From 33261017f89f18b810886df8b32e9f8917238530 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rapha=C3=ABl=20Vinot?= <Rafiot@users.noreply.github.com>
Date: Thu, 20 Jul 2023 12:58:59 +0200
Subject: [PATCH] chg: Update readme, add notes regarding HTTP2 and cURL

---
 README.md | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 16f2aaf..bf8ce2c 100644
--- a/README.md
+++ b/README.md
@@ -20,8 +20,10 @@ The HHHash value is the SHA256 of the list.
 
 ### Calculating HHHash from a curl command
 
-~~~
-$ curl -s -D - https://www.circl.lu/ -o /dev/null  | awk 'NR != 1' | cut -f1 -d: | sed '/^[[:space:]]*$/d' | sed -z 's/\n/:/g' | sed 's/.$//' | sha256sum | cut -f1 -d " " | awk {'print "hhh:1:"$1'}
+Curl will attempt to run the request using HTTP2 by default. In order to get the same hash as the python requests module (which doesn't supports HTTP2), you need to specify the version with the `--http1.1` switch.
+
+~~~bash
+curl --http1.1 -s -D - https://www.circl.lu/ -o /dev/null  | awk 'NR != 1' | cut -f1 -d: | sed '/^[[:space:]]*$/d' | sed -z 's/\n/:/g' | sed 's/.$//' | sha256sum | cut -f1 -d " " | awk {'print "hhh:1:"$1'}
 ~~~
 
 Output value
@@ -33,6 +35,8 @@ hhh:1:78f7ef0651bac1a5ea42ed9d22242ed8725f07815091032a34ab4e30d3c3cefc
 
 HHHash is an effective technique; however, its performance is heavily reliant on the characteristics of the HTTP client requests. Therefore, it is important to note that correlations between a set of hashes are typically established when using the same crawler or HTTP client parameters.
 
+HTTP2 requires the [headers to be lowercase](https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2). It will then changes the hash so you need to be aware of the HTTP version you're using.
+
 ### hhhash - Python Library
 
 The [hhhash package](https://pypi.org/project/hhhash/) can be installed via a `pip install hhhash` or build with Poetry from this repository `poetry build` and `poetry install`.