qmail is a mail transfer agent that runs on Unix. It was written, starting December 1995, by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program. qmail's source code is released to the public domain, making qmail free software.
When first published, qmail was the first security-aware mail transport agent; since then, other security-aware MTA
's have been published. The most popular predecessor to qmail, Sendmail
, was not designed with security as a goal, and as a result has been a perennial target for attackers. In contrast to sendmail, qmail has a modular architecture composed of mutually untrusting components; for instance, the SMTP
listener component of qmail runs with different credentials
than the queue manager, or the SMTP sender. qmail was also implemented with a security-aware replacement to the C standard library
, and as a result has not been vulnerable to stack and heap overflows, format string attacks, or temporary file race conditions..
When it was released, qmail was significantly faster than Sendmail, particularly for bulk mail tasks such as mailing list servers. qmail was originally designed as a way for managing large mailing lists.
Prior to qmail, the most popular Internet mail server was Sendmail. Sendmail configuration had been notoriously complex, involving one of the most complicated configuration file formats facing Unix systems administrators. In contrast, qmail was configured using a series of small flat files. For common configurations, in many circumstances, qmail was significantly easier to configure and deploy.
qmail encourages the use of several innovations in mail (some originated by Bernstein, others not): Maildir
- Bernstein invented the Maildir format for qmail, which splits individual email messages into separate files. Unlike the de facto standard Mbox format, which stored all messages in a single file, Maildir avoids many locking and concurrency problems, and can safely be provisioned over NFS. qmail also delivers to Mbox mailboxes. Wildcard mailboxes
- qmail introduced the concept of user-controlled wildcards. Out of the box, mail addressed to "user-wildcard" on qmail hosts is delivered to separate mailboxes, allowing users to publish multiple mail addresses for mailing lists and spam management.
qmail also introduces the Quick Mail Transport Protocol (QMTP) and Quick Mail Queuing Protocol (QMQP) protocols.
qmail is nearly a completely modular system in which each major function is separated from the other major functions. It is easy to replace any part of the qmail system with a different module as long as the new module retains the same interface as the original.
qmail was designed as a pointed response to Sendmail, an extremely popular and somewhat beloved piece of Unix software. Author Bernstein was not shy about pointing out the deficiencies in Sendmail's design and the superior characteristics of qmail, nor did he take pains to replicate Sendmail's behavior, which at the time was the de facto standard for Internet mail delivery. As a result, qmail came under unusually intense scrutiny.
Security reward and Georgi Guninski's vulnerability
Bernstein offered a US$500 reward for the first person to publish a verifiable security hole
in the latest version of the software.
In 2005, security researcher Georgi Guninski found an integer overflow in qmail. On 64-bit platforms, in certain configurations of disputed realism (including absence of resource limits and unusually large amounts of available virtual memory), the delivery of huge amounts of data to certain qmail components may allow remote code execution. Bernstein disputes that this is a practical attack, arguing that no real-world deployment of qmail would be susceptible. Configuration of resource limits for qmail components mitigates the vulnerability.
On November 1, 2007, Bernstein raised the reward to US$1000. At a slide presentation the following day, Bernstein stated that there were 4 "known bugs" in the ten year old qmail-1.03, none of which were "security holes." He characterized the bug found by Guniski as a "potential overflow of an unchecked counter." "Fortunately, counter growth was limited by memory and thus by configuration, but this was pure luck."
Frequency of updates
The core qmail package has not been updated for many years. New features are provided by third party patches, such as net-qmail. This is a benefit for some users, from not needing to apply patches continuously, and a liability for other users, particularly those who rely on authentication mechanisms that post-date qmail.
qmail was designed to replace Sendmail
, but does not behave exactly as Sendmail
did in all situations. In some cases, these differences in behavior have become grounds for criticism. For instance, qmail's approach to bounce messages (a format called QSBMF) differs from the standard format of delivery status notifications
specified by the IETF
in RFC 1894, meanwhile advanced to draft standard
as RFC 3464, and recommended in the SMTP
Furthermore, some qmail features have been criticized for introducing mail forwarding complications; for instance, qmail's "wildcard" delivery mechanism and security design prevents it from rejecting messages to nonexistent senders during SMTP transactions. In the past, these differences may have made qmail behave differently when abused as a spam relay, though modern spam delivery techniques are less influenced by bounce behavior.
Despite these concerns, qmail is among the five most popular Unix mail servers, forwarding a significant fraction of all of the email on the Internet.
qmail was released to the public domain
in November 2007. Formerly, qmail was license-free software
, which granted permission for distribution in source form or in pre-compiled form (a "var-qmail package") only if certain restrictions were met, primarily involving compatibility.
qmail is the only broadly deployed MTA in the public domain.