Re: My variable seems to never expire...

@christian yes, the initial workaround about `expirevar` is from @theseion (
https://github.com/SpiderLabs/ModSecurity/issues/1803#issuecomment-474884652).
In my conf, I've added a rule (see rule: 5, the second part) to init my
session.api_req_counter__expire_timestamp  variable once.

If my recipe can help somebody, here the modsec and nginx (to add X-Limit-*
headers) configuration:

ModSec conf :

#
# Rate-limiting rules identified by PHPSESSIONID on /api requests
#
# Variables:
# SESSION:API_REQ_COUNTER       counter of API requests
# TX:API_REQ_COUNTER_MAX        max number of API requests which can pass
in %{tx.api_req_counter_timeout} seconds
# TX:API_REQ_COUNTER_TIMEOUT    period in seconds a blocked PHP session id
will be blocked on API requests
#
# As a precondition for these rules, please set the following 2 variables:
#  - TX:API_REQ_COUNTER_MAX
#  - TX:API_REQ_COUNTER_TIMEOUT
#

# Define TX:API_REQ_COUNTER_MAX and TX_REQ_COUNTER_TIMEOUT
SecAction \
    "id:1,\
    phase:1,\
    nolog,\
    pass,\
    t:none,\
    setvar:tx.api_req_counter_timeout=60,\
    setvar:tx.api_req_counter_max=100"

#
# Define SESSION
#
SecRule REQUEST_COOKIES:PHPSESSID "!^$" \
    "id:2,\
    phase:1,\
    pass,\
    nolog,\
    setsid:%{REQUEST_COOKIES.PHPSESSID}"


###################################################
# TEMPORARY WORKAROUND FOR not working `expirevar`
###################################################
# Expire variables
SecRule &SESSION:API_REQ_COUNTER__EXPIRE_TIMESTAMP "@eq 1" \
  "id:3,\
  phase:1,\
  pass,\
  t:none,\
  nolog,\
  chain"
    SecRule SESSION:API_REQ_COUNTER__EXPIRE_TIMESTAMP "@lt %{TIME_EPOCH}" \
    "setvar:'!session.api_req_counter',\
    setvar:'!session.api_req_counter__expire_timestamp'"
###################################################
###################################################

#
# If it's an API request:
#   1. Identify API requests
#   2. update counter
#   3. if session:api_req_counter__expire_timestamp is not define, define it
#
SecRule REQUEST_URI "@beginsWith /api/" \
    "id:4,\
    phase:1,\
    pass,\
    nolog,\
    setvar:'tx.is_api_req=1',\
    setvar:'session.api_req_counter=+1',\
    expirevar:'session.api_req_counter=%{tx.api_req_counter_timeout}',\
    chain"
      SecRule &SESSION:API_REQ_COUNTER__EXPIRE_TIMESTAMP "@eq 0" \

"setvar:'session.api_req_counter__expire_timestamp=%{TIME_EPOCH}',\

setvar:'session.api_req_counter__expire_timestamp=+%{tx.api_req_counter_timeout}'"

#
# If max counter is reached AND it's an API request, deny it
#
SecRule SESSION:API_REQ_COUNTER "@gt %{tx.api_req_counter_max}" \
    "id:5,\
    deny,\
    status:429,\
    log,\
    msg:'RATELIMITED',\
    chain"
    SecRule TX:IS_API_REQ "@eq 1"

SecRule TX:IS_API_REQ "@eq 1" \
    "id:6,\
    phase:1,\
    pass,\
    nolog,\
    setenv:'ratelimit_limit=%{tx.api_req_counter_max}',\
    setenv:'ratelimit_counter=%{session.api_req_counter}',\
    setenv:'ratelimit_reset=%{session.api_req_counter__expire_timestamp}'"


Nginx Conf (i use the module lua
https://openresty.org/en/lua-nginx-module.html) :

...
    location /api/ {
      set_by_lua $ratelimit_limit 'return os.getenv("ratelimit_limit")';
      set_by_lua $ratelimit_remaining 'return os.getenv("ratelimit_limit")
- os.getenv("ratelimit_counter")';
      set_by_lua $ratelimit_reset 'return os.getenv("ratelimit_reset")';

      add_header X-RateLimit-Limit "$ratelimit_limit";
      add_header X-RateLimit-Remaining "$ratelimit_remaining";
      add_header X-RateLimit-Reset "$ratelimit_reset";
      ...
  }
...

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