DNS lookups fail

Question

DNS lookups fail

regnete opened this issue 4 years ago · 12 comments

docker-compose yml

services:
  smtp:
    image: juanluisbaptiste/postfix
    environment:
      SERVER_HOSTNAME: "xxx"
      SMTP_SERVER: 'smtp.mydomain.intranet'
      SMTP_PORT: '587'
      SMTP_USERNAME: 'someuser'
      SMTP_PASSWORD: "secret"
    extra_hosts:
      - "smtp.mydomain.intranet:192.168.5.1"

Describe the issue
postfix cannot resolve the host smtp.mydomain.intranet
I guess this happens due to the issue decribed here: https://serverfault.com/a/1004330

Expected behavior
postfix resolves the host smtp.mydomain.intranet to 192.168.5.1

juanluisbaptiste commented 4 years ago

Fixed.

Answer 1 · 2021-03-09T16:35:20.000Z

@regnete , for me resolution is working fine. Using your example compose file, entering a container and doing a ping to smtp.mydomain.intranet resolves correctly:

#  ping smtp.mydomain.intranet
PING smtp.mydomain.intranet (192.168.5.1): 56 data bytes

Answer 2 · 2021-03-10T05:38:30.000Z

Thx for the quick answer. PING is not the problem. Postfix cannot send emails to the given host. It fails with a DNS resolution error.

Answer 3 · 2021-03-10T16:15:52.000Z

Can you please post the error message you are getting ?

Answer 4 · 2021-03-11T13:56:14.000Z

Mar 11 13:53:41 smtp postfix/smtp[102]: 08E88B1F2CE: to=<xxx@gmail.com>, relay=none, delay=0.1, delays=0.07/0.02/0.01/0, dsn=4.3.5, status=deferred (Host or domain name not found. Name service error for name=smtp.mydomain.intranet type=A: Host not found)

As docker adds extra_hosts to the /etc/hosts, I guess this happens due to the issue decribed here: https://serverfault.com/a/1004330

Answer 5 · 2021-03-11T17:22:28.000Z

Mar 11 13:53:41 smtp postfix/smtp[102]: 08E88B1F2CE: to=<xxx@gmail.com>, relay=none, delay=0.1, delays=0.07/0.02/0.01/0, dsn=4.3.5, status=deferred (Host or domain name not found. Name service error for name=smtp.mydomain.intranet type=A: Host not found)

Ok I was able to reproduce it, I will look into it.

As docker adds extra_hosts to the /etc/hosts, I guess this happens due to the issue decribed here: https://serverfault.com/a/1004330

This solution does not work because it copies the resolv.conf file on image build, and every person resolv.conf file can be different.

Answer 6 · 2021-03-12T06:04:16.000Z

Correct. But what if you copy the file on container start? You could even link it.

Answer 7 · 2021-03-12T15:56:08.000Z

I tested that on a running container, symlinking /etc/resolv.conf and /etc/hosts to /var/spool/postfix/etc/ and restarted the postfix process but it did not work. I have to investigate this further.

Answer 8 · 2021-03-13T02:17:04.000Z

I tested that on a running container, symlinking /etc/resolv.conf and /etc/hosts to /var/spool/postfix/etc/ and restarted the postfix process but it did not work. I have to investigate this further.

This will not work in any case because:

a) The resolv.conf file is to set the DNS server to use, and this case is about adding an entry to /etc/hhosts using docker-compose's extra_hosts config parameter.
b) Adding /etc/hosts will not work either because postfx does not read it as it does name server queries directly, not through the os by default.

Answer 9 · 2021-03-15T10:38:24.000Z

http://www.postfix.org/postconf.5.html#smtp_host_lookup

Answer 10 · 2021-03-17T21:51:05.000Z

http://www.postfix.org/postconf.5.html#smtp_host_lookup

Yes, I found that took last weekend, tested it and it works. Now I need to make sure that changing smtp_host_lookup = native does not break anything else.

Answer 11 · 2021-03-18T06:05:33.000Z

Great. Thx. Works now as expected!