The following describes the authentication methods in more detail.
When trust authentication is specified,
PostgreSQL assumes that anyone who can
connect to the server is authorized to access the database as
whatever database user he specifies (including the database superuser).
This method should only be used when there is adequate system-level
protection on connections to the postmaster port.
trust authentication is appropriate and very convenient
for local connections on a single-user workstation. It is usually
not appropriate by itself on a multiuser machine.
However, you may be able to use trust even on a multiuser
machine, if you restrict access to the postmaster's socket file using
file-system permissions. To do this, set the parameter
unix_socket_permissions (and possibly
unix_socket_group) in postgresql.conf,
as described in Section 3.4.4. Or you could
set unix_socket_directory to place the socket file
in a suitably restricted directory.
Setting file-system permissions only helps for Unix-socket connections.
Local TCP connections are not restricted by it; therefore, if you want
to use permissions for local security, remove the host ...
127.0.0.1 ... line from pg_hba.conf, or change it to a
non-trust authentication method.
trust authentication is only suitable for TCP connections
if you trust every user on every machine that is allowed to connect
to the server by the pg_hba.conf lines that specify
trust. It is seldom reasonable to use trust
for any TCP connections other than those from localhost (127.0.0.1).
Password-based authentication methods include md5,
crypt, and password. These methods operate
similarly except for the way that the password is sent across the
connection. If you are at all concerned about password
"sniffing" attacks then md5 is preferred, with
crypt a second choice if you must support pre-7.2
clients. Plain password should especially be avoided for
connections over the open Internet (unless you use SSL, SSH, or
other communications security wrappers around the connection).
PostgreSQL database passwords are
separate from operating system user passwords. The password for
each database user is stored in the pg_shadow system
catalog table. Passwords can be managed with the query language
commands CREATE USER and ALTER
USER, e.g., CREATE USER foo WITH PASSWORD
'secret';. By default, that is, if no password has
been set up, the stored password is null and
password authentication will always fail for that user.
To restrict the set of users that are allowed to connect to certain
databases, list the users separated by commas, or in a separate
file. The file should contain user names separated by commas or one
user name per line, and be in the same directory as
pg_hba.conf. Mention the (base) name of the file
preceded with @ in the user column. The
database column can similarly accept a list of values or
a file name. You can also specify group names by preceding the group
name with +.
Kerberos is an industry-standard secure
authentication system suitable for distributed computing over a
public network. A description of the
Kerberos system is far beyond the scope
of this document; in all generality it can be quite complex (yet
powerful). The Kerberos
FAQ or MIT Project Athena can be
a good starting point for exploration. Several sources for
Kerberos distributions exist.
In order to use Kerberos, support for it must be
enabled at build time. See Chapter 1 for more
information. Both Kerberos 4 and 5 are supported, but only one
version can be supported in any one build.
PostgreSQL operates like a normal Kerberos service.
The name of the service principal is
servicename/hostname@realm, where
servicename is postgres (unless a
different service name was selected at configure time with
./configure --with-krb-srvnam=whatever).
hostname is the fully qualified domain name of the
server machine. The service principal's realm is the preferred realm
of the server machine.
Client principals must have their PostgreSQL user
name as their first component, for example
pgusername/otherstuff@realm. At present the realm of
the client is not checked by PostgreSQL; so if you
have cross-realm authentication enabled, then any principal in any
realm that can communicate with yours will be accepted.
Make sure that your server key file is readable (and preferably only
readable) by the PostgreSQL server
account (see Section 3.1). The location of the
key file is specified with the krb_server_keyfile run
time configuration parameter. (See also Section 3.4.) The default is /etc/srvtab
if you are using Kerberos 4 and
FILE:/usr/local/pgsql/etc/krb5.keytab (or whichever
directory was specified as sysconfdir at build time)
with Kerberos 5.
To generate the keytab file, use for example (with version 5)
kadmin% ank -randkey postgres/server.my.domain.org
kadmin% ktadd -k krb5.keytab postgres/server.my.domain.org
Read the Kerberos documentation for details.
When connecting to the database make sure you have a ticket for a
principal matching the requested database user name. An example: For
database user name fred, both principal
fred@EXAMPLE.COM and
fred/users.example.com@EXAMPLE.COM can be used to
authenticate to the database server.
If you use mod_auth_krb and
mod_perl on your
Apache web server, you can use
AuthType KerberosV5SaveCredentials with a
mod_perl script. This gives secure
database access over the web, no extra passwords required.
The ident authentication method works by inspecting the client's
operating system user name and determining the allowed database
user names by using a map file that lists the permitted
corresponding user name pairs. The determination of the client's
user name is the security-critical point, and it works differently
depending on the connection type.
The "Identification Protocol" is described in
RFC 1413. Virtually every Unix-like
operating system ships with an ident server that listens on TCP
port 113 by default. The basic functionality of an ident server
is to answer questions like "What user initiated the
connection that goes out of your port X
and connects to my port Y?".
Since PostgreSQL knows both X and
Y when a physical connection is established, it
can interrogate the ident server on the host of the connecting
client and could theoretically determine the operating system user
for any given connection this way.
The drawback of this procedure is that it depends on the integrity
of the client: if the client machine is untrusted or compromised
an attacker could run just about any program on port 113 and
return any user name he chooses. This authentication method is
therefore only appropriate for closed networks where each client
machine is under tight control and where the database and system
administrators operate in close contact. In other words, you must
trust the machine running the ident server.
Heed the warning:
| | The Identification Protocol is not intended as an authorization
or access control protocol.
| |
| --RFC 1413 | |
On systems supporting SO_PEERCRED requests for
Unix-domain sockets (currently Linux, FreeBSD,
NetBSD, and BSD/OS), ident authentication can also be applied
to local connections. In this case, no security risk is added by
using ident authentication; indeed it is a preferable choice for
local connections on such systems.
On systems without SO_PEERCRED requests, ident
authentication is only available for TCP/IP connections. As a
work around, it is possible to specify the localhost address 127.0.0.1 and make connections to this
address.
When using ident-based authentication, after having determined the
name of the operating system user that initiated the connection,
PostgreSQL checks whether that user is
allowed to connect as the database user he is requesting to connect
as. This is controlled by the ident map argument that follows the
ident keyword in the pg_hba.conf
file. There is a predefined ident map sameuser,
which allows any operating system user to connect as the database
user of the same name (if the latter exists). Other maps must be
created manually.
Ident maps
other than sameuser are defined in the file
pg_ident.conf in the data directory, which
contains lines of the general form:
map-name ident-username database-username
Comments and whitespace are handled in the usual way. The
map-name is an arbitrary name that will be used to
refer to this mapping in pg_hba.conf. The other
two fields specify which operating system user is allowed to connect
as which database user. The same map-name can be
used repeatedly to specify more user-mappings within a single map.
There is no restriction regarding how many database users a given
operating system user may correspond to and vice versa.
The pg_ident.conf file is read on start-up and
when the postmaster receives a
SIGHUP signal. If you edit the file on an
active system, you will need to signal the postmaster
(using pg_ctl reload or kill -HUP) to make it
re-read the file.
A pg_ident.conf file that could be used in
conjunction with the pg_hba.conf file in Example 6-1 is shown in Example 6-2. In this example setup, anyone
logged in to a machine on the 192.168 network that does not have the
Unix user name bryanh, ann, or
robert would not be granted access. Unix user
robert would only be allowed access when he tries to
connect as PostgreSQL user bob, not
as robert or anyone else. ann would
only be allowed to connect as ann. User
bryanh would be allowed to connect as either
bryanh himself or as guest1.
Example 6-2. An example pg_ident.conf file
# MAPNAME IDENT-USERNAME PG-USERNAME
omicron bryanh bryanh
omicron ann ann
# bob has user name robert on these machines
omicron robert bob
# bryanh can also connect as guest1
omicron bryanh guest1
This authentication type operates similarly to
password except that it uses PAM (Pluggable
Authentication Modules) as the authentication mechanism. The
default PAM service name is postgresql. You can
optionally supply you own service name after the pam
keyword in the file. For more information about PAM, please read
the Linux-PAM
Page and the Solaris PAM Page.
