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PPoossttffiixx PPeerr--CClliieenntt//UUsseerr//eettcc.. AAcccceessss CCoonnttrrooll
-------------------------------------------------------------------------------
PPoossttffiixx rreessttrriiccttiioonn ccllaasssseess
The Postfix SMTP server supports access restrictions such as reject_rbl_client
or reject_unknown_client_hostname on the right-hand side of SMTP server access
(5) tables. This allows you to implement different junk mail restrictions for
different clients or users.
Having to specify lists of access restrictions for every recipient becomes
tedious quickly. Postfix restriction classes allow you to give easy-to-remember
names to groups of UCE restrictions (such as "permissive", "restrictive", and
so on).
The real reason for the existence of Postfix restriction classes is more
mundane: you can't specify a lookup table on the right-hand side of a Postfix
access table. This is because Postfix needs to open lookup tables ahead of
time, but the reader probably does not care about these low-level details.
Example:
/etc/postfix/main.cf:
smtpd_restriction_classes = restrictive, permissive
# With Postfix < 2.3 specify reject_unknown_client.
restrictive = reject_unknown_sender_domain
reject_unknown_client_hostname ...
permissive = permit
smtpd_recipient_restrictions =
permit_mynetworks
# reject_unauth_destination is not needed here if the mail
# relay policy is specified with smtpd_relay_restrictions
# (available with Postfix 2.10 and later).
reject_unauth_destination
check_recipient_access hash:/etc/postfix/recipient_access
...
/etc/postfix/recipient_access:
joe@my.domain permissive
jane@my.domain restrictive
With this in place, you can use "restrictive" or "permissive" on the right-hand
side of your per-client, helo, sender, or recipient SMTPD access tables.
The remainder of this document gives examples of how Postfix access restriction
classes can be used to:
* Shield an internal mailing list from outside posters,
* Prevent external access by internal senders.
These questions come up frequently, and the examples hopefully make clear that
Postfix restriction classes aren't really the right solution. They should be
used for what they were designed to do, different junk mail restrictions for
different clients or users.
PPrrootteeccttiinngg iinntteerrnnaall eemmaaiill ddiissttrriibbuuttiioonn lliissttss
We want to implement an internal email distribution list. Something like
all@our.domain.com, which aliases to all employees. My first thought was to
use the aliases map, but that would lead to "all" being accessible from the
"outside", and this is not desired... :-)
Postfix can implement per-address access controls. What follows is based on the
SMTP client IP address, and therefore is subject to IP spoofing.
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
check_recipient_access hash:/etc/postfix/access
...the usual stuff...
/etc/postfix/access:
all@my.domain permit_mynetworks,reject
all@my.hostname permit_mynetworks,reject
Specify ddbbmm instead of hhaasshh if your system uses ddbbmm files instead of ddbb files.
To find out what map types Postfix supports, use the command ppoossttccoonnff --mm.
Now, that would be sufficient when your machine receives all Internet mail
directly from the Internet. That's unlikely if your network is a bit larger
than an office. For example, your backup MX hosts would "launder" the client IP
address of mail from the outside so it would appear to come from a trusted
machine.
In the general case you need two lookup tables: one table that lists
destinations that need to be protected, and one table that lists domains that
are allowed to send to the protected destinations.
What follows is based on the sender SMTP envelope address, and therefore is
subject to SMTP sender spoofing.
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
check_recipient_access hash:/etc/postfix/protected_destinations
...the usual stuff...
smtpd_restriction_classes = insiders_only
insiders_only = check_sender_access hash:/etc/postfix/insiders, reject
/etc/postfix/protected_destinations:
all@my.domain insiders_only
all@my.hostname insiders_only
/etc/postfix/insiders:
my.domain OK matches my.domain and subdomains
another.domain OK matches another.domain and subdomains
Getting past this scheme is relatively easy, because all one has to do is to
spoof the SMTP sender address.
If the internal list is a low-volume one, perhaps it makes more sense to make
it moderated.
RReessttrriiccttiinngg wwhhaatt uusseerrss ccaann sseenndd mmaaiill ttoo ooffff--ssiittee ddeessttiinnaattiioonnss
How can I configure Postfix in a way that some users can send mail to the
internet and other users not. The users with no access should receive a
generic bounce message. Please don't discuss whether such access
restrictions are necessary, it was not my decision.
Postfix has support for per-user restrictions. The restrictions are implemented
by the SMTP server. Thus, users that violate the policy have their mail
rejected by the SMTP server. Like this:
554 <user@remote>: Access denied
The implementation uses two lookup tables. One table defines what users are
restricted in where they can send mail, and the other table defines what
destinations are local. It is left as an exercise for the reader to change this
into a scheme where only some users have permission to send mail to off-site
destinations, and where most users are restricted.
The example assumes DB/DBM files, but this could also be done with LDAP or SQL.
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
check_sender_access hash:/etc/postfix/restricted_senders
...other stuff...
smtpd_restriction_classes = local_only
local_only =
check_recipient_access hash:/etc/postfix/local_domains, reject
/etc/postfix/restricted_senders:
foo@domain local_only
bar@domain local_only
/etc/postfix/local_domains:
this.domain OK matches this.domain and subdomains
that.domain OK matches that.domain and subdomains
Specify ddbbmm instead of hhaasshh if your system uses ddbbmm files instead of ddbb files.
To find out what map types Postfix supports, use the command ppoossttccoonnff --mm.
Note: this scheme does not authenticate the user, and therefore it can be
bypassed in several ways:
* By sending mail via a less restrictive mail relay host.
* By sending mail as someone else who does have permission to send mail to
off-site destinations.
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