The making of this blog, part 4 - Logging

Adding logs is a lot more complicated than I thought

python web making_of flask gunicorn docker nginx cloud

Part 3

As of this writing, I still haven't purchased a domain name and therefore the site is only accessible via direct IP address. Despite that, while I was looking at the output of nginx on the server, I saw that the IP was actually hit! Something requested the site!

My first thought - and still is my main thought - was that this is some sort of web crawler/scraper that just goes through all the IPs.. it could be Google, it could be some other search engine, or it could be private. In order to see who is it though, or if there is a pattern, I wanted to set up some IP address logging.

I thought this would be relatively straightforward, but ended up being anything but. I had to 1) get the real IP address from within my application and 2), log the IP address to a file. Seems simple right?

Getting the real IP address

The first thing I wanted to tackle was getting the real IP address. In Flask, there is an object you can import called request. One of its attributes is .remote_addr, which is billed as being "the remote address of the client". Straightforward, right? Import request and simply log this value!

nginx says not so fast

As the title suggest - this isn't really the case. Since in production we run our app using gunicorn behind an nginx reverse proxy, if we do this naively then we will just get the IP address of the reverse proxy, which (should be) unchanging. Luckily, proxies can keep track of the IP addresses they see as they pass through requests, and add this onto the request header. Unluckily, this can be spoofed and is therefore a security concern.

In order to get around this, you have to tell Flask how many proxies it is behind, using ProxyFix. This is... complicated when you don't know a ton about HTML headers. In the end I think I pieced it together, but if this was ever to be actually put into practice for a business, I would have someone more knowledgeable look at it.

The first step to telling Flask how many proxies there are is to correctly set up nginx. I set up the following nginx.conf file, setting the following headers: X-Forwarded-For, Host and X-Real-IP with variables provided by nginx. In the end, I think the one that matters the most is X-Real-IP, as it was unchanging during my testing.

Once those were set, you can now begin to access the IP(s) in the Flask app, with the previously mentioned request.remote_addr. To test that this was the correct way to go about it, I falsified some headers using curl (curl --header "X-Forwarded-For:" http://localhost:9000, where I tested falsifying X-Forwarded-For, Host and X-Real-IP), and saw what was returned. Many tutorials online say to look at the last (or correctly numbered) value in request.access_route, but this is where the falsified info went, and it never went into .remote_addr in my experience.

Things were moving in a positive direction and the correct IP addresses were being at least printed to stdout.

Logging says not so fast

Now - time to write to a file. Luckily Python has logging built into the standard library, unluckily, the Basic Config version prints too much other stuff (mostly from Matplotlib). To make a long story short, I had to initialize this bit of code in the beginning of my file:

import logging

logger = logging.getLogger(__name__)
fh = logging.FileHandler("./logs/ip_log.log", mode="a")
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
logger.addHandler(fh)"Log is working.")

It is relatively straightforward - import logging, create a logger instance, set the level for which warnings you want, make a FileHandler, and Formatter, and attach the formatter to the file handler (fh), and then the handler to the logger. Then you can just call"insert message here").

AFTER doing all of this, I found out that Flask has a logger built in. I guess room for future improvement.

Also room for improvement - change my logging call to a function decorator. Right now, to log the IP on each page of my site, I have"{request.remote_addr} - {request.full_path} - {request.referrer}") within every route that I have, so multiple times and multiple places for error. What I need to do in the future is change that line into a part of a @log decorator, so I just can decorate each of my routes.

At this point things are looking good and my text file is getting written with the IP addresses that access the site! (Let's be real... 99% only my IP).

Docker says not so fast

Here I am, thinking things are going great, composing my Docker image, running it.. and... nothing. No log files!

After looking into this, it turns out it is a noob mistake - containers are isolated, so if I don't provide a mount point outside of the container, everything inside of the container dies when it dies.

star wars gif
Depiction of a Docker container shutting down.

The solution to this is to provide a mount point, which in the end is pretty simple. Just adding

      - type: bind
        source: ./logs/
        target: /logs

to my compose file was enough. Care has to be taken with regards to paths, but remember, everything is at the root level in the container (at least how I've set it up), so it all works. Using a relative path in the Flask app is required to not go insane with paths. source is the path on the hardware, and target is the path in the container. Note the . indicating relative path for source!

Final note - this uses a Docker mount (as indicated by type: bind), not a Volume. From what I could figure out, mounts are good for getting data out of the container, while Volume is better for data that will stay inside the container/be read by a future container. Most of the Docker documentation recommends Volumes for their many benefits, but ease of access to the outside world is a big plus for a mounted drive.


No huge takeaways here - this was a more complicated problem than I initially expected, but by breaking it down in to small component parts, I could work my way through it until everything worked (again). This required reconfiguring my reverse proxy, understanding how that worked to access the outside users' IP address within Flask, learning about logging and how to set that up (with room for future improvements), and finally configuring Docker to have a persistent storage location. The combination of these things resulted in a somewhat basic IP address logging system.

To improve it, as previously mentioned, I want to turn it into a decorator based system, as well as keep more header information (particularly if the requests are from robots or actual people... not sure how to figure that out yet).

Part 4