A simple mod security config for IP Rate limiting Apache server behind a load balancer.

modsec-ratelimit.conf

# assumes libapache2-modsecurity is installed

# Reference Mannual - https://github.com/SpiderLabs/ModSecurity/wiki

SecRuleEngine On

<LocationMatch "^/.*">
  # initialise the state based on X-Forwarded-For ip address
  SecRule REQUEST_HEADERS:X-Forwarded-For "@unconditionalMatch" "phase:2,initcol:ip=%{MATCHED_VAR},pass,nolog,id:100"

  # if greater then burst_rate_limit then pause set RATELIMITED var and then return 503
  SecRule IP:ACCESS_COUNT "@gt {{ burst_rate_limit }}" "phase:2,pause:300,deny,status:503,setenv:RATELIMITED,skip:1,nolog,id:102"

  # if above rule doesnt match increment the count
  SecAction "phase:2,setvar:ip.access_count=+1,pass,nolog,id:103"

  # set the base rate to one per second
  SecAction "phase:5,deprecatevar:ip.access_count=1/1,pass,nolog,id:104"

  # set a header when ratelimited
  Header always set Retry-After "10" env=RATELIMITED
</LocationMatch>

ErrorDocument 503 "Service Unavailable"