fix(notify-*-mail): render Icinga logo inline on hosts with long FQDNs

On hosts with a long FQDN (>80 characters, typically with
multiple hyphens), the Icinga logo in notification mails used to
show up as a base64-encoded attachment instead of the embedded
banner image inside the HTML body.

Root cause: `email.utils.make_msgid()` defaults to
`socket.getfqdn()` for the Content-ID domain. Long FQDNs push
the resulting header past 80 characters, at which point Python's
quoted-printable encoder wraps it across multiple lines and
breaks the `cid:<...>` reference that the HTML body uses to pull
the inline logo image. See python/cpython#100293 for the
upstream limitation.

Fix: pin the domain to `socket.gethostname()` (the short
hostname), which keeps the Content-ID well under the 80-char
limit. The short hostname is already imported in both notify
plugins for the HTML template's `icinga_host=gethostname()`
substitution, so no new imports are needed.

Closes #790

Author: markuslf

CHANGELOG.md

@@ -163,6 +163,7 @@ Monitoring Plugins:
 * needs-restarting: the "Running Kernel X != Installed Kernel Y" line now shows up on Debian-based systems when `needrestart` reports a pending kernel upgrade
 * network-connections: the plugin now exits with the correct WARN/CRIT state when any threshold is violated; previously it always reported OK regardless of the loop's accumulated state ([#1070](https://github.com/Linuxfabrik/monitoring-plugins/issues/1070))
 * nextcloud-security-scan: only trigger a rescan for scans that are actually in the past; future-dated scans caused by clock skew no longer trigger an unnecessary rescan ([#1070](https://github.com/Linuxfabrik/monitoring-plugins/issues/1070))
+* notify-host-mail, notify-service-mail: the Icinga logo now renders inline again in notification mails sent from hosts with long FQDNs. On such hosts the logo used to appear as a base64 attachment instead of the embedded banner image ([#790](https://github.com/Linuxfabrik/monitoring-plugins/issues/790))
 * ntp-\*: prevent `TypeError: ''=' not supported between instances of 'int' and 'str'`
 * ntp-w32tm: show the correct state label (WARNING or CRITICAL) in the "Time since Last Good Sync Time" line; it was hardcoded to WARNING even when the actual state was CRITICAL ([#1070](https://github.com/Linuxfabrik/monitoring-plugins/issues/1070))
 * openstack-swift-stat: "bytes used" is now shown for any account with non-zero usage, regardless of whether a quota is set ([#1070](https://github.com/Linuxfabrik/monitoring-plugins/issues/1070))

notification-plugins/notify-host-mail/notify-host-mail

@@ -43,7 +43,7 @@ import lib.url     # pylint: disable=C0413
 from lib.globals import STATE_UNKNOWN # pylint: disable=C0413
 
 __author__ = 'Linuxfabrik GmbH, Zurich/Switzerland'
-__version__ = '2026040801'
+__version__ = '2026041301'
 
 DESCRIPTION = 'Sends notifications for hosts using mail.'
 
@@ -458,7 +458,13 @@ def main():
         mail_msg.set_content(plain)
 
     else: # no args.SHORT
-        logo_cid =  make_msgid()
+        # Pin the domain to the short hostname so make_msgid() does not
+        # fall back to socket.getfqdn(). On hosts with a long FQDN the
+        # resulting Content-ID header overflows 80 characters and gets
+        # wrapped by Python's quoted-printable encoder, which breaks the
+        # cid:<...> reference in the HTML body and turns the inline logo
+        # into an attachment (see python/cpython#100293 and issue #790).
+        logo_cid = make_msgid(domain=gethostname())
         plain, html = generate_mail_content(args, logo_cid)
 
         subject = f'Host "{args.HOST_DISPLAYNAME}" is {args.HOST_STATE}'

notification-plugins/notify-service-mail/notify-service-mail

@@ -43,7 +43,7 @@ import lib.url     # pylint: disable=C0413
 from lib.globals import STATE_UNKNOWN # pylint: disable=C0413
 
 __author__ = 'Linuxfabrik GmbH, Zurich/Switzerland'
-__version__ = '2026040801'
+__version__ = '2026041301'
 
 DESCRIPTION = 'Sends notifications for services using mail.'
 
@@ -478,7 +478,13 @@ def main():
         mail_msg.set_content(plain)
 
     else: # no args.SHORT
-        logo_cid =  make_msgid()
+        # Pin the domain to the short hostname so make_msgid() does not
+        # fall back to socket.getfqdn(). On hosts with a long FQDN the
+        # resulting Content-ID header overflows 80 characters and gets
+        # wrapped by Python's quoted-printable encoder, which breaks the
+        # cid:<...> reference in the HTML body and turns the inline logo
+        # into an attachment (see python/cpython#100293 and issue #790).
+        logo_cid = make_msgid(domain=gethostname())
         plain, html = generate_mail_content(args, logo_cid)
 
         subject = f'Service "{args.SERVICE_DISPLAYNAME}" on "{args.HOST_DISPLAYNAME}" is {args.SERVICE_STATE}'