postgrest/admin.rst

Hardening PostgREST
===================

PostgREST is a fast way to construct a RESTful API. Its default behavior is great for scaffolding in development. When it's time to go to production it works great too, as long as you take precautions. PostgREST is a small sharp tool that focuses on performing the API-to-database mapping. We rely on a reverse proxy like Nginx for additional safeguards.

The first step is to create an Nginx configuration file that proxies requests to an underlying PostgREST server.

.. code-block:: nginx

  http {
    # ...
    # upstream configuration
    upstream postgrest {
      server localhost:3000;
    }
    # ...
    server {
      # ...
      # expose to the outside world
      location /api/ {
        default_type  application/json;
        proxy_hide_header Content-Location;
        add_header Content-Location  /api/$upstream_http_content_location;
        proxy_set_header  Connection "";
        proxy_http_version 1.1;
        proxy_pass http://postgrest/;
      }
      # ...
    }
  }

.. note::

  For ubuntu, if you already installed nginx through :code:`apt` you can add this to the config file in
  :code:`/etc/nginx/sites-enabled/default`.

.. _block_fulltable:

Block Full-Table Operations
---------------------------

Each table in the admin-selected schema gets exposed as a top level route. Client requests are executed by certain database roles depending on their authentication. All HTTP verbs are supported that correspond to actions permitted to the role. For instance if the active role can drop rows of the table then the DELETE verb is allowed for clients. Here's an API request to delete old rows from a hypothetical logs table:

.. code-block:: http

  DELETE /logs?time=lt.1991-08-06 HTTP/1.1

However it's very easy to delete the **entire table** by omitting the query parameter!

.. code-block:: http

  DELETE /logs HTTP/1.1

This can happen accidentally such as by switching a request from a GET to a DELETE. To protect against accidental operations use the `pg-safeupdate <https://bitbucket.org/eradman/pg-safeupdate/>`_ PostgreSQL extension. It raises an error if UPDATE or DELETE are executed without specifying conditions. To install it you can use the `PGXN <http://pgxn.org/>`_ network:

.. code-block:: bash

  sudo -E pgxn install safeupdate

  # then add this to postgresql.conf:
  # shared_preload_libraries='safeupdate';

This does not protect against malicious actions, since someone can add a url parameter that does not affect the result set. To prevent this you must turn to database permissions, forbidding the wrong people from deleting rows, and using `row-level security <https://www.postgresql.org/docs/current/static/ddl-rowsecurity.html>`_ if finer access control is required.

Count-Header DoS
----------------

For convenience to client-side pagination controls PostgREST supports counting and reporting total table size in its response. As described in :ref:`limits`, responses ordinarily include a range but leave the total unspecified like

.. code-block:: http

  HTTP/1.1 200 OK
  Range-Unit: items
  Content-Range: 0-14/*

However including the request header :code:`Prefer: count=exact` calculates and includes the full count:

.. code-block:: http

  HTTP/1.1 206 Partial Content
  Range-Unit: items
  Content-Range: 0-14/3573458

This is fine in small tables, but count performance degrades in big tables due to the MVCC architecture of PostgreSQL. For very large tables it can take a very long time to retrieve the results which allows a denial of service attack. The solution is to strip this header from all requests:

.. code-block:: postgres

  -- Pending nginx config: Remove any prefer header which contains the word count

.. _https:

HTTPS
-----

PostgREST aims to do one thing well: add an HTTP interface to a PostgreSQL database. To keep the code small and focused we do not implement HTTPS. Use a reverse proxy such as NGINX to add this, `here's how <https://nginx.org/en/docs/http/configuring_https_servers.html>`_. Note that some Platforms as a Service like Heroku also add SSL automatically in their load balancer.

Rate Limiting
-------------

Nginx supports "leaky bucket" rate limiting (see `official docs <https://nginx.org/en/docs/http/ngx_http_limit_req_module.html>`_). Using standard Nginx configuration, routes can be grouped into *request zones* for rate limiting. For instance we can define a zone for login attempts:

.. code-block:: nginx

  limit_req_zone $binary_remote_addr zone=login:10m rate=1r/s;

This creates a shared memory zone called "login" to store a log of IP addresses that access the rate limited urls. The space reserved, 10 MB (:code:`10m`) will give us enough space to store a history of 160k requests. We have chosen to allow only allow one request per second (:code:`1r/s`).

Next we apply the zone to certain routes, like a hypothetical stored procedure called :code:`login`.

.. code-block:: nginx

  location /rpc/login/ {
    # apply rate limiting
    limit_req zone=login burst=5;
  }

The burst argument tells Nginx to start dropping requests if more than five queue up from a specific IP.

Nginx rate limiting is general and indiscriminate. To rate limit each authenticated request individually you will need to add logic in a :ref:`Custom Validation <custom_validation>` function.

Debugging
=========

Server Version
--------------

When debugging a problem it's important to verify the PostgREST version. At any time you can make a request to the running server and determine exactly which version is deployed. Look for the :code:`Server` HTTP response header, which contains the version number.

Logging
-------

The PostgREST server logs basic request information to stdout, including the requesting IP address and user agent, the URL requested, and HTTP response status. However this provides limited information for debugging server errors. It's helpful to get full information about both client requests and the corresponding SQL commands executed against the underlying database.

.. note::

   When running it in an SSH session you must detach it from stdout or it will be terminated when the session closes. The easiest technique is redirecting the output to a log file or to the syslog:

   .. code-block:: bash

     ssh foo@example.com \
       'postgrest foo.conf </dev/null >/var/log/postgrest.log 2>&1 &'

     # another option is to pipe the output into "logger -t postgrest"

HTTP Requests
-------------

A great way to inspect incoming HTTP requests including headers and query params is to sniff the network traffic on the port where PostgREST is running. For instance on a development server bound to port 3000 on localhost, run this:

.. code:: bash

  # sudo access is necessary for watching the network
  sudo ngrep -d lo0 port 3000

The options to ngrep vary depending on the address and host on which you've bound the server. The binding is described in the :ref:`configuration` section. The ngrep output isn't particularly pretty, but it's legible.

Database Logs
-------------

Once you've verified that requests are as you expect, you can get more information about the server operations by watching the database logs. By default PostgreSQL does not keep these logs, so you'll need to make the configuration changes below. Find :code:`postgresql.conf` inside your PostgreSQL data directory (to find that, issue the command :code:`show data_directory;`). Either find the settings scattered throughout the file and change them to the following values, or append this block of code to the end of the configuration file.

.. code:: sql

  # send logs where the collector can access them
  log_destination = "stderr"

  # collect stderr output to log files
  logging_collector = on

  # save logs in pg_log/ under the pg data directory
  log_directory = "pg_log"

  # (optional) new log file per day
  log_filename = "postgresql-%Y-%m-%d.log"

  # log every kind of SQL statement
  log_statement = "all"

Restart the database and watch the log file in real-time to understand how HTTP requests are being translated into SQL commands.

.. note::

  On Docker you can enable the logs by using a custom ``init.sh``:

  .. code:: bash

    #!/bin/sh
    echo "log_statement = 'all'" >> /var/lib/postgresql/data/postgresql.conf

  After that you can start the container and check the logs with ``docker logs``.

  .. code:: bash

    docker run -v "$(pwd)/init.sh":"/docker-entrypoint-initdb.d/init.sh" -d postgres
    docker logs -f <container-id>

.. _schema_reloading:

Schema Reloading
----------------

Users are often confused by PostgREST's database schema cache. It is present because detecting foreign key relationships between tables (including how those relationships pass through views) is necessary, but costly. API requests consult the schema cache as part of :ref:`resource_embedding`. However if the schema changes while the server is running it results in a stale cache and leads to errors claiming that no relations are detected between tables.

.. important::

   Since v5.0, PostgREST also makes use of the schema cache for stored functions metadata: parameters, return type, volatility.
   It also uses the schema cache for resolving overloaded functions. You should refresh the cache if a change in any of the prior is done.

To refresh the cache without restarting the PostgREST server, send the server process a SIGUSR1 signal:

.. code:: bash

  killall -SIGUSR1 postgrest

.. note::

   To refresh the cache in docker:

   .. code:: bash

     docker kill -s SIGUSR1 <container>

     # or in docker-compose
     docker-compose kill -s SIGUSR1 <service>

The above is the manual way to do it. To automate the schema reloads, use a database trigger like this:

.. code-block:: postgresql

  CREATE OR REPLACE FUNCTION public.notify_ddl_postgrest()
    RETURNS event_trigger
   LANGUAGE plpgsql
    AS $$
  BEGIN
    NOTIFY ddl_command_end;
  END;
  $$;

  CREATE EVENT TRIGGER ddl_postgrest ON ddl_command_end
     EXECUTE PROCEDURE public.notify_ddl_postgrest();

Then run the `pg_listen <https://github.com/begriffs/pg_listen>`_ utility to monitor for that event and send a SIGUSR1 when it occurs:

.. code-block:: bash

  pg_listen <db-uri> ddl_command_end $(which killall) -SIGUSR1 postgrest

Now, whenever the structure of the database schema changes, PostgreSQL will notify the ``ddl_command_end`` channel, which will cause ``pg_listen`` to send PostgREST the signal to reload its cache. Note that pg_listen requires full path to the executable in the example above.

Daemonizing
===========

For linux distros that use **systemd** (ubuntu, debian, archlinux) you can create a daemon in the following way.

First, create postgrest configuration in ``/etc/postgrest/config``

.. code-block:: ini

  db-uri = "postgres://<your_user>:<your_password>@localhost:5432/<your_db>"
  db-schema = "<your_exposed_schema>"
  db-anon-role = "<your_anon_role>"
  db-pool = 10

  server-host = "127.0.0.1"
  server-port = 3000

  jwt-secret = "<your_secret>"

Then create the systemd service file in ``/etc/systemd/system/postgrest.service``

.. code-block:: ini

  [Unit]
  Description=REST API for any Postgres database
  After=postgresql.service

  [Service]
  ExecStart=/bin/postgrest /etc/postgrest/config
  ExecReload=/bin/kill -SIGUSR1 $MAINPID

  [Install]
  WantedBy=multi-user.target

After that, you can enable the service at boot time and start it with:

.. code-block:: bash

  systemctl enable postgrest
  systemctl start postgrest

  ## For reloading the service
  ## systemctl restart postgrest

Alternate URL Structure
=======================

As discussed in :ref:`singular_plural`, there are no special URL forms for singular resources in PostgREST, only operators for filtering. Thus there are no URLs like :code:`/people/1`. It would be specified instead as

.. code:: http

  GET /people?id=eq.1 HTTP/1.1
  Accept: application/vnd.pgrst.object+json

This allows compound primary keys and makes the intent for singular response independent of a URL convention.

Nginx rewrite rules allow you to simulate the familiar URL convention. The following example adds a rewrite rule for all table endpoints, but you'll want to restrict it to those tables that have a numeric simple primary key named "id."

.. code-block:: nginx

  # support /endpoint/:id url style
  location ~ ^/([a-z_]+)/([0-9]+) {

    # make the response singular
    proxy_set_header Accept 'application/vnd.pgrst.object+json';

    # assuming an upstream named "postgrest"
    proxy_pass http://postgrest/$1?id=eq.$2;

  }

.. TODO
.. Administration
..   API Versioning
..   HTTP Caching
..   Upgrading