You can ascertain if a .db file is a BDB file using
the file command.
E.g., the
Sendmail email software uses .db files stored in /etc/mail
to control various aspects of the software's functionality. For instance,
you can specify the domains for which Sendmail will accept email in
the /etc/mail/local-host-names file or create "virtual users"
using the /etc/mail/virtusertable file. But to alter
Sendmail's behavior using these files, you need to generate .db files from
the text files using the
makemap command. E.g., makemap hash
/etc/mail/virtusertable < /etc/mail/virtusertable. If you check
the type of file for the .db file that will be created by the
makemap utility with the file command, you will
see the following information, if you check from the root account:
# file /etc/mail/virtusertable.db /etc/mail/virtusertable.db: Berkeley DB (Hash, version 9, native byte-order) #
Since a normal user account doesn't have read permission for that .db file, you will see the message below, if you check from a regular user account:
$ file /etc/mail/virtusertable.db /etc/mail/virtusertable.db: regular file, no read permission $ ls -l /etc/mail/virtusertable.db -rw-r-----. 1 root root 12288 Aug 19 12:41 /etc/mail/virtusertable.db $
Supposing you need to read the contents of a .db file, how do you do it? You can use the db_dump command, which is a command line interface (CLI) tool. You can determine if the utility is on the system using the which command.
$ which db_dump /usr/bin/db_dump $
On a CentOS
7 Linux system, the utility is provided with the
libdb-utils package, which can be installed with
yum install libdb-utils, if it is not present and you use
yum for
package
management.
$ rpm -q --whatprovides /usr/bin/db_dump libdb-utils-5.3.21-17.el7_0.1.x86_64
You may also find the utility in a db_utils or
db4-utils package.
To view the
records in a .db database with the
db_dump command you can issue a command in the form
db_dump -p db_file where db_file is the relevant
.db file. E.g., suppose I have a virtusertable file with
the following contents:
# cat /etc/mail/virtusertable debbie@example.com dsahlin pam@example.com pcullen gdavis@example.com gregg jgarland@example.com jan #
If I view the contents of the .db file generated by
makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
using the db_dump command, I see the following. The database records start
after the "HEADER=END" line.
# db_dump -p /etc/mail/virtusertable.db VERSION=3 format=print type=hash h_nelem=4100 db_pagesize=4096 HEADER=END debbie@example.com dsahlin jgarland@example.com jan gdavis@example.com gregg pam@example.com pcullen DATA=END #
If you don't specify the -p option with the dp_dump command,
you will see the data displayed in
hexadecimal
form, which for most people won't be easy to interpret. E.g., I would see
the following, instead:
# db_dump /etc/mail/virtusertable.db VERSION=3 format=bytevalue type=hash h_nelem=4100 db_pagesize=4096 HEADER=END 646562626965406578616d706c652e636f6d 647361686c696e 6a6761726c616e64406578616d706c652e636f6d 6a616e 676461766973406578616d706c652e636f6d 6772656767 70616d406578616d706c652e636f6d 7063756c6c656e DATA=END #
In the case of the Sendmail .db files, if you have the original text
files, you can always regenerate the .db files, but if one of those files
is accidentally deleted or corrupted, you can use the db_dump
command to recreate it by viewing the records in the database using that
command.