Translating Addresses

Transcription

1 Translating Addresses Reading: Sections 9.3.1, Slides by Princeton, slightly altered by M.D. Context Application DNS DHCP Transport TCP UDP Network ICMP IP ARP Physical Physical Network 2

2 Three Kinds of Identifiers Host name (e.g., Mnemonic name appreciated by humans Provides little (if any) information about location Hierarchical, based on organizations IP address (e.g., ) Numerical address appreciated by routers Hierarchical, based on organizations and topology MAC address (e.g., C A9) Numerical 48-bit address appreciated by adapters Non-hierarchical, unrelated to network topology 6 Three Hierarchical Assignment Processes Host name: Domain: registrar for each top-level domain (e.g.,.edu) Host name: local administrator assigns to each host IP addresses: Prefixes: ICANN, regional Internet registries, and ISPs Hosts: static configuration, or dynamic using DHCP MAC addresses: C A9 Blocks: assigned to vendors by the IEEE Adapters: assigned by the vendor from its block 7

3 Mapping Between Identifiers Domain Name System (DNS) Given a host name, provide the IP address Given an IP address, provide the host name Dynamic Host Configuration Protocol (DHCP) Given a MAC address, assign a unique IP address and tell host other stuff about the Local Area Network To automate the boot-strapping process Address Resolution Protocol (ARP) Given an IP address, provide the MAC address To enable communication within the Local Area Network 8 Domain Name System (DNS) Proposed in 1983 by Paul Mockapetris Slides by Princeton, slightly altered by M.D.

4 Domain Name System (DNS) Properties of DNS Hierarchical name space divided into zones Distributed over a collection of DNS servers Hierarchy of DNS servers Root servers Top level domain (TLD) servers Authoritative DNS servers Performing the translations Local DNS servers and client resolvers 10 DNS Root Servers 13 root servers (see Labeled A through M E NASA Mt View, CA F Internet Software C. Palo Alto, CA (and 17 other locations) A Verisign, Dulles, VA C Cogent, Herndon, VA (also Los Angeles) D U Maryland College Park, MD G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign, ( 11 locations) K RIPE London (also Amsterdam, Frankfurt) I Autonomica, Stockholm (plus 3 other locations) M WIDE Tokyo B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA 11

5 TLD Servers Top-level domain (TLD) servers Generic domains (e.g., com, org, edu) Country domains (e.g., uk, fr, ca, jp) Typically managed professionally Network Solutions maintains servers for com Educause maintains servers for edu 12 Exercise Use the whois utility to find out who is in charge of our villanova.edu site. Look up your site both by DNS name and by IP network number. Try other sites as well, such as cisco.com. 13

6 Distributed Hierarchical Database root com edu org ac uk zw arpa bar generic domains country domains ac inaddr Authoritative Servers west foo east my Provide public records for hosts at an organization cam usr my.east.bar.edu usr.cam.ac.uk /24 DNS Resource Records Slides by Princeton, slightly altered by M.D.

7 Resource Records (RR) RR format: (name, value, type, TTL) Type=A name is hostname value is IP address Type=NS name is domain (e.g. foo.com) value is hostname of authoritative name server for this domain Type=CNAME name is alias name for some canonical (the real) name is really servereast.backup2.ibm.com value is canonical name Type=MX value is name of mailserver associated with name 17 Inserting Resource Records into DNS Example: just created startup FooBar Register foobar.com at Network Solutions Provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts two RRs into the.com TLD server: (foobar.com, dns1.foobar.com, NS) (dns1.foobar.com, , A) Set up authoritative server dns1.foobar.com Type A record for Type MX record for foobar.com 18

8 nslookup Use to query DNS servers Interactive and Non-interactive modes Examples: nslookup nslookup query=mx villanova.edu nslookup enter interactive shell type a host name; get its IP address info ls <domain_name> exit 1 nslookup Example > nslookup -query=mx villanova.edu Server: ns1.villanova.edu Address: villanova.edu mail exchanger = mskan-in-01.villanova.edu villanova.edu mail exchanger = mskan-in-02.villanova.edu mskan-in-01.villanova.edu internet address = mskan-in-02.villanova.edu internet address =

9 DNS Queries Slides by Princeton, slightly altered by M.D. Using DNS Local DNS server ( default name server ) Usually near the end hosts who use it Local hosts configured with local server (e.g., /etc/resolv.conf) or learn the server via DHCP Client application Extract server name (e.g., from the URL) Do gethostbyname() to trigger resolver code 22

10 Identity of Root Servers All DNS servers are configured with a list of 13 root servers, a selection of which looks like: A.ROOT SERVERS.NET IN A B.ROOT SERVERS.NET IN A C.ROOT SERVERS.NET IN A M.ROOT SERVERS.NET IN A Query Example Host at cis.poly.edu wants IP address for local DNS server dns.poly.edu 2 root DNS server 3 TLD.edu server requesting host cis.poly.edu authoritative DNS server dns.umass.edu 25

11 Query Example: Root Server The root server does not know about but sends the TLD servers responsible for the.edu domain: EDU IN NS A.GTLD SERVERS.EDU IN NS B.GTLD SERVERS.EDU IN NS C.GTLD SERVERS.EDU A.GTLD SERVERS.EDU IN A B. GTLD SERVERS.EDU IN A C. GTLD SERVERS.EDU IN A Query Example: GTLD Server The GTLD server does not know about but sends the TLD servers responsible for the.edu domain: umass.edu IN NS dns.cs.umass.edu IN NS dnsec.umass.edu dns.cs.umass.edu IN A dnsec. umass.edu IN A

12 Recursive vs. Iterative Queries Recursive query Ask server to get answer for you E.g., request 1 and response 8 Iterative query Ask server who to ask next E.g., all other request-response pairs local DNS server dns.poly.edu 1 root DNS server 2 3 (delegation) TLD DNS server requesting host cis.poly.edu authoritative DNS server dns.cs.umass.edu 28 Questions Give two reasons why a local DNS server is not configured with the IP addresses of the top-level domain servers (e.g., for.edu or.org ). How does the local DNS server learn the addresses of the top-level domain servers? 29

13 DNS Caching Slides by Princeton, slightly altered by M.D. DNS Caching Performing all these queries take time And all this before the actual communication takes place E.g., 1-second latency before starting Web download Caching can substantially reduce overhead The top-level servers very rarely change Popular sites (e.g., visited often Local DNS server often has the information cached How DNS caching works DNS servers cache responses to queries Responses include a time to live (TTL) field Server deletes the cached entry after TTL expires 33

14 Setting the Time To Live (TTL) TTL trade offs Small TTL: fast response to change Large TTL: higher cache hit rate Following the hierarchy Top of the hierarchy: days or weeks Bottom of the hierarchy: seconds to hours Ex: Browsers cache for seconds 34 Negative Caching Remember things that don t work Misspellings like and These can take a long time to fail the first time Good to remember that they don t work so the failure takes less time the next time around 35

15 DNS Protocol Slides by Princeton, slightly altered by M.D. DNS Protocol Context DNS Header DNS Data TCP/UDP Header TCP/UDP Data IP Header IP Data Frame Header Frame Data 37

16 DNS Header Format Identification: 16-bit number Same for query/reply Flags: Query or reply Recursion desired Recursion available Reply is authoritative Truncated (if answer can t fit in the 512-byte UDP limit) 38 DNS Query Example This is a Query Standard Query Recursion Desired QID = Question Count = 1 Answer Count = 0 Authority Count = 0 Additional Count = 0 Qu What is the A record of 39

17 DNS Response Example This is a Response Authoritative! Recursion Unavailable QID = Question Count = 1 Answer Count = 1 Authority Count = 2 Additional Count = 2 Qu What is the A record of An A = hr Au umass.edu NS = dns.cs.umass.edu 2dy Au umass.edu NS = dnsec.umass.edu 2dy Ad dns.cs.umass.edu A = hr Ad dnsec.umass.edu A = hr 40 Learning Your Local DNS Server Slides by Princeton, slightly altered by M.D.

18 How To Bootstrap an End Host? What local DNS server to use? What IP address should the host use? How to send packets to remote destinations???? host host... DNS host host... DNS / / router router router 42 Manual Configuration Technician must configure each PC... IP Address and Netmask Gateway Address Hostname, Domain, DNS WINS Server n n Waste of staff time Possible errors or duplicate IP asignment 43

19 Avoiding Manual Configuration Dynamic Host Configuration Protocol (DHCP) End host learns how to send packets Learn IP address, DNS servers, and gateway Address Resolution Protocol (ARP) Others learn how to send packets to the end host Learn mapping between IP address & interface address??? host host... DNS host host... DNS / / router router router 44 Key Ideas in Both Protocols Broadcasting: when in doubt, shout! Broadcast query to all hosts in the local-area-network when you don t know how to identify the right one Caching: remember the past for a while Store the information you learn to reduce overhead Remember your own address & other host s addresses Soft state: but eventually forget the past Associate a time-to-live field with the information and either refresh or discard the information Key for robustness in the face of unpredictable change 45

20 Bootstrapping Problem Host doesn t have an IP address yet So, host doesn t know what source address to use Host doesn t know who to ask for an IP address So, host doesn t know what destination address to use Solution: shout to discover a server who can help Broadcast a DHCP server-discovery message Server sends a DHCP offer offering an address host host... host DHCP server 46 Broadcasting Broadcasting: sending to everyone Special destination address: FF-FF-FF-FF-FF-FF All adapters on the LAN receive the packet Who am I? Where am I? Your name is clueless, address , netmask , router , DNS server , etc. DHCP server 47

21 Response from the DHCP Server DHCP offer message from the server Configuration parameters (proposed IP address, mask, gateway router, DNS server,...) Lease time (the time the information remains valid) Multiple servers may respond Multiple servers on the same broadcast media Each may respond with an offer The client can decide which offer to accept Accepting one of the offers Client sends a DHCP request echoing the parameters The DHCP server responds with an ACK to confirm and the other servers see they were not chosen 48 Dynamic Host Configuration Protocol arriving client DHCP server

22 DHCP Discover 50 DHCP Offer 51

23 DHCP Request and ACK At this time, the DHCP client can start to use the IP address. 52 Deciding What IP Address to Offer Static Allocation All parameters are statically configured in the server E.g., a dedicated IP address for each MAC address Makes it easy to track a host over time Dynamic Allocation Server maintains a pool of available addresses and assigns them to hosts on demand Leads to less configuration complexity and more efficient use of the pool of addresses 53

24 Soft State: Refresh or Forget Why is a lease time necessary? Client can release the IP address (DHCP RELEASE) But, the host might not release the address E.g., the host crashes (blue screen of death!) E.g., buggy client software Don t want the address to be allocated forever Performance trade-offs Short lease: returns inactive addresses quickly Long lease: avoids overhead of frequent renewals 54 DHCP Lease Renewal Client Server T = 0 Typical lease time is 1 30 days. At T1 = 1/2 Lease time (default) client sends a unicast DHCP REQUEST to the server to renew the lease. Client repeats DHCP REQUEST until the lease is renewed. If no response from server: At T2 = 3/4 Lease time (default) client begins to broadcast DHCP DISCOVER to locate a new server. 55

25 DHCP on Single Network One DHCP server can configure all machines on a network. Faculty & Staff offices switch or hub DHCP provides each client: IP Address: xx.x Netmask: Router: Broadcast addr: WINS server: Domain-name: csc.villanova.edu DNS servers: , Lease Time: 10 days Other information Computer Lab WINS Server DHCP Server 56 DHCP on Multiple Networks One server can manage many networks using a Relay Agent. Network A Network B DHCP Request (broadcast) Network C File Server WINS Server DHCP Server Router configured as DHCP Relay Agent: router forwards DHCP DHCP Reply requests (broadcasts) to server router forwards DHCP replies to client No client reconfiguration is needed to use a relay agent. 57

26 DHCP on Remote Networks There can be other routers between the DCHP Relay Agent and the DHCP server. DHCP Relay Agent for Network A Network A Network B DHCP Relay Agent for Network B Relay Agent not necessary here. Network not using DHCP DHCP Server DHCP relaying is transparent to the clients. 58 Where is the Client? A DHCP Server can support multiple networks. Assigned IP addresses must match the clients network. How does the server know which network the client is on? Address requests Relay Agent /22 Net /22 Net DHCP OFFER IP addr = DHCP OFFER IP addr = DHCP Server

27 Relay Agent Operation (1) The Relay Agent inserts the IP address of its own interface into the broadcast request from the client, then forwards the request to a DHCP server Net Backbone DHCP Server Frame: src=00:01:02:03:04:05 dest=ff:ff:ff:ff:ff:ff Frame: src=router_macaddr dest= nexthop_macaddr IP Hdr: DHCP: src= dest= srcmac= IP Hdr: src = dest= DHCP: srcmac = RelayAddr = Options: msg type=1 (Discover) Options: msg type=1 (Discover) 60 Relay Agent Operation (2) The DHCP server uses the RelayAddr value in the DHCP message to derive the client s network address ( ) Net Backbone DHCP Server RelayAddr value indicates which network the client is on. This address is also used to reply. srcmac in DHCP message identifies the client. Frame: src=router_macaddr dest= server_macaddr IP Hdr: src= dest= DHCP: RelayIP = srcmac = Options: msg type=1 (Discover) 61

28 Relay Agent Operation (3) The DHCP server sends a reply addressed to the relay agent, using the RelayAddr as the destination IP address Net Backbone DHCP Server Server sends a unicast reply to the relay agent, using the RelayAddr value. Offered IP address and other data for the DHCP client. Server returns original RelayAddr and srcmac values to assist in relaying. Frame: src=server_macaddr dest= next_hop_macaddr IP Hdr: src= dest= DHCP: RelayIP = OfferIP= srcmac = Options: msg type=2 (Offer) 62 Relay Agent Operation (4) When the relay agent receives a reply with its own IP address, it relays the reply to the client Net Backbone DHCP Server Frame: IP Hdr: src= router-macaddr dest= 00:01:02:03:04:05 src= dest= Agent copies client s hardware address from the srcmac field to the destination field DHCP: RelayIP = OfferIP = srcmac = Options: msg type=2 (Offer) 63

29 What Happens When a Client Moves? If a client moves from one network to another, how does it discover that its old address is no longer valid? Network A Network B DHCP client addresses have a lease time, which is usually several hours or several days or weeks. A computer might move to a new network while its old lease is still valid. File Server WINS Server DHCP Server 64 DHCP Boot Confirmation Each time a DHCP client boots, it broadcasts a DHCP DISCOVER to verify that its current configuration is still valid. Network A Network B Client broadcasts a DHCP DISCOVER packet containing current configuration data. File Server WINS Server NACK DHCP Server Server sees that client s data is not valid for new location and returns a DHCP NACK message. 65

30 DHCP Reconfiguration When a client receives a NACK reply from server it discards old configuration and requests a new address. Network A NACK Network B When the client receives a NACK reply from server, it discards its old DHCP configuration. DHCP Server Client requests a new address and other data using DHCP broadcast. This sequence also applies to case where network addresses or other configuration data change while client is turned off. 66 Other Uses for Boot Confirmation During DHCP Boot Confirmation a client can renew its lease. The server can include new data values in the ACK message. Network A Network B When client broadcasts a DHCP DISCOVER packet, he also requests lease extension. ACK Server includes all configuration data, with new values, in ACK reply. File Server WINS Server DHCP Server 67

31 DHCP Performance Network traffic from DHCP is light. If lease time is 1 day, there would be 8 packets/client/day for DHCP A server handles thousands of clients, according to The DHCP Handbook Memory complexity: a server managing 10,000 client leases would consume up to 20MB of memory for the lease table 68 So, Now the Host Knows Things IP address Mask Gateway router DNS server And can send packets to other IP addresses But, how to learn the MAC address of the destination? 70

32 Sending Packets Over a Link IP packet host host... Web router Adapters only understand MAC addresses Translate the destination IP address to MAC address Encapsulate the IP packet inside a link-level frame 71 Address Resolution Protocol Every node maintains an ARP table (IP address, MAC address) pair Consult the table when sending a packet Map destination IP address to destination MAC address Encapsulate and transmit the data packet But, what if the IP address is not in the table? Sender broadcasts: Who has IP address ? Receiver responds: MAC address D7-FA-20-B0 Sender caches the result in its ARP table Receiver caches the sender info in its ARP table Cache table fills up => Least Recently Used policy used 72

33 ARP Request Argon :a0:24:71:e4:44 Router :e0:f9:23:a8:20 ARP Request: What is the MAC address of ? 73 ARP Reply ARP Reply: The MAC address of is 00:e0:f9:23:a8:20 74

34 ARP Table Use arp a to view the ARP cache on your own machine. Command available in both Unix and Windows 75 ARP Exercise IA A EA IB B EB IR1 ER1 Router R IR2 ER2 ED D ID IC C EC IA, IB, are IP addresses; EA, EB, are MAC addresses. Assume that all ARP tables are initially empty. Give the complete contents of the ARP table for A, B, C, D and R after each of the following operations. 76

35 ARP Exercise (1) IA IB A B EA EB IR1 ER1 Router R IR2 ER2 ED D ID IC C EC 1) Host A sends a packet to host C ARP Table of A ARP Table of C 77 ARP Exercise (2) IA IB A B EA EB IR1 ER1 Router R IR2 ER2 ED D ID IC C EC 2) Host B sends a packet to host D ARP Table of B ARP Table of D ARP Table of R 78

36 ARP Exercise (3) IA IB A B EA EB IR1 ER1 Router R IR2 ER2 ED D ID IC C EC 3) Host D sends a packet to host B 79 Question How could the ARP protocol be used to determine if another host on my network is using my IP address? 80

37 Putting it Together: A Sending a Packet to B How does host A send an IP packet to B ( A R B 82 Basic Steps Host A must learn the IP address of B via DNS Host A uses gateway R to reach external hosts Host A sends the frame to R s MAC address Router R inspects dest. IP address to decide where to send Router R learns B s MAC address and forwards frame A R B 83

38 Host A Learns the IP Address of B Host A does a DNS query to learn B s address Suppose gethostbyname() returns Host A constructs an IP packet to send to B Source , destination A R B 84 Host A Learns the IP Address of B IP header From A: To B: Ethernet frame From A: C-E8-FF-55 To gateway:???? A R B 85

39 Host A Sends Through R Host A has a gateway router R Used to reach destinations outside of /24 Address for R learned via DHCP But, what is the MAC address of the gateway? A R B 86 Host A Sends Packet Through R Host A learns the MAC address of R s interface ARP request: broadcast request for ARP response: R responds with E6-E BB-4B Host A encapsulates the packet and sends to R Host A also caches R s MAC in its ARP table A R B 87

40 Host A Sends Packet Through R IP header From A: To B: Ethernet frame From A: C-E8-FF-55 To R: E6-E BB-4B A R B 88 R Decides how to Forward Packet Router R s adapter receives the packet R extracts the IP packet from the Ethernet frame R sees the IP packet is destined to Router R consults its forwarding table Packet matches /24 via other adapter A R B 89

41 Router R Wants to Forward Packet IP header From A: To B: Ethernet frame From R: 1A-23-F9-CD-06-9B To B:??? A R B 90 R Sends Packet to B Router R s learns the MAC address of host B ARP request: broadcast request for ARP response: B responds with 49-BD-D2-C7-56-2A Router R encapsulates the packet and sends to B Router R also caches B s MAC in its ARP table A R B 91

42 Router R Forwards Packet IP header From A: To B: Ethernet frame From R: 1A-23-F9-CD-06-9B To B: 49-BD-D2-C7-56-2A A R B 92 Question Suppose a rogue computer sends an unsolicited ARP reply message, with its own MAC address and the IP address of the LAN gateway router, to another host on the LAN. What will happen when this host (i.e., the host who believed the bogus ARP reply) transmits an IP packet destined to an external Internet address? 93

43 Question ARP and DNS both depend on caches. ARP cach entry lifetimes are typically 10 mins, while DNS cache is on the order of days. Why this difference? 95 Conclusion Domain Name System Distributed, hierarchical database Distributed collection of servers Caching to improve performance Bootstrapping an end host Dynamic Host Configuration Protocol (DHCP) Address Resolution Protocol (ARP) Next class: Middleboxes Reading: Section 8.4 and Ch. 2 Network Address Translator (NAT) Firewalls 97