1. 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 http://linkedin.com/in/sayedjustetc sayed@justetc.net

2.  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.  How to configure DNS in Linux  http://www.tldp.org/LDP/lame/LAME/linux-admin- made-easy/domain-name-server.html http://www.tldp.org/LDP/lame/LAME/linux-admin- made-easy/domain-name-server.html  http://computernetworkingnotes.com/network- administrations/dns-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://en.wikipedia.org/wiki/Domain_Name_Syste m  http://wiki.bravenet.com/How_the_domain_name_ system_works http://wiki.bravenet.com/How_the_domain_name_ system_works

4.  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  nslookup yahoo.com  host -t A yahoo.com

6.  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. 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.commail exchanger = 10 mx2.redhat.com.  redhat.commail exchanger = 5 mx1.redhat.com.  Authoritative answers can be found from:  mx2.redhat.cominternet address = 66.187.233.33  mx1.redhat.cominternet address = 209.132.183.28

8.  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.comnameserver = ns4.redhat.com.  redhat.comnameserver = ns2.redhat.com.  redhat.comnameserver = ns1.redhat.com.  redhat.comnameserver = ns3.redhat.com.  Authoritative answers can be found from:  ns4.redhat.cominternet address = 209.132.188.218  ns2.redhat.cominternet address = 209.132.183.2  ns1.redhat.cominternet address = 209.132.186.218  ns3.redhat.cominternet address = 209.132.176.100

9. 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.  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.comnameserver = ns1.google.com.  google.comnameserver = ns2.google.com.

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

12.  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.arpaname = origin- www2.redhat.com.

13.  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.  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.  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.  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.  Code:  host 66.94.234.13  nslookup 66.94.234.13

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

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

20.  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.  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  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.  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 (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 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.  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.  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.  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.  > 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.  http://www.linuxjournal.com/article/4597?page=0,0 http://www.linuxjournal.com/article/4597?page=0,0  http://www.thegeekstuff.com/2012/07/nslookup-examples/ http://www.thegeekstuff.com/2012/07/nslookup-examples/