How do I read the contents of incoming network packets without using promiscuous mode?

To do this, make use of Netfilter module ip_queue. Using this module, packets are passed out of the stack for queueing to userspace. Applications can read from these queues, modify the packets and then re-insert it back into the kernel.

Load Module

modprobe iptable_filter

modprobe ip_queue

The following is an IPTables rule to take advantage of this...