MIME Headers¶

The Traffic Server **MIME header functions ** enable you to retrieve and modify information about HTTP MIME fields.

An HTTP request or response consists of a header, body, and trailer. The HTTP header contains a request (or response) line and a MIME header. A MIME header is composed of zero or more MIME fields. A MIME field is composed of a field name, a colon, and zero or more field values (values in a field are separated by commas).

In the example below: Foo is the MIME field name, bar is the first MIME field value, and car is the second MIME field value.

Foo: bar, car

The following example is an augmented Backus-Naur Form (BNF) for the form of a MIME header - it specifies exactly what was described above. A header consists of zero or more fields that contain a name, separating colon, and zero or more values. A name or value is simply a string of tokens that is potentially zero length; a token is any character except certain control characters and separators (such as colons). For the purpose of retrieving a field, field names are not case-sensitive; therefore, the field names Foo, foo and fOO are all equivalent.

MIME-header = *MIME-field
MIME-field = field-name ":" #field-value
field-name = *token
field-value = *token

The MIME header data structure is a parsed version of a standard Internet MIME header. The MIME header data structure is similar to the URL data structure (see URLs). The actual data is stored in a marshal buffer; the MIME header functions operate on a marshal buffer and a location (TSMLoc) within the buffer.

After a call to TSMimeHdrFieldDestroy, TSMimeHdrFieldRemove, or TSUrlDestroy is made, you must deallocate the TSMLoc handle with a call to TSHandleMLocRelease. You do not need to deallocate a NULL handles. For example: if you call TSMimeHdrFieldValueStringGet to get the value of the content type field and the field does not exist, then it returns TS_NULL_MLOC. In such a case, you wouldn’t need to deallocate the handle with a call to TSHandleMLocRelease.

The location (TSMLoc) in the MIME header functions can be either an HTTP header location or a MIME header location. If an HTTP header location is passed to these functions, then the system locates the MIME header associated with that HTTP header and executes the corresponding MIME header operations specified by the functions (see the example in the description of TSMimeHdrCopy()).

Note: MIME headers may contain more than one MIME field with the same name. Previous versions of Traffic Server joined multiple fields with the same name into one field with composite values, but this behavior came at a performance cost and caused compatability issues with older clients and servers. Hence, the current version of Traffic Server does not coalesce duplicate fields. Correctly-behaving plugins should check for the presence of duplicate fields and iterate over the duplicate fields by using TSMimeHdrFieldNextDup.

To facilitate fast comparisons and reduce storage size, Traffic Server defines several pre-allocated field names. These field names correspond to the field names in HTTP and NNTP headers.

TS_MIME_FIELD_ACCEPT: “Accept” TS_MIME_LEN_ACCEPT
TS_MIME_FIELD_ACCEPT_CHARSET: “Accept-Charset” TS_MIME_LEN_ACCEPT_CHARSET
TS_MIME_FIELD_ACCEPT_ENCODING: “Accept-Encoding” TS_MIME_LEN_ACCEPT_ENCODING
TS_MIME_FIELD_ACCEPT_LANGUAGE: “Accept-Language” TS_MIME_LEN_ACCEPT_LANGUAGE
TS_MIME_FIELD_ACCEPT_RANGES: “Accept-Ranges” TS_MIME_LEN_ACCEPT_RANGES
TS_MIME_FIELD_AGE: “Age” TS_MIME_LEN_AGE
TS_MIME_FIELD_ALLOW: “Allow” TS_MIME_LEN_ALLOW
TS_MIME_FIELD_APPROVED: “Approved” TS_MIME_LEN_APPROVED
TS_MIME_FIELD_AUTHORIZATION: “Authorization” TS_MIME_LEN_AUTHORIZATION
TS_MIME_FIELD_BYTES: “Bytes” TS_MIME_LEN_BYTES
TS_MIME_FIELD_CACHE_CONTROL: “Cache-Control” TS_MIME_LEN_CACHE_CONTROL
TS_MIME_FIELD_CLIENT_IP: “Client-ip” TS_MIME_LEN_CLIENT_IP
TS_MIME_FIELD_CONNECTION: “Connection” TS_MIME_LEN_CONNECTION
TS_MIME_FIELD_CONTENT_BASE: “Content-Base” TS_MIME_LEN_CONTENT_BASE
TS_MIME_FIELD_CONTENT_ENCODING: “Content-Encoding” TS_MIME_LEN_CONTENT_ENCODING
TS_MIME_FIELD_CONTENT_LANGUAGE: “Content-Language” TS_MIME_LEN_CONTENT_LANGUAGE
TS_MIME_FIELD_CONTENT_LENGTH: “Content-Length” TS_MIME_LEN_CONTENT_LENGTH
TS_MIME_FIELD_CONTENT_LOCATION: “Content-Location” TS_MIME_LEN_CONTENT_LOCATION
TS_MIME_FIELD_CONTENT_MD5: “Content-MD5” TS_MIME_LEN_CONTENT_MD5
TS_MIME_FIELD_CONTENT_RANGE: “Content-Range” TS_MIME_LEN_CONTENT_RANGE
TS_MIME_FIELD_CONTENT_TYPE: “Content-Type” TS_MIME_LEN_CONTENT_TYPE
TS_MIME_FIELD_CONTROL: “Control” TS_MIME_LEN_CONTROL
TS_MIME_FIELD_COOKIE: “Cookie” TS_MIME_LEN_COOKIE
TS_MIME_FIELD_DATE: “Date” TS_MIME_LEN_DATE
TS_MIME_FIELD_DISTRIBUTION: “Distribution” TS_MIME_LEN_DISTRIBUTION
TS_MIME_FIELD_ETAG: “Etag” TS_MIME_LEN_ETAG
TS_MIME_FIELD_EXPECT: “Expect” TS_MIME_LEN_EXPECT
TS_MIME_FIELD_EXPIRES: “Expires” TS_MIME_LEN_EXPIRES
TS_MIME_FIELD_FOLLOWUP_TO: “Followup-To” TS_MIME_LEN_FOLLOWUP_TO
TS_MIME_FIELD_FROM: “From” TS_MIME_LEN_FROM
TS_MIME_FIELD_HOST: “Host” TS_MIME_LEN_HOST
TS_MIME_FIELD_IF_MATCH: “If-Match” TS_MIME_LEN_IF_MATCH
TS_MIME_FIELD_IF_MODIFIED_SINCE: “If-Modified-Since” TS_MIME_LEN_IF_MODIFIED_SINCE
TS_MIME_FIELD_IF_NONE_MATCH: “If-None-Match” TS_MIME_LEN_IF_NONE_MATCH
TS_MIME_FIELD_IF_RANGE: “If-Range” TS_MIME_LEN_IF_RANGE
TS_MIME_FIELD_IF_UNMODIFIED_SINCE: “If-Unmodified-Since” TS_MIME_LEN_IF_UNMODIFIED_SINCE
TS_MIME_FIELD_KEEP_ALIVE: “Keep-Alive” TS_MIME_LEN_KEEP_ALIVE
TS_MIME_FIELD_KEYWORDS: “Keywords” TS_MIME_LEN_KEYWORDS
TS_MIME_FIELD_LAST_MODIFIED: “Last-Modified” TS_MIME_LEN_LAST_MODIFIED
TS_MIME_FIELD_LINES: “Lines” TS_MIME_LEN_LINES
TS_MIME_FIELD_LOCATION: “Location” TS_MIME_LEN_LOCATION
TS_MIME_FIELD_MAX_FORWARDS: “Max-Forwards” TS_MIME_LEN_MAX_FORWARDS
TS_MIME_FIELD_MESSAGE_ID: “Message-ID” TS_MIME_LEN_MESSAGE_ID
TS_MIME_FIELD_NEWSGROUPS: “Newsgroups” TS_MIME_LEN_NEWSGROUPS
TS_MIME_FIELD_ORGANIZATION: “Organization” TS_MIME_LEN_ORGANIZATION
TS_MIME_FIELD_PATH: “Path” TS_MIME_LEN_PATH
TS_MIME_FIELD_PRAGMA: “Pragma” TS_MIME_LEN_PRAGMA
TS_MIME_FIELD_PROXY_AUTHENTICATE: “Proxy-Authenticate” TS_MIME_LEN_PROXY_AUTHENTICATE
TS_MIME_FIELD_PROXY_AUTHORIZATION: “Proxy-Authorization” TS_MIME_LEN_PROXY_AUTHORIZATION
TS_MIME_FIELD_PROXY_CONNECTION: “Proxy-Connection” TS_MIME_LEN_PROXY_CONNECTION
TS_MIME_FIELD_PUBLIC: “Public” TS_MIME_LEN_PUBLIC
TS_MIME_FIELD_RANGE: “Range” TS_MIME_LEN_RANGE
TS_MIME_FIELD_REFERENCES: “References” TS_MIME_LEN_REFERENCES
TS_MIME_FIELD_REFERER: “Referer” TS_MIME_LEN_REFERER
TS_MIME_FIELD_REPLY_TO: “Reply-To” TS_MIME_LEN_REPLY_TO
TS_MIME_FIELD_RETRY_AFTER: “Retry-After” TS_MIME_LEN_RETRY_AFTER
TS_MIME_FIELD_SENDER: “Sender” TS_MIME_LEN_SENDER
TS_MIME_FIELD_SERVER: “Server” TS_MIME_LEN_SERVER
TS_MIME_FIELD_SET_COOKIE: “Set-Cookie” TS_MIME_LEN_SET_COOKIE
TS_MIME_FIELD_SUBJECT: “Subject” TS_MIME_LEN_SUBJECTTS_MIME_LEN_SUBJECT
TS_MIME_FIELD_SUMMARY: “Summary” TS_MIME_LEN_SUMMARY
TS_MIME_FIELD_TE: “TE” TS_MIME_LEN_TE
TS_MIME_FIELD_TRANSFER_ENCODING: “Transfer-Encoding” TS_MIME_LEN_TRANSFER_ENCODING
TS_MIME_FIELD_UPGRADE: “Upgrade” TS_MIME_LEN_UPGRADE
TS_MIME_FIELD_USER_AGENT: “User-Agent” TS_MIME_LEN_USER_AGENT
TS_MIME_FIELD_VARY: “Vary” TS_MIME_LEN_VARY
TS_MIME_FIELD_VIA: “Via” TS_MIME_LEN_VIA
TS_MIME_FIELD_WARNING: “Warning” TS_MIME_LEN_WARNING
TS_MIME_FIELD_WWW_AUTHENTICATE: “Www-Authenticate” TS_MIME_LEN_WWW_AUTHENTICATE
TS_MIME_FIELD_XREF: “Xref” TS_MIME_LEN_XREF

The header field names above are defined in ts.h as const char* strings. When Traffic Server sets the name portion of a header field (or any portion for that matter), it quickly checks to see if the new value is one of the known values. If it is, then Traffic Server stores a pointer into a global table instead of storing the known value in the marshal buffer. The header field names listed above are also pointers into this table, which enables simple pointer comparison of the value returned from TSMimeHdrFieldNameGet with one of the values listed above. It is recommended that you use the above values when referring to one of the known header field names to avoid the possibility of a spelling error.

Traffic Server adds one important feature to MIME fields that you may not know about: Traffic Server does not print a MIME field if the field name begins with the ‘@‘ symbol. For example: a plugin can add the field “@My-Field” to a header. Even though Traffic Server never sends that field out in a request to an origin server or in a response to a client, it can be printed to Traffic Server logs by defining a custom log configuration file that explicitly logs such fields. This provides a useful mechanism for plugins to store information about an object in one of the MIME headers associated with the object.

The MIME header functions are listed below: