Description of problem: When sendmail first tries to start right after system boot it does not appear to make use of the IPv6 nameservers configured on my system. Therefore any mail sent receives: 451 4.1.8 Domain of sender address dan does not resolve A restart of sendmail cures the problem. This occurs when resolv.conf contains only IPv6 nameservers. If IPv4 nameservers are added prior to the v6 listings the problem does not occur. I enabled the NetworkManager-wait-online.service but it did not help. I then booted the host using only the v6 nameservers, had tcpdump running, and saw that on a connection to port 25, the recipient destination was not being queried on the v6 DNS listed in resolv.conf. There was no DNS traffic. I restarted sendmail and then the v6 queries worked properly. The DNS servers, whether IPv4 or 6 are being provided by NetworkManager.
Confirmed that this is something between sendmail and NetworkManager. Disabled NetworkManager.service and enabled the older network.service and the problem evaporates.
I have a similar problem with only IPv4 name servers. After a reboot mails are deferred: From mailq: (host map: lookup (yahoo.com): deferred) The log shows: --- 451 yahoo.com: Name server timeout --- 050 <XXXXXXXXXX>... Transient parse error -- message queued for future delivery --- 451 yahoo.com: Name server timeout I too tried the NetworkManager-wait-online.service and increased the time out to 300 seconds. But that didn't help either. I also tried to increase the sendmail resolver retries: O Timeout.resolver.retry.first=40000 Assuming that this would cause sendmail to keep retying for 5*40000 seconds. I then disabled NetworkManager services and enabled the old network.service and now sendmail works without a restart after reboot. So there is definitely a problem with the interaction between sendmail and NetworkManager. Only problem is what and how to debug it further.
This problem is rather strange. I thought that the most likely reason for this problem to appear would be that the file /var/run/NetworkManager/resolv.conf (pointed to by the symbolic link /etc/resolv.conf) is created after sendmail starts thus making the resolver fail. That would explain why increasing the resolver retries didn't help. So I disabled network.service and enabled NetworkManager.service and rebooted. Now sendmail works without a restart and examining the time stamps of /var/run/NetworkManager/resolv.conf and the start up time of sendmail shows that sendmail starts after the creation of /var/run/NetworkManager/resolv.conf. Mar 20 17:09:47 www sm-msp-queue[2448]: starting daemon (8.15.2): queueing@01:00:00 Mar 20 17:09:49 www sendmail[2435]: NOQUEUE: stopping daemon, reason=signal Mar 20 17:09:49 www sendmail[2532]: starting daemon (8.15.2): SMTP+queueing@01:00:00 Mar 20 17:09:49 www sendmail[2532]: STARTTLS: CRLFile missing Mar 20 17:09:49 www sendmail[2532]: STARTTLS=server, Diffie-Hellman init, key=2048 bit (I) Mar 20 17:09:49 www sendmail[2532]: STARTTLS=server, init=1 Mar 20 17:09:49 www sendmail[2532]: started as: /usr/sbin/sendmail -bd -q1h Mar 20 17:09:49 www sm-msp-queue[2544]: starting daemon (8.15.2): queueing@01:00:00 # stat /var/run/NetworkManager/ Fil: '/var/run/NetworkManager/' Stør: 60 Blokke: 0 IO-blokke: 4096 katalog Enhed: 15t/21d Inode: 24823 Lænker: 2 Adgang: (0755/drwxr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root) Adgang: 2017-03-20 17:09:39.355124166 +0100 Redig.: 2017-03-20 17:09:42.728226631 +0100 Ændret: 2017-03-20 17:09:42.728226631 +0100 Opret.: - # stat /var/run/NetworkManager/resolv.conf Fil: '/var/run/NetworkManager/resolv.conf' Stør: 68 Blokke: 8 IO-blokke: 4096 almindelig fil Enhed: 15t/21d Inode: 30719 Lænker: 1 Adgang: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Adgang: 2017-03-20 17:13:55.974222842 +0100 Redig.: 2017-03-20 17:09:42.728226631 +0100 Ændret: 2017-03-20 17:09:42.728226631 +0100 Opret.: -
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