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Old 18th May 2011
lcxpics lcxpics is offline
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Question Help: NAT doesn't work on OpenBSD 4.9

Hi All,

I was doing the NAT testing(redirection) to access internal webserver from notebook with the diagram below:

notebook--->----em0[OpenBSD 4.9 PF]em1--->---webserver(TCP/443)

em0(external) is
notebook is
em1(internal) is
internal webserver is
Webserver gateway is em1(
Firewall default gateway is
notebook gateway is em0(
IP alias for NAT on em0 is (for redirecting the incoming traffic from to


My rule is:
# Tables: (1)
table <tbl.r0.d> { , , }

# Rule 0 (NAT)
match in on em0 proto {tcp udp icmp} from to rdr-to

# SSH mgmt rule
pass in quick inet proto tcp from to <tbl.r0.d> port 22

# Rule 0 (em0,em1) permit notebook to access https on internal webserver
pass log quick on { em0 em1 } inet proto tcp from to port 443

# Deny all
block quick inet from any to any no state

IP alias ( was configured on em0(external)

But the traffic can't pass through the internal webserver.
I was doing tcpdump on both interfaces:

tcpdump on em0(external):

# tcpdump -npi em0 host
tcpdump: listening on em0, link-type EN10MB
21:36:37.611311 arp who-has tell
21:36:37.611369 arp reply is-at 00:0c:29:97:2a:44
tcpdump: WARNING: compensating for unaligned libpcap packets
21:36:37.611708 > S 4176778738:4176778738(0                                                                             ) win 65535 <mss 1460,nop,wscale 1,nop,nop,sackOK> (DF)
21:36:37.611789 > R 0:0(0) ack 4176778739 w                                                                             in 0 (DF)
21:36:38.063449 > S 4176778738:4176778738(0                                                                             ) win 65535 <mss 1460,nop,wscale 1,nop,nop,sackOK> (DF)
21:36:38.063598 > R 0:0(0) ack 1 win 0 (DF)
21:36:38.566376 > S 4176778738:4176778738(0                                                                             ) win 65535 <mss 1460,nop,wscale 1,nop,nop,sackOK> (DF)
21:36:38.566453 > R 0:0(0) ack 1 win 0 (DF)
and tcpdump on em1(internal)

# tcpdump -npi em1 port 443
tcpdump: listening on em1, link-type EN10MB
It seemed that the translation didn't work and the request can't be redirected to internal webserver.

When I was doing the testing from notebook to internal webserver, the notebook was able to access internal webserver port 443. But when I was testing using rdr-to (NAT) it doesn't work.

Is there anything missing/misconfigured in my config?

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Old 18th May 2011
J65nko J65nko is offline
Join Date: May 2008
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I may be wrong, but I think the gotcha's explained in http://openbsd.org/faq/pf/rdr.html#reflect apply to your situation
You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump
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