On the smtp client package, there is no limit to the length of the
server's replies, so an evil server could cause a memory exhaustion DoS
by issuing very long lines.
This patch fixes the bug by limiting the total size of received data.
Ideally this would be done per-line instead, but gets much more complex,
so this is a compromise.
The limit chosen is 2 MiB, which should be plenty for any the total size
of server-side replies, considering we only send a single message per
connection anyway.
This is similar to 06d808c (smtpsrv: Limit incoming line length), which
was found and reported by Max Mazurov (fox.cpp@disroot.org).
This patch is the result of running go vet, go fmt -s and the linter,
and fixing some of the things they noted/suggested.
There shouldn't be any significant logic changes, it's mostly
readability improvements.
This patch makes the SMTP courier distinguish permanent errors, so the queue
can decide whether to retry or not.
It's based on the SMTP replies: 5xy is considerent permanent, anything else is
transient (including errors other than protocol issues).
This patch adds SMTPUTF8 to the SMTP courier.
It introduces a new internal/smtp package that extends Go's net/smtp with
SMTPUTF8 (in a very narrow way, not for general use), and then makes the
courier use it.
Also use an IDNA-safe version when saying hello, otherwise servers could
complain if the hostname is not ASCII, and at that stage we don't know if they
support it or not.
