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