At 8:58 AM 2/6/96, Ned Freed wrote:
1. I've already pointed out that filtering on specific origin addresses is
What about the following set of mechanisms, each of which I believe
I've seen in operation. In combination, they would seem to create a rather
powerful filter and control mechanism, I would think.
1. To subscribe to a list, the user sends a request. The list server
sends back a verification request. The subscriber returns a confirmation.
This provides reasonable assurance that the registered email address is
I've already described this mechanism and commented on it in some detail on the
Simply put, the problem is that list subscribers hate it and therefore most
list managers cannot reasonably use it. As such, it's a nonstarter more often
than not. It also has nothing to do with spamming; it's a mechanism intended to
defeat personal "subscribe him to every list in the world" attacks like the one
directed at Mark Crispin recently.
2. Those who are registered can send messages freely. Those who are not
have their messages channeled to the list moderate who decides whether to
forward the message or not. One could elaborate this, further, by having
non-registered email ALSO get a verification request from the server before
sending it on to the moderator. This would filter deprivation of service
attacks, I suppose.
I have also described this particular mechanism in an earlier message on this
The sender verification step suffers from the problems #1 has only more so --
people posting to lists *really* hate it and simply won't post if it is used.
(I personally can tolerate #1 but won't accept this variant of it.) As for
having a moderator review messages received from people not subscribed to the
list, this depends on having a moderator willing to perform the task. Such
moderator resources are in scarce supply. Also keep in mind that the burden
placed on the moderator is FAR greater than anticipated, as postings from
legitimate list members from addresses other than the address registered on the
list are incredibly common. (For example, I probably send out around 100
messages out every day, and at least half of them have this property.)
I also heard it suggested that lists maintain a separate set of "addresses that
can post to the list". This is a nice idea in theory, and some list managers,
including the one in our PMDF product, allow for it, but the problem is that
the information needed to maintain such tables is rarely available.
Finally, please note that the attack I previously described blows past all this
with ease, since it builds a list of addresses that are known to be subscribed
to the list in an automated way. I dealt with a use of this attack this morning
-- it is the first case I have seen. This will never be as common as more
simpleminded techniques, but it will not remain unknown for much longer.
In conclusion, my bottom line remains the same: In the lucky event that you can
find a moderator willing to look at every message, that is what you should do
and the more power to you. And if you can find a moderator willing to look
at exceptions defined by list-specific heuristics, then this is also
reasonable. But there are always going to be lists where these resources
don't exist, and the only viable approach then is to just delete the
offending spams and get on with life.