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pf Dropped Packets
I have a new firewall that is almost exactly the one show here
https://www.openbsd.org/faq/pf/example1.html I don't think it should matter but I have a couple of VLAN on em1 so my internal interfaces look like: int_if="{ em1 vlan101 vlan102 vlan103 }" As far as I can tell, everything is working. I can see dropped packets coming in from the internet and get out to everything so far. My LAN is on vlan102 and I'm getting some dropped packets coming from the LAN to the internet on the HTTPS port for services like dropbox. Dropbox seems to be working so I don't think it's a problem but I don't understand why these packets are being dropped when others are not. Here is an example: rule 4/(match) block in on vlan102: 10.x.x.x.54738 > 52.207.41.48.443: F 0:0(0) ack 1 win 254 (DF) The rule pass in on $int_if inet allows most everything in on the LAN but not these. Thanks in advance for any thoughts. |
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Thanks, rule 4 is
Quote:
Quote:
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I've been continuing to think about this. While I don't use the flags option on any of my own rule sets, and never have,
flags S/SA keep state is the default for a pass rule.My guess is this particular blocked packet is a TCP ACK which was received out of sequence, most likely after the TCP state had been torn down. This was an ACK to a remote port 443, so the likely originating application is a browser. I wouldn't necessarily be concerned about an errant out-of-sequence ACK from a browser originating on a local 10/8 network. (Yes, there are TCP injection attacks, but this doesn't look like one of them. They would normally originate on an external network, and contain data payloads.) |
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Just to see if I understand it, the line
Quote:
Quote:
Dropbox is trying to use a TCP connection that has expired in the state table so pf sees an ACK and drops it because it's not an S/SA. I'm guessing that when the connection gets dropped, Dropbox opens a new one so it's not an issue. |
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You can show the loaded ruleset with expansions and options by issuing
# pfctl -sr . (This doesn't show any anchor rulesets, if they are used. That requires the addition of -a <anchor> as described in the pfctl(8) man page.)Quote:
TCP is designed to provide end-to-end confirmation of transmissions, and include reassembly of packets that arrive at their destination in the wrong order, or eliminate duplicate packets that have already been confirmed to have arrived. In this instance, , it appears the local workstation on the local network issued a packet so out-of-order that the TCP session had already ended. The root cause could be many things. A browser, for example. |
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