|The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
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1.17.3 Protection provided in different modes
The different values for the
sslmode parameter provide different
levels of protection. SSL can provide
protection against three types of attacks:
|Eavesdropping|| If a third party can examine the network traffic between the
client and the server, it can read both connection information (including
the user name and password) and the data that is passed. SSL
uses encryption to prevent this.
|Man in the middle (MITM)|| If a third party can modify the data while passing between the
client and server, it can pretend to be the server and therefore see and
modify data even if it is encrypted. The third party can then
forward the connection information and data to the original server,
making it impossible to detect this attack. Common vectors to do this
include DNS poisoning and address hijacking, whereby the client is directed
to a different server than intended. There are also several other
attack methods that can accomplish this. SSL uses certificate
verification to prevent this, by authenticating the server to the client.
|Impersonation||If a third party can pretend to be an authorized client, it can simply access data it should not have access to. Typically this can happen through insecure password management. SSL uses client certificates to prevent this, by making sure that only holders of valid certificates can access the server.|
For a connection to be known secure, SSL usage must be configured
on both the client and the server before the connection
is made. If it is only configured on the server, the client may end up
sending sensitive information (e.g. passwords) before
it knows that the server requires high security. In libpq, secure
connections can be ensured
by setting the
sslmode parameter to
verify-ca, and providing the system with a root certificate to
verify against. This is analogous to using an
URL for encrypted web browsing.
Once the server has been authenticated, the client can pass sensitive data. This means that up until this point, the client does not need to know if certificates will be used for authentication, making it safe to specify that only in the server configuration.
All SSL options carry overhead in the form of encryption and
key-exchange, so there is a tradeoff that has to be made between performance
and security. The following table illustrates the risks the different
sslmode values protect against, and what statement they make
about security and overhead:
||Eavesdropping protection||MITM protection|| Statement
||No||No|| I don't care about security, and I don't want to pay the overhead
||Maybe||No|| I don't care about security, but I will pay the overhead of
encryption if the server insists on it.
||Maybe||No|| I don't care about encryption, but I wish to pay the overhead of
encryption if the server supports it.
||Yes||No|| I want my data to be encrypted, and I accept the overhead. I trust
that the network will make sure I always connect to the server I want.
||Yes|| || I want my data encrypted, and I accept the overhead. I want to be
sure that I connect to a server that I trust.
||Yes||Yes||I want my data encrypted, and I accept the overhead. I want to be sure that I connect to a server I trust, and that it's the one I specify.|
The difference between
depends on the policy of the root CA. If a public
CA is used,
verify-ca allows connections to a server
that somebody else may have registered with the CA.
In this case,
verify-full should always be used. If
a local CA is used, or even a self-signed certificate, using
verify-ca often provides enough protection.
The default value for
prefer. As is shown
in the table, this makes no sense from a security point of view, and it only
promises performance overhead if possible. It is only provided as the default
for backwards compatibility, and is not recommended in secure deployments.
|ISBN 9781906966065||The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide||See the print edition|