Dynamic Host Configuration Protocol (DHCP) 02 NAT and DHCP Tópicos Avançados de Redes

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1 Dynamic Host Configuration Protocol (DHCP) 1 1

2 Dynamic Assignment of IP addresses Dynamic assignment of IP addresses is desirable for several reasons: IP addresses are assigned on-demand Avoid manual IP configuration Support mobility of laptops 2 2

3 Solutions for dynamic assignment of IP addresses Reverse Address Resolution Protocol (RARP) Works similar to ARP Broadcast a request for the IP address associated with a given MAC address RARP server responds with an IP address Only assigns IP address (not the default router and subnetmask) IP address (32 bit) ARP RARP Ethernet MAC address (48 bit) 3 3

4 BOOTP BOOTstrap Protocol (BOOTP) From 1985 Host can configure its IP parameters at boot time. 3 services. IP address assignment. Detection of the IP address for a serving machine. The name of a file to be loaded and executed by the client machine (boot file name) Not only assign IP address, but also default router, network mask, etc. Sent as UDP messages (UDP Port 67 (server) and 68 (host)) Use limited broadcast address ( ): These addresses are never forwarded 4 4

5 DHCP Dynamic Host Configuration Protocol (DHCP) From 1993 An extension of BOOTP, very similar to DHCP Same port numbers as BOOTP Extensions: Supports temporary allocation ( leases ) of IP addresses DHCP client can acquire all IP configuration parameters needed to operate DHCP is the preferred mechanism for dynamic assignment of IP addresses DHCP can interoperate with BOOTP clients. 5 5

6 BOOTP Interaction Argon 00:a0:24:71:e4:44 BOOTP Request 00:a0:24:71:e4:44 Sent to (a) BOOTP Server Argon :a0:24:71:e4:44 (b) BOOTP Response: IP address: Server IP address: Boot file name: filename DHCP Server Argon :a0:24:71:e4:44 TFTP filename (c) DHCP Server BOOTP can be used for downloading memory image for diskless workstations Assignment of IP addresses to hosts is static 6 6

7 DHCP Interaction (simplified) Argon 00:a0:24:71:e4:44 DHCP Server DHCP Request 00:a0:24:71:e4:44 Sent to Argon :a0:24:71:e4:44 DHCP Response: IP address: Default gateway: Netmask: DHCP Server 7 7

8 BOOTP/DHCP Message Format OpCode Number of Seconds Hardware Type Hardware Address Hop Count Length Unused (in BOOTP) Flags (in DHCP) Transaction ID Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server host name (64 bytes) Boot file name (128 bytes) Options (There are >100 different options) 8 8

9 BOOTP/DHCP OpCode: 1 (Request), 2(Reply) Note: DHCP message type is sent in an option Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to boot Client IP address, Your IP address, server IP address, Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank 9 9

10 DHCP Message Type Message type is sent as an option. Value Message Type 1 DHCPDISCOVER 2 DHCPOFFER 3 DHCPREQUEST 4 DHCPDECLINE 5 DHCPACK 6 DHCPNAK 7 DHCPRELEASE 8 DHCPINFORM 10 10

11 Other options (selection) Other DHCP information that is sent as an option: Subnet Mask, Name Server, Hostname, Domain Name, Forward On/Off, Default IP TTL, Broadcast Address, Static Route, Ethernet Encapsulation, X Window Manager, X Window Font, DHCP Msg Type, DHCP Renewal Time, DHCP Rebinding, Time SMTP-Server, SMTP-Server, Client FQDN, Printer Name, 11 11

12 DHCP Operation DHCP Client 00:a0:24:71:e4:44 DHCP Server DCHP DISCOVER DHCPDISCOVER Sent to DHCP Server DHCP Client 00:a0:24:71:e4:44 DHCPOFFER DHCP Server DCHP OFFER DHCPOFFER DHCP Server 12 12

13 DHCP Operation DCHP DISCOVER DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCPREQUEST At this time, the DHCP client can start to use the IP address DHCPACK DHCP Server Renewing a Lease (sent when 50% of lease has expired) If DHCP server sends DHCPNACK, then address is released. DHCP Client 00:a0:24:71:e4:44 DHCPREQUEST DHCPACK DHCP Server DHCP Server 13 13

14 DHCP Operation DHCP Client 00:a0:24:71:e4:44 DHCP Server DCHP RELEASE DHCPRELEASE At this time, the DHCP client has released the IP address DHCP Server 14 14

15 NAT Network Address Translation 15 15

16 Private Network Private IP network is an IP network that is not directly connected to the Internet IP addresses in a private network can be assigned arbitrarily. Not registered and not guaranteed to be globally unique Generally, private networks use addresses from the following experimental address ranges (non-routable addresses):

17 Private Addresses H1 H2 H3 H Private network Private network 1 R1 Internet R H

18 Network Address Translation (NAT) NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are replaced at the boundary of a private network NAT is a method that enables hosts on private networks to communicate with hosts on the Internet NAT is run on routers that connect private networks to the public Internet, to replace the IP address-port pair of an IP packet with another IP address-port pair

19 Basic operation of NAT Private network Internet Source = Destination = Source = Destination = private address: public address: NAT device public address: H1 Source = Destination = Source = Destination = H5 Private Address Public Address NAT device has address translation table 19 19

20 Pooling of IP addresses Scenario: Corporate network has many hosts but only a small number of public IP addresses NAT solution: Corporate network is managed with a private address space NAT device, located at the boundary between the corporate network and the public Internet, manages a pool of public IP addresses When a host from the corporate network sends an IP datagram to a host in the public Internet, the NAT device picks a public IP address from the address pool, and binds this address to the private address of the host 20 20

21 Pooling of IP addresses Private network Internet H1 Source = Destination = private address: public address: NAT device Source = Destination = public address: H5 Private Address Public Address Pool of addresses:

22 Supporting migration between network service providers Scenario: In CIDR, the IP addresses in a corporate network are obtained from the service provider. Changing the service provider requires changing all IP addresses in the network. NAT solution: Assign private addresses to the hosts of the corporate network NAT device has static address translation entries which bind the private address of a host to the public address. Migration to a new network service provider merely requires an update of the NAT device. The migration is not noticeable to the hosts on the network. Note: The difference to the use of NAT with IP address pooling is that the mapping of public and private IP addresses is static

23 Supporting migration between network service providers Source = Destination = Source = Destination = ISP 1 allocates address block /24 to private network: H1 private address: public address: Private network NAT device Source = Destination = ISP 2 allocates address block /24 to private network: Private Address Public Address

24 IP masquerading Also called: Network address and port translation (NAPT), port address translation (PAT). Scenario: Single public IP address is mapped to multiple hosts in a private network. NAT solution: Assign private addresses to the hosts of the corporate network NAT device modifies the port numbers for outgoing traffic 24 24

25 IP masquerading Source = Source port = 2001 Source = Source port = 2100 private address: H1 Private network NAT device Internet private address: H2 Source = Source port = 3020 Source = Destination = 4444 Private Address Public Address / / / /

26 Load balancing of servers Scenario: Balance the load on a set of identical servers, which are accessible from a single IP address NAT solution: Here, the servers are assigned private addresses NAT device acts as a proxy for requests to the server from the public network The NAT device changes the destination IP address of arriving packets to one of the private addresses for a server A sensible strategy for balancing the load of the servers is to assign the addresses of the servers in a round-robin fashion

27 Load balancing of servers Private network Source = Destination = S Source = Destination = NAT device Source = Destination = Internet S2 S Source = Destination = Private Address Inside network Public Address Outside network Public Address

28 Concerns about NAT Performance: Modifying the IP header by changing the IP address requires that NAT boxes recalculate the IP header checksum Modifying port number requires that NAT boxes recalculate TCP checksum Fragmentation Care must be taken that a datagram that is fragmented before it reaches the NAT device, is not assigned a different IP address or different port numbers for each of the fragments

29 Concerns about NAT End-to-end connectivity: NAT destroys universal end-to-end reachability of hosts on the Internet. A host in the public Internet often cannot initiate communication to a host in a private network. The problem is worse, when two hosts that are in a private network need to communicate with each other

30 Concerns about NAT IP address in application data: Applications that carry IP addresses in the payload of the application data generally do not work across a privatepublic network boundary. Some NAT devices inspect the payload of widely used application layer protocols and, if an IP address is detected in the application-layer header or the application payload, translate the address according to the address translation table

31 Configuring NAT in Linux Linux uses the Netfilter/iptable package to add filtering rules To application From application to the IP module filter INPUT nat OUTPUT Yes Destination is local? No filter FORWARD filter OUTPUT nat PREROUTING (DNAT) nat POSTROUTING (SNAT) Incoming datagram Outgoing datagram 31 31

32 Configuring NAT with iptable First example: iptables t nat A POSTROUTING s j SNAT --to-source Pooling of IP addresses: iptables t nat A POSTROUTING s /24 j SNAT --to-source ISP migration: iptables t nat R POSTROUTING s /24 j SNAT --to-source IP masquerading: iptables t nat A POSTROUTING s /24 o eth1 j MASQUERADE Load balancing: iptables -t nat -A PREROUTING -i eth1 -j DNAT --todestination

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