coms w4995-1 coms w4995-1 lecture 8. nat, dhcp & firewalls
TRANSCRIPT
Outline
Network Address Translation (NAT)
Dynamic Host Configuration Protocol (DHCP)
Firewalls
Typical Application Domains where we use them
A hack to fix the IP address depletion problem. NAT is a router function where IP addresses (and possibly port
numbers) of IP datagrams are replaced at the boundary of a private network.
Breaks the End-to-End argument. But it became a standard: RFC 1631 - The IP Network Address
Translator (NAT)
Provides a form security by acting as a firewall home users. Small companies.
Network Address Translation: a hack
Is there any other solution to the IP address problem?
Basic operation of NAT
NAT device stores the address and port translation tables In the this example we mapped only addresses.
•Host
•private address: 10.0.1.2•public address: 128.143.71.21
Public Host
•Private Network •Internet
64.236.24.4
NATDevice
PrivateAddress
PublicAddress
10.0.1.1 128.59.16.21
•Source = 10.0.1.2•Destination = 64.236.24.4
•Source = 10.0.1.2•Destination = 64.236.24.4•Source = 128.143.71.21•Destination = 64.236.24.4
•Source = 128.143.71.21•Destination = 64.236.24.4
•Source = 64.236.24.4•Destination = 128.59.16.21
•Source = 64.236.24.4•Destination = 10.0.0.2
•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 10.0.0.2
Private Network
Private IP network is an IP network with Private IP Addresses (Can it be connected directly to the Internet?)
IP addresses in a private network can be assigned arbitrarily but they are usually picked from the reserved pool (can we use any?) Not registered and not guaranteed to be globally unique Question: how is public IP address assigned?
Generally, private networks use addresses from the following experimental address ranges (non-routable addresses): 10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255
Main uses of NAT
Pooling of IP addresses
Supporting migration between network service providers
IP masquerading and internal firewall
Load balancing of servers
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.
Pooling of IP addresses
•Host
•private address: 10.0.1.2•public address: 128.143.71.21
•Private Network •Internet
Public Host64.236.24.4
NATDevice
PrivateAddress
PublicAddress
10.0.1.2 128.59.16.21
•Source = 10.0.1.2•Destination = 64.236.24.4
•Source = 10.0.1.2•Destination = 64.236.24.4•Source = 128.143.71.21•Destination = 64.236.24.4
•Source = 128.143.71.21•Destination = 64.236.24.4
Supporting migration between network service providers
Scenario: In practice (using 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 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.
Supporting migration between network service providers
Host
private address: 10.0.1.2public address: 128.14.71.21
Source = 10.0.1.2Destination = 213.168.112.3
NAT device
Private Address
PublicAddress
10.0.1.2 128.14.71.21
128.14.71.21
Source = 128.14.71.21Destination = 213.168.112.3
Private network
ISP 1allocates address
block 128.14.71.0/24 to private network:
Supporting migration between network service providers
Host
private address: 10.0.1.2public address: 128.14.71.21
150.140.4.120
Source = 10.0.1.2Destination = 213.168.112.3
NAT device
Private Address
PublicAddress
10.0.1.2128.14.71.21150.140.4.120
128.14.71.21150.140.4.120
Source = 150.140.4.120Destination = 213.168.112.3
ISP 2allocates address block
150.140.4.0/24 to private network:
Private network
ISP 1allocates address
block 128.14.71.0/24 to private network:
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
IP masquerading
NAT device
Host 2
private address: 10.0.1.2
Private network
Source = 10.0.1.2Source port = 2001
Source = 128.59.71.21Source port = 80
Private Address
PublicAddress
10.0.1.2/2001 128.143.71.21/80
10.0.1.3/3020 128.143.71.21/4444
Host 1
private address: 10.0.1.3
Source = 10.0.1.3Source port = 3020
Internet
Source = 128.59.71.21Destination = 4444
128.16.71.2110.0.0.1
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.
Load balancing of servers
Private network
Source = 101.248.22.3Destination = 128.16.71.21
NAT device
Private Address
PublicAddress
10.0.1.2 128.59.71.21
Inside network
10.0.1.4 128.59.71.21
Internet128.59.71.21
S1
S2
S3
10.0.1.4
10.0.1.3
10.0.1.2
PublicAddress
64.30.4.120
Outside network
101.248.22.3
Source = 64.30.4.120Destination = 128.16.71.21
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.
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.
NAT and FTP
H1 H2
public address:128.143.72.21
FTP client FTP server
PORT 128.143.72.21/1027
200 PORT command successful
public address:128.195.4.120
RETR myfile
150 Opening data connection
establish data connection
Normal FTP operation
NAT and FTP
NAT device with FTP support
H1
Private network
NATdevice
H2
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PORT 10.0.1.3/1027 PORT 128.143.72.21/1027
200 PORT command successful200 PORT command successful
RETR myfile
establish data connection
RETR myfile
150 Opening data connection150 Opening data connection
establish data connection
NAT and FTP
FTP in passive mode and NAT.
H1
Private network
NATdevice
H2
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PASV PASV
Entering Passive Mode128.195.4.120/10001
Entering Passive Mode128.195.4.120/10001
public address:128.195.4.120
Establish data connection Establish data connection
Configuring NAT in Linux
Linux uses the Netfilter/iptable Kernel package
Incomingdatagram
filterINPUT
Destinationis local?
filterFORW ARD
natOUTPUT
To application From application
Outgoingdatagram
natPOSTROUTING
(SNAT)
No
Yes filterOUTPUT
natPREROUTING
(DNAT)
Configuring NAT with iptable
First example:iptables –t nat –A POSTROUTING –s 10.0.1.2 –j SNAT --to-source 128.16.71.21
Pooling of IP addresses:iptables –t nat –A POSTROUTING –s 10.0.1.0/24 –j SNAT --to-source 128.16.71.0–128.16.71.30
IP masquerading:
iptables –t nat –A POSTROUTING –s 10.0.1.0/24 –o eth1 –j MASQUERADE
Load balancing:
iptables -t nat -A PREROUTING -i eth1 -j DNAT --to-destination 10.0.1.2-10.0.1.4
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 Wireless networking and Home NATs
No static IP means that we have to depend on DNS for the packet routing Use of a DDNS (Dynamic DNS entry) Free sites for that service in the internet
Dynamic Host Configuration Protocol (DHCP)
Designed in 1993
Requires a server and free IP address space
Supports temporary allocation (“leases”) of IP addresses
DHCP client can acquire all IP configuration parameters
Any potential security risks?
Can we use something that can prevent unauthorized users?
DHCP Interaction (simplified)
Argon 00:a0:24:71:e4:44 DHCP Server
DHCP Request 00:a0:24:71:e4:44Sent to 255.255.255.255
Argon 128.16.23.144
00:a0:24:71:e4:44 DHCP ServerDHCP Response: IP address: 128.16.23.144Default gateway: 128.16.23.1Netmask: 255.255.0.0
DHCP Message Format
Number of Seconds
OpCode Hardware Type
Your IP address
Unused (in BOOTP)Flags (in DHCP)
Gateway IP address
Client IP address
Server IP address
Hardware Address Length
Hop Count
Server host name (64 bytes)
Client hardware address (16 bytes)
Boot file name (128 bytes)
Transaction ID
Options
(There are >100 different options)
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
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
DHCP operations
Src: 0.0.0.0, 68Dest: 255.255.255.255, 67DHCPDISCOVERYYiaddr: 0.0.0.0Transaction ID: 654
Src:128.195.31.1, 67
DHCPOFFERYiaddr: 128.59.20.147Transaction ID: 654
Dest: 255.255.255.255, 68
Lifetime: 3600 secsServer ID: 128.59.18.1
Src: 0.0.0.0, 68Dest: 255.255.255.255, 67DHCPREQUESTYiaddr: 128.59.20.147Transaction ID: 655server ID: 128.195.31.1Lifetime: 3600 secs
Src:128.59.18.1, 67
DHCPACKYiaddr: 128.59.20.147Transaction ID: 655
Dest: 255.255.255.255, 68
Lifetime: 3600 secsServer ID: 128.59.18.1
DHCP operations
More on DHCP operations
A client may receive DCHP offers from multiple servers
The DHCPREQUEST message accepts offers from one server.
Other servers who receive this message considers it as a decline
A client can use its address after receiving DHCPACK
DHCP replies can be unicast, depending on implmentation
DHCP relay agent
DHCPDISCOVERGiaddr: 0
Src: 0.0.0.0., 68Dest: 255.255.255.255, 67
128.16.31.1 128.16.41.1
DHCPDISCOVERGiaddr: 128.16.41.1
Src: 0.0.0.0., 68Dest: 255.255.255.255, 67
DHCPOFFER
……
Giaddr: 128.16.41.1
Src: 128.16.31.10, 67Dest: 128.16.41.1, 67
DHCPOFFER
……
Giaddr: 128.16.41.1
Src: 128.16.41.1, 67Dest: 255.255.255.255, 68
128.16.31.10
History of DHCP
Three Protocols: RARP (until 1985, no longer used) BOOTP (1985-1993) DHCP (since 1993) Secure DHCP – not a standard yet…
Only DHCP is widely used today.
Solutions for dynamic assignment of IP addresses
Reverse Address Resolution Protocol (RARP) RARP is no longer used 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)
RARP
Ethernet MACaddress(48 bit)
ARPIP address(32 bit)
BOOTP
BOOTstrap Protocol (BOOTP) 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 assigns IP address, but also default router, network mask, etc.
Sent as UDP messages (UDP Port 67 (server) and 68 (host))
Use limited broadcast address (255.255.255.255): These addresses are never forwarded
BOOTP Interaction
BOOTP can be used for downloading memory image for diskless workstations
Assignment of IP addresses to hosts is static
Argon00:a0:24:71:e4:44 BOOTP Server
BOOTP Request00:a0:24:71:e4:44Sent to 255.255.255.255
Argon128.143.137.144
00:a0:24:71:e4:44 DHCP ServerBOOTP Response:IP address: 128.143.137.144Server IP address: 128.143.137.100Boot file name: filename
(a) (b)
Argon128.143.137.14400:a0:24:71:e4:44 DHCP Server
128.143.137.100
TFTP“filename”
(c)
Lab errata
In Figure 7.1, the private network interface of Router2 should be labeled with IP address "10.0.1.1/24" (instead of 10.0.0.1/24).