Re: BCP38 on public-facing Ubuntu servers

[Stephen Satchell <list () satchell net>]

On 6/8/21 2:38 PM, Fran via NANOG wrote:
> Hey,
>
> to my knowledge there is no IPv6 equivalent for 
> net.ipv4.conf.all.rp_filter.
>
> Therefore I use netfilter to do the RP filtering for both address families.
>
> ip(6)tables -t raw -I PREROUTING -m rpfilter --invert -j DROP
>
> Using the raw tables less resources are used, but you could also choose 
> other tables.
> Details abour rpfilter can be found here [1].
>
> This can also be achieved using nftables [2].

I've been in discussions on how to filter packets with bad source 
addresses on several mailing lists, including this one.  For the last 
few weeks, I've been search for all the information I can find for how 
Linux implements rp_filter...which appears to have some holes.

Looking at /proc/sys/net/ipv6, there is no knob for rp_filter, so if 
your system is IPv6 enabled you have to use the built-in firewall.

For IPv4, I found kernel documentation, but it doesn't tell the whole 
story.  For that, I had to comb the kernel sources to find out all the 
details of rp_filter.  I've prepared a RFC letter of what I think I 
found, to be sent to the kernel developers.  Here is the text of what 
I'll be sending, with any constructive criticism I get from here:

Letter begins:

After looking at the source that appears to implement rp_filter
    linux/net/ipv4/fib_frontend.c
I believe that I now understand the tests rp_filter performs to
validate the source address when net.ipv4.conf.*.rp_filter is
set to one or two for a given interface.

Does the new paragraph I have written accurately reflect what
happens?  If so, then I find out how to submit a patch to add the
clarification to the kernel document.

Description of rp_filter from
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
--------------------------------------------------------------------
rp_filter - INTEGER
        0 - No source validation.
        1 - Strict mode as defined in RFC3704 Strict Reverse Path
            Each incoming packet is tested against the FIB and if the
            interface is not the best reverse path the packet check will
            fail. By default failed packets are discarded.
        2 - Loose mode as defined in RFC3704 Loose Reverse Path
            Each incoming packet's source address is also tested against
            the FIB and if the source address is not reachable via any
            interface the packet check will fail.

        [*new text here]
        
        Current recommended practice in RFC3704 is to enable strict mode
        to prevent IP spoofing from DDos attacks. If using asymmetric
        routing or other complicated routing, then loose mode is
        recommended.

        The max value from conf/{all,interface}/rp_filter is used
        when doing source validation on the {interface}.

        Default value is 0. Note that some distributions enable it
        in startup scripts.
--------------------------------------------------------------------

Recommended addition where marked with "[*new text here]":
    rp_filter will examine the source address of an incoming IP
    packet by performing an FIB lookup.  In loose mode (value 2),
    the packet is rejected if the source address is neither
    UNICAST nor LOCAL nor IPSEC.  For strict mode (value 1) the
    interface indicated by the FIB entry must also match the
    interface on which the packet arrived.