Re: pf starts blocking all traffic after a short while

From: Doug Hardie <>
Date: Fri, 4 Jun 2021 10:03:00 -0700
> On 4 June 2021, at 09:43, Chris <> wrote:
> On 2021-06-04 02:43, Pete French wrote:
>> I thought I understood pf pretty well, but this one puzzles me.
>> I have a very simple setup here - a machine I omnly want to allow
>> public IPv6 in from one place, allow private Ipv4 from its local
>> neighbours, and be able to connect out to anywhere.
>> Seems to work, I boot it up, I can ssh in. After about five
>> minutes it just starts blocking all traffic.  I have serial
>> console access, so I can still examine the machine, and if, when
>> it is stuck, I load a pf config files which allows everything, then
>> traffic resumesd again, which is what makes me think pf is doing this.
>> Heres the rules, all eleven of them...
>> root_at_joanna-may:~ # pfctl -s rules
>> scrub all max-mss 1200 fragment reassemble
>> block return all
>> pass quick proto icmp all keep state
>> pass quick proto ipv6-icmp all keep state
>> pass in inet from to any flags S/SA keep state
>> pass in inet from to any flags S/SA keep state
>> pass in inet from to any flags S/SA keep state
>> pass in inet from to any flags S/SA keep state
>> pass in inet6 from 2001:470:6cc4::/48 to any flags S/SA keep state
>> pass in inet6 from 2001:470:1f08:1771::2 to any flags S/SA keep state
>> pass out all flags S/SA keep state
>> Nothing particularly controversial there I think!
>> I've checked the states table, theres a handful in there, and they look fine.
>> If I ssh in and run top, then that connection eventually drops when the packet
>> flow ceases. The pf table is left with a state of TIME_WAIT in it.
>> Any ideas ? This is a mchine inside AWS, so not real hardware, but that
>> should not make a difference I think... Its also the only time I have used pf
>> without using NAT, so maybe I have issed something, but really, this was
>> supposd to be a very simple ruleset to do a very simple job.
> OK I may be completely off the mark here. But I seem to remember something
> about potential problems with fragment reassembly on IPv6. Just for kicks,
> does the problem still manifest if you comment
> scrub all max-mss 1200 fragment reassemble
> Again, I may be off the mark here, as I don't exactly remember where/when
> I read about it. But just thought I'd throw it out there in case it helped.

What IP address is the client you SSH from using?  I get the impression that it is not one of the private IP addresses listed in the rules.  If that is the case, there is no rule for incoming SSH from public IPs.  Hence, the default block all will apply.  I suspect you need another rule like "pass in quick port 22 all keep state".  I understand "keep state" is the default and doesn't need to be included on each rule.

-- Doug
Received on Fri Jun 04 2021 - 17:03:00 UTC

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