Built-in Header Semantics

Not all header semantics are defined in the configuration file. Header lines that should only be included by certain mailers (as well as other more obscure semantics) must be specified in the HdrInfo table in conf.c. This table contains the header name (which should be in all lower case) and a set of header control flags (described below), The flags are:

H_ACHECK
Normally when the check is made to see if a header line is compatible with a mailer, sendmail will not delete an existing line. If this flag is set, sendmail will delete even existing header lines. That is, if this bit is set and the mailer does not have flag bits set that intersect with the required mailer flags in the header definition in sendmail.cf, the header line is always deleted.
H_EOH
If this header field is set, treat it like a blank line, i.e., it will signal the end of the header and the beginning of the message text.
H_FORCE
Add this header entry even if one existed in the message before. If a header entry does not have this bit set, sendmail will not add another header line if a header line of this name already existed. This would normally be used to stamp the message by everyone who handled it.
H_TRACE
If set, this is a timestamp (trace) field. If the number of trace fields in a message exceeds a preset amount the message is returned on the assumption that it has an aliasing loop.
H_RCPT
If set, this field contains recipient addresses. This is used by the -t flag to determine who to send to when it is collecting recipients from the message.
H_FROM
This flag indicates that this field specifies a sender. The order of these fields in the HdrInfo table specifies sendmail's preference for which field to return error messages to.
H_ERRORSTO
Addresses in this header should receive error messages.
H_CTE
This header is a Content-Transfer-Encoding header.
H_CTYPE
This header is a Content-Type header.
H_STRIPVAL
Strip the value from the header (for Bcc:).

Let's look at a sample HdrInfo specification:


struct hdrinfo HdrInfo[] =
{
/* originator fields, most to least significant */
"resent-sender", H_FROM,
"resent-from", H_FROM,
"sender", H_FROM,
"from", H_FROM,
"full-name", H_ACHECK,
"errors-to", H_FROM|H_ERRORSTO,
/* destination fields */
"to", H_RCPT,
"resent-to", H_RCPT,
"cc", H_RCPT,
"bcc", H_RCPT|H_STRIPVAL,
/* message identification and control */
"message", H_EOH,
"text", H_EOH,
/* trace fields */
"received", H_TRACE|H_FORCE,
/* miscellaneous fields */
"content-transfer-encoding", H_CTE,
"content-type", H_CTYPE,


NULL, 0,
};

This structure indicates that the To:, Resent-To:, and Cc: fields all specify recipient addresses. Any Full-Name: field will be deleted unless the required mailer flag (indicated in the configuration file) is specified. The Message: and Text: fields will terminate the header; these are used by random dissenters around the network world. The Received: field will always be added, and can be used to trace messages.

There are a number of important points here. First, header fields are not added automatically just because they are in the HdrInfo structure; they must be specified in the configuration file in order to be added to the message. Any header fields mentioned in the configuration file but not mentioned in the HdrInfo structure have default processing performed; that is, they are added unless they were in the message already. Second, the HdrInfo structure only specifies cliched processing; certain headers are processed specially by ad hoc code regardless of the status specified in HdrInfo. For example, the Sender: and From: fields are always scanned on ARPANET mail to determine the sender[22]; this is used to perform the return to sender function. The From: and Full-Name: fields are used to determine the full name of the sender if possible; this is stored in the macro $x and used in a number of ways.


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