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Network Applications:DNS, UDP Socket
1/24/2012
2
Outline
Recap DNS Network application programming: UDP
3
Recap: The Big Picture of the Internet Hosts and routers:
>850 mil. hosts (2011) organized roughly hierarchical backbone links 10~40Gbps
Software: datagram switching with virtual
circuit support at backbone layered network architecture
• use end-to-end arguments to determine the servicesprovided by each layer
the hourglass architecture of the Internet
IP
Ethernet Cable/DSLWireless
TCP UDP
TelnetEmail FTPWWWSSL
4
Protocol Formats
5
Recap: Client-Server Paradigm
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
request
reply
The basic paradigmof network applications is the client-server(C-S) paradigm a client/server is a
process at a port number of a host
Key design questionsof a C-S application: protocol extensibility scalability robustness security
6
Recap: Email App
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
POP3 orIMAPSMTP
Some nice protocol extensibility design features
•separate protocols for different functions
•simple/basic (smtp) requests to implement basic control; fine-grain control through ASCII header and message body
•status code in response makes message easy to parse
Scalability/Robustness
High scalability and robustness fundamentally require multiple email servers to serve the same email address
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clientneed a email
server IP address
mailserver
mailserver mail
server
yale.edu
yale.eduyale.edu
130.132.50.7130.132.50.8 130.132.50.9
mapping
8
Mapping Functions Design Alternatives
Map from an email address server name to IP address of email server
mapping
name (e.g., yale.edu)
1 IP
mapping
multiple IPs
mapping
multiple IPs
name (e.g., yale.edu)
9
Mapping Functions Design Alternatives
load balancer
switch
mapping
name (e.g., yale.edu)
1 IP
mapping
name (e.g., yale.edu)
1 IP
10
DNS: Domain Name System
Function map between (domain
name, service) to value, e.g.,
• (www.cs.yale.edu, Addr) -> 128.36.229.30
• (cs.yale.edu, Email) -> netra.cs.yale.edu
Many benefits of introducing the mapping
routers
DNS
Hostname, Service
Address
servers
clients
11
Dummy Design
DNS itself can be considered as a client-server system as well
How about a dummy design: introducing one super Internet DNS server?THE DNS server of the
Internetre
gis
ter
<n
am
e>
resolve <name>
OK
/use
d a
lready
IP address
12
DNS: Distributed Management of the Domain Name Space
A distributed database managed by authoritative name servers divided into zones, where each zone is a sub-tree of the global
tree each zone has its own authoritative name servers an authoritative name server of a zone may delegate a subset
(i.e. a sub-tree) of its zone to another name server
called a zone
13
Email Architecture + DNS
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
POP3 orIMAPSMTP
DNS
14
Root Zone and Root Servers
The root zone is managed by the root name servers 13 root name servers worldwide
See http://root-servers.org/ for more details
15
Linking the Name Servers
Each name server knows the addresses of the root servers
Each name server knows the addresses of its immediate children (i.e., those it delegates)
Top level domain(TLD)
Q: how to query a hierarchy?
16
DNS Message Flow: Two Types of Queries
Recursive query: Puts burden of name resolution on contacted
name server the contacted name server resolves the name
completely
Iterated query: Contacted server replies with name of server
to contact “I don’t know this name, but ask this server”
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Two Extreme DNS Message Flows
client
cicada.cs.yale.edu
root name server
12
3
authoritative name server
56
TLD name server
4
client
cicada.cs.yale.edu
root name server
16
2
authoritative name server
4 3
TLD name server5
Issues of the two approaches?
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Typical DNS Message Flow: The Hybrid Case
requesting hostcyndra.cs.yale.edu
gaia.cs.umass.edu
root name server
1
23
4
authoritative name serverdns.cs.umass.edu
56
TLD name server
7
8
iterated query
local name server130.132.1.9
• Host knows only local name server
• Local name server is learned from DHCP, or configured, e.g. /etc/resolv.conf
• Local DNS server helps clients resolve DNS names
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Typical DNS Message Flow: The Hybrid Case
requesting hostcyndra.cs.yale.edu
gaia.cs.umass.edu
root name server
1
23
4
authoritative name serverdns.cs.umass.edu
56
TLD name server
7
8
iterated query
local name server130.132.1.9
• Host knows only local name server
• Local name server is learned from DHCP, or configured, e.g. /etc/resolv.conf
• Local DNS server helps clients resolve DNS names
• Benefits of local name servers
• simplifies client• caches results
20
DNS Records
DNS: distributed db storing resource records (RR)
Type=NS name is domain (e.g.
yale.edu) value is the name of
the authoritative name server for this domain
RR format: (name, type, value, ttl)
Type=A name is hostname value is IP address
Type=CNAME name is an alias name
for some “canonical” (the real) name
value is canonical name
Type=MX value is hostname of mail
server associated with name Type=SRV
general extension
21
DNS Protocol, Messages
DNS protocol : typically over UDP (can use TCP); query and reply messages, both with the same message format
DNS Msg header: identification: 16 bit #
for query, the reply to a query uses the same #
flags: query or reply recursion desired recursion available reply is authoritative
22
Observing DNS
Use the command dig: force iterated query to see the trace:%dig +trace www.cnn.com
• see the manual for more details
Capture the messages DNS server is at port 53
23
Evaluation of DNS
Key questions to ask about a C-S application
- extensible?- scalable?- robust? - security?
24
What DNS did Right?
Hierarchical delegation avoids central control, improving manageability and scalability
Redundant servers improve robustness see
http://www.internetnews.com/dev-news/article.php/1486981 for DDoS attack on root servers in Oct. 2002 (9 of the 13 root servers were crippled, but only slowed the network)
Caching reduces workload and improve robustness
25
Problems of DNS
Domain names may not be the best way to name other resources, e.g. files
Relatively static resource types make it hard to introduce new services or handle mobility
Although theoretically you can update the values of the records, it is rarely enabled
Simple query model makes it hard to implement advanced query
Early binding (separation of DNS query from application query) does not work well in mobile, dynamic environments e.g., load balancing, locate the nearest printer
Outline
Recap Email DNS Network application programming
26
27
Socket Programming
Socket API introduced in
BSD4.1 UNIX, 1981
Two types of sockets Connectionless
(UDP) connection-
oriented (TCP)
an interface (a “door”) into which one
application process can both send and
receive messages to/from another (remote
or local) application
process
socket
28
Services Provided by Transport
Transmission control protocol (TCP)
multiplexing/demultiplexing reliable data transfer rate control: flow controland congestion control
User data protocol (UDP) multiplexing/demultiplexing
Host A
Hello
Host B
I am ready
DATA
ACK
29
Big Picture: Socket
buffers,states
buffers,states
30
UDP Java API
buffers,states
buffers,states
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DatagramSocket(Java)
DatagramSocket() constructs a datagram socket and binds it to any available port on the local host
DatagramSocket(int lport) constructs a datagram socket and binds it to the specified port on the local host
machine. DatagramSocket(int lport, InetAddress laddr)
creates a datagram socket and binds to the specified local port and laddress. DatagramSocket(SocketAddress bindaddr)
creates a datagram socket and binds to the specified local socket address.
DatagramPacket(byte[] buf, int length) constructs a DatagramPacket for receiving packets of length length.
DatagramPacket(byte[] buf, int length, InetAddress address, int port) constructs a datagram packet for sending packets of length length to the specified
port number on the specified host.
receive(DatagramPacket p)receives a datagram packet from this socket.
send(DatagramPacket p)sends a datagram packet from this socket.
close() closes this datagram socket.
Connectionless UDP: Big Picture (Java version)
closeclientSocket
Server (running on hostid)
read reply fromclientSocket
create socket,clientSocket = DatagramSocket()
Client
Create datagram using (servhost, x) as (dest addr. port),send request using clientSocket
create socket,port=x, forincoming request:serverSocket = DatagramSocket( x )
read request fromserverSocket
write reply toserverSocket
generate reply, create datagram using clienthost address, port number
Create socket with port number:DatagramSocket sSock = new DatagramSocket(9876);
If no port number is specified, the OS will pick one
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Example: UDPClient.java
A simple UDP client which reads input from keyboard, sends the input to server, and reads the reply back from the server.
34
Example: Java client (UDP)
import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); String sentence = inFromUser.readLine(); byte[] sendData = new byte[1024];
sendData = sentence.getBytes();
DatagramSocket clientSocket = new DatagramSocket(); InetAddress sIPAddress = InetAddress.getByName(“servname");
Createinput stream
Create client socket
Translate hostname to IP
address using DNS
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Example: Java client (UDP), cont.
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, sIPAddress, 9876); clientSocket.send(sendPacket); byte[] receiveData = new byte[1024]; DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }
}
Create datagram with data-to-send,
length, IP addr, port
Send datagramto server
Read datagramfrom server
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Example: UDPServer.java A simple UDP server which changes any received
sentence to upper case.
37
Example: Java Server (UDP)
import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
String sentence = new String(receivePacket.getData()); sendData = capitalizedSentence.getBytes();
Createdatagram socket
at port 9876
Create space forreceived datagram
Receivedatagra
m
38
UDP Connectionless Demux
DatagramSocket serverSocket = new DatagramSocket(9876);
ClientIP:B
P2
client IP: A
P1P1P3
serverIP: S
SP: 9876
DP: 9157
SP: 9157
DP: 9876
SP: 9876
DP: 5775
SP: 5775
DP: 9876
Source Port (SP) provides “return address”
39
Example: Java server (UDP), cont
InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }
}
Get IP addrport #, of
sender
Write out datagramto socket
End of while loop,loop back and wait foranother datagram
Create datagramto send to client
Discussion
Are there any problems with the program?
40
Discussion
What are challenges in implementing DNS using UDP?
41
Discussion
Are there any problems with the program?
42
Backup Slides
44
UDP Provides Multiplexing/Demultiplexing
server clientUDP socket space
address: {*:9876}snd/recv buf:
128.36.232.5128.36.230.2
address: {128.36.232.5:53}snd/recv buf:
UDP socket space
address: {198.69.10.10:1500}snd/recv buf:
address: {198.69.10.10:4343}snd/recv buf:
198.69.10.10
Packet demutiplexing is based on (dst address, dst port) at dst%netstat –u –n -a
local addresslocal port
45
UDP Port Provides Multiplexing/Demultiplexing
server clientUDP socket space
address: {*:9876}snd/recv buf:
128.36.232.5128.36.230.2
address: {128.36.232.5:53}snd/recv buf:
UDP socket space
address: {*:1500}snd/recv buf:
address: {198.69.10.10:4343}snd/recv buf:
198.69.10.10
Packet demutiplexing is based on (dst address, dst port) at dst%netstat --udp –n -a
local addresslocal port