Transcript tut8 - Webcourse

Internet Networking
Spring 2006
Tutorial 8
DNS and DHCP
as UDP applications
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Domain Name System - DNS
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Provides mapping from ASCII domain names to IP
addresses
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Hierarchical naming system.
The set of names is divided into mutually exclusive
parts.
Supplies mechanism for global data storage and
information retrieval. Most important principles:
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Distributed system – set of servers sharing
information.
Efficiency - most of the requests resolved by local
servers.
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Use of Caching.
Reliability – works even if some of the servers fail.
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DNS tree
root
com
ibm
org
gov
net
cnn
il
ac
technion
co
gov
tau
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Reliability over UDP
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DNS is an application which runs over UDP
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Advantage: not as complex as TCP.
Disadvantage: requires reliability
implementation at application level .
Reliability scheme:
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Time out.
Retransmissions - resends timed out query to
a different server.
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DNS Terms
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Domain name: any name represented in the DNS
format
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DNS label: each string between two ".“
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ac.il – 2 labels
DNS zone: a set of names that are under the same
authority
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mail.yahoo.com
\.name.example
cs.technion.ac.il, ee.technion.ac.il and
www.technion.ac.il
Delegation: transfer of authority for a domain
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example.org is a delegation from org.
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DNS Elements
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Resolver:
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stub: simple, only asks questions;
recursive: takes simple query and makes all
necessary steps to get the full answer.
Server (some perform both roles at the same time):
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authoritative: the servers that contain the zone file
for a zone, one Primary, one or more Secondaries;
caching: a recursive resolver that stores prior
results and reuses them.
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How Does DNS Work?
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The resolver sends a DNS request message over UDP to a local
domain name server.
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DNS caches store data for a short time defined by TTL on the
Record.
When a name server does not have the requested information,
it starts at longest match on query name it has when looking for
data.
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The resolver is configured with the address of a local domain
name server.
Every domain name server knows the IP address of the root server.
The root server knows the IP address of each second-level
domain name server. This info is forwarded back to the
contacted server.
Consequently, every server knows how to reach servers that are
authorities for names further down the hierarchy. The resolver
follows delegations until it receives an answer.
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DNS query example
(from IETF slides)
Root Server
Ask org NS
www.ietf.org
Org Server
Ask ietf.org NS
Stub resolver
www.ietf.org A
65.256.255.51
Recursive
Resolver
Ietf.org Server
www.ietf.org A
65.256.255.51
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Dynamic Host Configuration
Protocol - DHCP
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RFC 2131
Runs over UDP as well
Provides automatic configuration of the host
connected to network or booted
Provides hosts with initial configuration
information upon bootup:
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IP address with subnet mask,
default gateway,
IP address of the DNS server .
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DHCP (cont.)
server A
(not selected)
DHCP discover
determine
configuration
DHCP offer
client
server B
(selected)
DHCP discover
DHCP offer
determine
configuration
select configuration
DHCP request
DHCP request
DHCP ack
Initialization completes
using the allocated configuration
graceful shutdown
DHCP release
discard lease
DHCP Messages
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DHCPDISCOVER
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DHCPOFFER
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The client broadcasts to the server, requesting offered parameters
from one server specifically.
Confirms correctness of previously allocated address after, e.g.,
system reboot.
Extends the lease on a particular network address.
DHCPACK
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The server response to the client DHCPDISCOVER with offer of
configuration parameters .
DHCPREQUEST
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The client broadcasts message in search of available DHCP servers.
The server-to-client communication with configuration parameters,
including committed network address.
DHCPRELEASE
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The client-to-server communication, relinquishing network address
and canceling remaining lease.
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DHCP Messages (Cont.)
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DHCPNAK
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DHCPDECLINE
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Server to client indicating client's notion of
network address is incorrect (e.g., client has
moved to new subnet) or client's lease as expired
The client-to-server communication, indicating
that the network address is already in use.
DHCPINFORM
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The client-to-server communication, asking for
only local configuration parameters that the client
already has externally configured as an address.
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DHCP message format
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xid –
Transaction ID.
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ciaddr –
Client IP address
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yiaddr –
Your IP address.
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siaddr
Server IP address.
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chaddr
Client MAC address
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DHCP message format (cont.)
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Transaction ID - a random number that is chosen by the client
and is used by the client to match incoming DHCP messages with
pending requests. All DHCP messages exchanged for a given
DHCP transaction use the xid.
Client IP address - used when client knows its IP address.
Your IP address - the DHCP server will place the offered client
IP address in this field, if the client IP address is 0.0.0.0.
Server IP address - if the client knows the IP address of the
DHCP server, this field will be populated with the DHCP server
address. Otherwise, it is used in DHCPOFFER and DHCPACK from
DHCP server.
options - DHCP defines a 'client identifier' option that is used to
pass an explicit client identifier to a server. (This option
eliminates the overloading of the 'chaddr' field, which is used for
identifying transaction otherwise).
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DHCP message filtering
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DHCPOFFER: only the client waiting for this message
will accept it in UDP layer. But what if two clients are
waiting for the offer simultaneously?
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In this situation, the filtering is possible only at the
application layer. The transaction ID field in the DHCP
message is responsible for this filtering.
DHCPREQUEST: the message is broadcasted to all
the servers.
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The server knows if its offer was accepted only upon reading
the server identifier field in the DHCP message (application
layer).
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DHCP as UDP application
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DHCP server - port 67, client - port 68
Reliability is not provided by UDP.
Client is responsible for reliability
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The client implements timer to measure timeout for
the messages that were not responded.
The client adopts a retransmission strategy that
incorporates a randomized exponential backoff
algorithm to determine the delay between
retransmissions.
Every next message acts as an acknowledgment for
the previous step.
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For example, DHCPREQUEST is an ACK for DHCPOFFER.
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