====== Docker Troubleshooting ======

===== Disk Usage (MergeDir) =====

If you run out of disk usage and it's not the data from your volumes, maybe a container is writing data to the overlay file system. This data is placed in the merge directory. Try the following snippets to show the size of the merge directory for all the containers. 

<code bash>
while read -r name mergedir _; do
    size="$([ -d "${mergedir}" ] && du -s "${mergedir}" | cut -f 1)"
    size_hr="$(awk -v s="${size:=0}" 'BEGIN { printf "%.2f\n", s/1024 }')"
    echo "${name} ${mergedir} ${size:=0} ${size_hr}"
done <<<"$(docker inspect $(docker ps -qa) | jq -r 'map([.Name, .GraphDriver.Data.MergedDir]) | .[] | "\(.[0])\t\(.[1])"')" | sort -nk3 | awk 'BEGIN{ print "NAME","MERGE_DIR","SIZE_B","SIZE_MB" } { print $0 }' | column -t
</code>

The same as a one-liner.
<code bash>
while read -r name mergedir _; do size="$([ -d "${mergedir}" ] && du -s "${mergedir}" | cut -f 1)"; size_hr="$(awk -v s="${size:=0}" 'BEGIN { printf "%.2f\n", s/1024 }')"; echo "${name} ${mergedir} ${size:=0} ${size_hr}"; done <<< "$(docker inspect $(docker ps -qa) |  jq -r 'map([.Name, .GraphDriver.Data.MergedDir]) | .[] | "\(.[0])\t\(.[1])"')" | sort -nk3 | awk 'BEGIN{ print "NAME","MERGE_DIR","SIZE_B","SIZE_MB" } { print $0 }' | column -t
</code>

This is how the output could be displayed.
<code - Example>
root@server:~# while read -r name mergedir _; do size="$([ -d "${mergedir}" ] && du -s "${mergedir}" | cut -f 1)"; size_hr="$(awk -v s="${size:=0}" 'BEGIN { printf "%.2f\n", s/1024 }')"; echo "${name} ${mergedir} ${size:=0} ${size_hr}"; done <<< "$(docker inspect $(docker ps -qa) |  jq -r 'map([.Name, .GraphDriver.Data.MergedDir]) | .[] | "\(.[0])\t\(.[1])"')" | sort -nk3 | awk 'BEGIN{ print "NAME","MERGE_DIR","SIZE_B","SIZE_MB" } { print $0 }' | column -t
NAME                              MERGE_DIR                                                                                         SIZE_B  SIZE_MB
/zabbix-db_data_pgsql-1           /var/lib/docker/overlay2/93e67764c8b6f74e5686a5421c2aa3e79e8a11e81eb5869fbb13ad13ccb46130/merged  0       0.00
/zabbix-zabbix-server-1           /var/lib/docker/overlay2/9e1ecf3b598501ff76c28264c73cd34bf9697fdf7605ad467f8b39a7f8f36fca/merged  63604   62.11
/zabbix-zabbix-web-nginx-pgsql-1  /var/lib/docker/overlay2/4942fc913e4546c7090fba3614c9718607f7b0274abbc50ea6feed252efed58a/merged  179516  175.31
/zabbix-postgres-server-1         /var/lib/docker/overlay2/d7918d5dea06a3df43c847c91368341e56bda720a68cd6a7be6296dcb9a97f27/merged  272720  266.33
/zabbix-zabbix-web-service-1      /var/lib/docker/overlay2/eebdcc8ee3b0b2fc871db6c32a8988a52b4b200996ff28aa015e2bec8f039eb0/merged  754740  737.05
</code>

===== About Namespaces =====

[[https://www.redhat.com/sysadmin/container-namespaces-nsenter]]

===== Network =====

==== Enter Namespace ====

<code bash>
container_id="mycontainerid"
container_pid="$(docker inspect --format "{{ .State.Pid }}" "${container_id}")"
nsenter -n -t "${container_pid}"
exit
</code>

After executing nsenter all commands that support namespaces see the network interfaces like they are in the container. Example commands: ip, tcpdump. With exit the namespace can be left to see the network interfaces from the host system again.

==== tcpdump in Container ====

tcpdump has to be installed on the host system and not in the container.

<code bash>
container_id="mycontainerid"
container_pid="$(docker inspect --format "{{ .State.Pid }}" "${container_id}")"
nsenter -n -t "${container_pid}" tcpdump -nn -i eth0
</code>

===== Logs =====

==== Add Local Timestamp to Container Logs ====

If a container is logging without a timestamp or with an unknown time zone, you can add a timestamp while watching the logs.
<code bash>
docker logs mycontainer -n 0 -f | xargs -IL date +'[%Y-%m-%dT%H:%M:%S%:z] L'
</code>

In case you want to catch the stderr logs, too
<code bash>
docker logs mycontainer -n 0 -f 2>&1 | xargs -IL date +'[%Y-%m-%dT%H:%M:%S%:z] L'
</code>

<code - Example>
root@server:~# docker logs mycontainer -n 0 -f 2>&1 | xargs -IL date +'[%Y-%m-%dT%H:%M:%S%:z] L'
[2025-02-04T16:55:01+01:00] 2025-02-04T15:55:01+0000 INFO main.py:24 send request for url http://server/ping.
[2025-02-04T16:55:01+01:00] 2025-02-04T15:55:01+0000 INFO main.py:29 Received answer with HTTP status code 200 and content: GET: pong
[2025-02-04T16:55:03+01:00] 2025-02-04T15:55:03+0000 INFO main.py:24 send request for url http://server/ping.
[2025-02-04T16:55:03+01:00] 2025-02-04T15:55:03+0000 INFO main.py:29 Received answer with HTTP status code 200 and content: GET: pong
[2025-02-04T16:55:05+01:00] 2025-02-04T15:55:05+0000 INFO main.py:24 send request for url http://server/ping.
[2025-02-04T16:55:05+01:00] 2025-02-04T15:55:05+0000 INFO main.py:29 Received answer with HTTP status code 200 and content: GET: pong
</code>