mknull - Make remote letters addressed to null


mknull [ -a ] [ -f ] [ site… ]


Mknull allows the system administrator to make “null” files in the mdss directories of directly connected sites. The presence or absence of a file allows scan to determine whether to poll a connecting site.

If the -a option is provided, null files are created for all sites directly serviced by the system. If the -f option is provided, null files are created only for sites which already have traffic pending.

The Null File

A “null” file contains a simple mail header, addressed to “null” at the remote site. Null files are sent from the system administrator to the null alias at connecting sites. They are handled by the sendmail program at connecting sites just like letters.

Here is a sample null file produced by mknull at a site (pro-example) that has a direct connection with pro-sol:

From pro-example!root Sat Jun 22 11:23:37 1991 Date: Sat, 22 Jun 91 11:23:37 EDT Ppath: pro-sol!null From: (System Administrator) To: null@pro-sol Subject: null X-Mailer: Mknull (1.1 8dec91)

This null file is named $/mdss/pro.sol/a0. If it were directed to pro-carolina, it would be named $/mdss/pro.carolina/a0, and the lines of the null file which contain “pro-sol” would instead contain “pro-carolina”.

Scanning for Null Files

Null files can be used by scan to determine whether or not to poll another site. Scan causes poll to call all sites that have null files in their $/mdss directories if it is called in the following manner:

scan -il -f a0

Assuming the null file, shown above, exists, and the system polls pro-sol, the null file would be sent to pro-sol!null, and it would no longer exist in the $/mdss/pro.sol/ directory. Thus, the next time scan runs, pro-sol would not be polled. In fact, pro-sol would not be polled again until another file is created. Thus, null files can be used as one method to LIMIT how often to poll another site.

Resource File

If no site arguments are given, mknull reads the resource file $/etc/rsrc/mknull.rsrc. Each line of this resource file contains the name of a connecting site for which a null file should be made. Example:

pro-carolina pro-sol

This tells mknull to make the files $/mdss/pro.carolina/a0, and $/mdss/pro.sol/a0.


If files already exist for a given site, mknull renames an existing queued file to “a0” instead of producing a null file. If only rnews batch files are queued, mknull produces a separate null file.

Notify the system administrator of each connecting site that you intend to send null files to their site. They must have an alias which directs all null files to a mail file that is regularly deleted, preferably by a maintenance script.


null files produced by mknull,
resource file.

See Also

aliases(NET), scan(NET), sendmail(C), rmnull(NET)


Dean Fick (dean@pro-electric)