mirror of
https://github.com/linuxkit/linuxkit.git
synced 2025-07-27 04:28:20 +00:00
793dd1b257
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
|
||
|
||
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:
|
||
|
||
1. `memlogd`: an in-memory circular buffer which receives logs (including
|
||
all the early `onboot` logs)
|
||
2. 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`.
|
||
|
||
## memlogd: an in-memory circular buffer
|
||
|
||
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.
|
||
|
||
### Message format
|
||
|
||
The format used to read logs is similar to [kmsg](https://www.kernel.org/doc/Documentation/ABI/testing/dev-kmsg):
|
||
```
|
||
<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 `;`.
|
||
|
||
## logwrite: writing logs to disk
|
||
|
||
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:
|
||
```
|
||
# cat /var/log/onboot.001-dhcpcd.out
|
||
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
|
||
```
|
||
|
||
## Current issues and limitations:
|
||
|
||
- 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` doesn’t 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)
|
||
|