ctrld/docs/runtime-internal-logging.md

Runtime internal logging

Purpose

ctrld writes its own log when the configuration sets no log file. The log gives Control D support and ctrld developers the evidence they need after a problem. Two of the three streams are files on disk. They keep their content across a restart and a self-upgrade.

When it is on

Internal logging is on when these three conditions are true at the same time:

• ctrld runs in Control D mode, with the --cd flag
• The configuration sets no log_path
• Silent mode is off

When log_path is set, ctrld writes that file and starts no internal stream. The journal needs internal logging, so log_path mode has no journal.

Streams and files

ctrld writes three streams. The debug file and the journal file live in the ctrld home directory, next to each other.

Stream	Level filter	File	Desktop budget	Router budget	Survives a restart
Console	Notice and above. The -v flag adds the lower levels	none	none	none	No
Debug	Every level	ctrld.log	10 MB, 4 backup files	5 MB, 1 backup file	Yes
Journal	Warning and above, plus every event with journal=true	ctrld-journal.log	2 MB, 2 backup files	1 MB, 1 backup file	Yes

The -v flag changes the level of every stream, the files included. Without the flag, ctrld sets the level to debug and holds the console at notice. With -v the level is info, so no debug line reaches the debug file. With -vv the level is debug again, and the console shows every line.

A backup file carries a number suffix. On a desktop the debug backup files are ctrld.log.1 to ctrld.log.4, and ctrld.log.1 holds the newest rotated content. The journal backup files are ctrld-journal.log.1 and ctrld-journal.log.2.

Rotation is by size. A write that crosses the size limit rotates the file first. ctrld deletes the file beyond the backup count.

Each stream also keeps a memory buffer: 5 MB for the debug stream and 1 MB for the journal. These buffers serve ctrld log tail and the fallback path when a file cannot open. The files hold the history.

A restart deletes the memory buffers and keeps the files. The new process appends a header line, which marks the restart point inside each file.

Header and rotation events

Every debug file and every journal file starts with one JSON line, the header. ctrld writes the header when it opens a file at start and after every rotation. On an existing file at start, ctrld appends the header.

The message of the line is Log header. The line carries these fields:

• Build: version and commit
• Host: os, arch, pid, and start_time
• Mode: intercept_mode, listeners, upstream_count, and upstream_types
• Identity: resolver_uid
• Files: log_files
• Network: network with default_route_interface, have_v4, have_v6, and interfaces

The header holds no provisioning token, no upstream endpoint URL, and no query name. upstream_types holds the type words, for example doh or os. listeners holds the listen addresses.

After each rotation ctrld logs one Log rotated event at info level, with the fields file, bytes_written, first_event_at, last_event_at, and backups. This event is a journal event, so it lands in the debug stream and in the journal.

Sampling of per-query errors

A broken network produces one error line per query. ctrld bounds this flood for each error class and each upstream. The first 5 lines in a 60 s window stay at error level. Later lines in the same window drop to debug level, so the debug stream stays complete. A run with -v holds every stream at info, and the debug stream then loses these lines.

When a window closes, ctrld logs one Per-query errors sampled summary at error level. The summary carries class, upstream, count, suppressed, and window_s. ctrld logs the summary only when the window suppressed at least one line.

An upstream logs one Upstream state changed event per transition. The down event is at warn level and carries state down, failure_count, and reason. The up event is at info level and carries state up, down_for_ms, and failure_count. Both are journal events.

Configuration keys

The log_level key does not change the internal streams. The debug file keeps every level, and the journal keeps the warnings, the errors, and the journal events. The log_level key changes the console output and the log_path file. The -v flag changes every stream, as the section "Streams and files" explains.

Two keys in the [service] section size the debug stream:

• The key log_max_size_mb sets the size limit of ctrld.log in MB. The default is 10 on a desktop and 5 on a router.
• The key log_max_backups sets the number of backup files. The default is 4 on a desktop and 1 on a router. The value 0 keeps no backup file.

Both keys also apply to the log_path file, which rotates the same way. The journal budget is fixed per platform.

Uploads

ctrld log send uploads the log to Control D. The body holds these parts, in this order:

• One header line rendered at send time, with the field trigger set to send
• The newest 10 MB of the debug files, oldest file first, cut at a line boundary
• The marker === LOG_END ===
• Every journal file, oldest first

The network object of the send-time header comes from a fresh read of the interfaces and the default route. ctrld log send --full sends every debug file in place of the newest 10 MB. The reported size equals the number of bytes uploaded.

ctrld log view prints the same composition on the terminal and takes the same --full flag. ctrld accepts one upload per minute. The body is plain text without gzip.

In log_path mode the body holds the send-time header, then the log_path file and its backup files, oldest first. This body has no marker and no journal.

ctrld log tail follows the live debug stream and prints each new line, journal events included.

Notes

• Internal logging is not a replacement for a configured log_path file in production.
• The journal is small on purpose. It keeps the events that describe a problem, not the events of each query.
• Support reads the upload without access to the host. The header line names the build, the mode, and the network, so support does not have to ask for them.