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

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

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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