[CentOS] sendmail mail relay backscatter issue.

Les Mikesell lesmikesell at gmail.com
Fri Feb 5 14:49:35 UTC 2010


Simon Billis wrote:
> Les Mikesell sent a missive on 2010-02-05:
> 
>> Simon Billis wrote:
>>>> The point would be able to include a default reject rule for each
>>>> domain, which means that you have to supply valid forwards for all
>>>> addresses you don't want to reject at the relay.  (You could default
>>>> to forwarding, but that doesn't help with the backscatter issue). But
>>>> that doesn't change the ability to queue/deliver except that the relay
>>>> has to accept the domains as local to do the virtuser lookup so the
>>>> new target has to have a different name for the delivery host.   I'm
>>>> not sure how that relates to your distinction between forwarding and
>>>> queuing. Sendmail has local and remote addresses, but remote ones all
>>>> go through the same steps.
>>> I am queuing and delivering using mailertable currently - hence the
>>> issue with backscatter as some of the domains do not have catch-all
>>> accounts. I am able to produce a list of valid email accounts and
>>> domains without a catch-all account so I should be able to create a
>>> virtusertable with the required entries to either accept all mail
>>> for a domain and then forward it to a specific account (the
>>> catch-all
>>> account) or to only accept mail for a specific account and then
>>> forward it to the same address (is this valid?) by again using
>>> mailertable(?). I think that using access.db and relay-domains may
>> also work as needed.
>>
>> Sendmail will only look in virtusertable if it considers the address
>> local (i.e.
>> you've added the target domain to local-host-names).  That means
>> you'll have to use some other name for the delivery target in the
>> virtusertable expansion side to get it to forward on.  Probably
>> whatever you are using in mailertable will work.  You might be able to
>> use user@[host.domain] notation or user@[IP_address] there to avoid
>> another MX lookup that would come back to the relay - I'm not sure
>> about that.  You'll probably have to do some testing with this part
>> since it is a fairly drastic change to make the targets local - but
>> you can do it one domain at a time.
>>
> 
> I don't think that this is going to work for me then... I'm not able to
> change the envelope address for the onward delivery. The final mail server
> will reject the mail if it is not the original email address that I'm
> accepting the mail for on the mail scanners. Also I understand from the
> documentation that mailertable is not used for class {w}, i.e. local host
> names so I think that I'm stuck with the following choices...
> 
> 1) getting access.db and relay-domains working correctly with:
>    (a) the _RELAY_FULL_ADDR_ feature
>    (b) without the above feature (which works but without the ability to
> send mail from our networks from email addresses in the access.db map but I
> think that this is because I need to add specific hosts to the access map.)
> 
> 2) utilising a milter.
> 
> Is this a fair conclusion in your opinion?

What are you currently using in mailertable to get there?  If you use [domain] 
and go to the A record of the same name it might be a problem - but that might 
work if you try it.  Where I've used it, the delivery hosts had their own names 
that they'd accept in the envelope and the [IP.address] form would also work.

-- 
   Les Mikesell
    lesmikesell at gmail.com


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