Network address translation

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What is Network Address Translation?
For each card, you can individually select what kind of hardware will be presented to the virtual machine. When the show folded, Cole and NBC expressed some optimism about reviving it if a national sponsor could be found, but that never happened. Also on hand are movie stars Wilde, King, and Ray, plugging their own pictures. It is the responsibility of the user to provide vmname. If you are uncomfortable with iptables, you might prefer to stick with the default NAT-based network. It is possible to have more than one such limit. Rather than creating the bridge manually, follow the instructions for Red Hat Enterprise Linux , Fedora , or Debian so that the bridge is created at every boot.

CCNA 200-125

Set up a NAT network

The IP address of a public server is also important, similar in global uniqueness to a postal address or telephone number. Both IP address and port number must be correctly known by all hosts wishing to successfully communicate. Private IP addresses as described in RFC are significant only on private networks where they are used, which is also true for host ports. Ports are unique endpoints of communication on a host, so a connection through the NAT device is maintained by the combined mapping of port and IP address.

PAT Port Address Translation resolves conflicts that would arise through two different hosts using the same source port number to establish unique connections at the same time. A NAT device is similar to a phone system at an office that has one public telephone number and multiple extensions. Outbound phone calls made from the office all appear to come from the same telephone number. However, an incoming call that does not specify an extension cannot be transferred to an individual inside the office.

In this scenario, the office is a private LAN, the main phone number is the public IP address, and the individual extensions are unique port numbers.

With NAT, all communications sent to external hosts actually contain the external IP address and port information of the NAT device instead of internal host IP addresses or port numbers.

NAT only translates IP addresses and ports of its internal hosts, hiding the true endpoint of an internal host on a private network. Typically the NAT device may function as the default gateway for the internal host. However the external host is only aware of the public IP address for the NAT device and the particular port being used to communicate on behalf of a specific internal host.

Hosts behind NAT-enabled routers do not have end-to-end connectivity and cannot participate in some Internet protocols. Services that require the initiation of TCP connections from the outside network, or stateless protocols such as those using UDP , can be disrupted. Unless the NAT router makes a specific effort to support such protocols, incoming packets cannot reach their destination.

Some protocols can accommodate one instance of NAT between participating hosts "passive mode" FTP , for example , sometimes with the assistance of an application-level gateway see below , but fail when both systems are separated from the Internet by NAT.

Use of NAT also complicates tunneling protocols such as IPsec because NAT modifies values in the headers which interfere with the integrity checks done by IPsec and other tunneling protocols. End-to-end connectivity has been a core principle of the Internet, supported for example by the Internet Architecture Board.

Current Internet architectural documents observe that NAT is a violation of the end-to-end principle , but that NAT does have a valid role in careful design. An implementation that only tracks ports can be quickly depleted by internal applications that use multiple simultaneous connections such as an HTTP request for a web page with many embedded objects. Because the internal addresses are all disguised behind one publicly accessible address, it is impossible for external hosts to initiate a connection to a particular internal host without special configuration on the firewall to forward connections to a particular port.

IP packets have a checksum in each packet header, which provides error detection only for the header. IP datagrams may become fragmented and it is necessary for a NAT to reassemble these fragments to allow correct recalculation of higher-level checksums and correct tracking of which packets belong to which connection.

This is not a completely solved problem. One solution is for the receiving NAT to reassemble the entire segment and then recompute a checksum calculated across all packets.

The originating host may perform Maximum transmission unit MTU path discovery to determine the packet size that can be transmitted without fragmentation, and then set the don't fragment DF bit in the appropriate packet header field.

Of course, this is only a one-way solution, because the responding host can send packets of any size, which may be fragmented before reaching the NAT. Destination network address translation DNAT is a technique for transparently changing the destination IP address of an end route packet and performing the inverse function for any replies.

Any router situated between two endpoints can perform this transformation of the packet. DNAT is commonly used to publish a service located in a private network on a publicly accessible IP address. The meaning of the term SNAT varies by vendor. Dynamic NAT, just like static NAT, is not common in smaller networks but is found within larger corporations with complex networks.

NAT loopback , also known as NAT hairpinning or NAT reflection , [11] is a feature in many consumer routers [12] which permits the access of a service via the public IP address from inside the local network. This eliminates the need for using separate domain name resolution for hosts inside the network than for the public network for a website.

If a packet is sent to the public address by a computer at A router with the NAT loopback feature detects that It determines the destination for that packet, based on DNAT port forwarding rules for the destination. If the data were sent to port 80 and a DNAT rule exists for port 80 directed to If no applicable DNAT rule is available, the router drops the packet. If any DNAT rules were present, address translation is still in effect; the router still rewrites the source IP address in the packet.

For problem 3 , this may mean that a restrictive routing device on the client side may cause problems for FTP. The classic example of a problem with this case is the common occurrence where a lengthy download finishes and the client wishes to start another download, but the routing device has timed-out the control connection since no activity took place for 15 minutes.

The client program then locks up waiting for the server to reply to a message it never received because the routing device did not route it to the server. Therefore, all modern FTP clients negotiate with the server on where the data is sent and who initiates the connection. The client program can specify active mode by sending the "PORT" command to instruct that the server should connect back to a specified IP address and port number and then send the data.

Or, a client program can choose passive mode by using the "PASV" command to ask that the server tell the client an IP address and port number that the client can connect to and receive the data.

Since the client connects to the server to establish the control connection, it would seem logical that the client should connect to the server to establish the data connection, which would imply that PASV would be preferred and at the same time eliminate the single biggest problem with FTP and firewalls. Example Sessions Using Active and Passive Data Transfers [ Contents ] At this point it might be helpful to see how the client and server are communicating for each type of data transfer.

The first example is an Active session that logs in anonymously and does a single active data transfer, a directory listing. Note that a directory listings are treated as data transfers just like uploading and downloading of files!

For restrictive firewalls, it is desirable to forbid all incoming connections, so using PORT would cause the connection incoming from the server to fail. Another big problem is that when a client program is using network address translation to hide behind a routing device on an internal network, when using PORT the client tells a server on the external network to connect to an address on the client's internal network. That almost always results in the routing device denying the connection, or the connection to fail completely if the IP address is a RFC compliant reserved address i.

In either case, the client user will typically experience a discarded connection that is very frustrating since the client program will just lock up until the connection is considered permanently timed-out. Using passive mode may not solve the problem if there is a similar restrictive firewall on the server side. A better solution is for the network administrator of the client network to use high-quality network address translation software.

Devices can keep track of FTP data connections, and when a client on a private network uses "PORT" with an internal network address, the device should dynamically rewrite the packet containing the PORT and IP address and change the address so that it refers to the external IP address of the routing device.

The device would then have to route the connection incoming from the remote FTP server back to the internal network address of the client. When the packet containing this PORT reaches the routing device, it should be rewritten like this, assuming the external address is The remote server would then attempt to connect to The routing device in this example would then forward all traffic for this connection to and from the client address at The most common problem is when the firewall the FTP server is behind is strict, i.

The network administrator of the server network can configure the firewall to allow in the entire ephemeral port range. The range of ephemeral ports that need to be opened up is dependent on the configuration of the server machine that is running the FTP server software -- not the ephemeral ports on the firewall! So, find out how the FTP server machine has configured the ephemeral port range whose default range varies with the operating system and then open those ports on the firewall.

Ideally, the firewall should be configured so that only that range of ports is accessible to the FTP server machine. Also double check to be sure that there aren't any other TCP services with port numbers in the ephemeral port range listening on the FTP server machine. The network administrator of the server network can consult the firewall vendor's documentation to see if FTP connections can be dynamically monitored and ports dynamically opened when a passive FTP connection is detected.

This is similar to what intelligent network address translation software can do on the client side for PORT -- the FTP control connections are monitored, and when a packet containing "PASV" from an FTP session is detected, the firewall can automatically open the port.

The firewall would then parse the request and find that the client will be instructed to connect to port on the address The firewall would then add a temporary rule that would allow exactly one connection to port only from the same IP address that the FTP control connection is connected from. Because a server response from PASV includes an IP address and port number, if this IP address corresponds to a private network then the client will not be able to connect to that private address.

From our PASV example above, we have:. The big red circle represents the virtual adapter to which NAT-based virtual machines connect You can see that there are two such virtual machines with IP addresses of The third virtual machine Your email address will not be published. Notify me of follow-up comments by email. Notify me of new posts by email.

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