Thanks Theo for the answer!
I'm still having difficulty wrapping my head around it.
I have two packets: DHCPREQUEST and DHCPACK
{timestamp} {my_ip}.68 > {ip1}.67: xid:0xfe51c9a3 [|bootp]
{timestamp} {ip2}.67 > {my_ip}.68: xid:0xfe51c9a3 Y:{my_ip} G:{ip1}[|bootp]
I get that tcpdump taps to bpf so it can see both packets.
And my understanding of your answer is that pf doesn't see the
first packet (DHCPREQUEST) since it's being sent using bpf.
The second packet (DHCPACK) -- although dhcpleased has unfiltered
access to -- is eventually visible to pf, thus will be blocked by
pf and should show up on the pflog0 interface as per the following
rule:
> block drop in log (all) on $ext_if inet proto udp \
> from port 67 to port 68
However, it seems that nothing appears on pflog0 in my case, which
is what I still can't comprehend.
Am I missing something here?
Best Regards
On 7/29/21 11:37 PM, Theo de Raadt wrote:
dhcpleased (and a few other daemons) use bpf, thus see raw packets
from the wire before pf can block them. Most daemons of this type
also use bpf to send packets, and pf doesn't see these either.
This behaviour is intentional, and useful.
beebeet...@posteo.de wrote:
Hi all,
I'm running OpenBSD 6.9 as a home router, and observed some behavior of
pf that I can't really make sense of.
The router runs dhcpleased to obtain its IP address from the ISP, and
I have
the following pf rules (only the relevant ones are shown):
block drop all
pass out on $ext_if inet from ($ext_if) to ! <ipv4_reserved>
block drop in log (all) on $ext_if inet proto udp from port 67 to port
68
pass in on $ext_if inet proto udp from port 67 to 255.255.255.255 port
68
(I need to mention here that after digging into some old discussions
on the
mailing list, I realize that the last two rules are unnecessary
because DHCP
traffic is supposedly processed by dhcpleased though bpf regardless of
pf's
decision, but my question is something else)
With tcpdump on the external interface, I see packets similar to the
following
for lease renewal:
{timestamp} {my_ip}.68 > {ip1}.67: xid:0xfe51c9a3 [|bootp]
{timestamp} {ip2}.67 > {my_ip}.68: xid:0xfe51c9a3 Y:{my_ip} G:{ip1}
[|bootp]
Note that DHCP renew request is sent to {ip1}, but the DHCP
acknowledgment
is from {ip2}, so I guess {ip1} is a DHCP relay?
The problems is, with my existing pf rules I expect the second packet
to be
blocked and logged to pflog0, but in reality, a tcpdump on pflog0
shows that no
packets are being blocked:
tcpdump -l -n -i pflog0
Why is the second packet not blocked by pf when its source ip address
{ip2} is
supposedly not in the state table?
I would greatly appreciate any help on this.
Best Regards
