Project: LinuxKit (Provisioning / Automation & Configuration)
File: logging.md

By default LinuxKit will write onboot and service logs directly to files in `/var/log` and `/var/log/onboot`. It is tricky to write the logs to a disk or a network service as no disks or networks are available until the `onboot` containers run. We work around this by splitting the logging into 2 pieces: `memlogd`: an in-memory circular buffer which receives logs (including all the early `onboot` logs) a log writing `service` that starts later and can download and process the logs from `memlogd` To use this new logging system, you should add the `memlogd` container to the `init` block in the LinuxKit yml. On boot `memlogd` will be started from `init.d` and it will listen on a Unix domain socket: ``` /var/run/linuxkit-external-logging.sock ``` The `init`/`service` process will look for this socket and redirect the `stdout` and `stderr` of both `onboot` and `services` to `memlogd`. The `memlogd` daemon reads the logs from the `onboot` and `services` containers and stores them together with a timestamp and the name of the originating container in a circular buffer in memory. The contents of the circular buffer can be read over the Unix domain socket ``` /var/run/memlogq.sock ``` The circular buffer has a fixed size (overridden by the command-line argument `-max-lines`) and when it fills up, the oldest messages will be overwritten. To store the logs somewhere more permanent, for example a disk or a remote network service, a service should be added to the yaml which connects to `memlogd` and streams the logs. The `logwrite` service described below shows how to do this. The format used to read logs is similar to : ``` <timestamp>,<log>;<body> ``` where `<timestamp>` is an RFC3339-formatted timestamp, `<log>` is the name of the log (e.g. `docker-ce.out`) and `<body>` is the output. The `<log>` must not contain the character `;`. The service `pkg/logwrite` connects to `memlogd` and streams the logs to files in `/var/log`. The logs are automatically rotated; by default each file has a maximum size of 1 MiB and up to 10 files are kept per log. The arguments `-max-log-files` and `-max-log-size` can be used to override these defaults. Here is an example log file: ``` 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: waiting for carrier 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: carrier acquired 2018-07-08T09:16:53Z onboot.001-dhcpcd.out DUID 00:01:00:01:22:d4:93:05:02:50:00 :00:00:06 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: IAID 00:00:00:06 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: adding address fe80::f346:56a6:590d:5ea4 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: soliciting an IPv6 router 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: soliciting a DHCP lease 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: offered 192.168.65.8 from 192.168.65.1 `vpnkit' 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: leased 192.168.65.8 for 7200 se conds 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: adding route to 192.168.65.0/24 2018-07-08T09:16:53Z onboot.001-dhcpcd.out eth0: adding default route via 192.16 8.65.1 2018-07-08T09:16:53Z onboot.001-dhcpcd.out exiting due to oneshot 2018-07-08T09:16:53Z onboot.001-dhcpcd.out dhcpcd exited ``` No docker logger plugin support yet - it could be nice to add support to memlogd, so the docker container logs would also be gathered in one place No syslog compatibility at the moment and `/dev/log` doesnt exist. This socket could be created to keep syslog compatibility, e.g. by using https://github.com/mcuadros/go-syslog. Processes that require syslog should then be able to log directly to memlogd. Currently no direct external hooks exposed - but options available that could be added. Should also be possible to pipe output to e.g. `oklog` from `logread` (https://github.com/oklog/oklog)