PMDF System Manager's Guide

Although fragmentation may be used to break messages into smaller pieces automatically, it may also be appropriate in some cases to simply reject outright messages larger than some administratively defined limit, (e.g., so as to avoid service denial attacks).

The blocklimit, linelimit and sourceblocklimit keywords are used to impose absolute size limits. Each of these keywords must be followed by a single integer value. blocklimit specifies the maximum number of blocks allowed in a message. PMDF will reject attempts to queue messages containing more blocks than this to the channel. The sourceblocklimit specifies the maximum number of blocks allowed in an incoming message. PMDF will reject attempts to submit a message containing more blocks than this to the channel. In other words, blocklimit applies to destination channels; sourceblocklimit applies to source channels. A PMDF block is normally 1024 bytes; this can be changed with the BLOCK_SIZE option in the PMDF option file. linelimit specifies the maximum number of lines allowed in a message. Note that linelimit counts both header lines and body lines of a message. PMDF will reject attempts to queue messages containing more than this number of lines to the channel. These limits can be imposed simultaneously if necessary.

Note that the PMDF options LINE_LIMIT and BLOCK_LIMIT can be used to impose similar limits on all channels. These limits have the advantage that since they apply across all channels PMDF's servers can make them known to mail clients prior to obtaining message recipient information. This simplifies the process of message rejection in some protocols.

The nolinelimit and noblocklimit channel keywords are the default and mean that no limits are imposed, other than any global limits imposed via the LINE_LIMIT or BLOCK_LIMIT PMDF options.