Debug Tags

Use the API macro Dbg(ctl, const char *format_str, ...) to add traces in your plugin. In this macro:

  • ctl is the Traffic Server parameter that enables Traffic Server to print out format_str. It is an instance of the class DbgCtl.

  • ... are variables for format_str in the standard printf style.

The DbgCtl control is enabled when debug output is enabled globally by configuration, and tag matches a configured regular expression.

Run Traffic Server with the -Ttag option. For example, if the tag is my-plugin, then the debug output goes to traffic.out.See below:

traffic_server -T"my-plugin"

Sets the following variables in records.yaml (in the Traffic Server config directory):

      enabled: 1
      tags: debug-tag-name

In this case, debug output goes to traffic.out.

The statement "Starting my-plugin at <time>" appears whenever you run Traffic Server with the my-plugin tag:

traffic_server -T"my-plugin"


DbgCtl my_dbg_ctl{"my-plugin"}; // Non-local variable.
Dbg(my_dbg_ctl, "Starting my-plugin at %d", the_time);
Dbg(my_dbg_ctl, "Later on in my-plugin");

Other Useful Internal Debug Tags

Embedded in the base Traffic Server code are many debug tags for internal debugging purposes. These can also be used to follow Traffic Server behavior for testing and analysis.

The debug tag setting (-T and proxy.config.diags.debug.tags) is a anchored regular expression (PCRE) against which the tag for a specific debug message is matched. This means the value “http” matches debug messages with the tag “http” but also “http_seq” and “http_trans”. If you want multiple tags then use a pipe symbol to separate the tags. For example “http_tproxy|dns|hostdb” will match any of the message tags “http_tproxy”, “dns”, “hostdb”, or “dns_srv” (but not “http_trans” nor “splitdns”).

Some of the useful HTTP debug tags are:

  • http_hdrs - traces all incoming and outgoing HTTP headers.

  • http_trans - traces actions in an HTTP transaction.

  • http_seq - traces the sequence of the HTTP state machine.

  • http_tproxy - transparency related HTTP events

  • dns - DNS operations

  • hostdb - Host name lookup

  • iocore_net - Socket and low level IO (very voluminous)

  • socket - socket operations

  • ssl - SSL related events

  • cache - Cache operations (many subtags, examine the output to narrow the tag set)

  • cache_update - Cache updates including writes

  • cache_read - Cache read events.

  • dir_probe - Cache searches.

  • sdk - gives some warning concerning API usage.