cse 124 networked services fall 2010 lecture 5: domain...
Post on 15-Aug-2020
3 Views
Preview:
TRANSCRIPT
CSE 124 Networked Services
Fall 2010
Lecture 5: Domain Name System
B. S. Manoj, Ph.D
http://cseweb.ucsd.edu/classes/fa10/cse124
10/7/2010 CSE 124 Networked Services Fall 2010 1
General updates
• Writeup-1 Due: This Saturday
• Homework-1 Due: This Friday
• The programming project-1
– An HTTP Web Server
– %./your_server PortNo ./file_location/ options
– Deadline: November 1st, 2010
10/7/2010 CSE 124 Networked Services Fall 2010 2
DNS
10/7/2010 CSE 124 Networked Services Fall 2010 3
Application Layer Services: Domain Name System
• Provides many services including the name-to-address translation
• Why DNS– Names are easy for human reading, however, IP addresses are
required for Internet routers– Names can be used for routing, but with high in efficiency
• In original ARPANET,– hosts.txt file contained host-to-address translation– A central server maintained and managed the changes in hosts.txt file– Hosts downloaded the hosts.txt file periodically (e.g., midnight )
• With the growth of ARPANET, the above method proved unscalable
10/7/2010 4CSE 124 Networked Services Fall 2010
Domain Name System
• One of the most important application layer protocol
– Every other application layer protocol needs DNS
• Provides many services• Name to address resolution
– Host name to IP address translation
• Name aliasing– Host name aliasing
• Service aliasing– Mail services aliasing
• Information services• Performance optimization
– Traffic load distribution, Improving response time, and Improving scalability10/7/2010 5CSE 124 Networked Services Fall 2010
Name to Address resolution• Translation from human-readable name to router-readable address• Example: Name Server Lookup
# nslookup> cseweb.ucsd.eduServer: 132.239.0.252Address: 132.239.0.252#53
Name: cseweb.ucsd.eduAddress: 132.239.51.6> > serverDefault server: 132.239.0.252Address: 132.239.0.252#53Default server: 128.54.16.2Address: 128.54.16.2#53
• New alternatives to nslookup: Domain Information Groper (dig) or host10/7/2010 6CSE 124 Networked Services Fall 2010
Domain Name
IP address
Primary Local DNS server
Secondary Local DNS server
10/7/2010 CSE 124 Networked Services Fall 2010 7
>dig any cseweb.ucsd.edu
; <<>> DiG 9.3.3rc2 <<>> any cseweb.ucsd.edu
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16514
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 4, ADDITIONAL: 8
;; QUESTION SECTION:
;cseweb.ucsd.edu. IN ANY
;; ANSWER SECTION:
cseweb.ucsd.edu. 43200 IN MX 50 cseweb.ucsd.edu.
cseweb.ucsd.edu. 43200 IN MX 75 inbound.ucsd.edu.
cseweb.ucsd.edu. 43200 IN A 132.239.51.6
;; AUTHORITY SECTION:
ucsd.edu. 43200 IN NS ns0.ucsd.edu.
ucsd.edu. 43200 IN NS ns1.ucsd.edu.
;; Query time: 0 msec
;; SERVER: 132.239.0.252#53(132.239.0.252)
;; WHEN: Thu Oct 7 07:16:18 2010
;; MSG SIZE rcvd: 309
Host Name aliasing • Translation from complex canonical names to simple mnemonic names• e.g., cluster0028.asia.microsoft.com • Relay8951.cluster392.utopia.com
# nslookup> docs.google.comServer: 132.239.0.252Address: 132.239.0.252#53
Non-authoritative answer:docs.google.com canonical name = writely.l.google.com.Name: writely.l.google.comAddress: 66.102.7.139>PING google.com (74.125.67.100) 56(84) bytes of data.64 bytes from gw-in-f100.google.com (74.125.67.100): icmp_seq=1 ttl=46 time=88.9 ms64 bytes from gw-in-f100.google.com (74.125.67.100): icmp_seq=2 ttl=46 time=88.8 ms
10/7/2010 8CSE 124 Networked Services Fall 2010
Mnemonic domain name
Canonical domain name
Service Aliasing
• DNS can also be used for service aliasing– e.g., mail services aliasing – IP address aliasing (reverse address lookup) – SRV extensions (which port a particular service is running)
• Mail services are typically not provided by the web server
• Web server is more popular than mail servers (www.microsoft.com) – It is better served when associated with the same server name for both– somebody@ucsd.edu is better than somebody@receive-
mailserver001.cse.ucsd.edu
• The DNS uses a special record to provide the domain name of the mail server associated with a domain name
• Helps multiple services such as web and mail using same domain name
10/7/2010 9CSE 124 Networked Services Fall 2010
Features of DNS
• A hierarchical naming system• e.g., cseweb.ucsd.edu
• A distributed, federated, hierarchically organized, server system
• An application layer protocol
– Simple, Query-Reply-based, On UDP
10/7/2010 10CSE 124 Networked Services Fall 2010
Features of DNS
• A hierarchical naming system• e.g., cseweb.ucsd.edu
CSEWEB
• Subdomain
• Under the control of the domain (e.g., images.google.com)
UCSD
• Domain name for the organization
.EDU
• Top level domain
10/7/2010 11CSE 124 Networked Services Fall 2010
Root DNS servers
.com
Google.com
Docs images
Microsoft.com
.gov .org
Gnu.org
.edu
ucsd
blink cse cseweb
ucla
.Uk .no
Hierarchical naming/server system13 root domain servers
(10 in US, 2 in Europe, 1 in Asia)Top Level Domains
(TLD) (> 258)
TLD for country domains
Sub domain
10/7/2010 12CSE 124 Networked Services Fall 2010
Examples of Generic Top Level Domains (gTLDs)
• .aero Aviation• .asia Asia• .biz Business Organizations• .cat Catalan language and
culture• .com Commercial• .coop Co-Operative
Organizations• .edu Education• .gov US Government• .info Open TLD• .int International
Organizations• .jobs Jobs• .mil US Department of
Defense
• .mobi Mobile devices• .museum Museums• .name Personal• .net Networks• .org Organizations• .pro Credentialed
professionals and related entities
• .tel Publishing of contact data
• .travel Travelling
10/7/2010 13CSE 124 Networked Services Fall 2010
DNS Server Hierarchy• Root DNS (rDNS) servers
– 13 of them (10 in US, 2 in Europe, and 1 in Asia), each is a collection of servers– Do not maintain A records– Maintain records for locating Top level DNS servers
• Top level domain (TLD) DNS servers– Do not maintain A records– Responsible for the top level domains such as .com, .org, .edu etc – Also responsible for the country level domains such as .us, .uk, .no, .jp, .cn, etc. – More than 258 domains exist today, (likely to grow even further)
• Authoritative DNS server– Provided by the organization which hosts the web servers and hosts– Maintains A record for many hosts (in some cases there may be hierarchy of aDNS
servers)– Either hosted within the organization or through a DNS service provider
• Local DNS server– Plays the intermediary role between client and the hierarchy of DNS servers– Not part of the hierarchy of the DNS
10/7/2010 14CSE 124 Networked Services Fall 2010
How DNS works
• A client calls resolver function– A software library that implements the DNS client in an end-
user machine
– Resolver sends a DNS query message to its local DNS (LDNS) server
– LDNS replies immediately if it has a cached information available
– If LDNS does not have the IP address, then it tries• First the root DNS servers• Then the general TLD servers• Then one or more of the authoritative DNS servers
10/7/2010 15CSE 124 Networked Services Fall 2010
DNS APIs
• For unix-based systems, the client calls – struct hostent *gethostbyname(const char *name);
• returns a structure of type hostent for the given host name.• Name is a character array contains the host name
– struct hostent *gethostbyaddr(const char *addr, int len, inttype);
• Used for reverse lookup (IP address to hostname)
– struct hostent { char *h_name; /* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* host address type */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses */ }
10/7/2010 16CSE 124 Networked Services Fall 2010
A client side code example……int sfd;struct sockaddr_un addr; char host_name[256]; struct hostent *host_addr;sprintf(host_name,”www.google.com”); /* copies the name to the host_name array*/
host_addr = gethostbyname(host_name);If(!host_addr){
fprintf(stderr,”Unknown host: %s\n”,host_name);exit(1);
}
sfd = socket(AF_UNIX, SOCK_STREAM, 0); /* socket is opened*/
if (sfd == -1) {perror("socket");exit(EXIT_FAILURE);
}
memset(&addr, 0, sizeof(struct sockaddr_un)); /* Clear structure */addr.sun_family = AF_UNIX;strncpy(addr.sun_path, MY_SOCK_PATH,
sizeof(addr.sun_path) - 1);10/7/2010 17CSE 124 Networked Services Fall 2010
How DNS works
Enterprise/Campus/ Local
Network
Local DNS (lDNS) server
The Internet
1
2
3
4
5
6
8
7
Root DNS
(rDNS) servers
General Top Level Domain
(gTLD) DNS servers
Authoritative DNS
(aDNS) servers10/7/2010 18CSE 124 Networked Services Fall 2010
docs.google.com
docs.google.com
NS: .com
docs.google.com
NS: dns.google.com
A: 66.102.7.113 and …..
66.102.7.113 and …..
DNS resource record types• A Resource Record (RR) is a basic data element in a DNS database• DNS server stores different types of RRs• Each RR corresponds to a set of information for a particular service
DNS provides• A record consists of multiple values that contain {Name, TTL, Class,
Type, Value}
10/7/2010 19CSE 124 Networked Services Fall 2010
Name of the primary source of information about name server’s zone (email), serial number, timeout
DNS record types
• Record: {Name, Value, Class, Type, TTL}
• Type A: Name=Hostname, Value= IP address, TTL=time to live– Type A record gives hostname-to-address resolution– E.g; {cseweb.ucsd.edu, 132.239.51.6, A, 19800}
• Type NS: Name=domain (e.g., google.com), Value = address of the authoritative DNS (aDNS) server – aDNS server can help with resolution of hostnames in the domain
• Type CNAME: Name=hostname, Value=canonical host name for hostname– Provides hostname aliasing service– e.g., {docs.google.com, writely.l.google.com, CNAME}
• Type MX: Name=hostname, Value=canonical name of the mail server associated with the hostname– e.g., {ucsd.edu, mail-jsoe.ucsd.edu, MX}
10/7/2010 20CSE 124 Networked Services Fall 2010
An estimate of DNS queries (2003)
• Organizations managing rDNS servers report 100s of millions of queries per day
• Traces from F-root server
– San Francisco and Palo Alto
– 4 root server machines
– 152 million queries/24 hours
10/7/2010 21CSE 124 Networked Services Fall 2010 Source: D. Wessels and M. Fomenkov,
“WoW, That’s a lot of packets,” ACM PAM 2003
CSE 124 Networked Services Fall 2010 22
DNS protocol, messagesDNS protocol : query and reply messages, both with same
message formatmsg header
identification: 16 bit # for
query, reply to query uses
same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative
10/7/2010
DNS query flags
10/7/2010 CSE 124 Networked Services Fall 2010 23
CSE 124 Networked Services Fall 2010 24
DNS protocol, messages
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used
10/7/2010
CSE 124 Networked Services Fall 2010 25
Inserting records into DNS
• example: new startup “Network Utopia”• register name networkuptopia.com at DNS registrar (e.g.,
Network Solutions, godaddy.com, yahoo domains)– provide names, IP addresses of authoritative name server (primary
and secondary)– registrar inserts two RRs into com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
• Similarly, create authoritative server Type A record for www.networkuptopia.com; Type MX record for networkutopia.com
10/7/2010
DNS-based Performance optimization services
• Not originally intended • However, today it is a very important service
• Main performance optimization– Load Distribution– Provisioning Reliability– Content Distribution network services
• Load distribution – DNS is used to provide load balancing among multiple servers that
serve the same domain– Many popular web servers use server replication– A set of IP addresses is associated with a canonical name– When a client queries, DNS server replies with the entire set of IP
addresses– Each client makes use of the first IP address in the set– The set of IP addresses is rotated in order when subsequent request– Therefore, the load is almost equally balanced among the set of
servers10/7/2010 26CSE 124 Networked Services Fall 2010
An example
>docs.google.comServer: 132.239.0.252Address:
132.239.0.252#53
Non-authoritative answer:docs.google.com canonical
name = writely.l.google.com.
Address: 66.102.7.113Address: 66.102.7.138Address: 66.102.7.139Address: 66.102.7.100Address: 66.102.7.101Address: 66.102.7.102
> docs.google.comServer: 132.239.0.252Address:
132.239.0.252#53
Non-authoritative answer:docs.google.com canonical
name = writely.l.google.com.
Address: 66.102.7.102Address: 66.102.7.113Address: 66.102.7.138Address: 66.102.7.139Address: 66.102.7.100Address: 66.102.7.101
> docs.google.comServer: 132.239.0.252Address:
132.239.0.252#53
Non-authoritative answer:docs.google.com canonical
name = writely.l.google.com.
Address: 66.102.7.139Address: 66.102.7.100Address: 66.102.7.101Address: 66.102.7.102Address: 66.102.7.113Address: 66.102.7.138
1 2 3
10/7/2010 27CSE 124 Networked Services Fall 2010
Content Distribution Network services
• DNS is effectively used to provide content distribution services
• When a client queries for an address of a host, the DNS server provide the name of a CDN
• The CDN’s DNS replies with the address of the web cache nearest to the client’s location
• The client receives content from a nearby Web cache than the main web server– Quick response– Better load balancing– High server scalability
10/7/2010 28CSE 124 Networked Services Fall 2010
CDN example
origin server (www.cnn.com)
• distributes HTML
• replaces:http://www.cnn.com/sports.ruth.gif
withhttp://www.cdn.com/www.cnn.com/sports/ruth.gif
HTTP request for
www.cnn.com/sports/sports.html
DNS query for www.cdn.com
HTTP request for
www.cdn.com/www.cnn.com/sports/ruth.gif
1
2
3
origin server
CDN’s authoritative
DNS server
CDN server near client
CDN company (cdn.com)
• distributes gif files
• uses its authoritative
DNS server to route
redirect requests
client
10/7/2010 29CSE 124 Networked Services Fall 2010
Additional sources• Domain names are sold by domain name registrars
– Very competitive business models exist for domain names– Domain names are one of the most sought after real-estates in the web
today– Some simple names are worth several millions
• Business.com: $8 million• Wine.com: $2.9 million• Drugs.com: $830,000
• Important– When you register valuable names, register for longer duration
• Internet Corporation of Assigned Names and Numbers (ICANN)– Responsible for accrediting various domain name registrars
• http://www.internic.net/– For the list of accredited registrars
10/7/2010 30CSE 124 Networked Services Fall 2010
Attacks on DNS
• Distributed Denial of Service– Bandwidth flooding or resource consumption– Flooding Ping messages
• Distributed DNS query attack
• Man-in-the middle attack– Capture DNS queries and generate bogus DNS replys
• DNS poisoning– Erroneously populate the DNS caches by originating bogus replys
• Reflection attacks against hosts by – Generate spoofed DNS queries and thus create large number of replys– Targetting a host with DSN replys– Querys can be ANY? So that replys will be bigger packets
10/7/2010 31CSE 124 Networked Services Fall 2010
Summary• DNS is an important service on Internet
• Every other service depends much on it
• DNS is scalable, distributed, hierarchical, and federated
• Many performance optimization strategies are used with the help of DNS
• References:
– Kurose and Rose (Chapter 1-3)
– Andrew Tannenbaum (Chapter 7)
10/7/2010 CSE 124 Networked Services Fall 2010 32
top related