maildropgdbm - GDBM/DB support in maildrop
gdbmopen(filename, mode)
gdbmclose
gdbmfetch(key [,default])
gdbmstore(key,value)
The gdbm family of functions provides access to the GDBM library - a library of
routines that manage simple database files. The library provides a way of
quickly storing and looking up key/data pairs.
GDBM support in
maildrop is optional, and may not be available to you.
GDBM support in
maildrop can optionally be implemented using the DB
library. This option is selected by the system administrator. If this is the
case, these functions still work exactly as described below, except that they
will operate on DB hash files, instead of GDBM files.
To see whether GDBM or DB support is used, run the command "
maildrop
-v".
GDBM support is minimal, and simplistic. A filter file may have only one gdbm
file open at the same time. However, the filter file can close the current
gdbm file, and open another one. If another filter file is included using the
include statement, the included filter file may open its own, separate, gdbm
file.
A GDBM file contains a list of key/value pairs. All keys in the GDBM file are
unique. After storing an arbitrary key/value pair in the GDBM file, the value
associated with the given key can be quickly located and retrieved.
This function closes the current GDBM file.
gdbmfetch ( key [, options] [, default])
This function retrieves the data for the given key.
key is the key to
retrieve. The
gdbmfetch function returns the data associated with this
key. If the key does not exist in the GDBM file,
gdbmfetch returns the
default argument. If the
default argument is not specified,
gdbmfetch returns empty text. Please note that the
default
argument is not actually evaluated unless the key does not exist in the GDBM
file.
The
options argument specifies additional
maildrop value-added
features. The following functionality is not available in the GDBM library,
but is rather provided by
maildrop.
If the
options argument is set to "D", and the key could not be
found in the GDBM database, and the key is of the form
"user@domain",
maildrop will then attempt to look up the key
"user@". If that key is also not found,
maildrop finally
looks up the key "domain".
If "domain" is also not found, and domain is of the form
"a.b.c.d.tld" (with variable number of period-separated sections),
maildrop then attempts to look up the key "b.c.d.tld". If
that key is not found,
maildrop tries "c.d.tld", and so on,
until a key is found, or there are no more subdomains to remove, at which
point
gdbmfetch will return either the
default argument, or
empty text.
If the
options argument is set to "D", and the key could not be
found in the GDBM database, and the key is of the form "a.b.c.d.tld"
(with variable number of period-separated sections),
maildrop will also
attempt to look up keys for successive higher-level domains in the GDBM
database.
Note
GDBM databases are case sensitive. Make sure that the GDBM database is created
using lowercase letters only, and use the
tolower[1] function to
convert the key to lowercase.
If the
options argument is "I", and the key is not in the GDBM
database, and the key is of the form "w.x.y.z" (with variable number
of period-separated sections),
maildrop then tries to look up the key
"w.x.y", then "w.x", until a key is found, or there are no
more sections to remove. Use this feature to look up IP-address based GDBM
lists.
Note
These features are implemented by brute force: if the query doesn't succeed, try
again. Take note of potential denial-of-service attacks where key is set to a
long text string consisting mostly of periods, which will result in numerous
GDBM queries that will take an excessive amount of time to complete.
gdbmopen ( file [, mode])
gdbmopen opens the indicated GDBM file. The optional second argument
specifies the following:
"R"
Open this GDBM file for reading.
"W"
Open this GDBM file for reading and
writing.
"C"
Open this GDBM file for reading and writing.
If the GBDM file doesn't exist, create it.
"N"
Create a new GDBM file. If the file exists,
the existing file is deleted. The file is opened for reading and
writing.
The
mode argument defaults to "R" is used. In embedded mode,
only "R" is allowed.
The GDBM library allows multiple processes to read the same GDBM file at the
same time, but it does not allow multiple access when the GDBM file is open
for writing. Using
flock[2] or
dotlock[3] is
highly recommended.
In delivery mode,
maildrop runs from the recipient's home directory. Keep
that in mind while specifying the filename.
The gdbmopen function returns 0 if the GDBM file was successfully opened,
non-zero otherwise.
key is the key value to store in the GDBM file.
value is the value
to store. If
key already exists in the GDBM file,
value
replacest the old value. The
gdbmstore function is only permitted if
the GDBM file is opened for writing. If
gdbmopen opened the GDBM file
for reading only,
gdbmstore will return -1. Otherwise,
gdbmstore
returns 0.
Sam Varshavchik
Author
- 1.
- tolower
- 2.
- flock
- 3.
- dotlock