forwarding http requests with ipfw

Glenn Dawson glenn at
Fri Dec 30 22:13:48 PST 2005

At 09:07 PM 12/30/2005, Robert Collins wrote:
>I've got a situation where I've got an internal host using a private 
>ip/domainname. Let's say for the sake of this discussion the host is 
> privatehost isn't running a 
>webserver. But I would like machines on the 
>network to query privatehost as if it was. When one of these 
>machines queries privatehost I would like privatehost to forward 
>those requests to my webserver,, so that it can 
>handle the request. In order to accomplish that I have done the following:
>My kernel was compiled with these options:
>options         IPFIREWALL
>options         IPFIREWALL_FORWARD
>"ipfw list" looks like this:
>00100 allow ip from any to any via lo0
>00110 deny ip from any to
>00120 deny ip from to any
>10000 fwd tcp from any to me dst-port 80
>65000 allow ip from any to any
>65535 deny ip from any to any
>The problem I am having is that it seems the packets never leave 
>privatehost. tcpdump shows packets coming in destined for port 80. 
>"ipfw show" shows that packets are matching my rule, but tcpdump 
>never shows any packets going out to tcpdump on 
>  also shows that no packets are being recieved. I 
>did a tcpdump on lo0 just for kicks, and that didn't show anything. 
>It seems as if the packets are just disappearing. Someone on 
>#freebsdhelp suggested doing a "sysctl -w net.inet.ip.forwarding=1" 
>but that didn't help the situation. Is there something minor I'm 
>missing here...or am I totally off in my understanding of how "ipfw fwd" works?

To quote the ipfw man page:

"The fwd action does not change the contents of the packet at 
all.  In particular, the destination address remains unmodified, so 
packets forwarded to another system will usually be rejected by that 
system unless there is a matching rule on that system to capture them."

You probably need to re-think what you are trying to do.


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