I am trying to test some research ideas and am not able to perform NAT
translation on incoming packets only. I am trying to explain why, and am
looking for a confirmation (or correction) of what I think is happening. I
have two Routers (B and C) in my network. I am trying to send packets directly
from A to D through B, but route the replies through C and B using NAT
translation.
D
/ \
/ \
A --- B ---- C
The gateway route of Host D is to Router C. Router C has the following
iptables entries in the NAT PREROUTING and NAT POSTROUTING table.
iptables -t nat -A PREROUTING -s D -d A -j DNAT --to B
iptables -t nat -A POSTROUTING -s D -d B -j SNAT --to C
Router B has the following entries in its iptables NAT PREROUTING and
POSTROUTING tables.
iptables -t nat -A PREROUTING -s C -d B -j LOG
iptables -t nat -A PREROUTING -s C -d B -j DNAT --to A
iptables -t nat -A POSTROUTING -s C -d A -j LOG
iptables -t nat -A POSTROUTING -s C -d A -j SNAT --to D
I am able to see the first TCP SYN packet travel from A through B to D. At
this point Router B has an ip_conntrack entry from A to D. I then see the
reply travel from D to C. C successfully performs NAT translation, and the
packet is sent to Router B with a source of C and a destination of B. I added
logging to the iptables entries in Router B, and I see the packet get
translated in both the PREROUTING and the POSTROUTING tables, but the packet
is never sent. I never see a new conntrack entry for this packet.
I think that Router B is dropping the packet through connection tracking since
it cannot add a new conntrack entry for this packet because it already has an
entry for packets from A to D that do not require NAT translation. If it added
a new entry, then it would not know whether to translate the next packet from
A to D or send it directly to D.
Is this analysis correct? Is there anyway to send packets directly to D and
router them back to A through C and B using NAT translation the way I have
described using iptables or otherwise? Any explanation on what criteria
connection tracking uses to drop packets in this case would be a great help.
Thanks,
Tish Best
