# Customize log driver output The `tag` log option specifies how to format a tag that identifies the container's log messages. By default, the system uses the first 12 characters of the container ID. To override this behavior, specify a `tag` option: ```console $ docker run --log-driver=fluentd --log-opt fluentd-address=myhost.local:24224 --log-opt tag="mailer" ``` Docker supports some special template markup you can use when specifying a tag's value: | Markup | Description | | ------------------ | ---------------------------------------------------- | | `{{.ID}}` | The first 12 characters of the container ID. | | `{{.FullID}}` | The full container ID. | | `{{.Name}}` | The container name. | | `{{.ImageID}}` | The first 12 characters of the container's image ID. | | `{{.ImageFullID}}` | The container's full image ID. | | `{{.ImageName}}` | The name of the image used by the container. | | `{{.DaemonName}}` | The name of the Docker program (`docker`). | For example, specifying a `--log-opt tag="{{.ImageName}}/{{.Name}}/{{.ID}}"` value yields `syslog` log lines like: ```text Aug 7 18:33:19 HOSTNAME hello-world/foobar/5790672ab6a0[9103]: Hello from Docker. ``` At startup time, the system sets the `container_name` field and `{{.Name}}` in the tags. If you use `docker rename` to rename a container, the new name isn't reflected in the log messages. Instead, these messages continue to use the original container name.