Specifications:

The IP layer object will be that same as the IP lab, with the exception of the NAT functionality in the pop method:

• init ( char *name, protocol *higher, protocol *lower, char *addr, int port, int client); The initialization code will be called with the addr field corresponding to your machine's IP address.  You should save this away so that you can send it as the source address in your packets.
• push ( char *buf, int len, char *addr);The push method will
• Look at the len field.  If it is greater than 30 bytes, then you should fragment the segment.  You will send the first packet with the M bit on and only the last packet of the segment will have the M bit off.  Remember that the segment must be fragmented on 8 byte boundaries because of the Offset field.
• pop ( char *buf, int len);When this method is called, you should
• Make sure that your machine is the destination of the packet; if not, process it through the NAT scheme described below.
• Use the ident field to identify which segment it is associated with. If this is a new ident value, allocate a record-keeping structure for this segment.
• See if the M bit is on.  If so, put the fragment in the correct offset in the segment (you will need to declare an array to store the fragments in as they are received).
• If the M bit is not on, then you can determine the total length of the segment (Offset*8+length-HLen*4) to pass up to the next layer.
• When all of the holes (fragments) have been received, pass the segment up to the next layer.
• NAT scheme
• If the destination address does not match your configured IP address for your stack, use the proxy_subnet_mask to determine if the address is in the proxy_input_subnet. The table structure defined in nat.h and declared in main.cc will give you the subnet and mask values.  If there is a match, change the network part of the source IP address to have the proxy_output_src_subnet and the network part of the destination addresses to use the proxy_output_dest_subnet, leaving the host part of the address unchanged.  You will need to recompute the checksum before sending the packet out since the addresses have changed.  You may want to change the values in the table if you are conflicting with other people's labs.  
• If the destination address doesn't match anything that you have configured, discard it.

    struct nat {
      unsigned char proxy_input_subnet[IP_ADDRESS_LEN];
      unsigned char proxy_output_src_subnet[IP_ADDRESS_LEN];
      unsigned char proxy_output_dest_subnet[IP_ADDRESS_LEN];
      unsigned char proxy_subnet_mask[IP_ADDRESS_LEN];
    }
For example, if the following NAT table were compiled into main.cc

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.16	192.168.102.32	192.168.102.240	255.255.255.240
192.168.102.32	192.168.102.16	192.168.102.240	255.255.255.240

• Push the packet back down.
• You should also respond to ARP requests for any address that matches the proxy_input_subnet, responding with your configured ethernet address.  
• The following example should illustrate the process:
  • Your switch is running on cs460-3 (192.168.102.243)
  • From cs460-2 (192.168.102.242) you run "ping -n 192.168.102.17".
  • cs460-2 will then issue a ARP request for 192.168.102.17.  Since this machine (17) does not really exist, it will not respond to the ARP request.
  • Your NAT program will mask (AND) this address (192.168.102.17) with the proxy_subnet_mask (255.255.255.240) to get (192.168.102.16) which is in your NAT table.
  • Your ARP layer should then respond to the ARP request, using the destination protocol address (192.168.102.17) in the request ARP packet as the source protocol address in the response packet your ARP code generates. The source hardware address should be the ethernet address configured in the init routine for your ARP layer. The destination protocol and hardware addresses in the response ARP packet should be identical to the source protocol and hardware addresses in the request ARP packet.
  • Since this is one of your proxy_input_subnets, you will perform the translation.  
    • Change the source address from 192.168.102.242 to 192.168.102.34
      1. source host = (ipaddress AND ( INVERSE proxy_subnet_mask)
      2. source host = 192.168.102.242 & ~255.255.255.240) = 2
      3. add the host to the proxy_output_src_subnet 192.168.102.32 + 2
    • Change the destination address from 192.168.102.17 to 192.168.102.241
    1. dest host = (ipaddress AND ( INVERSE proxy_subnet_mask)
    2. dest host = 192.168.102.17 & ~255.255.255.240) = 1
    3. add the host to the proxy_output_dest_subnet 192.168.102.240 + 1
    • Recalculate the checksum 
    • Push the IP packet back down (this may generate a ARP request for the destination machine)
  • cs460-1 (192.168.102.241) will respond to the ping packet and will send the response back to the source address in the IP packet (192.168.102.34).  
  • Your switch will mask this address with the proxy_subnet_mask and finds that this is host number 2 in subnet 192.168.102.32.  Since this is one of your proxy_input_subnets, you will perform the translation.  
    • Change the source address from 192.168.102.241 to 192.168.102.17
    • Change the destination address from 192.168.102.34 to 192.168.102.242
    • Recalculate the checksum 
    • Push the IP packet back down (this may generate a ARP request for the destination machine)

  You don't need the TA version of the code for the NAT features. If you want an example of how the NAT process works, you can use the lab code to generate an ethereal trace. The TA version of the lab code uses different NAT tables, so dont try the following steps with your own code. The lab code will forward using NAT if you follow the following process

  • DONT TRY THESE STEPS WITH YOUR CODE, IT ONLY WORKS WITH THE TA CODE
  • start up the lab code on cs460-3
  • run "ping -n 192.168.102.66" from cs460-5
  • cs460-6 should respond to the ping and send the response to 128.168.102.67
  • the labcode NAT will then send the response to cs460-5

Tips:

• You will probably want to turn on "Update list of packets in real time" and "Automatic scrolling in live capture" in the ethereal capture window so you can see packets as they arrive. Do not enable name resolution.
• You should use ping -n so that ping doesnt do name resolution.
• Use different IP addresses and subnet masks than those specified in this lab writeup. If you use the same values, other student's code will respond to your arp requests and you will become confused.
In order to keep you from using the same IP addresses as other students in the class, you could use the following values at random.

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.48	192.168.102.64	192.168.102.240	255.255.255.240
192.168.102.64	192.168.102.48	192.168.102.240	255.255.255.240

Ping 192.168.102.50 to get to machine cs460-2

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.80	192.168.102.96	192.168.102.240	255.255.255.240
192.168.102.96	192.168.102.80	192.168.102.240	255.255.255.240

Ping 192.168.102.82 to get to machine cs460-2

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.112	192.168.102.128	192.168.102.240	255.255.255.240
192.168.102.128	192.168.102.112	192.168.102.240	255.255.255.240

Ping 192.168.102.114 to get to machine cs460-2

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.144	192.168.102.160	192.168.102.240	255.255.255.240
192.168.102.160	192.168.102.144	192.168.102.240	255.255.255.240

Ping 192.168.102.146 to get to machine cs460-2

proxy_input_subnet	proxy_output_src_subnet	proxy_output_dest_subnet	proxy_subnet_mask
192.168.102.176	192.168.102.192	192.168.102.240	255.255.255.240
192.168.102.192	192.168.102.176	192.168.102.240	255.255.255.240

Ping 192.168.102.178 to get to machine cs460-2

• if you want to clear the ARP table on a machine run "ifdown eth1; ifup eth1". This will bring the interface down and up and will clear the arp cache
• Please note that if you want this to work using these or different numbers for the nat table IP addresses, you will need to modify the #defined values at the top of main.cc.
• You only need to run one copy of your code as server (with the -s flag).  There is no passoff code for this lab except for the ping command.

Example:

The arguments to the binary are the same as ARP. The arguments are (host where daemon is running[this machine]) (port daemon is listening to[1234]) (src IP address) (src ETH address)  (dst IP address) (dst ETH address) ( -s) (-v optional debug flag)(0xff debug level). The additional arguments were included so that you can run your code without interfering with other students.  Pick IP addresses and Ethernet addresses with the same top bytes as this example, then change the bottom byte to a value between 0 and 200.

It doesnt matter what IP addresses you assign to run the ip layer for this lab. You could use the following line to start your code, but the IP addresses dont matter. The MAC addresses dont matter either, pick something that you think is unique.

On the machine cs460-7 run

./ip cs460-7 1234 192.168.102.7 00:a0:cc:66:99:33 192.168.102.2 00:a0:cc:66:99:35 -s -v 0xff

While you are getting your code working, you will probably want to turn on some serious debug statements.  If you run the lab code with something like "./ip cs460-7 1234 192.168.102.1 00:a0:cc:66:99:33 192.168.102.2 00:a0:cc:66:99:35 -s -v 0xff" it will turn on a lot of debug in the lower layers.  Bit 0 turns on the physical layer printfs.  Bit 1 (0x2) turns on the ethernet debug.  Bit 2 (0x4) turns on a printf when something is not destined for your ethernet address.  Bit 3 (0x8) turns on the basic arp debug.  Bit 4 (0x10) just shows what is being put into the arp table.

Other Changes:

In addition to changing internet.cc, you will need to change your arp.cc to make NAT work.  You should respond to the ARP if any of the proxy_input_subnet values match the dst address of the packet masked with the subnet mask.  You should respond with the same ethernet address you would for a normal ARP.

Passoff:

The TA will ping one of the machines in your NAT table subnet (proxy_input_subnet) from one of the real machines in the lab.  If all goes well, the ping should be successful and the response should proceed through your NAT table as well.  In essence, ping should not be able to detect that you are doing the translation.   Be aware that the TA may ping the selected machine using either(or both) of your subnets (i.e. test your code to make sure you can use all the entries in your NAT Table)
 

The following passoff levels will apply to this lab:
 

Passoff Level	Behavior	Points
Minimal Passoff	The ping packet reaches the correct machine	2 Points
NAT	Ping does not notice your NAT presence	8 Points