INTRODUCTION(UNIX)

OVERVIEW
This section outlines the interaction between Postgres and the operating system. In particular, this section describes the Postgres support programs that are executable as Unix commands.

TERMINOLOGY
In the following documentation, the term site may be interpreted as the host machine on which Postgres is installed. Since it is possible to install more than one set of Postgres databases on a single host, this term more precisely denotes any particular set of installed Postgres binaries and databases.

The Postgres super-user is the user named “postgres” who owns the Postgres binaries and database files. As the database super-user, all protection mechanisms may be bypassed and any data accessed arbitrarily. In addition, the Postgres super-user is allowed to execute some support programs which are generally not available to all users. Note that the Postgres super-user is not the same as the Unix super-user, root, and should have a non-zero userid for security reasons.

The database base administrator or DBA, is the person who is responsible for installing Postgres to enforce a security policy for a site. The DBA can add new users by the method described below and maintain a set of template databases for use by createdb(1).

The postmaster is the process that acts as a clearing-house for requests to the Postgres system. Frontend applications connect to the postmaster, which keeps tracks of any system errors and communication between the backend processes. The postmaster can take several command-line arguments to tune its behavior. However, supplying arguments is necessary only if you intend to run multiple sites or a non-default site. See postmaster(1) for details.

The Postgres backend (the actual executable program called "postgres") may be executed directly from the user shell by the Postgres super-user (with the database name as an argument). However, doing this bypasses the shared buffer pool and lock table associated with a postmaster/site, therefore this is not recommended in a multiuser site.

NOTATION
“.../” at the front of a file name is used to represent the path to the Postgres super-user's home directory. Anything in brackets (“[” and “]”) is optional. Anything in braces (“{” and “}”) can be repeated 0 or more times. Parentheses (“(” and “)” ) are used to group boolean expressions. “|” is the boolean operator OR .

USING Postgres FROM Unix
All Postgres commands that are executed directly from a Unix shell are found in the directory “.../bin”. Including this directory in your search path will make executing the commands easier.

A collection of system catalogs exist at each site. These include a class (“pg_user”) that contains an instance for each valid Postgres user. The instance specifies a set of Postgres privileges, such as the ability to act as Postgres super-user, the ability to create/destroy databases, and the ability to update the system catalogs. A Unix user cannot do anything with Postgres until an appropriate instance is installed in this class. Further information on the system catalogs is available by running queries on the appropriate classes.

Security

USER AUTHENTICATION
Authentication is the process by which the backend server and postmaster ensure that the user requesting access to data is in fact who he/she claims to be. All users who invoke Postgres are checked against the contents of the “pg_user” class to ensure that they are authorized to do so. However, verification of the user's actual identity is performed in a variety of ways.

  From the user shell
A backend server started from a user shell notes the user's (effective) user-id before performing a setuid(3) to the user-id of user “postgres”. The effective user-id is used as the basis for access control checks. No other authentication is conducted.

  From the network
If the Postgres system is built as distributed, access to the Internet TCP port of the postmaster process is available to anyone. However, Postgres offers optional host-based authentication where only access from certain hosts are allowed. Of course, host-based authentication is not fool-proof in Unix, either. It is possible for determined intruders to also masquerade the origination host. Those security issues are beyond the scope of Postgres.

If greater security is desired, Postgres and its clients may be modified to use a network authentication system. For example, the postmaster, psql and the libpq library have already been configured to use either Version 4 or Version 5 of the Kerberos authentication system from the Massachusetts Institute of Technology. For more information on using Kerberos with Postgres, see the appendix below.

ACCESS CONTROL
Postgres provides mechanisms to allow users to limit the access to their data that is provided to other users.

  Database superusers
Database super-users (i.e., users who have “pg_user.usesuper” set) silently bypass all of the access controls described below with two exceptions: manual system catalog updates are not permitted if the user does not have “pg_user.usecatupd” set, and destruction of system catalogs (or modification of their schemas) is never allowed.

  Access Privilege
The use of access privilege to limit reading, writing and setting of rules on classes is covered in grant/revoke(l).

  Class removal and schema modification
Commands that destroy or modify the structure of an existing class, such as alter, drop table, and drop index, only operate for the owner of the class. As mentioned above, these operations are never permitted on system catalogs.

FUNCTIONS AND RULES
Functions and rules allow users to insert code into the backend server that other users may execute without knowing it. Hence, both mechanisms permit users to trojan horse others with relative impunity. The only real protection is tight control over who can define functions (e.g., write to relations with SQL fields) and rules. Audit trails and alerters on “pg_class”, “pg_user” and “pg_group” are also recommended.

  Functions
Functions written in any language except SQL run inside the backend server process with the permissions of the user “postgres” (the backend server runs with its real and effective user-id set to “postgres”). It is possible for users to change the server's internal data structures from inside of trusted functions. Hence, among many other things, such functions can circumvent any system access controls. This is an inherent problem with user-defined C functions.

  Rules
Like SQL functions, rules always run with the identity and permissions of the user who invoked the backend server.

SEE ALSO
postmaster(1), alter(l), insert(l), grant/revoke(l), copy(l), create(l), delete(l), drop table(l), drop index(l), drop rule(l), update(l), select(l), kerberos(1), kinit(1), kerberos(3)

CAVEATS

There are no plans to explicitly support encrypted data inside of Postgres (though there is nothing to prevent users from encrypting data within user-defined functions). There are no plans to explicitly support encrypted network connections, either, pending a total rewrite of the frontend/backend protocol.

User names, group names and associated system identifiers (e.g., the contents of “pg_user.usesysid”) are assumed to be unique throughout a database. Unpredictable results may occur if they are not.

APPENDIX: USING KERBEROS

  Availability
The Kerberos authentication system is not distributed with Postgres, nor is it available from the University of California at Berkeley. Versions of Kerberos are typically available as optional software from operating system vendors. In addition, a source code distribution may be obtained through MIT Project Athena by anonymous FTP from ATHENA-DIST.MIT.EDU (18.71.0.38). (You may wish to obtain the MIT version even if your vendor provides a version, since some vendor ports have been deliberately crippled or rendered non-interoperable with the MIT version.) Users located outside the United States of America and Canada are warned that distribution of the actual encryption code in Kerberos is restricted by U. S. government export regulations.

Any additional inquiries should be directed to your vendor or MIT Project Athena (“info-kerberos@ATHENA.MIT.EDU”). Note that FAQLs (Frequently-Asked Questions Lists) are periodically posted to the Kerberos mailing list, “kerberos@ATHENA.MIT.EDU” (send mail to “kerberos-request@ATHENA.MIT.EDU” to subscribe), and USENET news group, “comp.protocols.kerberos”.

  Installation
Installation of Kerberos itself is covered in detail in the Kerberos Installation Notes. Make sure that the server key file (the srvtab or keytab) is somehow readable by user “postgres”.

Postgres and its clients can be compiled to use either Version 4 or Version 5 of the MIT Kerberos protocols by setting the KRBVERS variable in the file “.../src/Makefile.global” to the appropriate value. You can also change the location where Postgres expects to find the associated libraries, header files and its own server key file.

After compilation is complete, Postgres must be registered as a Kerberos service. See the Kerberos Operations Notes and related manual pages for more details on registering services.

  Operation
After initial installation, Postgres should operate in all ways as a normal Kerberos service. For details on the use of authentication, see the manual pages for postmaster(1) and psql(1).

In the Kerberos Version 5 hooks, the following assumptions are made about user and service naming: (1) user principal names (anames) are assumed to contain the actual Unix/Postgres user name in the first component; (2) the Postgres service is assumed to be have two components, the service name and a hostname, canonicalized as in Version 4 (i.e., all domain suffixes removed).

user example: frew@S2K.ORG
user example: aoki/HOST=miyu.S2K.Berkeley.EDU@S2K.ORG
host example: postgres_dbms/ucbvax@S2K.ORG

Support for Version 4 will disappear sometime after the production release of Version 5 by MIT.