Re: [PATCH] sysctl.conf: Enable Loose Reverse Path Filter according to RFC 3704

[Michael Tremer <michael.tremer@ipfire.org>]

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Hello,

Okay. Let’s give it a try. I will refer anyone running into problems to you :)

-Michael

> On 18 Jan 2022, at 21:18, Peter Müller <peter.mueller(a)ipfire.org> wrote:
> 
> Hello Michael,
> 
> thanks for your reply.
> 
> Well, besides amending the patch for Core Update 164 (or beyond), mention it in the changelog,
> and encourage people running special setups to test this update, I have no idea.
> 
> Since the vast majority of IPFire installations will have a default route set, Loose Reverse
> Path Filtering cannot break anything for them. Therefore, I am willing to risk the procedure
> mentioned above.
> 
> Thanks, and best regards,
> Peter Müller
> 
> 
>> Hello,
>> 
>> How are we going to test this with a wider audience?
>> 
>> I do not expect this to break anything, but I would like to make sure that this assumption holds true.
>> 
>> -Michael
>> 
>>> On 16 Jan 2022, at 14:47, Peter Müller <peter.mueller(a)ipfire.org> wrote:
>>> 
>>> For historical reasons, we were always reluctant to reverse path
>>> filtering, since configuration changes were tricky to evaluate for a
>>> larger userbase, IPFire permits a number of complex scenarios, and due
>>> to limited resources.
>>> 
>>> As a compromise, this patch suggests to enable Loose Reverse Path
>>> Filtering, as specified in RFC 3704 (section 2.4), to gain at least some
>>> security achievement on this end.
>>> 
>>> To quote from that:
>>> 
>>>  Loose Reverse Path Forwarding (Loose RPF) is algorithmically similar
>>>  to strict RPF, but differs in that it checks only for the existence
>>>  of a route (even a default route, if applicable), not where the route
>>>  points to.  Practically, this could be considered as a "route
>>>  presence check" ("loose RPF is a misnomer in a sense because there is
>>>  no "reverse path" check in the first place).
>>> 
>>>  The questionable benefit of Loose RPF is found in asymmetric routing
>>>  situations: a packet is dropped if there is no route at all, such as
>>>  to "Martian addresses" or addresses that are not currently routed,
>>>  but is not dropped if a route exists.
>>> 
>>> There is no legitimate reason why we cannot enable this: If IPFire
>>> receives a packet on some interface it cannot route on _any_ interface
>>> at all, there is no sense in processing it.
>>> 
>>> While testing this change, I was unable to produce a situation where it
>>> actually causes any harm. In theory, it shouldn't do so anyways.
>>> 
>>> In the future, we will hopefully be able to set these sysctl's to "1",
>>> using Strict Reverse Path Filtering, as specified in RFC 3704 (section
>>> 2.2). Doing so was found to work fine in my testing environment as well,
>>> but there is no asymmetric routing in place there.
>>> 
>>> Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
>>> ---
>>> config/etc/sysctl.conf | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/config/etc/sysctl.conf b/config/etc/sysctl.conf
>>> index bc2d21c93..c8c775d13 100644
>>> --- a/config/etc/sysctl.conf
>>> +++ b/config/etc/sysctl.conf
>>> @@ -12,13 +12,13 @@ net.ipv4.tcp_syn_retries = 3
>>> net.ipv4.tcp_synack_retries = 3
>>> 
>>> net.ipv4.conf.default.arp_filter = 1
>>> -net.ipv4.conf.default.rp_filter = 0
>>> +net.ipv4.conf.default.rp_filter = 2
>>> net.ipv4.conf.default.accept_redirects = 0
>>> net.ipv4.conf.default.accept_source_route = 0
>>> net.ipv4.conf.default.log_martians = 1
>>> 
>>> net.ipv4.conf.all.arp_filter = 1
>>> -net.ipv4.conf.all.rp_filter = 0
>>> +net.ipv4.conf.all.rp_filter = 2
>>> net.ipv4.conf.all.accept_redirects = 0
>>> net.ipv4.conf.all.accept_source_route = 0
>>> net.ipv4.conf.all.log_martians = 1
>>> -- 
>>> 2.31.1
>>