Firewall & Proxy server (IP forwarding)

Firewall & Proxy server (IP forwarding)

Post by Lasse K. Christians » Wed, 24 Apr 1996 04:00:00

Hi everyone,

A couple of days ago i wrote about not being able to find out whether
or not my firewalling problems would be solved with msaquerading or a
proxy server.

After getting some quite informative mails back i decided on using a
proxy server (socks).

Now I have run into a strange kind of problem.

My Linux box (slackware 1.2.13) is configured with two IP interfaces.
A token-ring interface connected to the protected network has the IP
addresse (mask while the other interface,
connected to the unsecure (internet) network, an etherlink III card
has the IP addresse Before enabling the Firewalling
option and disabling the IP forwarding/gatewaying function in the
kernel it was possible for me to ping the both interfaces from the
secure and the unsecure network. Then I recompiled the kernel with
firewalling on and disabled forwarding and gatewaying.

Thereafter i installed SOCKS 4.2b to act as my connection from the
inside network to the unsecure network. It needed three files


Which should route all requests to the outgoing interfade (on the
unsecure net).


Which should give me access from all machines on the local net to the
outside world.

Now the funny things comes. When i use netscape (from windows 95) to
get access to the outside network everything seems to work just fine.

But when i use ping from the internal network then i can ping both
interfaces on the firewall. The same is possible from the unsecure
network. This should not be possible when firewalling is on and
forwarding is off.

I then recompiled the kernel again, after reading the Firewalling
howto which stated that if this was possible then forwarding was not
disabled. this time making sure that forwarding was off. But to no
avail. It is still possible toe ping both interfaces from both the
"secure" and "insecure" side of the net ????

Do anyone have any ideas as to what might be wrong. What am i missing
here ???

Thanks in advance

Lasse K. christiansen


1. IP Forwarding vs Proxy/Firewall

Yes, it sounds like ip masquerading is the best choice here.  It acts like
a proxy server, but on the network level, instead of for just http
connects.   You can find a howto on setting it up at

Frank Sweetser rasmusin at fsweetser at | PGP key available RedHat 4.9.1 Linux 2.0.31 i586     | at public servers
"If the future navigation system [for interactive networked services on
the NII] looks like something from Microsoft, it will never work."
(Chairman of Walt Disney Television & Telecommunications)

2. Mail Problems on SCO 5

3. Using SSH / Port Forwarding to get around local proxies & firewalls

4. Data corruption with Maxtor & Western Digital on same IDE port?

5. Setting Up Proxy Server & Firewall (Linux server or workstations)

6. Telnet problems

7. ip-forwarding && dns-forwarding

8. Writing the Redhat CD's

9. IP forwarding, accounting & firewalling in kernel?

10. ip forwarding - hiding public servers behind a firewall

11. Setting Up Proxy Server & Firewall

12. Firewalls & Proxy/Tunneling Servers . . . UDP

13. Setting Up Proxy Server & Firewall