Postfix "Name service error for name=dovecot" in docker-compose

Losmoges

12/16/23, 8:02 PM

Context

My docker-compose --version is 2.13.0

I have created a reproducer in https://github.com/Losmoges/postfix-dovecot-lmtp-reproducer

My trial has three docker services in the compose file: postfix, dovecot, and client. They share a network through the bridge driver.

Postfix is configured to forward incoming emails to dovecot through the virtual_transport = lmtp:dovecot:24 setting in the main.cf configuration.
Dovecot is configured to receive connections through the service lmtp / inet_listener lmtp / port = 24 setting in the dovecot.conf configuration.
I can send an email through Postfix by executing echo test | msmtp [email protected] in the client container. Its msmtp is configured to connect to host postfix through a msmtprc configuration file.

This yields the following result in the Postfix queue, which I can inspect by running postqueue -p in the Postfix container.

Error

-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
DEC4F3DCBE      245 Fri Dec 16 19:41:17  [email protected]
(Host or domain name not found. Name service error for name=dovecot type=A: Host not found, try again)
                                         [email protected]

Workaround

When I find the IP address of the Dovecot container through docker inspect and enter it into the Postfix configuration, for example as virtual_transport = lmtp:172.18.0.4:24, everything works as expected. In this case, the command postqueue -p gives Mail queue is empty (after sending an email through msmtp)

Questions

Why does virtual_transport = lmtp:dovecot:24 not work? Did I misconfigure something? Any form of name-lookup, such as nslookup dovecot (after installing dnsutils) in the Postfix container works just fine and returns the IP address of the Dovecot container. Does Postfix do its own separate domain-name resolution? If so, how can I force it to use the default domain-name resolution?

Edit: Solution in Answer

I added the following line to the Dockerfile for the Postfix image

RUN sed -i'' -e 's/^lmtp .*/lmtp      unix  -       -       n       -       -       lmtp/g' /etc/postfix/master.cf

152

1 + 0

postfix

dovecot

docker

docker-compose

Score:1

Server

larsks

12/16/23, 9:45 PM

Why does virtual_transport = lmtp:dovecot:24 not work?

In your master.cf, you have lmtp configured to run in a chroot environment:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (no)    (never) (100)
lmtp      unix  -       -       n       -       -       lmtp

The chroot environment doesn't have access to the container's /etc/resolv.conf, so it doesn't know how to resolve hostnames. The lmtp configuration defaults to run in a chrooted environment because a typical configuration uses unix sockets to communicate, so hostname resolution isn't an issue.

The simplest fix is just to configure lmtp to not run in chroot environment:

lmtp      unix  -       -       y       -       -       lmtp

With this change, mail is delivered correctly:

postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/smtpd[598]: connect from unknown[172.28.0.1]
postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/smtpd[598]: 9131638672DA: client=unknown[172.28.0.1]
postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/qmgr[582]: 9131638672DA: from=<[email protected]>, size=434, nrcpt=1 (queue active)
postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/smtpd[598]: disconnect from unknown[172.28.0.1] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
dovecot_1  | Dec 16 21:47:15 lmtp(16): Info: Connect from 172.28.0.3
dovecot_1  | Dec 16 21:47:15 lmtp([email protected])<16><+A19J2PnnGMQAAAAqj6rOA>: Info: msgid=<20221216164715.079491@rocket>: saved mail to INBOX
postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/lmtp[597]: 9131638672DA: to=<[email protected]>, relay=dovecot[172.28.0.2]:24, delay=0.09, delays=0.07/0/0/0.01, dsn=2.0.0, status=sent (250 2.0.0 <[email protected]> +A19J2PnnGMQAAAAqj6rOA Saved)
dovecot_1  | Dec 16 21:47:15 lmtp(16): Info: Disconnect from 172.28.0.3: Logged out (state=READY)
postfix_1  | Dec 16 21:47:15 8d74be7a5951 postfix/qmgr[582]: 9131638672DA: removed

One alternative would be to configure a shared unix socket between the dovecot and postfix containers, and use that for communication rather than an inet connection.

A second alternative would be to run the lmtp container in the same network namespace as the postfix container, in which case you could simpler use the hostname localhost.

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