New SA TrainingTopic 7: DNS and DHCP

To implement the underlying basis for our organizations networking, we rely on two fundamental services

DNS – the hierarchical system by which host (computer, printer, etc.) names can be are translated to IP addresses – and IP addresses to names.

DHCP – the protocol which allows hosts to receive basic configuration information (most commonly, the IP address) necessary for communication on a given network.

DNS - Overview

Three components: resolver (client) name server (named) zone files (or some form of database, like AD). A

zone corresponds roughly to a domain.

Resolver is set in platform-specific ways. Though other information can be configured as well, the two key items are: What domain am I in by default? What servers should I get DNS information from?

DNS - Overview (cont.)

Servers can be: Masters that are the authoritative source

for a domain Slaves that download information from a

master via zone transfers Caching-only servers that only cache

queries and don't pre-fetch Zone files are databases -- we'll discuss this

in a bit more detail when we see actual named.conf and zone files

DNS - Platform-specific Issues

Windows client configuration is handled via DHCP or through the GUI

Linux client is managed in /etc/resolv.conf Windows server DNS is usually managed

through the Microsoft Management Console (MMC) and can be configured as part of Active Directory (AD)

Linux server is named and base configuration is in /etc/named.conf, but the actual zone files themselves are typically in /var/named

DNS - sample named.confoptions {

directory "/var/named"; }; // a master server configuration zone "." {

type hint; file "named.ca";

}; zone "0.0.127.in-addr.arpa" {

type master; file "named.local";

}; zone "mylab.net" {

type master; file "mylab.net.hosts";

}; zone "168.192.in-addr.arpa" {

type master; file "192.168.reverse";

};

DNS - sample zone file$TTL 86400 @ IN SOA mylab.net. ( 2001061401 ; Serial

21600 ; Slave Refresh 1800 ; Slave Retry 604800 ; Slave Expire 900 ) ; Negative cache

TTL IN NS ns1.mylab.net. ; Defines a name serverIN NS ns2.mylab.net. ; Defines a name serverIN NS ns-server.myisp.com. ; Defines a name serverIN MX 10 mail.mylab.net. ; Defines a mail serverIN MX 20 backup-mail.mylab.net. ; Defines a mail server

localhost IN A 127.0.0.1 ; Defines the local hostsocrates IN A 192.168.1.1 ; Defines a host in this zone plato IN A 192.168.1.2 ; Defines a host in this zone ns1 IN A 192.168.1.3 ; Defines a host in this zone ns2 IN A 192.168.1.4 mail IN A 192.168.1.5 loghost IN CNAME plato.mylab.net. ; Defines a host in this zone backup-mail IN CNAME ns2.mylab.net. ; Defines a host in this zone aristotle IN A 192.168.1.6 ; Defines a host in this zone alexander IN A 192.168.1.7

DNS - A few things to note

Each zone needs a separate zone file, which holds the actual DNS contents

Types of records/entries header/localhost/host order header contents Order of entries is arbitrary, but it helps to be human-

friendly Without PTR records (in the .in-addr.arpa domain),

reverse lookups won't work. This is where DHCP breaks static DNS and makes DDNS useful.

DNS – sample reverse zone file

$TTL 86400 ; Addresses and other host information for the domain 168.192.in-

addr.arpa @ IN SOA mylab.net. (

2001061401 ; Serial 21600 ; Refresh 1800 ; Retry 604800 ; Expire 900 ) ; Negative cache TTL

1.1 IN PTR socrates.mylab.net. 2.1 IN PTR plato.mylab.net. 3.1 IN PTR ns1.mylab.net. 4.1 IN PTR ns2.mylab.net. 5.1 IN PTR mail.mylab.net. 6.1 IN PTR aristotle.mylab.net. 7.1 IN PTR alexander.mylab.net.

DNS Exercise

Let’s think about adding a domain to DNS Let’s think about looking up a host’s IP

Organizational addressing

For the initial setup of our new business locations, you will use the 10.1.x.x range 10.1.10.x – Abingdon, VA 10.1.20.x – Caro, WV 10.1.30.x – Chattanooga, TN 10.1.40.x – Kiawah, SC 10.1.50.x – Mortimer, NC 10.1.60.x – York, PA 10.1.70.x – Cleveland, OH

DHCP - Background

Basic idea is that systems will lease an IP address rather than be assigned one statically. This simplifies a traditional problem: keeping track of which IP addresses in a set are free.

Leasable parameters include: IP addresses and netmasks Gateways (default routes) DNS name servers Logging hosts

DHCP – How it works

Client: who am I? Server: Would you like a lease for IP address

BLAH? Client: I would like to lease IP address BLAH Server: Ok

Note the four step process:Discovery/Offer/Request/Acknowledge

DHCP - Platform-specific Issues

Windows client is handled through the ipconfig command or through the GUI

Linux client is handled through pump as part of /etc/sysconfig/network-scripts/ifcfg*

Windows server is managed through the Microsoft Management Console (MMC)

Linux server is dhcpd and configuration is in /etc/dhcpd.conf

DHCP - sample dhcpd.conf

# dhcpd.conf # # global options option domain-name "mylab.test"; option domain-name-servers ns1.mylab.test; option subnet-mask 255.255.255.0 default-lease-time 600; max-lease-time 7200; subnet 192.168.1.0 netmask 255.255.255.0 {

range 192.168.1.51 192.168.1.60; option broadcast-address

192.168.1.255; option routers gateway.mylab.test;

}

DHCP Exercise

Question: What would happen if there were two DHCP servers on a single network, with a client attempting to lease an IP address? Describe a potential successful scenario and a potential failure.