Debug level messages are written to this file. In particular if a debug tag is enabled output for that tag is placed in this file. This applies to both the Traffic Server core and plugins. This file also contains some generic operational messages which track Traffic Server starting and stopping. If Traffic Server crashes a stack trace should be put in this file.

Most messages will have a timestamp, process tag, and a thread pointer.:

[Oct 13 14:40:46.134] Server {0x2ae95ff5ce80} DIAG: (header_freq.init) initializing plugin

The process tag is “Server” indicating traffic_server. It was logged from the thread with an instance address of 0x2ae95ff5ce80 and had a priority level of “DIAG” (“diagnostic”). The message is from a plugin, logged because the debug tag “header_freq.init” was active.


Diagnostic output file. Logging messages with a priority higher than “DEBUG” are sent to this file. If there is an problem that may cause Traffic Server to malfunction or not start up this is the first file that should be checked for logging messages.

Messages in this file follow the same general format as for traffic.out.


Operational error messages are placed here. These messages are about errors in transactions, not Traffic Server itself. For instance if a user agent request is denied that will be logged here.