Request for comment -- BCP38

Hugo Slabbert hugo at
Mon Sep 26 15:12:54 UTC 2016

On Mon 2016-Sep-26 10:47:24 -0400, Ken Chase <math at> wrote:

>This might break some of those badly-behaving "dual ISP" COTS routers out there
>that use different inbound from outbound paths since each is the fastest of
>either link.

As it should.

If you have links from both ISP A and ISP B and decide to send traffic out 
ISP A's link sourced from addresses ISP B allocated to you, ISP A *should* 
drop that traffic on the floor.  There is no automated or scalable way for 
ISP A to distinguish this "legitimate" use from spoofing; unless you 
consider it scalable for ISP A to maintain thousands if not more 
"exception" ACLs to uRPF and BCP38 egress filters to cover all of the cases 
of customers X, Y, and Z sourcing traffic into ISP A's network using IPs 
allocated to them by other ISPs?

If you want to play asymmetry tricks, get some PI space and make 
arrangements.  If that's outside your wheelhouse, get an ISP that will sell 
this to you as a service either with dissimilar links they provide to you 
or over-the-top with tunnels etc.

Playing NAT games with different classes of traffic to e.g. send traffic 
type 1 over ISP A and traffic type 2 over ISP B *BUT* using the 
corresponding source addresses in each case and having the traffic return 
back over the same links is fine and dandy.  If you send traffic into an 
ISP-provided link using addresses from another provider, though, that ISP 
*should* be dropping that traffic.  If they don't, send them here so we can 
yell at them.

>I did this manually when I was messing around with multiple broadband links on
>a fbsd router years ago, was glad it worked at the time.
>On Mon, Sep 26, 2016 at 07:11:42AM -0700, Paul Ferguson said:
>  >No -- BCP38 only prescribes filtering outbound to ensure that no packets leave your network with IP source addresses which are not from within your legitimate allocation.
>  >
>  > - ferg
>  >
>  >
>  >On September 26, 2016 7:05:49 AM PDT, Stephen Satchell <list at> wrote:
>  >>Is this an accurate thumbnail summary of BCP38 (ignoring for the moment
>  >>
>  >>the issues of multi-home), or is there something I missed?
>  >>
>  >>>     The basic philosophy of BCP38 boils down to two axioms:
>  >>>
>  >>>         Don't let the "bad stuff" into your router
>  >>>         Don't let the "bad stuff" leave your router
>  >>>
>  >>>     The original definition of "bad stuff" is limited to source-
>  >>>     address grooming both inbound and outbound.  I've expanded on the
>  >>>     original definition by including rule generation to control
>  >>>     broadcast address abuse.
>  >
>  >--
>  >Sent from my Android device with K-9 Mail. Please excuse my brevity.
>Ken Chase - math at Toronto Canada

Hugo Slabbert       | email, xmpp/jabber: hugo at
pgp key: B178313E   | also on Signal
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