Postfix in a container
I want to run postfix in a container as a mail relay for my network. Here are the problems I ran in to and how I solved them.
I want to run postfix in a container as a mail relay for my network. Here are the problems I ran in to and how I solved them.
I had a couple of requirements, run the latest postfix and store the configuration outside of the container so it would be easy to run in my Kubernetes cluster.
My Dockerfile
was a simple one, based on Debian 10.8. Ran apt update
, apt upgrade -y
and apt install -y postfix
then set the container to run postfix in the foreground.
Here it is:
FROM debian:10.8
RUN apt update && \
apt upgrade -y && \
apt install -y postfix
RUN mkdir /config && \
mv /etc/postfix/main.cf /config/main.cf && \
mv /etc/postfix/master.cf /config/master.cf && \
ln -s /config/main.cf /etc/postfix/main.cf && \
ln -s /config/master.cf /etc/postfix/master.cf
CMD ["postfix", "start-fg"]
You may notice that I moved main.cf
and master.cf
into a /config
directory. This is so I can mount a config map from Kubernetes into the container without blowing up the rest of /etc/postfix
.
Next, you want to get it to log to the console, in your main.cf
put in this line:
maillog_file=/dev/stdout
I created my image and pushed it to my private registry. And now postfix started in a container.
I wanted it to be a relay for everything on my network so I built my configuration to do the relay but it didn't work. I was getting this cryptic message:
smtp postfix/qmgr[105]: warning: private/relay socket: malformed response
smtp postfix/master[1]: warning: process /usr/lib/postfix/sbin/smtp pid 111 exit status 1
smtp postfix/master[1]: warning: /usr/lib/postfix/sbin/smtp: bad command startup -- throttling
smtp postfix/qmgr[105]: warning: transport relay failure -- see a previous warning/fatal/panic logfile record for the problem description```
With that I started ramping up the logging. In the master.cf
file I started my troubleshooting by adding -v
to qmgr
hoping to find something there. Not much, but some additional detail.
smtp postfix/qmgr[105]: warning: private/relay socket: malformed response
smtp postfix/master[1]: warning: process /usr/lib/postfix/sbin/smtp pid 111 exit status 1
smtp postfix/master[1]: warning: /usr/lib/postfix/sbin/smtp: bad command startup -- throttling
smtp postfix/qmgr[105]: qmgr_transport_throttle: transport relay: status: 4.3.0 reason: unknown mail transport error
smtp postfix/qmgr[105]: warning: transport relay failure -- see a previous warning/fatal/panic logfile record for the problem description
smtp postfix/qmgr[105]: defer transport relay: 4.3.0 unknown mail transport error
smtp postfix/qmgr[105]: defer site example-com.mail.protection.outlook.com: 4.3.0 unknown mail transport error
There wasn't anything that helped, just thought it was interesting that I'm now seeing a new error. Next I removed the -v
from qmgr
and added -v
to the smtp
entries. That yielded an error that helped out, I started seeing this:
fatal: unknown service: smtp/tcp
After some hunting, I saw some places that said that we need to copy the /etc/services
into /var/spool/postfix/etc
. So I did that. Then I started getting these next errors (some lines are omitted).
smtp postfix/relay/smtp[118]: dns_query: example-com.mail.protection.outlook.com (MX): Host not found, try again
.
.
.
smtp postfix/relay/smtp[118]: send attr reason = Host or domain name not found. Name service error for name=example-com.mail.protection.outlook.com type=MX: Host not found, try again
.
.
.
smtp postfix/relay/smtp[118]: CDEFD2D83: to=<xxx@example.com>, relay=none, delay=0.03, delays=0.02/0.01/0/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=example-com.mail.protection.outlook.com type=MX: Host not found, try again)
.
.
.
smtp postfix/relay/smtp[118]: deliver_request_final: send: "Host or domain name not found. Name service error for name=example-com.mail.protection.outlook.com type=MX: Host not found, try again" -1
.
.
.
smtp postfix/relay/smtp[118]: send attr reason = Host or domain name not found. Name service error for name=example-com.mail.protection.outlook.com type=MX: Host not found, try again
These messages tell me that it cannot resolve my relay address. I then copied my /etc/resolv.conf
into that same directory, /var/spool/postfix/etc
. And suddenly it all worked. I did not like this option, it would mean needing a startup script in my container and thus increasing complexity and maintenance. After thinking about it, it seemed like it was being chroot
ed into /var/spool/postfix
. After some digging in to how to chroot
postfix processes I found that you do it in the master.cf
. All I needed to do was change the chroot
field of the smtp
command entries to n
, restart my container, and finally everything worked.
Configuration files
I have documented my config files below. These are very minimal and work as a simple relay in a Debian container with no frills, no authentication, no SSL, etc.
main.cf
:
compatibility_level = 2
queue_directory = /var/spool/postfix
data_directory = /var/lib/postfix
mail_owner = postfix
maillog_file = /dev/stdout
mynetworks = 127.0.0.0/8,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16
relayhost = example-com.mail.protection.outlook.com
myhostname = relay.example.com
setgid_group = postdrop
inet_protocols = ipv4
mail_name = My mail relay
master.cf
:
smtp inet n - y - - smtpd
pickup unix n - y 60 1 pickup
cleanup unix n - y - 0 cleanup
qmgr unix n - n 300 1 qmgr
#qmgr unix n - n 300 1 oqmgr
tlsmgr unix - - y 1000? 1 tlsmgr
rewrite unix - - y - - trivial-rewrite
bounce unix - - y - 0 bounce
defer unix - - y - 0 bounce
trace unix - - y - 0 bounce
verify unix - - y - 1 verify
flush unix n - y 1000? 0 flush
proxymap unix - - n - - proxymap
proxywrite unix - - n - 1 proxymap
smtp unix - - n - - smtp
relay unix - - n - - smtp
-o syslog_name=postfix/$service_name
showq unix n - y - - showq
error unix - - y - - error
retry unix - - y - - error
discard unix - - y - - discard
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - y - - lmtp
anvil unix - - y - 1 anvil
scache unix - - y - 1 scache
postlog unix-dgram n - n - 1 postlogd
maildrop unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/bin/maildrop -d ${recipient}
uucp unix - n n - - pipe
flags=Fqhu user=uucp argv=uux -r -n -z -a$sender - $nexthop!rmail ($recipient)
ifmail unix - n n - - pipe
flags=F user=ftn argv=/usr/lib/ifmail/ifmail -r $nexthop ($recipient)
bsmtp unix - n n - - pipe
flags=Fq. user=bsmtp argv=/usr/lib/bsmtp/bsmtp -t$nexthop -f$sender $recipient
scalemail-backend unix - n n - 2 pipe
flags=R user=scalemail argv=/usr/lib/scalemail/bin/scalemail-store ${nexthop} ${user} ${extension}
mailman unix - n n - - pipe
flags=FR user=list argv=/usr/lib/mailman/bin/postfix-to-mailman.py
${nexthop} ${user}
Conclusion
It took some time to figure out everything I needed to do to get the relay working. Now that it is, I'm happy with it, I think it will suit my needs very well. Now to move it in to my Kubernetes cluster.