[gentoo-user] Sendmail confused by network change

[gentoo-user] Sendmail confused by network change

[Bas Zoutendijk]

Dear Gentoo Users,

On my  new Gentoo laptop  installation I recently  installed Sendmail in
order to receive messages from Cron on the root account.  I noticed that
when I connect my laptop to a different network than the one I connected
to during booting,  Sendmail does not know what to do with the Cron mail
any more.

For the purpose  of clarity,  let’s say the host name  of this laptop is
‘hostname’.  I  did not  configure the  domain  part  of  the  host name
because of the mobile nature of this machine.

When  I  boot at  home,  Cron  sends  mail  to root@hostname.homedomain.
‘homedomain’ is automatically added to all host names on my home network
by the router.  It can only be resolved inside the network;  it is not a
registered domain name.  I can receive mail from Cron just fine.

When I boot  at work,  Cron sends mail to  root@hostname.  Note that the
domain name  ‘workdomain’ is not added  to the  host name.  I  can still
receive  Cron  mail.  However,  when  I  take  the  laptop  home without
rebooting and connect to the home network, Sendmail is unable to deliver
the Cron  mail for root@hostname.homedomain  and sends  notifications of
this  to  root@hostname.homedomain,  which  somehow  do  seem  to arrive
without problems.  The error message  is “config error:  mail loops back
to me”.

Based on what I can find about this error on the internet, it looks like
Sendmail does not  know where hostname.homedomain is and  asks my router
to resolve that.  When it finds out it is localhost, it thinks something
is wrong and  does  not  deliver  the  mail.  A  possible solution is to
register hostname.homedomain as an alias of hostname or localhost, but I
would rather  not do that,  since hard  coding domain names  on a laptop
seems kludgy to me.

Does anyone know  a more elegant solution?  Some way  to inform Sendmail
about changes  to the domain name,  or  configure it to  check for these
changes?  I  would rather  not have  to reboot.  Restarting  Sendmail is
acceptable, I guess, but perhaps not the most elegant solution.

Sincerely,

Bas

-- 
Sebastiaan L. Zoutendijk | slzoutendijk@gmail.com

Re: [gentoo-user] Sendmail confused by network change

[John Covici]

On Wed, 20 Sep 2017 05:45:07 -0400,
Bas Zoutendijk wrote:
> 
> Dear Gentoo Users,
> 
> On my  new Gentoo laptop  installation I recently  installed Sendmail in
> order to receive messages from Cron on the root account.  I noticed that
> when I connect my laptop to a different network than the one I connected
> to during booting,  Sendmail does not know what to do with the Cron mail
> any more.
> 
> For the purpose  of clarity,  let’s say the host name  of this laptop is
> ‘hostname’.  I  did not  configure the  domain  part  of  the  host name
> because of the mobile nature of this machine.
> 
> When  I  boot at  home,  Cron  sends  mail  to root@hostname.homedomain.
> ‘homedomain’ is automatically added to all host names on my home network
> by the router.  It can only be resolved inside the network;  it is not a
> registered domain name.  I can receive mail from Cron just fine.
> 
> When I boot  at work,  Cron sends mail to  root@hostname.  Note that the
> domain name  ‘workdomain’ is not added  to the  host name.  I  can still
> receive  Cron  mail.  However,  when  I  take  the  laptop  home without
> rebooting and connect to the home network, Sendmail is unable to deliver
> the Cron  mail for root@hostname.homedomain  and sends  notifications of
> this  to  root@hostname.homedomain,  which  somehow  do  seem  to arrive
> without problems.  The error message  is “config error:  mail loops back
> to me”.
> 
> Based on what I can find about this error on the internet, it looks like
> Sendmail does not  know where hostname.homedomain is and  asks my router
> to resolve that.  When it finds out it is localhost, it thinks something
> is wrong and  does  not  deliver  the  mail.  A  possible solution is to
> register hostname.homedomain as an alias of hostname or localhost, but I
> would rather  not do that,  since hard  coding domain names  on a laptop
> seems kludgy to me.
> 
> Does anyone know  a more elegant solution?  Some way  to inform Sendmail
> about changes  to the domain name,  or  configure it to  check for these
> changes?  I  would rather  not have  to reboot.  Restarting  Sendmail is
> acceptable, I guess, but perhaps not the most elegant solution.

Restarting sendmail seems fine with me, if you want to have something
that works everywhere, why not get a domain name from ddns or
somewhere and use a full fqdn all the time -- you can put  your home
machine on another host in that domain and you will be good to go.

-- 
Your life is like a penny.  You're going to lose it.  The question is:
How do
you spend it?

         John Covici
         covici@ccs.covici.com

[gentoo-user] Re: Sendmail confused by network change

[Ian Zimmerman]

On 2017-09-20 11:45, Bas Zoutendijk wrote:

> When I boot at home, Cron sends mail to root@hostname.homedomain.
> ‘homedomain’ is automatically added to all host names on my home
> network by the router.  It can only be resolved inside the network; it
> is not a registered domain name.  I can receive mail from Cron just
> fine.
> 
> When I boot at work, Cron sends mail to root@hostname.  Note that the
> domain name ‘workdomain’ is not added to the host name.  I can still
> receive Cron mail.  However, when I take the laptop home without
> rebooting and connect to the home network, Sendmail is unable to
> deliver the Cron mail for root@hostname.homedomain and sends
> notifications of this to root@hostname.homedomain, which somehow do
> seem to arrive without problems.  The error message is “config error:
> mail loops back to me”.

According to crontab(5), you can configure where the mail is sent, by
setting the MAILTO variable in the crontab file.  [This is for cronie, I
am not sure if other cron variants do this.]  So, you could tell it to
send to <me@localhost>; presumably this would solve the problem.

Despite being one of the retro/traditionalist guys on this list, I have
to say installing sendmail just for this purpose doesn't seem
proportionate.  Even if you don't want to look into the smallish MTA
packages (nullmailer etc.), you can make /usr/sbin/sendmail to be a
pretty trivial script (or a link to one) that just locks the destination
mailbox and appends the input to it.  Heck, procmail could do that.

-- 
Please don't Cc: me privately on mailing lists and Usenet,
if you also post the followup to the list or newsgroup.
Do obvious transformation on domain to reply privately _only_ on Usenet.

Re: [gentoo-user] Sendmail confused by network change

[Bas Zoutendijk]

Dear John,

On Wed 20 Sep 2017 at 07:05:11 -0400, John Covici wrote:
> Restarting sendmail seems fine with me, if you want to have something
> that works everywhere, why not get a domain name from ddns or
> somewhere and use a full fqdn all the time -- you can put  your home
> machine on another host in that domain and you will be good to go.

Thank you for  this suggestion.  I added ‘.localdomain’ to  my host name
and now Sendmail  uses  this  domain  name  on  all networks.  Cron mail
arrives again at home.

I did  not want to  try a ‘real’  domain name,  as  this could  make the
system  accessible  from outside  the  local  network,  if  I understand
correctly.  Right now  it is  hidden behind  the router  and only  has a
local IP address.  I would like to  keep it that way,  because then I do
not have to worry about network security settings.  Nevertheless you led
me to a solution.

Thank you,

Bas

-- 
Sebastiaan L. Zoutendijk | slzoutendijk@gmail.com

Re: [gentoo-user] Re: Sendmail confused by network change

[Bas Zoutendijk]

Dear Ian,

On Wed 20 Sep 2017 at 08:23:33 -0700, Ian Zimmerman wrote:
> Despite being one of the retro/traditionalist guys on this list, I have
> to say installing sendmail just for this purpose doesn't seem
> proportionate.  Even if you don't want to look into the smallish MTA
> packages (nullmailer etc.), you can make /usr/sbin/sendmail to be a
> pretty trivial script (or a link to one) that just locks the destination
> mailbox and appends the input to it.  Heck, procmail could do that.

I  appreciate  your  opinion.  I  normally  seek  to  avoid  unnecessary
complication and  find the  right tool  for the  job.  Since this  is my
first Gentoo install,  this  is the first time  I had to pick  an MTA by
myself.   Not  knowing  much   about  MTAs  I  though   Sendmail  was  a
common/traditional choice and would therefore probably serve my need.

I will check out nullmailer and other  MTAs to see if they better fit my
need without  adding extra complexity.  As I  wrote John  I was  able to
resolve the issue so I can take my time now.

Thank you,

Bas

-- 
Sebastiaan L. Zoutendijk | slzoutendijk@gmail.com

Re: [gentoo-user] Sendmail confused by network change

[J. Roeleveld]

On Wednesday, September 20, 2017 6:50:54 PM CEST Bas Zoutendijk wrote:
> Dear John,
> 
> On Wed 20 Sep 2017 at 07:05:11 -0400, John Covici wrote:
> > Restarting sendmail seems fine with me, if you want to have something
> > that works everywhere, why not get a domain name from ddns or
> > somewhere and use a full fqdn all the time -- you can put  your home
> > machine on another host in that domain and you will be good to go.
> 
> Thank you for  this suggestion.  I added ‘.localdomain’ to  my host name
> and now Sendmail  uses  this  domain  name  on  all networks.  Cron mail
> arrives again at home.

Glad it works now.

> I did  not want to  try a ‘real’  domain name,  as  this could  make the
> system  accessible  from outside  the  local  network,  if  I understand
> correctly.

To say it bluntly, you don't :)

> Right now  it is  hidden behind  the router  and only  has a
> local IP address.  I would like to  keep it that way,  because then I do
> not have to worry about network security settings.  Nevertheless you led
> me to a solution.

This is how it works.

To clarify:
The IP-address determines how it is accessible. If the IP is not accessible 
from the rest of the world, then you don't have to worry too much about 
securing your mailserver.

The domain name is only an address-book entry. Unless it is fully entered into 
the public one, noone will see the IP you are using. And as mentioned above, 
even if they can, the IP is sitting behind a router and is most likely in a 
private range.

--
Joost

Re: [gentoo-user] Sendmail confused by network change

[Bas Zoutendijk]

On Wed 20 Sep 2017 at 19:02:52 +0200, J. Roeleveld wrote:
> To clarify:
> The IP-address determines how it is accessible. If the IP is not accessible 
> from the rest of the world, then you don't have to worry too much about 
> securing your mailserver.
> 
> The domain name is only an address-book entry. Unless it is fully entered into 
> the public one, noone will see the IP you are using. And as mentioned above, 
> even if they can, the IP is sitting behind a router and is most likely in a 
> private range.

Good to know, thanks.

Sincerely,

Bas

-- 
Sebastiaan L. Zoutendijk | slzoutendijk@gmail.com