This patch adds support for TLS-wrapped submission connections.
Instead of clients establishing a connection over plain text and then
using STARTTLS to switch over a TLS connection, this new mode allows the
clients to connect directly over TLS, like it's done in HTTPS.
This is not an official standard yet, but it's reasonably common in
practice, and provides some advantages over the traditional submission
port.
The default port is 465, commonly used for this; chasquid defaults to
systemd file descriptor passing as for the other protocols (for now).
The server is written assuming there's at least one valid SSL/TLS
certificate. For example, it unconditionally advertises STARTTLS, and
only supports AUTH over TLS.
This patch makes the server fail to listen if there are no certificates
configured, so the users don't accidentally run an unsupported
configuration.
Picking the domain used in the DSN message "From" header is more
complicated than it needs to be, causing confusing code paths and having
different uses for the hostname, which should be purely aesthetic.
This patch makes the queue pick the DSN "From" domain from the message
itself, by looking for a local domain in either the sender or the
original recipients. We should find at least one, otherwise it'd be
relaying.
This allows the code to be simplified, and we can narrow the scope of
the hostname option even further.
glog works fine and has great features, but it does not play along well
with systemd or standard log rotators (as it does the rotation itself).
So this patch replaces glog with a new logging module "log", which by
default logs to stderr, in a systemd-friendly manner.
Logging to files or syslog is still supported.
The default INFO logs are more oriented towards debugging and can be
a bit too verbose when looking for high-level information.
This patch introduces a new "maillog" package, used to log messages of
particular relevance to mail transmission at a higher level.
This patch implements a post-DATA hook, which is run after receiving the
data but before sending a reply.
It can be used to implement content filtering when receiving email, for
example for passing the email through an anti-spam or an anti-virus.
Today, we pick the domain used to send the DSN from based on what we
presented to the client at EHLO time, which itself may be based on the
TLS negotiation (which is not necessarily trusted).
This is complex, not necessarily correct, and involves passing the
domain around through the queue and persisting it in the items.
So this patch simplifies that handling by always using the main domain
as specified by the configuration.
This patch moves chasquid's Server and Conn structures to their own
smtpsrv package, to make chasquid.go a bit more readable. It also helps
clarify the relation between Server and Conn.
There are no functional changes.
Note that git can still track the history across this commit (e.g. git
gui blame shows the right data).
