What is DBM?

DBM files are a simple and relatively standard method of storing information for quick retrieval. Each item of information stored in a DBM file consists of two parts: a key and a value. If you know the key you can access the value very quickly. The DBM file maintains an 'index' of the keys, each of which points to where the value is stored within the file, and the index is usually arranged such that values can be accessed with the minimum number of file system accesses even for very large numbers of keys.

In practice, on many systems a DBM 'file' is actually stored in two files on the disk. If, for example, a DBM file called 'users' is created, it will actually be stored in files called users.pag and users.dir. If you ever need to rename or delete a DBM from the command line, remember to change both the files, keeping the extensions (.pag and .dir) the same. Some newer versions of DBM only create one file.

Provided the key is known in advance DBM format files are a very efficient way of accessing information associated with that key. For web user authentication, the key will be the username, and the value will store their (encrypted) password. Looking up usernames and their passwords in a DBM file will be more efficient than using a plain text file when more than a few users are involved. This will be particularly important for sites with lots of users (say, over 10,000) or where there are lots of accesses to authenticated pages.

	Preparing Apache for DBM Files

If you want to use DBM format files with Apache, you will need to make sure it is compiled with DBM support. By default, Apache cannot use DBM files for user authentication, so the optional DBM authentication module needs to be included. Note that this is included in addition to the normal user authentication module (which uses plain text files, as explained in the previous article). It is possible to have support for multiple file formats compiled into Apache at the same time.

To add the DBM authentication module, edit your Configuration file in the Apache src directory. Remove the comment from the line which currently says # Module dbm_auth_module mod_auth_dbm.o To remove the comment, delete the # and space character at the right-hand end of the line. Now update the Apache configuration by running ./Configure, then re-make the executable with make.

However, before compiling you might also need to tell Apache where to find the DBM functions. On some systems this is automatic. On others you will need to add the text -lndbm or -ldbm to the EXTRA_LIBS line in the Configuration file. (Apache 1.2 will attempt to do this automatically if needed, but you might still need to configure it manually in some cases). If you are not sure what your system requires, try leaving it blank and compiling. If at the end of the compilation you see errors about functions such as _dbm_fetch() not being found, try each of these choices in turn. (Remember to re-run ./Configure after changing Configuration). If you still cannot get it to compile, you might have a system where the DBM library is installed in a non-standard directory, or where the there is no DBM library available. You could either contact you system administrator, or download and compile your own copy of the DBM libraries (a good choice might be GDBM: read about it or download it).