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Help with ipchains on Potato -- problem with -s?

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I've been trying to configure a firewall using ipchains on a machine
running pretty much a stock installation of Potato -- I've done the apt-get
upgrade but not dist-upgrade.  My kernel is the default 2.2.19pre17, and
given that /proc/net/ip_fwchains exists, I figure it has the appropriate
support for using ipchains.

My problem is this:  whenever I use ipchains to try to filter by source
address, i.e. with -s x.x.x.x/x as an option, something goes wrong.

Details (I'll use *chain* to stand for any one of the chains):

All the following ipchains commands work properly (i.e. checking with
ipchains -L returns an intelligible response, and the packet filter seems
to behave as it should given the ipchains commands):

ipchains -F *chain*
ipchains -P *chain* DENY
ipchains -A input -i lo -j ACCEPt
ipchains -A input -i eth0 -p tcp ! -y -j ACCEPT
ipchains -A input -i eth0 -p icmp --destination-port 0 -j ACCEPT
ipchains -A input -j DENY -l

BUT when I try to filter by source address, e.g.

ipchains -A input -i eth0 -s -j DENY

and check with ipchains -L to see my rule set, ipchains -L just seems to
hang, and prints out just this:

Chain input (policy ACCEPT):
target     prot opt     source                destination           ports

I have to hit ctrl-c to get the prompt back.

When I look at /proc/net/ip_fwchains, it seems that rules with -s options
make a change there (i.e. it looks like the rule gets registered there,
when I check that file with more), but ipchains -L just hangs there.

One more complication:  this doesn't happen every time.  Oddly, sometimes
my whole firewall script runs and everything works -- I get a proper
response from ipchains -L.  But sometimes it doesn't.  I've tried to
establish a pattern, but other than noting that it seems to be using
ipchains with the -s option that triggers it, I can't seem to detect
anything that might indicate why it works sometimes but not others.
Absolutely maddening.

Any ideas?  Anything I'm missing, or anywhere else I can check?


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