Previously, our rate-limiting code trusted the entire `X-Forwarded-For`
header, allowing a malicious client to spoof that header and evade
rate-limiting. This commit introduces a new module and setting
allowing us to make a more conservative choice of IPs.
- Create new `openedx.core.djangoapps.util.ip` module for producing
the IP "external chain" for requests based on the XFF header and the
REMOTE_ADDR.
- Include a function that gives the safest choice of IPs.
- Add new setting `CLOSEST_CLIENT_IP_FROM_HEADERS` for configuring how
the external chain is derived (i.e. setting the trust
boundary). Currently has a default, but we may want to make it
mandatory in the future.
- Change `django-ratelimit` code to use the proximate IP in the external
chain -- the one just outside the trust boundary.
Also:
- Change `XForwardedForMiddleware` to use more conservative choice for
its `REMOTE_ADDR` override
- Other adjustments to `XForwardedForMiddleware` as needed in order to
initialize new module and support code that needs the real
`REMOTE_ADDR` value
- Metrics for observability into the change (and XFF composition)
- Feature switch to restore legacy mode if needed
This also gives us a path forward to removing use of the django-ipware
package, which is no longer maintained and has a handful of bugs that make it
difficult to use safely.
Internal ticket: ARCHBOM-2056
a251d18281