/host IP address
 nslookup yahoo.com
 host -t A yahoo.com

6. NSLOOKUP
 1. nslookup – Simple Example
 nslookup followed by the domain name will
display the “A Record” ( IP Address ) of the
domain
 $ nslookup redhat.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 Name: redhat.com
 Address: 209.132.183.181

7. NSLOOKUP AND QUERY THE MX RECORD
2. Query the MX Record using -query=mx
 MX ( Mail Exchange ) record maps a domain name to a list of mail exchange
servers for that domain. The MX record tells that all the mails sent to
“@redhat.com” should be routed to the Mail server in that domain.
 $ nslookup -query=mx redhat.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 redhat.com mail exchanger = 10 mx2.redhat.com.
 redhat.com mail exchanger = 5 mx1.redhat.com.
 Authoritative answers can be found from:
 mx2.redhat.com internet address = 66.187.233.33
 mx1.redhat.com internet address = 209.132.183.28

8. QUERY THE NS RECORD USING -QUERY=NS
 3. Query the NS Record using -query=ns
 NS ( Name Server ) record maps a domain name to a list of DNS servers authoritative for
that domain. It will output the name serves which are associated with the given domain
 nslookup -type=ns redhat.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 redhat.com nameserver = ns4.redhat.com.
 redhat.com nameserver = ns2.redhat.com.
 redhat.com nameserver = ns1.redhat.com.
 redhat.com nameserver = ns3.redhat.com.
 Authoritative answers can be found from:
 ns4.redhat.com internet address = 209.132.188.218
 ns2.redhat.com internet address = 209.132.183.2
 ns1.redhat.com internet address = 209.132.186.218
 ns3.redhat.com internet address = 209.132.176.100

9. QUERY THE SOA RECORD USING -QUERY=SOA
4. Query the SOA Record using -query=soa
 SOA record ( start of authority ), provides the authoritative information about the
domain, the e-mail address of the domain admin, the domain serial number,
etc…
 $ nslookup -type=soa redhat.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 redhat.com
 origin = ns1.redhat.com
 mail addr = noc.redhat.com
 serial = 2012071601
 refresh = 300
 retry = 180
 expire = 604800
 minimum = 14400

10. ALL THE AVAILABLE DNS RECORDS USING -QUERY=ANY
 Authoritative answers can be found from:
 ns1.redhat.com internet address = 209.132.186.218
5. View available DNS records using -query=any
 We can also view all the available DNS records using -query=any option.
 $ nslookup -type=any google.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 Name: google.com
 Address: 173.194.35.7
 Name: google.com
 Address: 173.194.35.8
 google.com nameserver = ns1.google.com.
 google.com nameserver = ns2.google.com.

11. ALL THE AVAILABLE DNS RECORDS USING -QUERY=ANY
 google.com
 origin = ns1.google.com
 mail addr = dns-admin.google.com
 serial = 2012071701
 refresh = 7200
 retry = 1800
 expire = 1209600
 minimum = 300
 google.com mail exchanger = 20 alt1.aspmx.l.google.com.
 google.com mail exchanger = 30 alt2.aspmx.l.google.com.
 google.com mail exchanger = 40 alt3.aspmx.l.google.com.
 google.com mail exchanger = 50 alt4.aspmx.l.google.com.
 google.com mail exchanger = 10 aspmx-v4v6.l.google.com.
 google.com has AAAA address 2a00:1450:4002:801::1004
 Authoritative answers can be found from:
 ns4.google.com internet address = 216.239.38.10
 ns3.google.com internet address = 216.239.36.10

12. REVERSE DNS LOOKUP
 6. Reverse DNS lookup
 You can also do the reverse DNS look-up by
providing the IP Address as argument to
nslookup.
 $ nslookup 209.132.183.181
 Server: 192.168.19.2
 Address: 192.168.19.2#53
 Non-authoritative answer:
 181.183.132.209.in-addr.arpa name = origin-
www2.redhat.com.

13. USING SPECIFIC DNS SERVER
 7. Using Specific DNS server
 Instead of using default DNS server’s for querying, you can
also specify a particular name server to resolve the domain
name.
 $ nslookup redhat.com ns1.redhat.com
 Server: 209.132.186.218
 Address: 209.132.186.218#53
 Name: redhat.com
 Address: 209.132.183.181
 In the above command, we have used the
ns1.redhat.com as the DNS server. Here you may notice
that, we don’t get any “Non-authoritative answer:” header,
since ns1.redhat.com has all the zone information of
redhat.com

14. PORT FOR DNS
 8. Change the port number to connect with
 By default DNS servers uses the port number 53.
If for any reasons, the port number got changed,
then we can specify the port number using -port
option
 $ nslookup -port 56 redhat.com

15. DNS TIMEOUT
 9. Change timeout interval to wait for a reply
 You can change the default timeout to wait for a
reply using -timeout option
 $ nslookup -timeout=10 redhat.com

16. ENABLING DEBUG MODE USING -DEBUG
 10. Enabling debug mode using -debug
 You can turn on/off the debugging using -debug
option in the command line
 $ nslookup -debug redhat.com
 Server: 192.168.19.2
 Address: 192.168.19.2#53

17. FIND IP MAPPING
 Code:
 host 66.94.234.13
 nslookup 66.94.234.13

18. DIG
 You can also use dig
 Code:
 dig yahoo.com
 dig 66.94.234.13

19. NAMESERVER FOR YOUR HOST
 Your name server listed in /etc/resolv.conf file
 Code:
 more /etc/resolv.conf
 cat /etc/resolv.conf
 vi /etc/resolv.conf

20. HOST
 host
 This is the simplest of the DNS commands. It is a
quick way to determine the IP address of a
hostname:
 host www.linuxjournal.com
 www.linuxjournal.com has address 207.178.22.49
 www.linuxjournal.com mail is handled (pri=80)
 by www.ssc.com
 www.linuxjournal.com mail is handled (pri=10)
 by mail.ssc.com
 www.linuxjournal.com mail is handled (pri=40)
 by cascadia.a42.com

21. REVERSE LOOKUP
 Now that you know the IP address for
www.linuxjournal.com, you might want to
make sure the reverse lookup works.
 The reverse lookup checks to see if the reverse
zone file maps the IP address to the hostname:
 host 207.178.22.49 49.22.178.207.IN-ADDR.ARPA
 domain name pointer www.linuxjournal.com

22. LISTING 1. DNS INFO IN VERBOSE FORMAT WITH -A OPTION
 Listing 1. DNS Info in Verbose Format with -a Option
 host -a www.linuxjournal.com
 Trying null domain
 rcode = 0 (Success), ancount=4
 The following answer is not authoritative:
 The following answer is not verified as authe
 by the server:
 www.linuxjournal.com 19788 IN MX 80 www.ssc.com
 www.linuxjournal.com 19788 IN MX 10 mail.ssc.com
 www.linuxjournal.com 85833 IN A 207.178.22.49
 www.linuxjournal.com 19788 IN MX 40
cascadia.a42.com

23. LISTING 1. DNS INFO IN VERBOSE FORMAT WITH -A OPTION
 For authoritative answers, see:
 linuxjournal.com 85833 IN NS NS2.ssc.com
 linuxjournal.com 85833 IN NS cascadia.a42.com
 linuxjournal.com 85833 IN NS
NS2.RACKSPACE.com
 Additional information:
 www.ssc.com 226 IN A 207.178.22.48
 mail.ssc.com 86261 IN A 216.39.159.17
 cascadia.a42.com 72030 IN A 216.39.159.18
 NS2.ssc.com 88090 IN A 209.61.155.155
 NS2.RACKSPACE.com 172308 IN A
207.71.44.121

24. DIG
 dig (domain information groper)
 This powerful command gathers and returns DNS
information in a format the name server can use directly.
For this reason, dig is particularly useful in scripts. You
will find it easy to query specific name servers with dig,
making it a useful tool for narrowing down the source of
DNS problems.
 Suppose you have just transferred your domain name
hosting from old-host.com to new-host.com. A customer
sends you an e-mail saying he cannot reach your web
site when he is logged into his ISP. You suspect the zone
information simply has not had time to propagate. So,
you find out what the NS records are for the ISP in
question:

25. DIG
 dig ns isp-in-question.com
 ;; ANSWER SECTION:
 isp-in-question.com. 10H IN NS
ns1.hugeupstream.com.
 isp-in-question.com. 10H IN NS isp-in-
question.com.
 isp-in-question.com. 10H IN NS ns.isp-in-
question.com.
 isp-in-question.com. 10H IN NS
ns.goodnameserver.com.

26. DIG
 Then you check your company's web site
against the ISP's name servers:
 dig www.yourcompany.com @ns.isp-in-
question.com
 ;; ANSWER SECTION:
 www.yourcompany.com. 59m53s IN A
192.168.5.10

27. .NSLOOKUPRC
 Just as you can issue commands to
nslookup interactively, you can also change
the initial defaults by starting a .nslookuprc
file. The format of the .nslookup is one
command per line:
 set type=NS
 set domain=srvns.new-host.com
 set timeout=10

28. NSLOOKUP
 Listing 2. Output with nslookup
 nslookup
 Default Server: server.randomisp.com
 Address: 172.16.53.130
 > set all
 Default Server: server.randomisp.com
 Address: 172.16.53.130
 Set options:
 nodebug defname search recurse
 nod2 novc noignoretc port=53
 querytype=A class=IN timeout=10 retry=2
 root=a.root-servers.net.
 domain=randomisp.com
 srchlist=randomisp.com

29. NSLOOKUP
 > set type=mx
 > server srvns.new-host.com
 Default Server: srvns.new-host.com
 Address: 192.168.200.145
 > yourcompany.com
 Server: webns.new-host.com
 Address: 192.168.200.144
 yourcompany.com preference = 10, mail exchanger =<
 mail.new-host.com
 yourcompany.com nameserver = srvns.new-host.com
 yourcompany.com nameserver = webns.new-host.com
 mail.new-host.com internet address = 192.168.200.72
 srvns.new-host.com internet address = 192.168.200.145
 webns.new-host.com internet address = 192.168.200.144
 > exit

30. REFERENCES
 http://www.linuxjournal.com/article/4597?page=0,0
 http://www.thegeekstuff.com/2012/07/nslookup-examples/