ULOG is the netfilter/iptables logging daemon. It connects to the netlink device of the Linux kernel and reads messages from the netfilter that get queued with the iptables ULOG target. It can output to plain text, MySQL, Postgres and PCAP.
apt-get install ulogd-pcap
Configure ulogd to output in PCAP format
plugin="/usr/lib/ulogd/ulogd_PCAP.so", restart ulog. Then add a log rule to iptables before the final drop rule.
-A INPUT -j ULOG
The PCAP log file should be located at
23 Mar 2014