More Internet Support Protocols

Transcription

1 Domain Name System (DNS) Ch 2.5 More Internet Support Protocols Problem statement: Average brain can easily remember 7 digits On average, IP addresses have digits We need an easier way to remember IP addresses Solution: Use alphanumeric names to refer to hosts Add a distributed, hierarchical protocol (called DNS) to map between alphanumeric host names and IP addresses We call this Address Resolution Domain Name Space Domain Name Service yahoo com edu net gov int mil org ae... us... zw cnn rutgers cs yale eng Generic Domains Country Domains The domain name service consists of Domain name space Name servers In each zone, there is a primary name server and one or more secondary name servers Name servers contain two kinds of address mappings: Authoritative mappings: For hosts within the zone Cached mappings: For previously requested mappings to hosts not in the zone Resolvers Programs that extract information from name servers in response to client requests

2 Domain Name Hierarchy DNS Protocol yahoo com edu net gov int mil org ae... us... zw cnn rutgers cs yale eng When client wants to know an IP address for a host name Client sends a DNS query to the primary name server in its zone If name server contains the mapping, it returns the IP address to the client Otherwise, the name server forwards the request to the root name server The request works its way down the tree toward the host until it reaches a name server with the correct mapping DNS Protocol Example DNS Protocol Another Example remus.rutgers.edu remus.rutgers.edu Scenario: 1 8 Scenario: 1 2 remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu using a recursive query ns-lcsr.rutgers.edu 2 7 remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu using an iterative query 3 ns-lcsr.rutgers.edu 4 a.root-servers.net a.root-servers.net yale.edu yale.edu cs.yale.edu cs.yale.edu

3 DNS message DNS Message Header In DNS, all communications use a single format called a message. The top level format of message is divided into 5 sections (some of which are empty in certain cases) RR( Resource record) Header Question Answer Authority Additional the question for the name server RRs answering the question RRs pointing toward an authority RRs holding additional information The answer section contains RRs that answer the question; the authority section contains RRs that point toward an authoritative name server; the additional records section contains RRs which relate to the query, but are not strictly answers for the question. Clients communicate with DNS servers using either TCP or UDP on port Transaction Identification Flags Number of Questions Number of Answer RRs Number of Authoritative RRs Number of Additional RRs Questions Answer Resource Records Authoritative Resource Records Additional Resource Records DNS Message Fields DNS Packet Fields (cont d) Transaction Identification: Random number used to match client queries with name server responses Flags: QR opcode AA TC RD RA (unused) rcode QR: 0=Query, 1=Response opcode: 0=standard query, 1=inverse query, 2=status request AA: Authoritative answer TC: Truncated DNS packet RD: Recursion desired RA: Recursion available rcode: Return code. 0=no error, 3=name error Transaction Identification: Random number used to match client queries with name server responses Number of Questions: Number of DNS queries in the packet Number of Answer RRs: Number of non-authoritative DNS responses in the packet Number of Authoritative RRs: Number of authoritative DNS responses in the packet Number of Additional RRs: Number of other DNS responses in the packet (usually contains other DNS servers in domain) Questions & Answers: Variable length fields to store DNS queries and DNS server responses

4 DNS Queries Encoding Query Names DNS Packet Question field contains a sequence of queries: Query Type Query name Query Class Query Name: Contains an encoded form of the name for which we are seeking an IP address Query Type: 1=IP address, 2=name server, 12=pointer record, etc. Query Class: 1=Internet address (IN) DNS queries must be encoded in a special way Divide host address into segments whenever a period appears For each segment, store a byte representing the length of the segment followed by the letters in the segment Store a zero byte at the end of the query DNS Responses DNS Caching DNS Packet RR fields contain a sequence of resource records: Type Resource data length Domain name Time-to-live Class Resource Data Domain Name: Encoded domain name for query Type & Class: Same as for query (1=IP; 1=Internet) Time-to-Live: How long this responses will be useful Resource Data: Contains the four-byte IP address Going to the root server and then down the tree every time we need to resolve an address is inefficient Introduce address caching at name servers Store host-to-ip-address mappings from recently requested host names at name server When the same address is requested later, use the cached version at the local name server instead of recursively querying other name servers again

5 DNS Caching Example DHCP (Ch 4.4.6) First time: remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu using a recursive query remus.rutgers.edu 1 8 ns-lcsr.rutgers.edu 2 7 a.root-servers.net 3 6 yale.edu Later: venus.cs.yale.edu has been cached at ns-lcsr. remus.rutgers.edu (and any other host that uses ns-lcsr) will receive the cached IP address for venus.cs.yale.edu remus.rutgers.edu 1 2 ns-lcsr.rutgers.edu DHCP stands for dynamic host configuration protocol DHCP is client-server DHCP offers a number of more features Dynamic IP address allocation IP addresses can be leased for a certain time Useful where there are a limited number of IP addresses Useful for temporary connections (testing, laptops, mobile networks) 4 5 cs.yale.edu DHCP (cont d) Address Allocation Modes DHCP has two components: A protocol for delivering bootstrap information from the server to the clients An algorithm for dynamically assigning addresses to clients DHCP supports three modes of allocation Automatic allocation: Server assigns a permanent address to a host Dynamic allocation: Server assigns a host an IP address with a finite lease Manual allocation: Server assigns host an IP address chosen by the network administrator

6 DHCP Packets (cont d) Hardware address Request/Reply Hardware type length in bytes Hop count Transaction ID Number of seconds Flags Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server hostname (64 bytes) Boot filename (128 bytes) Definitions of address fields ciaddr Client IP address; only filled in if client is in BOUND, RENEW or REBINDING state and can respond to ARP requests. yiaddr 'your' (client) IP address. siaddr 4 IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK by server. giaddr Relay agent IP address, used in booting via a relay agent. chaddr Client hardware address. Options (312+ bytes) DHCP Packet Fields DHCP Message types All fields are same as BOOTP except: Flags: One flag currently defined Broadcast (bit 0): Clients can request that all DHCP server messages be broadcast to it Options: All DHCP packets must use the DHCP message type option, which defines the type of DHCP message being sent: DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK DHCPNAK etc. DHCP message types DHCP Discover: Client broadcasts to locate a server DHCP Offer: Server responds with proposal of parameters DHCP Request: Client broadcasts its choice of server. All other servers are implicitly declined. DHCP ACK: Selected server responds to client with address DHCP NAK: Selected server rejects the client s request DHCP Decline: Client declines server s parameters DHCP Release: Client releases its assigned address

7 DHCP Protocol Server 1 Client Server 2 DHCPDISCOVER DHCPOFFER DHCPREQUEST Collects replies Selects server 2 DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK DHCP Protocol (cont d) DHCP client broadcasts a DHCP Discover message Client may specify preference of a lease and/or IP address Many servers may respond with offers Client chooses one server from them Client broadcasts DHCP request with id of chosen server Selected server sends DHCP ACK or NAK Client begins using offered IP address once it receives ACK If the client finds a problem, it sends a DHCP Decline message to the server and starts over again Client may choose to release the address before lease expires by sending a DHCP Release message to the server DHCP Relay Agents Summary Similar to BOOTP Relay Agents DHCP relay agents allow DHCP servers to handle requests from other subnets DHCP allow ignorant hosts to receive IP addresses (and more) at start-up time IP addresses don t have to be manually configured into hosts Client DHCP Relay Agent IP Gateway Router IP Gateway Router DHCP Server

8 Network address Translators (NAT) Ch Private IP addreses Every host needs an IP address IPv4 address space is limited It is expensive to get an IP address for every device that may be connected to the internet IETF has set aside private IP address for use within a network but can be translated into a fixed public address by a special router NAT box (10/8 prefix) (172.16/12 prefix) ( /16 prefix) These addresses can be assigned to any of the machines within a network but will be translated to a public address by the NAT router Ports are used to distinguish among multiple addresses that need to be mapped from one public address to mutiple private addresses NAT Disadvantages NAT provides mapping functions between public address and a private address Keep a table of internal addresses/ports and external hosts/ports contacted from the internals (we can map multiple internals to a single public address as long as they're coming from distinct ports) / / / / / /8004 It is a hack Works only with TCP/UPD port connections NAT has to understand all higher layer application protocols to correctly map the port Every packet needs to be remapped Widespread deployment of IPv6 should help Private LAN side Public WAN side

9 Mobile-IP Chapter 4.9 Mobile users Explosion in usage of hand helds Anytime, anywhere wireless services Some connectivity everywhere Many-time, many-where (Infostations) Users can be connected when moving Users can be connect and disconnect to different networks Mobility vs connectivity IP address problem New research problems Continuous connectivity for a mobile host Seamless movement between networks Mobile systems Move from place to place while being wireless Move from place to place by plugging-in at different attachment points Why maintain connectivity? Avoid restarting applications/networks Internet hosts/interfaces are identified by IP address Domain name service translates host name to IP address IP address identifies host/interface and locates its network Mixes naming and location Moving to another network requires different network address But this would change the host s identity How can we still reach that host?

10 Basic idea Basic idea Home Agent = correspondent HOST Foreign Agent = Mobile Host Mobile hosts attaches to foreign network and obtains guest address Via DHCP Via Foreign agent Registration with local agent LA has list of all foreign hosts visiting the network Routing for mobile hosts Use Arp = mobile host A designated router proxy-arps for mobile host = correspondent host How to direct packets to moving hosts transparently? H4 I have 1 Who has 1? Know? mh1@h4 1

11 Basic Mobile IP to mobile hosts = mobile host = correspondent host = home agent FA = foreign agent (We ll see later that FA is not necessary or even desirable) FA registers new care-of address (FA) with tunnels packets to FA FA decapsulates packets and delivers them to IP-in-IP (Packet encapsulation) Packet from to Source address = address of Destination address = home IP address of Payload Home agent intercepts above packet and tunnels it Source address = address of Destination address = care-of address of Source address = address of Destination address = home IP address of Original payload When mobile host moves again #1 FA #1 FA #2 #2 registers new address (FA #2) with & FA #1 tunnels packets to FA #2, which delivers them to Packets in flight can be forwarded from FA #1 to FA #2 Basic Mobile IP - from mobile hosts Mobile hosts also send packets FA Mobile host uses its home IP address as source address -Lower latency as can send packets directly to -Still transparent to correspondent host This is called a triangle route or a dog-leg route

12 Problems with Foreign Agents Assumption of support from foreign networks A foreign agent exists in all networks you visit The foreign agent is robust and up and running The foreign agent is trustworthy Solution Mobile host is responsible for itself -(With help from infrastructure in its home network) -Mobile host decapsulates packets -Mobile host sends its own packets - Co-located FA on must acquire its own IP address in foreign network This address is its new care-of address Mobile IP spec allows for this option Obtaining a foreign IP address Problems with ingress/egress filtering How to get a new IP address? DHCP Dynamic IP address binding like some dialup services Mobile host uses its home IP address as source address Security-conscious boundary routers will drop this packet An egress router will see a packet with source address that does not belonging to its network

13 Solution: bi-directional tunnel Solution: yet more flexibility Provide choice of safe route through home agent both ways Use encapsulation in both directions ( and ) Use current care-of address and send packet directly -This is regular IP! -This is not mobility but portability Do we need Mobile IP When do we really need this Mobile clients have short lived sessions Reconnect on move Most mobile users are in private net Mobile servers? Cellphone (IP enabled)