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Author image Juerd Waalboer


Qmail::Deliverable::Comparison - Overview of recipient checkers for qmail


    There is more than one way to do it
    -- Larry Wall


Qmail::Deliverable is not the only software for qmail that stops backscatter by checking recipient addresses. Other solutions have existed for a long time, but after evaluating them, I felt more comfortable rolling my own.

While exploring the existing options, I learned their strengths and weaknesses. I revisited them all again after writing Q::D, and now share my findings.

It is most amazing, though, that no other solution provides a re-usable library. One comes very close, by providing a daemon that you can query via a very simple protocol. Q::D has both a library and a daemon. Also, good support for dot-qmail files like .qmail-ext-default, is rare, but absolutely essential for interoperability with, for example, ezmlm.

The main problem with checking address validity during the SMTP transaction, is one of permissions and separation. Root access is required to peek into home directory of system users. Privilege separation is one of the classic strengths of qmail, and it would be unwise to break that. Additional mechanisms are needed to keep things separate, but as you can see, this wheel has been invented plenty of times.

Qmail's modular design made replacing the SMTP daemon very simple, and because development on qmail stopped with version 1.03 in 1998, several replacement daemons have been released. There is a patch for qmail-smtpd, to make it extensible with plugins, called qmail-spp. Then, there is a daemon called magic-smtpd. My favourite, however, is the very extensible Perl based qpsmtpd.





Using qmail logic

Any solution that is to integrate well with existing qmail systems, has to copy the delivery logic that qmail itself uses.

They need to use the files control/virtualdomains, and users/assign or users/assign.cdb, and some mechanism to detect local users (getpw). For use on a relaying server, or "smart host", accepting all email for domains not listed in control/virtualdomains or control/locals is also required.

Vpopmail support can be added by checking dot-qmail files for the presence of vdeliver and bounce-no-mailbox. In a similar fashion, fastforward and bouncesaying can be supported at SMTP level. Bouncesaying is used by Plesk.

List of patches and plugins for qmail SMTP daemons

I've divided the solutions into qmail support levels. Level 0 supports only lists of specific email addresses, level 4 solutions are (almost) drop-in compatible with existing setups.

Before installing any solution, carefully read the documentation (if there is any) and/or source code to determine if everything you need is sufficiently supported.

As you know, programmers start counting at 0 :)

Level 0

Level 0 solutions are limited to checking specific user@domain, without support for catch-all addresses.

db/user plugin for qpsmtpd

Written in Perl, checks against an SQL database


magic-smtpd, support built in

Written in C, checks against a Berkely DB.


check_user_cdb plugin for qpsmtpd

Written in Perl, checks against a CDB


check_vuser plugin for qpsmtpd

Written in Perl, checks against a directory tree.


Level 1

Level 1 solutions are limited to checking user@domain and also provide support for catch-all domains (anything@domain) and sometimes catch-all extensions (user-anything@domain), but not specific user extensions (user-extension@domain).

goodrcptto patch for qmail-smtpd

Written in C, checks against a text file or CDB.


validrcppto patch for qmail-smtpd

Written in C, checks against a text file.


validrcptto.cdb patch for qmail-smtpd

Written in C, checks against a CDB.


aliases plugin for qpsmtpd

Written in Perl, checks against a text file with aliases and expands them.


check_dns_user plugin for qpsmtpd

Written in Perl, checks against a DNS zone. Supports catch-all domains if you use wildcard hostnames in the zone.


check_goodrcptto plugin for qpsmtpd

Written in Perl, checks against a text file. Supports catch-all extensions, but not specific extensions.


qmail-ldap suite

Written in C, checks against an LDAP database.


Level 2

Level 2 solutions do everything that level 1 solutions do, but also provide support for prefixed wildcards, like user-anything@domain. Level 2 is the minimum level of support needed for good integration with existing qmail installations and things like EZMLM, but may still require a certain specific setup like a user database or vpopmail.

Dot-qmail files belonging to users may not be accessible to the SMTP daemon. This limitation can be lifted by pregenerating a list or database, by using a special purpose setuid root program, or a daemon that runs as root.

vpopmail_check_recipient-wm plugin for qmail-spp

Written in sh, checks against vpopmail executables and dot-qmail.

Supports .qmail-default, of course handling vpopmail's vdeliver, and has hard coded support for ezmlm, and resulting from the way that was written, limited support for .qmail-ext-default. It assumes a strictly vpopmail-ized setup with no local users or local aliases. Requires that the vpopmail utilities are setuid, including vuserinfo which displays user passwords, clear text if available. That makes it unsafe for use on any system that has local users other than the system administrators.


local_check_recipient plugin for qmail-spp

Written in sh, checks against a directory tree and dot-qmail.

This is like vpopmail_check_recipient, but for local users only. It does not actually query /var/qmail/users/assign or getpw for home directories, but requires /home/username, where username is directly taken from the email address. Probably needs to be suid root to function correctly.


check_recipient plugin for qmail-spp

Written in Perl, checks against partial qmail-local logic.

Uses virtualdomains and users/assign logic, but does not query getpw for local users not listed there. With its limited set of allowed characters, it would immediately disqualify a local-part like the ##### in my email address. Having ~alias/.qmail-default turns everything into one big catch all with this script, disregarding users/assign logic and .qmail-ext for local users. Supports .qmail-default, but not .qmail-ext-default.

Cannot be used for relay servers, because a bug causes it to disallow all remote addreses. (It should read locals and virtualdomains to find out if a domain is local).

Not networked.


vpopmail-check-user script for magic-smtpd

Written in sh, checks against vpopmail executables.

Appears to be a proof of concept only, since it does not support domain catch-alls. (If I were strict, it wouldn't even classify as level 1.) Has to run setuid vpopmail.


rcpt_regexp plugin for qpsmtpd.

Written in Perl, checks against a text file with regular expressions or static strings.


rcpt_ldap plugin for qpsmtpd

Written in Perl, checks against an LDAP database. Supports extensions if you craft an ldap filter that does.


Level 3

Level 3 solutions do everything that level 2 solutions do, but fully support qmail-local logic, and can be added to existing local delivery mailservers without changing their setup. This does, of course, assume that the mail setup is based on qmail's structure and not a heavily patched system that changes the delivery rules, as is the case with qmail-ldap (although that has its own recipient checking).

check_delivery plugin for qpsmtpd

Written in Perl, checks against qmail-local logic using a suid root program.

No longer developed or maintained. The documentation says that .qmail-default files are only fully supported for the alias user, but there does seem to be code for supporting .qmail-ext-default.

It requires an external suid root program, dot-qmail-exists.pl, that seems to have vanished from the web. Even a re-implementation because the original could not be found, is either no longer there, or very well hidden.

Does not reach level 4 because it checks rcpthosts, not locals+virtualhosts (which would be okay if it also checked smtproutes, but it doesn't).



Level 4

Level 4 solutions do everything that level 3 solutions do, but also support qmail-send logic. That is: non-local email is accepted, so that it can be relayed to another server. This makes the solutions (almost) drop-in compatible with most qmail setups.

qmail-realrcptto patch for qmail-smtpd

Written in C, checks against qmail-local logic.

Runs under the same user as qmail-smtpd and as such may not be able to access users' files. It does not have any special support for vpopmail, fastforward, or bouncesaying, i.e. all .qmail-default files are considered catch all.


qmail-verify daemon + patch for qmail-smtpd

Written in C, checks against qmail-local logic and dot-qmail using a daemon.

In hindsight, my solution was probably mostly inspired by this nice piece of software. A light weight daemon works together with a patched qmail-smtpd, and they communicate through UDP. A single bit, 0 or 1, determines the fate of the current message. While possibly susceptible to spoofing and race conditions, that's unlikely because all the malicious hacker can probably accomplish, is that email is rejected.

Unfortunately it has no support for vpopmail, bouncesaying, or fastforward.

This solution is networked and lets you pick the server based on the connecting host. That's almost sufficient for using the client on a mail hub, which requires some kind of domain to qmail-verify host mapping.


check_validrcptto_cdb plugin for qpsmtpd

Written in Perl, checks against a CDB. Together with mkvalidrcptto, qualifies as level 4.


check_qmail_deliverable plugin for qpsmtpd

Written in Perl, checks against qmail-send and qmail-local logic and dot-qmail using a daemon.

This is the plugin that comes with the Qmail::Deliverable distribution. It uses the bundled qmail-deliverabled daemon which is like qmail-verify, but heavier. It inherits all supported features from Qmail::Deliverable::deliverable, which means that it works well with vpopmail and bouncesaying, but not fastforward.

Its networking can be configured to look in the smtproutes configuration file to find the server to query, so it can easily be used on a mail hub.

This is the only solution that I know of, that separates the core functionality and provides it as a software library -- in this case, a Perl module. This makes writing a similar plugin for qmail-spp or magic-smtpd relatively simple.


Unknown support level

chkrcptto plugin for qmail-spp

Closed source, distributed with Plesk. I have no idea how well it would work with a non-plesk setup.


Recipient list generators

These programs read your qmail configuration files and generate a list of valid recipients.


Written in Perl, generates according to qmail-local logic and dot-qmail.

Has support for vpopmail and fastforward, but not for bouncesaying. It understands domain aliases, but keeps .qmail-ext-default as ext-default@domain, which only the check_validrcptto_cdb plugin for qpsmtpd handles.



Please contact me if any information listed here is wrong, or if you know of other alternatives for qmail that reduce backscatter by checking if a recipient is valid.


Thanks to all the authors of the tools listed on this page. I've used them for inspiration while thinking about writing my solution, and their software has already prevented a lot of CPU usage, memory consumption, queue capacity, data traffic, and personal annoyance that spam backscatter causes.

Thanks to people who have contributed information for this document. They are, in chronological order: Robin Bowes, Hanno Hecker, Guy Hulbert, Ernesto, Peter J. Holzer.


Juerd Waalboer <#####@juerd.nl>


Qmail::Deliverable, http://web.netdevice.com:9080/qmail/rcptck/, http://spamlinks.net/prevent-secure-backscatter.htm