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DHCP, DNS
Introduction
 Client administration:
 IP address management:
 They need to ease the process of joining the network and they
do not want users to do any special configuration (DHCP)
 They want to network boot their workstations
 i.e. Diskless workstations or remote OS installation (acquiring the
network setting during boot process)
 Solution: Deploy a DHCP server
 Machine names management:
 They need to be able to name machines and access them by
names instead of IP addresses
 Solution: Deploy A DNS server
DHCP
 DHCP - Dynamic Host Configuration Protocol
 Provides configuration parameters specific to the DHCP client host
requesting, information required by the client host to participate on
an IP network
 Method of IP allocation
 Manual
 Only requesting clients with a MAC address listed in the table (MAC-
IP pairs) get the IP address according to the table
 Automatic
 DHCP server permanently assigns to a requesting client a free IP-
address from a range given by the administrator
 Dynamic
 The only method which provides dynamic re-use of IP addresses
 The request-and-grant process uses a lease concept with a
controllable time period.
DHCP cont.
 DHCP server can provide optional configuration
 e.g. Subnet Mask, Router, Name Server, …
 RFC 2132 defines DHCP options Usage
 DHCP relay agent (mostly in network routers/high-end switches)
 Relays DHCP Discover broadcasts from a LAN without DHCP to
a network which has one
 Usage
 US
 Cable Internet providers use DHCP
 DSL providers prefer PPPoE
 UK
 Many broadband ISP networks use DHCP
 XDSL providers use infinite lease  Semi-static IPs
 Office networks, public internet access
 Places where there are mobile nodes that want to access the net
DHCP Implementations
 Microsoft introduced DHCP on their NT server with Windows NT 3.5
in late 1994
 DHCP did not originate from Microsoft
 Internet Software Consortium published DHCP for Unix variants
 Version 1.0.0 released on December 6, 1997
 Version 2.0 on June, 1999 – A more RFC-compliant one
 Novell included a DHCP server in NetWare OS since v. 5, 1999
 It integrates with Novell eDirectory
 Weird solutions introducing a variety of multiplatform DHCP
implementations since 1997
 Cisco since Cisco IOS 12.0 in February 1999
 Sun added DHCP support in Solaris 8, July 2001
DHCP Anatomy
 Uses the same IANA assigned ports as BOOTP
 67/udp for the server, 68/udp for the client
 DHCP Messages
 Discover
 Client broadcasts on the local physical subnet to find servers
 UDP packet (broadcast dest. 255.255.255.255)
 Also request last-known IP address (optional parameter)
 Offer
 Server determines the configuration based on the client’s MAC addr.
 Server specifies the IP address and put optional parameters
 Request
 Client selects a configuration out the DHCP Offer packet and
broadcasts it again
 Acknowledge
 Server acknowledges the request and sends the ack to the client
DHCP Anatomy cont.
 Inform
 Client requests more information than the server sent with the
DHCPACK, or to repeat data for a particular application (e.g.
to obtain web proxy settings by a browser)
 Release
 Client requests the server to release the DHCP and the client
unconfigures its IP address
 Sending this message is not mandatory (unplug or …)
BOOTP
 BOOTstrap Protocol (RFC 951)
 UDP
 Used to obtain IP address automatically
 Usually in booting process of computers or OSs
 Diskless workstations
 Historically used for UNIX-like diskless workstations
 Also obtains the locations of the boot image
 Also can be used for installing a pre-configured OS
 Protocol became embedded in the BIOS of some NICs
 Allowing direct network booting without need for a floppy
BOOTP cont.
 Recently used for booting a Windows OS in diskless
standalone media center PCs
 DHCP is a more advanced protocol base on BOOTP
 Far more complex to implement than BOOTP
 Most DHCP servers also offer BOOTP support
 Duration based leases is the fundamental addition in DHCP
 Dynamic in DHCP is for this
RARP
 ARP
 Address Resolution Protocol
 Resolve a hardware address from a given IP address
 Try arp command in both Windows and Linux
 RARP
 Reverse Address Resolution Protocol (RFC 903)
 Complement of ARP
 Resolve an IP address from a given hardware address
 Needs manual configuration on a central server
 Not scalable
 Obsoleted by BOOTP and the more modern DHCP
 Try rarp command in Linux (if supported by Kernel), and RARP
daemon - RARPd
DNS
 Domain Name System (RFC 1034, 1035)
 RFC 1034 and 1035 made RFC 882, 883 obsolete
 A system that stores info associated with domain names
in a distributed database on networks (such as Internet)
 Many types of information for the domain provided by
DNS
 Most important, IP address associated with domain name
 Mail eXchange servers accepting e-mail for each domain
 Mainly UDP
 TCP only when response data size exceeds 512 bytes or
for things like zone transfer
DNS is Decentralized
 No single point of failure
 Less traffic volume
 Easier maintenance
 Scalable
 Less distant (delay) issues
 Delegation
Resolvers
 Clients that access name servers
 Querying a name server
 Interpreting responses
 Returning the information to the programs that requested it
 In BIND, the resolver is just a set of library routines that
is linked into programs
 Not even a separate process
 Most of the burden of finding an answer to the query is
placed on the name server
 The DNS specs call this kind of resolver a stub resolver
Types of DNS Servers
 Primary master
 Reads the data for the zone from a file on its host
 Secondary master (Slave)
 Gets the zone data from another ns that is authoritative for the
zone (master server)
 Often, master server is the zone’s primary master
 Not always the case
 Secondary master may get the info from another secondary server
 Zone transfer
 Contacting master ns and if necessary pulling the zone data
 Redundancy
 An authoritative ns may be master for some of its zones and be
slave for some others
 It’s imprecise to call an ns, master or slave!
DNS Applications
 Attach IP addresses to domain names (ease of use)
 Many to many mapping
 Virtual Hosting
 Sender Policy Framework
 Makes it possible for people to assign authoritative
names, without needing to communicate with a central
registrar
 Load balancing between hosts
DNS History
 Idea in ARPAnet
 Originally, each computer retrieved a file called
HOSTS.TXT from SRI which contained the mappings
 Hosts file exists today (Looked up before querying DNS)
 /etc/hosts, C:WINDOWSsystem32driversetchosts
 Limitations
 Not scalable
 Each time a given computer’s address changed, all computers
should update their Hosts file
 DNS invented by Paul Mockapetris in 1983
 First implementation was called JEEVES by himself
Parts of a Domain Name
 Domain name consists of two or more parts separated
by dots (here ce.sharif.edu for example)
 Rightmost label: Top-level domain (edu)
 Each label to the left specifies a subdomain of the domain
above it.
 Relative dependence, not absolute dependence
 sharif is a subdomain of the edu domain
 ce is a subdomain of the sharif.edu domain
 Theoretical limits: 127 level, each level 63 chars, total domain
name 255 chars
 A domain name with one or more IP addresses is called a
hostname (sharif.edu, ce.sharif.edu but not edu)
A Distributed Hierarchical Database
 Root Servers (13 root
servers worldwide)
 TLD Servers (.com, .org,
.net, .uk, .ir, …)
 Authoritative DNS Servers
(organization’s DNS
server)
Local DNS Server
 Does not belong to hierarchy
 Also called default name server
 Acts as a proxy (forwarder), forwards query into
hierarchy
 Caches the results if of interest
DNS Queries
 Recursive
 Contacted name server should recurs and find the mapping
for the requesting host
 Heavy load on the servers
 Iterative
 Contacted server replies with the name of the server to
contact
 An ns provides the name of the next ns
 Bootstrapping problem (another query is required and …)
 So the IP of the next ns is provided
 Glue record
DNS Queries
 Recursive query example
DNS Queries
 DNS in the real world
DNS Caching and Updating Records
 Once a name server learns mapping, it caches it
 It’ll expire (TTL defined by the authoritative server)
 TLD servers typically cached in local name server
 Root name servers not often visited
 Update/Notify Mechanisms
 RFC 2136
 TTL is specified in the Start Of Authority (SOA) record
 Serial – Incremented when the zone file modified, others know
when the zone has been changed and should be reloaded
 Refresh – Number of seconds between update requests
 Retry – Number of seconds between retries (if a request failed)
 Expire – Number of seconds before considering the data stale
 Minimum – Used for minimum TTL, used for negative caching
DNS Records
 Resource Records
 Tuples which are stored in the distributed database
 (name, value, type, ttl)
 Types
 There are many types, most famous ones (IPv4 mostly)
 A: Maps a hostname to an IPv4 address
 NS: Maps a domain name to a list of authoritative DNS
servers
 CNAME: Makes one domain name an alias of another
 MX: Maps a domain name to a list of mail exchange servers
 PTR: Maps an IPv4 address to canonical name for that host
 SOA: Specifies the authoritative DNS server
 Info like email of the domain administrator, serial number, …
Advanced Features of DNS Servers
 Address Match Lists and Address Control Lists
 i.e. defining a network and referring to it with the name we
defined. e.g.
 acl “ce” {
{ 81.31.164.0/24; 81.13.179.0/24; };
};
 DNS Notify
 Notify the listed servers on zone change
 DNS Dynamic Update
 This permits authorized updaters to add and delete
resource records from a zone for which the server is
authoritative
 Used in DNS, DHCP servers integration
Legal Users of Domains
 Registrant
 Administrative contact
 Technical contact
 Billing contact
 Name servers
 Try whois in Linux and see these information for different
hosts
DNS - BIND
 BIND (Berkeley Internet Name Domain) written for
Berkeley’s 4.3BSD UNIX OS by Kevin Dunlap
 It is not maintained by Internet Software Consortium
 The most popular implementation of DNS today
 Ported to many flavors of UNIX
 Shipped as a standard part of most vendors’ UNIX
offerings
 Has even been ported to Microsoft Windows

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Session_2.ppt

  • 2. Introduction  Client administration:  IP address management:  They need to ease the process of joining the network and they do not want users to do any special configuration (DHCP)  They want to network boot their workstations  i.e. Diskless workstations or remote OS installation (acquiring the network setting during boot process)  Solution: Deploy a DHCP server  Machine names management:  They need to be able to name machines and access them by names instead of IP addresses  Solution: Deploy A DNS server
  • 3. DHCP  DHCP - Dynamic Host Configuration Protocol  Provides configuration parameters specific to the DHCP client host requesting, information required by the client host to participate on an IP network  Method of IP allocation  Manual  Only requesting clients with a MAC address listed in the table (MAC- IP pairs) get the IP address according to the table  Automatic  DHCP server permanently assigns to a requesting client a free IP- address from a range given by the administrator  Dynamic  The only method which provides dynamic re-use of IP addresses  The request-and-grant process uses a lease concept with a controllable time period.
  • 4. DHCP cont.  DHCP server can provide optional configuration  e.g. Subnet Mask, Router, Name Server, …  RFC 2132 defines DHCP options Usage  DHCP relay agent (mostly in network routers/high-end switches)  Relays DHCP Discover broadcasts from a LAN without DHCP to a network which has one  Usage  US  Cable Internet providers use DHCP  DSL providers prefer PPPoE  UK  Many broadband ISP networks use DHCP  XDSL providers use infinite lease  Semi-static IPs  Office networks, public internet access  Places where there are mobile nodes that want to access the net
  • 5. DHCP Implementations  Microsoft introduced DHCP on their NT server with Windows NT 3.5 in late 1994  DHCP did not originate from Microsoft  Internet Software Consortium published DHCP for Unix variants  Version 1.0.0 released on December 6, 1997  Version 2.0 on June, 1999 – A more RFC-compliant one  Novell included a DHCP server in NetWare OS since v. 5, 1999  It integrates with Novell eDirectory  Weird solutions introducing a variety of multiplatform DHCP implementations since 1997  Cisco since Cisco IOS 12.0 in February 1999  Sun added DHCP support in Solaris 8, July 2001
  • 6. DHCP Anatomy  Uses the same IANA assigned ports as BOOTP  67/udp for the server, 68/udp for the client  DHCP Messages  Discover  Client broadcasts on the local physical subnet to find servers  UDP packet (broadcast dest. 255.255.255.255)  Also request last-known IP address (optional parameter)  Offer  Server determines the configuration based on the client’s MAC addr.  Server specifies the IP address and put optional parameters  Request  Client selects a configuration out the DHCP Offer packet and broadcasts it again  Acknowledge  Server acknowledges the request and sends the ack to the client
  • 7. DHCP Anatomy cont.  Inform  Client requests more information than the server sent with the DHCPACK, or to repeat data for a particular application (e.g. to obtain web proxy settings by a browser)  Release  Client requests the server to release the DHCP and the client unconfigures its IP address  Sending this message is not mandatory (unplug or …)
  • 8. BOOTP  BOOTstrap Protocol (RFC 951)  UDP  Used to obtain IP address automatically  Usually in booting process of computers or OSs  Diskless workstations  Historically used for UNIX-like diskless workstations  Also obtains the locations of the boot image  Also can be used for installing a pre-configured OS  Protocol became embedded in the BIOS of some NICs  Allowing direct network booting without need for a floppy
  • 9. BOOTP cont.  Recently used for booting a Windows OS in diskless standalone media center PCs  DHCP is a more advanced protocol base on BOOTP  Far more complex to implement than BOOTP  Most DHCP servers also offer BOOTP support  Duration based leases is the fundamental addition in DHCP  Dynamic in DHCP is for this
  • 10. RARP  ARP  Address Resolution Protocol  Resolve a hardware address from a given IP address  Try arp command in both Windows and Linux  RARP  Reverse Address Resolution Protocol (RFC 903)  Complement of ARP  Resolve an IP address from a given hardware address  Needs manual configuration on a central server  Not scalable  Obsoleted by BOOTP and the more modern DHCP  Try rarp command in Linux (if supported by Kernel), and RARP daemon - RARPd
  • 11. DNS  Domain Name System (RFC 1034, 1035)  RFC 1034 and 1035 made RFC 882, 883 obsolete  A system that stores info associated with domain names in a distributed database on networks (such as Internet)  Many types of information for the domain provided by DNS  Most important, IP address associated with domain name  Mail eXchange servers accepting e-mail for each domain  Mainly UDP  TCP only when response data size exceeds 512 bytes or for things like zone transfer
  • 12. DNS is Decentralized  No single point of failure  Less traffic volume  Easier maintenance  Scalable  Less distant (delay) issues  Delegation
  • 13. Resolvers  Clients that access name servers  Querying a name server  Interpreting responses  Returning the information to the programs that requested it  In BIND, the resolver is just a set of library routines that is linked into programs  Not even a separate process  Most of the burden of finding an answer to the query is placed on the name server  The DNS specs call this kind of resolver a stub resolver
  • 14. Types of DNS Servers  Primary master  Reads the data for the zone from a file on its host  Secondary master (Slave)  Gets the zone data from another ns that is authoritative for the zone (master server)  Often, master server is the zone’s primary master  Not always the case  Secondary master may get the info from another secondary server  Zone transfer  Contacting master ns and if necessary pulling the zone data  Redundancy  An authoritative ns may be master for some of its zones and be slave for some others  It’s imprecise to call an ns, master or slave!
  • 15. DNS Applications  Attach IP addresses to domain names (ease of use)  Many to many mapping  Virtual Hosting  Sender Policy Framework  Makes it possible for people to assign authoritative names, without needing to communicate with a central registrar  Load balancing between hosts
  • 16. DNS History  Idea in ARPAnet  Originally, each computer retrieved a file called HOSTS.TXT from SRI which contained the mappings  Hosts file exists today (Looked up before querying DNS)  /etc/hosts, C:WINDOWSsystem32driversetchosts  Limitations  Not scalable  Each time a given computer’s address changed, all computers should update their Hosts file  DNS invented by Paul Mockapetris in 1983  First implementation was called JEEVES by himself
  • 17. Parts of a Domain Name  Domain name consists of two or more parts separated by dots (here ce.sharif.edu for example)  Rightmost label: Top-level domain (edu)  Each label to the left specifies a subdomain of the domain above it.  Relative dependence, not absolute dependence  sharif is a subdomain of the edu domain  ce is a subdomain of the sharif.edu domain  Theoretical limits: 127 level, each level 63 chars, total domain name 255 chars  A domain name with one or more IP addresses is called a hostname (sharif.edu, ce.sharif.edu but not edu)
  • 18. A Distributed Hierarchical Database  Root Servers (13 root servers worldwide)  TLD Servers (.com, .org, .net, .uk, .ir, …)  Authoritative DNS Servers (organization’s DNS server)
  • 19. Local DNS Server  Does not belong to hierarchy  Also called default name server  Acts as a proxy (forwarder), forwards query into hierarchy  Caches the results if of interest
  • 20. DNS Queries  Recursive  Contacted name server should recurs and find the mapping for the requesting host  Heavy load on the servers  Iterative  Contacted server replies with the name of the server to contact  An ns provides the name of the next ns  Bootstrapping problem (another query is required and …)  So the IP of the next ns is provided  Glue record
  • 21. DNS Queries  Recursive query example
  • 22. DNS Queries  DNS in the real world
  • 23. DNS Caching and Updating Records  Once a name server learns mapping, it caches it  It’ll expire (TTL defined by the authoritative server)  TLD servers typically cached in local name server  Root name servers not often visited  Update/Notify Mechanisms  RFC 2136  TTL is specified in the Start Of Authority (SOA) record  Serial – Incremented when the zone file modified, others know when the zone has been changed and should be reloaded  Refresh – Number of seconds between update requests  Retry – Number of seconds between retries (if a request failed)  Expire – Number of seconds before considering the data stale  Minimum – Used for minimum TTL, used for negative caching
  • 24. DNS Records  Resource Records  Tuples which are stored in the distributed database  (name, value, type, ttl)  Types  There are many types, most famous ones (IPv4 mostly)  A: Maps a hostname to an IPv4 address  NS: Maps a domain name to a list of authoritative DNS servers  CNAME: Makes one domain name an alias of another  MX: Maps a domain name to a list of mail exchange servers  PTR: Maps an IPv4 address to canonical name for that host  SOA: Specifies the authoritative DNS server  Info like email of the domain administrator, serial number, …
  • 25. Advanced Features of DNS Servers  Address Match Lists and Address Control Lists  i.e. defining a network and referring to it with the name we defined. e.g.  acl “ce” { { 81.31.164.0/24; 81.13.179.0/24; }; };  DNS Notify  Notify the listed servers on zone change  DNS Dynamic Update  This permits authorized updaters to add and delete resource records from a zone for which the server is authoritative  Used in DNS, DHCP servers integration
  • 26. Legal Users of Domains  Registrant  Administrative contact  Technical contact  Billing contact  Name servers  Try whois in Linux and see these information for different hosts
  • 27. DNS - BIND  BIND (Berkeley Internet Name Domain) written for Berkeley’s 4.3BSD UNIX OS by Kevin Dunlap  It is not maintained by Internet Software Consortium  The most popular implementation of DNS today  Ported to many flavors of UNIX  Shipped as a standard part of most vendors’ UNIX offerings  Has even been ported to Microsoft Windows