Proxy Server, Network Address Translator, Firewall

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1 For Summer Training on Computer Networking visit Proxy Server, Network Address Translator, Firewall Prepared by : Swapan Purkait Director Nettech Private Limited

2 Proxy Server

3 Introduction What is a proxy server? Acts on behalf of other clients, and presents requests from other clients to a server. Acts as a server while talking with a client, and as a client while talking with a server. Commonly used HTTP proxy server: Squid available on all platforms.

4 What is it really? It is a server that sits between a client application (Web browser), and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server.

5 Mainly serves two purposes: Improve performance Can dramatically improve performance for a group of users. It saves all the results of requests in a cache. Can greatly conserve bandwidth. Filter requests Prevent users from accessing a specific set of web sites. Prevent users for accessing pages containing some specified strings. Prevent users from accessing video files (say).

6 Anonymous Proxy Servers Hide the user s IP address, thereby preventing unauthorized access to user s computer through the Internet. All requests to the outside world originate with the IP address of the proxy server. Very convenient for group subscription: On-line journals. Digital library.

7 Where it is located? User agent User agent PROXY SERVER Origin server User agent Access Rules Cache

8 Functions of a HTTP Proxy Request forwarding Primary function. Acts as a rudimentary firewall. Access control Allow or deny accesses, based on Contents Location Cache management Efficient utilization of bandwidth. Faster access.

9 Network Address Translator (NAT)

10 What is NAT? Allows a single device (router or a dedicated box) to act as an agent between the Internet (public network) and a local (private) network. Tries to address the IP address distribution problem. RFC Only one unique IP address is required to represent an entire group of computers. Several variations possible.

11 Private Addresses

12 Basic operation of NAT Private network Internet Sourc e = Destination = Sourc e = 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

13 Various Forms of NAT Static NAT Used to map an unregistered IP address to a registered IP address. One-to-one mapping. N registered addresses for N machines. Dynamic NAT Used to map an unregistered IP address to a registered IP address. From a given pool of registered IP addresses. Addresses are assigned dynamically. Any number of internal computers. A limit N to the number communicating at a time.

14 Various Forms of NAT (contd.) Overloading A special form of dynamic NAT. Used to map multiple unregistered IP addresses to a single registered IP address by using different ports. Also called port address translation (PAT). Each computer on the private network gets translated to the same IP address, but with a different port number assignment. Widely used.

15 NAT Overloading. Utilizes the multiplexing feature of TCP/IP stack. A computer maintains several concurrent connections with a remote computer, using different port numbers. The header of a TCP/IP packet contains: Source IP address (32 bit) Source port number (16 bit) Destination IP address (32 bit) Destination port number (16 bit) The combination of above four elements define a TCP/IP connection.

16 Notations: Stub domain: the internal or the private network. Address translation table (ATT): maintained by router/nat for address and port mapping. Easy to implement dynamic NAT. Address translation table need only contain IP address mappings. Private to public, and vice versa. No port numbers needed.

17 How NAT overloading works? The scenario: Internal network has non-routable IP addresses. NAT-enabled router contains a registered IP address assigned by IANA. An internal host X tries to connect to, say, an outside Web server. The router receives the packet from X.

18 The router will now: Save IP address and port number from X s packet to an ATT. In the packet, replace the IP address with the router s IP address. Replace the port number with a port number from the ATT (look for match). For new connection, generate a unique port number.

19 When a packet comes back. Its destination port is used to search ATT. Source IP address and port numbers can be obtained. Addresses changed accordingly.

20 The Address Translation Table (ATT) looks like: Source Computer Source IP address Source port number NAT IP address NAT port number A B C D

21 Capability Limit of a NAT Maximum number of concurrent translations: Mainly determined by the size of the memory to store the ATT. Typical entry in the ATT takes about 160 bits. Memory size of 8 Mbyte will support about 8 x 1024 x 1024 x 8 / 160 = 4,19,000 concurrent translations.

22 Which addresses to use inside? Private address classes. Set aside by IANA an non-routable. These addresses are considered unregistered. Routers discard these addresses, if used as destination. A packet from a host with a private unregistered address can reach a registered destination host, but not the reverse.

23 The Private Address Classes Class A (one) to Class B (sixteen) to Class C (256) to

24 Main uses of NAT Pooling of IP addresses Supporting migration between network service providers IP masquerading Load balancing of servers

25 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.

26 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.

27 Pooling of IP addresses

28 Migration Between 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.

29 Migration to a new network service provider merely requires an update of the NAT device. This migration is not noticeable to the hosts on the network. Note: The difference to the use of NAT with IP address pooling is that in the present case mapping of public and private IP addresses is static.

30 Supporting Migration Sourc e = Destination = Source = Destination = ISP 1 allocates address block /24 to private netw ork: H1 private address: public address: Private network NAT device Source = Destination = ISP 2 allocates address block /24 to private netw ork: Private Address Public Address

31 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.

32 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 / / / /4444

33 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 roundrobin fashion.

34 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

35 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.

36 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.

37 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 private-public network boundary. Some NAT devices inspect the payload of widely used application layer protocols and, if an IP address is detected in the applicationlayer header or the application payload, translate the address according to the address translation table.

38 Other Benefits of NAT Use of NAT automatically creates a firewall between the internal and external networks. NAT will only allow connections that has originated from within the internal network. An outside host cannot initiate a connection with an internal host. Inbound mapping requires static NAT.

39 Is NAT a Proxy Server? The answer is NO. NAT is transparent to both source and destination hosts. But a proxy server is not transparent. NAT is a layer 3 (network) protocol. In contrast, a proxy server works at layer 4 (transport) or higher.

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

41 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

42 Firewall Design

43 Why Firewalls? Firewalls are effective to protect local systems; protect network-based security threats; provide secured and controlled access to Internet; provide restricted and controlled access from the Internet to local servers.

44 Firewall Characteristics Design goals: All traffic from inside to outside must pass through the firewall (physically blocking all access to the local network except via the firewall). Only authorized traffic (defined by the local security police) will be allowed to pass. The firewall itself is immune to penetration (use of trusted system with a secure operating system).

45 Types of Firewalls 1. Packet filters. 2. Application-level gateways. 3. Circuit-level gateways.

46 Packet Filtering Router INTERNET Packet Filtering router PRIVATE NETWORK Some of the attacks that can be made on packet filtering routers: IP address spoofing Source Routing attacks Tiny fragment attacks

47 Packet Filtering Firewall

48 Packet Filtering Router (contd.) Applies a set of rules to each incoming IP packet and then forwards or discards the packet. Typically based on IP addresses and port numbers. Filter packets going in both directions. The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header. Two default policies (discard or forward).

49 Packet Filtering Router (contd.) Advantages: Simplicity Transparency to users High speed Disadvantages: Difficulty of setting up packet filter rules Lack of authentication

50 Application-Level gateway Outside Host Outside connection TELNET FTP SMTP HTTP Inside connection Inside Host Application Level Gateway Also called a Proxy Server; acts as relay of application level traffic. It is service specific.

51 Application Level Gatway

52 Application-level Gateway (contd.) Application-level Gateway Also called proxy server Acts as a relay of application-level traffic Advantages: Higher security than packet filters Only need to scrutinize a few allowable applications Easy to log and audit all incoming traffic Disadvantages: Additional processing overhead on each connection (gateway as splice point)

53 Circuit-Level gateway Outside connection Out In Outside Host Out Out In In Inside connection Circuit Level Gateway Inside Host This can be a standalone system / specialized system. It does not permit an end-to-end TCP connection; rather the gateway sets up two TCP connections. Once the TCP connections are established, the Gateway relays TCP segments from one connection to the other without examining the contents.

54 Circuit Level Gateway

55 Circuit-level Gateway (contd.) Stand-alone system, or specialized function performed by an Application-level Gateway. Sets up two TCP connections: The gateway typically relays TCP segments from one connection to the other without examining the contents. The security function consists of determining which connections will be allowed. Typically use is a situation in which the system administrator trusts the internal users. An example is the SOCKS package.

56 Bastion Host It is a system identified by the firewall administrator as a critical point in the network s security. It executes a secure version of its OS and is trusted. It consists of services which are essential. Requires additional authentication before access is allowed.

57 Firewall Configurations In addition to the use of simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible Three common configurations are in popular use.

58 Screened Host Firewall (Single-homed host) Bastion Host Private Network Hosts Internet Packet filtering router Information Server Private Network Hosts

59 Firewall consists of two systems: A packet-filtering router A bastion host Configuration for the packet-filtering router: Only packets from and to the bastion host are allowed to pass through the router. The bastion host performs authentication and proxy functions.

60 Greater security than single configurations because of two reasons: Implements both packet-level and application-level filtering (allowing for flexibility in defining security policy). An intruder must generally penetrate two separate systems.

61 Screened Host Firewall (dual-homed host) Bastion Host Internet Packet filtering router Information Server Private Network Hosts This configuration physically prevents security breach.

62 The packet-filtering router is not completely compromised. Traffic between the Internet and other hosts on the private network has to flow through the bastion host.

63 Screened Subnet Firewall Bastion Host Internet Outside router Inside router Private Network Modem Information Server

64 Most secure configuration of the three. Two packet-filtering routers are used. Creation of an isolated sub-network. Advantages: Three levels of defense to thwart intruders. The outside router advertises only the existence of the screened subnet to the Internet (internal network is invisible to the Internet). The inside router advertises only the existence of the screened subnet to the internal network. The systems on the inside network cannot construct direct routes to the Internet.

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