Apache: The Definitive Guide, 3rd Edition

ContentDigest

This directive enables the generation of Content-MD5 headers as defined in RFC1864 and RFC2068.

ContentDigest on|off
Default: ContentDigest off
server config, virtual host, directory, .htaccess

MD5, as described earlier in this chapter, is an algorithm for computing a “message digest” (sometimes called “fingerprint”) of arbitrary-length data, with a high degree of confidence that any alterations in the data will be reflected in alterations in the message digest. The Content-MD5 header provides an end-to-end message integrity check (MIC) of the entity body. A proxy or client may check this header for detecting accidental modification of the entity body in transit. See the following example header:

   Content-MD5: AuLb7Dp1rqtRtxz2m9kRpA==

Note that this can cause performance problems on your server since the message digest is computed on every request (the values are not cached).

Content-MD5 is only sent for documents served by the core and not by any module. For example, SSI documents, output from CGI scripts, and byte-range responses do not have this header.