Headerlimit Channel Option

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Limiting header storage (headerlimit)

The MTA stores outermost message headers in memory. This means that a single extremely large header could potentially consume all available memory, possibly leading to a denial of service attack. When placed on a source channel, the headerlimit channel option provides a means to prevent such attacks. It accepts a single integer argument specifying the maximum number of MTA blocks that can be consumed by the message header. Messages with headers that exceed this limit will be silently truncated.

Note that the header_limit MTA option can be used to implement the same limit for all channels. The lower of the two option values is used as the actual limit.

The default value for headerlimit is infinite if the channel option isn't specified. Note, however, that since the default value for header_limit MTA option is 2000 blocks, the effective default limit on the size of a header if no limiting options are specified is 2000 blocks.

The headerlimit channel option initially appeared in Messaging Server 6.1.