How to add response headers with HAproxy 1.6 based on request URI?

I use HAproxy 1.6 as load balancer in front of tomcat servers.

I need to add response headers based on the request URI.

For e.g., I'd like to add the response header Cache-Control public,max-age="600" when the request uri is /api but not when the request uri is something else.

• My first try was using acl based on path to add the headers to http-response:

  acl api path_reg ^/api/(.*)$
  http-response add-header Cache-Control public,max-age="600" if api
  

  When I start haproxy with -d, I have warning saying that path_reg (or path) is incompatible with http-response:

  Dec  6 15:22:29 ip-10-30-0-196 haproxy-systemd-wrapper[315]: 
  [WARNING] 340/152229 (2035) : parsing 
  [/etc/haproxy/haproxy.cfg:78] : acl 'api' will never match because 
  it only involves keywords that are incompatible with 'backend 
  http-response header rule'
  

• I tried to add the header in http-request instead of http-response:

  acl api path_reg ^/api/(.*)$
  http-request add-header Cache-Control public,max-age="600" if api
  

  That worked but I need it in the response

• I also tried to use haproxy variables:

  http-request set-var(txn.path) path
  acl path_acl %[var(txn.path)] -m ^/api/(.*)$
  http-response add-header Cache-Control public,max-age="600" if path_acl
  

  But when I try HAproxy doesn't event start and I have the following error:

  [ALERT] 340/162647 (2241) : parsing [/etc/haproxy/haproxy.cfg:48] 
  : error detected while parsing ACL 'path_acl' : unknown fetch 
  method '%[var' in ACL expression '%[var(txn.path)]'.
  

How can I use the request path into an acl to set the response header?

Solution 1:

Try this:

http-response set-header Cache-Control no-cache,\ max-age=600 if { capture.req.uri -m beg /api/ }

capture.req.uri persists until the response is processed, unlike path, which does not.

A few notes:

This example uses an anonymous ACL. You could also do it with a named ACL, but that takes 2 lines.

There is no reason I am aware of why you should quote the max-age value.

You probably don't want to add-header, you want to set-header, which ensures that if one is already present, it will be removed.

acl path_acl %[var(txn.path)] -m ^/api/(.*)$ is probably correctly written as acl path_acl var(txn.path) -m ^/api/(.*)$. HAProxy is a little finicky about when it expects %[ ] and when it doesn't. I'm sure there's a pattern, but I'm not clear just what is is.