So reading this also this type of spam bounce has a common name, its
called backscattering
(https://en.wikipedia.org/wiki/Backscatter_%28email%29). There is
actually a blacklist for this and quite a few companies use it, as well
as greylisting software will check for it. We for sure want to make sure
that we are able to turn off backscatter and have it off by default.
Honestly users should beable to turn this on and off based on the
direction of the mail. internally it is useful to have it spit back the
message, headers, and the raw error that the MTA returned. Externally we
should just spit back at most headers but honestly what is internal and
external needs to be set by the user since there is no good way to
determine this on its own and the MTA honestly should handle this all as
part of doing the bounce. It makes no sense to make a filter or have a
second function handle it since we already have a function to handle
bounces.
On 03/03/14 09:16, Jason A. Donenfeld wrote:
Hi folks,
Spammers have an easy trick against OpenSMTPD: they send a message
that bounces for some reason (say, it's forwarded to another MTA that
rejects it on on the basis of it being spam), and the bounce message
then contains the original spam message. Egress spam filters on
various hosting networks -- such as OVH -- then will spot that bounce
message as spam, and block the IP on the basis of it being the
spammer.
The solution is obvious, and other MTAs have incorporated this: for
networks with egress filters like this, opensmtpd should have a
configuration option to only send headers, not bodies, in bounce
messages. This has the additional benefit too of lowering bandwidth
usage.
For the same reason that mask-source was added as a config flag, not
sending bodies in bounce messages should also be added as a config
flag: opensmtpd generates this data in a very unstructured way (random
text in a message body that's then queued like other messages), and
then ships it off. It would be wasteful implement this as a filter
using the filter api, which would parse an unstructured format, and
remove bits of data based on a buggy heuristic, when opensmtpd is the
one adding it in the first place. The obvious solution is to simply
add a flag so that this information doesn't get added in the first
place.
Thus, I propose a configuration flag for not adding bodies to bounce messages.
Thanks
Jason
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