Thank you for explaining it in detail.
One quick check is to type:
iptables -j ULOG -h
which should show the help text for the ULOG module at the end if your binary has support for ULOG.
As you write above, the command shows the help.
The iptables binary support is generally provided as a module. Where this is found depends on the compile time options used, but on a Redhat stock system it's found as /lib/iptables/libipt_ULOG.so (worst case, do a "locate libipt_ULOG.so"). That module not only provides the help text but also all the code necessary to parse command line options and actually insert your request to _use_ the ULOG kernel module into the kernel.
I checked that there was /lib/iptables/libipt_ULOG.so.
Judging from the two points that you teached me, iptables in my system seems to support ULOG.
If you don't have that file, you'll need to compile the iptables binary. The source code is available from ftp://ftp.netfilter.org/pub/iptables/ (I personally get the snapshots, but for critical systems you might want to stick with the last released 1.2.9). The INSTALL file should provide enough hints of what you need to do.
The other day, I updated iptables to 1.2.9. But, I haven't resolved the problem yet.
Best regards.
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