[SOLVED] Sending email without MTA?

littlebigman · 09-29-2021, 05:26 AM

Hello,

I use a cheap Debian computer as NAS to back up important files daily over the LAN.

I'd like to add a cron job so that the NSA checks disk usage, and sends an email the day the disk is 80% full.

Is there a way to send an email without setting up a whole MTA server (sendmail, Postfix, etc.)?

Thank you.

Code:

nas@debian:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       885G  395G  454G  47% /

wpeckham · 09-29-2021, 08:49 AM

It is possible, so long as it is possible to access a mail server directly. I have done it, by hand, using telnet, so it can be done. Certainly it should therefor be possible to code the process. Issues will come in if the mail server in question uses some of the new encryption and security features, because those cannot easily be coded or scripted.

Possible does not mean smart. The smarter option is to install and configure a minimal MTA set up to use your mail server as a SMARTHOST. MSMTP and EXIM are reasonable options, but there are others. That way proper logging and handling outbound email is easier and you can use standard tools instead of reinventing the horse.

littlebigman · 09-29-2021, 09:36 AM

Thank you.

I'll experiment, and see if I can send the email either directly to the target server, or through my ISP's SMTP server.

descendant_command · 09-29-2021, 01:27 PM

Yes, there are some very small, basic MTA's for just this reason.

ssmtp, msmtp, esmtp are examples.

wpeckham · 09-29-2021, 01:50 PM

Quote:

Originally Posted by descendant_command

Yes, there are some very small, basic MTA's for just this reason.

ssmtp, msmtp, esmtp are examples.

I liked SSMTP, but I do not think it is still under development and may not work on some recent distro releases. There are a lot of MTA options.

shruggy · 09-29-2021, 02:47 PM

Well, I'm still using SSMTP on CentOS 8 as it is packaged for Fedora and can be easily rebuilt from Fedora sources.

For a similar setting on Ubuntu 20.04 LTS, I prefer dma.

littlebigman · 11-07-2021, 03:12 PM

For some reason, cron doesn't send the email, while the script works OK if run manually:

Code:

~# crontab -l
50 11 * * Sun /root/show.free.disk.space.bash

~# ll /root/show.free.disk.space.bash
-rwxr-xr-x 1 root 166 Oct  4 11:45 /root/show.free.disk.space.bash*

~# cat /root/show.free.disk.space.bash
#!/bin/bash

CURRENT=$(df --output=pcent / | sed 1d)
printf 'To: me@isp.com\nSubject: Disk space\n\nRoot partition %s full.'  "$CURRENT" | ssmtp me@isp.com

Is it a security reason?

shruggy · 11-07-2021, 03:51 PM

A couple of wild guesses.

1) From the crontab(5) man page:

Quote:

cron requires that each entry in a crontab end in a newline character. If the last entry in a crontab is missing a newline (i.e. terminated by EOF), cron will consider the crontab (at least partially) broken. A warning will be written to syslog.

2) Try sun or 0 instead of Sun. It shouldn't matter, but there're different implementations of cron: Vixie cron, cronie, etc. POSIX only mandates numbers as days of week.

littlebigman · 11-08-2021, 05:27 AM

Thanks for the tips.

The last line did end with a carriage return, so that wasn't it.

I should have checked /var/log/sys

Code:

Nov  8 12:25:01 debian CRON[2845]: (root) CMD (/root/show.free.disk.space.bash)
Nov  8 12:25:01 debian cron[2852]: sendmail: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com
Nov  8 12:25:01 debian sSMTP[2852]: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com
Nov  8 12:25:01 debian CRON[2844]: (root) MAIL (mailed 66 bytes of output but got status 0x0001 from MTA#012)

Heres's how I configured ssmtp:

Code:

~# cat /etc/ssmtp/ssmtp.conf
root=postmaster
mailhub=smtp.isp.com
hostname=isp.com
FromLineOverride=YES

~# cat /etc/ssmtp/revaliases
root:me@isp.com:smtp.isp.com

Changing ownership of the script makes no difference:

Code:

chown fred.fred show.free.disk.space.bash

shruggy · 11-08-2021, 06:08 AM

Quote:

Originally Posted by littlebigman

Changing ownership of the script makes no difference:

Code:

chown fred.fred show.free.disk.space.bash

No, this is wrong. You are executing a cronjob for root. cron checks that root owns the file and will refuse to open it otherwise.

Rather, try to explicitly specify the From: name either in the body of the message or on the ssmtp command line with -fname.

littlebigman · 11-08-2021, 07:10 AM

I tried those, one after the other, still NOK:

Code:

printf 'From: me@isp.com\nTo: me@isp.com\nSubject: Disk space\n\nRoot partition %s full.'  "$CURRENT" | ssmtp me@isp.com

printf 'From: me@isp.com\nTo: me@isp.com\nSubject: Disk space\n\nRoot partition %s full.'  "$CURRENT" | ssmtp -f"me@isp.com" me@isp.com

Output:

Code:

cron[266]: sendmail: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com
sSMTP[24771]: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com
CRON[24759]: (root) MAIL (mailed 61 bytes of output but got status 0x0001 from MTA#012)

Since ssmtp is deadware since 2013, I'll experiment with msmtp.

shruggy · 11-08-2021, 07:12 AM

Then run it not as a root, but as a regular user cronjob.

littlebigman · 11-08-2021, 08:16 AM

It doesn't work either: For some reason, ssmtp is invisible to a non-root user:

Code:

fred@debian:~$ ./test.bash: line 9: ssmtp: command not found
fred@debian:~$ which ssmtp
fred@debian:~$

Besides, I get the same error in syslog even after creating /etc/cron.d/cron.allow to include root + fred, and adding a job as "fred":

Code:

CRON[25475]: (fred) CMD (test.bash)
cron[266]: sendmail: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com
sSMTP[25476]: 550 5.7.1 <root@isp.com>: Sender address rejected: you are not root@isp.com

wpeckham · 11-08-2021, 08:22 AM

Have you considered modifying the script to use sudo or doas for the parts that need to run as root, but having it scheduled, run, and send mail under your personal account?

This presumes that your personal account has no problem sending email.

shruggy · 11-08-2021, 08:44 AM

Why explicitly run ssmtp from the script at all? If ssmtp is set up as your MTA, sending mails with mailx from the command line should also work.