Você pode pular o proxy e apenas usar o tcpdump com a opção -A e um filtro?
# capture everything destined for port 80
tcpdump -qni eth0 -s 0 -A port 80
# capture everything destined for port 80 on 192.168.32.1
tcpdump -qni eth0 -s 0 -A port 80 and host 192.168.32.1
# capture everything destined for port 80 and display only the interesting bit.
tcpdump -qni eth0 -s 0 -A port 80 | grep "Text I'm interested in"