Hi, ulog_packet_msg is the struct (defined in ipt_ULOG.h) which determines the format of the data passed by kernel to ulogd via netlink socket whenever ULOG iptables target is triggered. The problem is that the fields are defined using data types, like long and size_t, which are not guaranteed to have the same storage size in kernel- and userspace (for example, they will be different if kernel is 64-bit, but userspace is 32-bit). Other similar data structures defined in ipt_*.h header files use types with explicit storage size, like uint32_t, etc, and it would be nice if the same would be done for ULOG. Thanks.
We can't change this without breaking backwards compatibility. ULOG is obsoleted by nfnetlink_log, so we won't fix this. IIRC ulogd also includes a hack to work properly in a mixed 32/64 bit environment.