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Host and Domain Name Resolution


A session layer that resides in the application layer of the TCP/IP stack. ... If unique, WINS adds the NetBIOS and IP pair to its database and sends a ... – PowerPoint PPT presentation

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Title: Host and Domain Name Resolution

Host and Domain Name Resolution
  • Domain Name System (DNS)
  • NetBIOS

DNS Names
  • DNS is a name resolution method.
  • Invented to overcome the problems with host
  • Hierarchical distributed database.
  • A domain name is a two (three) tier name.
  • Administered centrally (eg InterNIC)
  • First part identifies the organisation. Eg Curtin
  • Second part is a classification. Eg edu

DNS Names
  • Last comes the country. Eg au
  • Also referred to as top level domains (TLD)
  • DNS consists of thousands of servers.
  • Each contains its own part of the database.
  • Example
  • Hostname mycomputer
  • domain
  • FQDN

Host Files
  • InterNICs host.txt file is no longer maintained.
  • Host files are still relevant.
  • Linux, UNIX, NT and static networks.
  • Points to note when editing/creating host files.
  • Names must be separated by at least one space.
  • Additional names on a line become aliases.

Host File
  • The file is parsed from top to bottom. Ie when
    the first match is found parsing stops.
  • Therefore place server entries at the top.
  • is the comment symbol.
  • FQDNs are allowed but not encouraged.
  • They can cause difficult to trace problems if
  • Tools to test DNS include
  • ping, nslookup, telnet, ftp etc

  • Have a look at domtools - good open source
  • When checking DNS, make sure you disable the
    hosts file - its checked first.
  • Ping returns
  • IP Number
  • FQDN
  • Packet size
  • Round trip delay
  • TTL setting

PING (Microsoft)
Usage ping -t -a -n count -l size -f
-i TTL -v TOS -r count -s
count -j host-list -k host-list
-w timeout destination-list Options -t
Ping the specified host until
interrupted. -a Resolve addresses
to hostnames. -n count Number of echo
requests to send. -l size Send buffer
size. -f Set Don't Fragment flag
in packet. -i TTL Time To Live.
-v TOS Type Of Service. -r count
Record route for count hops. -s count
Timestamp for count hops. -j host-list
Loose source route along host-list. -k
host-list Strict source route along host-list.
-w timeout Timeout in milliseconds to wait
for each reply.
  • Local administrators maintain their part of the
  • This must be accessible to the rest of the DNS
  • Individual servers handle queries.
  • Authoritative and non authoritative answers.
  • Allows rapid, up to date answers to name queries.

How DNS Works
  • Remember, DNS is a hierarchical multi layered
    name space.
  • Dot (.) character is used as a separator between
  • Names farthest to the right (com,edu etc) are
    known as TLDs.
  • Used for broad classification.
  • Next are the registered domain names.
  • Eg Curtin.

How DNS Works
  • Once a domain is registered, the organisation may
    subdivide it as they see fit.
  • Eg ece, cs, atri, csp etc
  • Few servers handle the TLDs
  • eg internic
  • Known as root level servers.

How DNS Works
  • Once a domain is registered
  • Entries in your DNS zone file tell the servers
    how to respond to queries.
  • Authoritative or non authoritative.

How DNS Works
An Example
  • How an application uses DNS to resolve a name.
  • Looking up by entering the following in a
  • http//
  • http states the protocol to be used.
  • www denotes what?????
  • Default.html is the required document.

An Example
  • The browser extracts the domain name impressions
    from the address.
  • The local DNS server is contacted.
  • If no match is found
  • Query is sent to the root TLD com.
  • The root server does not have an entry for
  • but does have an entry for
  • responds with the address of
  • the local DNS contacts DNS server

An Example
  • Asks for the IP of the computer www.home
  • Local DNS now has the IP of www.home.impressions.c
  • 3 types of query are involved.
  • Client makes a recursive query to the local DNS.
  • An iterative query- takes place between local and
    other DNS servers
  • An inverse query. A client provides the IP and
    requests the Domain name in a reverse lookup

Zone Files
  • Zone files contain the information that tells the
    server how to respond to DNS queries.
  • Defines the servers zone of authority.
  • Standard text file.
  • Contains records for all nodes that the server is
    responsible for.
  • Zones and domains are NOT the same!
  • Several servers (zone files) for one domain.

Zone Files
Zone file for linux.bogus The full zone
file _at_ IN SOA
ns.linux.bogus. hostmaster.linux.bogus. (
199802151 serial,
todays date todays serial
8H refresh, seconds
2H retry,
seconds 1W
expire, seconds 1D )
minimum, seconds
NS ns Inet Address of name
server MX 10
mail.linux.bogus Primary Mail Exchanger
MX 20 mail.friend.bogus.
Secondary Mail Exchanger localhost A ns A mail A
Resource records
  • Different resource records describe the type of
    node or service each entry represents.
  • Cname An alias record.
  • www CNAME bauhaus
  • Note resource records do not contain the FQN.
  • MX Mail exchange record.
  • MX 10 mail Primary Mail Exchanger
  • NS Name Server record.
  • NS ns.friend.bogus.
  • A A record
  • gw A
  • HINFO "Cisco" "IOS"
  • TXT "The router"

Resource Records
  • Every DNS server must contain a SOA record.
  • Always the first entry.
  • Defines which entry is responsible from this
    point of the hierarchy down.
  • Identifies
  • The servername
  • contact email (note the different form of email

Reverse Lookup Zone
  • Another type of zone file is the Reverse Lookup
  • Used when a client supplies the IP.
  • Note that in an IP address
  • Left portion is general.
  • Right portion is specific.
  • Opposite of domain names.
  • The reverse zone file has reverse
  • is entered as
  • All resource records are appended with
  • A holdover from the original ARPAnet.

NSLookup Utility
  • Available on most platforms. (NT,UNIX,Linux etc)
  • Enables the querying of DNS servers.
  • Two modes
  • Batch
  • Interactive
  • You will use this in the Lab. A very useful

Caching Server
  • Responds to queries from clients.
  • Stores the returned information.
  • Are not responsible for zones.
  • Used to reduce the load on DNS servers, and to
    speed responses.
  • Other DNS servers do not know of these servers.

NetBios Name Resolution
  • Network Basic Input/Output System.
  • A session layer that resides in the application
    layer of the TCP/IP stack.
  • Uses NETbios names instead of IP address.
  • Gave rise to NetBEUI
  • NetBios extended user interface.
  • Microsoft use NetBIOS to connect between nodes
    across a network with NetBEUI as the standard

  • NetBEUI is a non routable protocol.
  • MS developed NetBIOS over TCP/IP (NetBT) to
    overcome this.
  • NetBIOS names may be up to 15 characters.
  • Technically 16 characters, with the last
    character being used by the underlying
  • Does not allow duplicate names.

  • Think of our schools network - 350 unique
    computer names !
  • NetBIOS is a flat namespace.
  • Cannot qualify names.
  • Methods of resolution include
  • Broadcast based name resolution.
  • LMHosts file name resolution.
  • WINS name resolution.

NetBIOS Broadcast
  • Broadcast Based Resolution.
  • A node braodcasts to all other nodes on its
    segment that it needs the address of a particular
  • All nodes examine the broadcast.
  • The node specified responds if it exists.
  • Also know as B-Node resolution.
  • Does not work in larger networks, as routers
    block broadcasts.

  • LMHosts File.
  • Similar to Hosts file.
  • MS places a simple LMHosts file on the host when
    networking is installed.
  • LMHosts.sam - remove the .sam to enable.
  • marketserver description
  • LMHosts requires manual editing.
  • Recently resolved NetBIOS names are stored in the
    NetBIOS cache.

  • Names may be loaded into the cache by including
    the PRE keyword.
  • These files are located on each node.
  • High maintenance.
  • May be centralised by use of the INCLUDE
  • UNC names are used.
  • \\bauhaus\dirname\LMHosts

  • Windows Internet Name Service.
  • Installed on NT/2000 machines.
  • Described is RFCs1001 1002
  • Known as NBNS.
  • Also known as P-node resolution.
  • Other resolution modes are
  • M-node - First use broadcast then WINS
  • H-node - Use WINS first then broadcast.
  • Windows PCs default to H-node.

  • WINS maintains a database of registered NetBIOS
  • Includes users, computers, services and
  • Unlike DNS, the database is dynamically updated.
  • Clients register their name and IP at startup.
  • TASK Find out if Linux supports WINS.

  • When a WINS client starts the following processes
  • Service startup.
  • Some of which must be made known to other nodes.
  • Printserver, file server etc.
  • Registration Request.
  • The node must register itself with the WINS
  • The WINS client packages the NetBIOS name and IP
    address inside a name registration request.
  • This is sent to the WINS server, where it is
    checked against the database.(for duplicate

  • If unique, WINS adds the NetBIOS and IP pair to
    its database and sends a registration response
    indicating success.
  • If the request is not unique, the WINS challenges
    the node currently registered. If it responds, a
    negative acknowledgment is sent to the requesting
  • If the challenge is not responded to, the old
    entry is overwritten by the new request.
  • Lease.
  • Successful registrations are considered leased.
  • Ie valid for a limited time only. (eg 6 days)
  • Client will typically renew the lease at half the
    total lease time.(eg 3 days)

  • Remember the 16th character in a NetBIOS name?
  • During WINs registration the 16th character is
    appended to the name based on the type of
  • In a workgroup, it is not unusual for a
    workstation to have 10-15 entries.
  • Wins may be integrated with DNS.

Dynamic Host Configuration Protocol (DHCP)
  • Allows a set of hosts to share a pool of IP
  • Newly booted computer broadcasts to discover
  • Datagram destined for UDP port 68.
  • This port is reserved for bootp and DHCP.
  • This contains the MAC address of the DHCP Client.
  • And other configuration information.

Dynamic Host Configuration Protocol (DHCP)
  • DHCP servers reply with offers of IP addresses.
  • Only if the server has unleased IPs available.
  • Sent via broadcast to the node that issued the
    DHCP discover.
  • Sent to UDP port 67.
  • Contains the MAC address of the DHCP Client.
  • Also contains the IP and physical address of the
    DHCP server.

DHCP Cont.
  • Contains the IP address being offered and the
    subnet mask for this particular network.
  • Note The client may receive many offers at this
  • Host picks one and broadcasts a request to a
    particular server.
  • This contains the IP address of the server and
    the MAC address of the client.
  • This performs two functions.
  • Notifies the selected DHCP server that the IP is
  • Notifies all other DHCP servers that they may
    retract their offers.

DHCP Cont.
  • All other servers withdraw offers, and selected
    server sends an ack.
  • This is the final datagram of the DHCP
  • Includes the IP and subnet mask for the client.
  • May also include default gateway and WINS server
  • Other fields include
  • Lease period.
  • T1 and T2 - used when the client attempts to
    renew the lease.

DHCP Cont.
  • When done, host sends a release.
  • Server reuses IP addresses when their lease is
  • Time Fields
  • T1 indicates when the client should begin the
    process of renewing the lease.(usually 50)
  • T2 is the time to start querying other DHCP
    servers if the original server does not respond.
  • Renewals are not broadcast.