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--- linux [2023/11/09 09:12] – created nine
+++ linux [2024/02/22 21:11] (current) – nine
@@ Line 5: / Line 5: @@
 Clean unused/disabled snap packages
   snap list --all | while read snapname ver rev trk pub notes; do if [[ $notes = *disabled* ]]; then snap remove "$snapname" --revision="$rev"; fi; done
+===== iptables/netfilter =====
+Restrict to single host, e.g. for prometheus node exporter
+  iptables -N prometheus
+  iptables -A prometheus --src <server-ip> -j ACCEPT
+  iptables -A prometheus -j DROP
+  iptables -I INPUT -m tcp -p tcp --dport 9100 -j prometheus
+===== docker =====
+open shell in a running container
+  docker exec -ti <container-name> /bin/bash
+===== email =====
+==== sender guidelines ====
+  - https://support.google.com/a/answer/14229414
+  - https://senders.yahooinc.com/best-practices/
+  - https://senders.yahooinc.com/faqs/
+  - https://www.bsi.bund.de/dok/TR-03182
+===== Video =====
+compress video for email sending
+  ffmpeg -i input.mp4 output_compressed.mp4
+adjust volume
+  ffmpeg -i input.avi -af "volumedetect" -vn -sn -dn -f null /dev/null
+gives:
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] n_samples: 1839104
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] mean_volume: -43.4 dB
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] max_volume: -31.5 dB
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_31db: 64
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_32db: 455
+  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_33db: 1884
+increase max volume:
+  ffmpeg -i input.avi -af "volume=30dB" -c:v copy -c:a libmp3lame -q:a 2 output.mp4
+flip image
+  ffmpeg -i input.mp4 -vf "transpose=2" output.mp4
+transpose options:
+= 90° counter-clockwise and vertical flip (default)
+= 90° clockwise
+= 90° counter-clockwise
+= 90° clockwise and vertical flip