(Ubuntu) DNS Notes

Contact

• Slack: @manwthglasses on slack
• Email: wmurphy1@mail.usf.edu

Table

1. Prerequisites
2. Summary
3. Background
4. Installation
5. Configuration
6. Helpful links

Prerequisites

1. Download virtualization program
2. Download ISO images
3. Setup virtual environment
4. Setup Ubuntu ISP
5. Setup PFsense box

Summary

Background

DMZ (Demilitarized Zone)

• Outward facing network inbetween trusted internal network (LAN) and untrusted external network such as the internet
• Segregated from personal files
• Typically containing devices accessible to internet traffic, such as Web and DNS servers

DNS (Domain Name Server)

• DNS stands for Domain Name Servers and serve the purpose of maintaining a directory of domain names and translating them to IP addresses.
• All websites you visit are normally accessed through domain names such as “www.google.com” but are then translated to the actual IP address that will let you reach that site.
• DNS are the servers that translate the domain names to the IP addresses and even vice versa if needed.
  • www.google.com -> 201.23.52.1
  • 201.23.52.1 -> www.google.com
• Domain Name Space
  • The Domain Name Space is a term for the hierarchal structure in which the domain names are distributed.
  • The Domain Name Space is an inverted tree that would read a domain name starting from right to left, and branch down by every designated dot.
    • ex: “my.usf.edu” would be found in the domain name space beginning at the root node (instead of a “/” like in UNIX filesystems, it is designated with a “.”), then would branch down to the “.edu” branch, and then further to the “usf.edu” branch where the domain name would ultimately be held.
• DNS Zone: a set of DNS records for all the domain names within the specified branch, but excluding any children branches that are designated their own zones
• Configuration
  • Primary Master Server: term for reading data for a zone from a file on the server
  • Secondary Master: term for getting the zone data from another DNS server that is the Primary Master for that zone
  • Resolvers: a term for the clients that access nameservers
    • often a very simple set of libray routines that perform the basic task of querying a name server, interpreting the response from that server, and then returning the information to the program requesting it.
    • Queries is the term for when a resolver requests an IP address or a domain name from a server. There are two different types of queries:
      • Recursive: a recursive query is one that takes the domain requested and queries it to the root zone name server, that would then refer it to another server further down the branch, that would then keep referring it until it reached the zone for the domain. ex: a recursive query for the address of “my.usf.edu” would first query the address to the root server, who would refer it to the “.edu” server, who would refer it to the “.usf” server that would be authorative for that domain and return the address.
      • Iterative” an iterative query is the component of the recursive query that gives the best answer, whether that be a referral to another server further down the branch or the actual address requested.
• DNS Record : single entry of instructions on handling requests based on types for a zone
  • A Record : Specifies IPv4 Address for a given host
    • www.google.com -> 201.23.52.1
  • AAAA Record (quad-A record): specifies IPv6 address for given host
    • www.google.com -> 2001:db8::7348
  • CNAME Record: specifies a domain name that has to be queried in order to resolve the original DNS query
    • also used to create aliases of domain names
    • same server can be accessesed through documents.example.com and docs.example.com because of CNAME
  • MX Record: specifies a mail exchange server for a DNS domain name
    • the information is used by Simple Mail Transfer Protocol (SMTP) to route emails to proper hosts
  • PTR Record: (reverse of A and AAAA DNS Records) used to look up domain names based on IP addresses

Installation

• Install Ubuntu
• Change network options to Host-Only, DMZ
  • 
  • Select the correct Virtual Machine, click on settings, click on Network, change adapter 1 settings to Host-only and then change to your corresponding DMZ adapter
• Type in ip link show into terminal and you should see: lo, enp0s3
  • ip link show : shows information for all interfaces
  • lo : loopback
  • enp0s3 : virtual network driver
• Add another interface
  • type sudo nano /etc/network/interfaces to edit the network interfaces configuration file with console based text editor nano
  • add

      auto enp0s3
      iface enp0s3 inet static
      address 127.20.240.23
      netmask 255.255.255.0
      gateway 127.20.240.254
      dns-nameservers 8.8.8.8
    

  • type sudo service networking restart
  • verify netowrk interfaces with ifconfig
  • verify connectivity with ping 8.8.8.8
  • should return with statistics for packets transmitted and recieved, and can exit with Ctrl+C
    • 8.8.8.8 is google’s DNS server

Configuration

• Helpful link for configuration process
• Helpful link for understanding bind
• Update and install bind
  • sudo apt-get update
  • sudo apt-get upgrade
  • sudo apt-get install bind9 bind9utils bind9doc dnsutils
  • bind is a widely used domain name system software for your server to become a DNS for your network
  • dnsutils is a package for testing and troubleshooting DNS related issues, including tools such as dig and nslookup
• Configure caching name server
  • sudo nano /etc/bind/named.conf.options
  • uncomment and change the lines

      //forwarders {
      //	0.0.0.0;
      //};
    

    to

      forwarders {
      8.8.8.8;
      8.8.4.4;
      };
    

  • Because of this, if our cached server fails to recognize a IP address, our server will forward the request to 8.8.8.8 and 8.8.4.4.
    • 8.8.8.8 and 8.8.4.4 are both public Google DNS servers.
• Change dns-nameservers so that bind will handle it
  • sudo nano /etc/network/interfaces
  • change to

      auto enp0s3
      iface enp0s3 inet static
      address 127.20.240.23
      netmask 255.255.255.0
      gateway 127.20.240.254
      dns-nameservers 127.0.0.1
    

• Refresh
  • sudo ip addr flush enp0s3
    • ip addr flush removes all addresses for the interface enp0s3
  • sudo systemctl restart networking.service
    • systemctl is the system manager for services on machine
    • restart networking.service will restart the networking service
    • verify internet connection again with ping 8.8.8.8
• Creating zones
  • Adding a DNS zone, and therefore making this a Primary Master Server.
  • sudo nano /etc/bind/named.conf.local
  • add ``` zone “wcsc.com” { type master; file “/etc/bind/forward.wcsc.com”; allow-transfer { 127.20.241.27; }; also-notify { 127.20.241.27; }; };

  zone “20.127.in-addr.arpa” { type master; file “/etc/bind/reverse.wcsc.com”; allow-transfer { 127.20.241.27; }; also-notify { 127.20.241.27; }; }; ```

  • create the zone files mentioned in the configuration
    • sudo touch /etc/bind/forward.wcsc.com
    • sudo touch /etc/bind/forward.wcsc.com
  • add zone content
    • add lines sudo nano /etc/bind/forward.wcsc.com
    • 
    • The ‘@’ notation delineates the name of the zone and can be used whenever the domain name is the same as the origin
    • The ‘IN’ stands for Internet and specifies the class of data
    • The next column specifies the type of record, and therefore ‘SOA’ indicates that this name server is authorative for this zone
    • The next column is the name of the primary master name server
    • The last field is the mail address of the person in charge of the data (DNS syntax formats the “@” as a “.” so “root.wcsc.com.” translates to “root@wcsc.com.”)
      • It is common practice to use “root” as the email address
    • The parentheses allow the record to expand the fields past one line for readability purposes
      • ”;” indicate the beginning of a comment so name servers will ignore anything past “;” on a line
      • The serial field is meant to indicate how updated a name server is so that slave name servers are able to decide if their zone data is out of date by comparing their serial values to the primary name server’s serial values
        • The serial field isn’t crucial to this lab but it is important to know that the common practices are
          1. Starting the serial field at 1 and incrementing it every update
          2. Using the format YYYYMMDDNN (YYYY is the year, MM is the month, DD is the day, and NN is the number of times the data was changed within that day)
      • The refresh field tells the slave name servers how often to check to see if their data is updated (The number 3600 specifies seconds so therefore the slave name servers will check to see if their data is updated every hour)
      • The retry field specifies the intervals between how often the slave name server would attempt to reconnect with the primary name server if it were to fail to connect initially
      • The expire field tells the slave name server when to stop giving out zone data if the primary server could not be reached in that amount of seconds
      • TTL stands for ‘time to live’ specifying how long other servers may cache the data for this zone
    • The NS and PTR records follow the initial SOA record and are seperated by lines
      • add lines sudo nano /etc/bind/reverse.wcsc.com
      • 
• Verify
  • sudo systemctl restart bind9
  • verify with nslookup 172.20.240.11
    • (in this case) nslookup is used as a command to print the name and requested information for a domain (or host) nslookup 172.20.240.23 nslookup WEB.wcsc.com nslookup DNS.wcsc.com ping WEB.wcsc.com

Troubleshooting

- 'sudo named-checkconf /etc/bind/named.conf' will output the erros for fixing

Helpful Links