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Full Discussion: Routing ICMP echo requests
Special Forums IP Networking Routing ICMP echo requests Post 302949421 by dschlic1 on Friday 10th of July 2015 12:04:12 PM
Old 07-10-2015
Routing ICMP echo requests

I have an application where I need to configure a router to pass through ping requests (ICMP type Image through to the LAN port. I have a Linksys WRT54GS with tiny DD-WRT V24 SP2 installed. I am adding the following iptables rules:
Code:
iptables -t nat -I PREROUTING -p icmp --icmp-type 8 -s 72.64.140.50 -j DNAT --to-destination 192.168.1.200 

iptables -t filter -I FORWARD -p icmp --icmp-type 8 -s 72.64.140.50 -d 192.168.1.200 -j ACCEPT 

iptables -t nat -I POSTROUTING -p icmp --icmp-type 8 -s 72.64.140.50 -d 192.168.1.200 -j ACCEPT

The intent is that the first rule will change an incoming echo request destination to the unit on the LAN which I want to receive the ping request.

The second rule allows the modified echo request to pass through the FORWARD table.

And the last one allows the modified echo request to pass through the POSTROUTING table.

When I send a ping to the router with four tries, I get no pings out the LAN. Using iptables -L -v -n I can see were rule #1 passes one packet (but not four), rule #2 passes four packets (good!) and rule #3 passes 1 packet.

At this point I am at loss as to why this is not working. Can someone help me out here?

Last edited by Don Cragun; 07-10-2015 at 04:56 PM.. Reason: Add CODE tags.
 

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ping(8) 						      System Manager's Manual							   ping(8)

Name
       ping - send ICMP ECHO_REQUEST packets to network hosts

Syntax
       /etc/ping [ options ] host [ datasize [ npackets ]]

Description
       The  DARPA  Internet  is  a large and complex network of hardware connected together by gateways.  The command utilizes the ICMP protocol's
       mandatory ECHO_REQUEST datagram to elicit an ICMP ECHO_RESPONSE from a host or gateway.	ECHO_REQUEST datagrams (pings) have an IP and ICMP
       header,	followed  by  a  struct timeval, and then an arbitrary number of pad bytes used to fill out the packet.  The length of the default
       datagram 64 bytes, but this may be changed using the command-line option.

       Typing ``ping host'' without any options will either report ``host is alive'' or ``no answer from host''.  To get more statistics  use  the
       -l option or one of the other options.

       When  using  for  fault	isolation,  it should first be run on the local host to verify that the local network interface is up and running.
       Then, hosts and gateways further and further away should be pinged.  The command with options sends one datagram per second and prints  one
       line  of  output  for every ECHO_RESPONSE returned.  No output is produced if there is no response.  If an optional npackets is given, only
       that number of requests is sent.  Round-trip times and packet loss statistics are computed.  When all responses have been received  or  the
       program times out with npackets specified, or if the program is terminated with a SIGINT, a brief summary is displayed.

Options
       -d   Turns on SO_DEBUG flag on the socket.

       -l   Gives more statistics than if is used without options.  Long output.

       -r   Bypasses  the  normal  routing  tables and sends directly to a host on an attached network.  If the host is not on a directly-attached
	    network, an error is returned.  This option can be used to ping a local host through an interface that has no route through  it.   For
	    example, after the interface was dropped by

       -v   Lists ICMP packets other than ECHO RESPONSE that are received. Verbose output.

Restrictions
       This  program is intended for use in network testing, measurement, and management.  It should be used primarily for manual fault isolation.
       Because of the load it could impose on the network, it is unwise to use during normal operations or from automated scripts.

See Also
       netstat(1), ifconfig(8c)

																	   ping(8)
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