I have a proxy that is injecting some headers but I want to modify it so it only sets the headers if they are not already present:

<Location /api>    
    RequestHeader set MY_HEADER "value"


Is this possible?


In Apache 2.4.7, x86_64, Ubuntu 14.04 LTS

I have found that this works

RequestHeader setIfEmpty X-Forwarded-For ""

works all day long. However if one tries to use a dynamic value,

RequestHeader setIfEmpty X-Forwarded-For "%{REMOTE_ADDR}e"

does not work. I have found that you need the help of mod_rewrite to harvest the value. My configuration now looks like this, and it works.

RewriteRule . - [E=noxff:%{REMOTE_ADDR}] RequestHeader setIfEmpty X-Forwarded-For "%{noxff}e"

I know it is stoopid, but it works.


How about something like:

RequestHeader set X-My-Header "value" expr="req('X-My-Header')==''"

Consult http://httpd.apache.org/docs/current/expr.html for more info about the expr= syntax.

Edit: In Apache 2.2 you should be able to do this:

SetEnvIf X-My-Header "" no_my_header
RequestHeader set X-My-Header "value" env=no_my_header
  • I seem to be having some problems with that in apache 2.2.24. Is it just a 2.4 thing? – Hobozilla Jul 3 '13 at 15:16
  • Doesn't appear to exist in Apache 2.2. – Michael Hampton Jul 3 '13 at 17:31
  • See my edits above for Apache 2.2. – MLu Jul 3 '13 at 22:13
  • 2
    For some reason it doesn't like "" as a regular expression but ^$ seemed to do the trick. – Hobozilla Jul 4 '13 at 7:53

Apache 2.4.7

Header setifempty Access-Control-Allow-Origin "*"

Apache 2.2.4 below

Header append Access-Control-Allow-Origin ""
Header edit Access-Control-Allow-Origin "^$" "*"
  • The solution for Apache 2.2.4 and below listed here Header append Access-Control-Allow-Origin "" Header edit Access-Control-Allow-Origin "^$" "*" may have a side effect of appending , to a non-empty header value because it's how append action works. It's the case, at least, in Apache 2.4.6. – Artem Shafranov Feb 8 at 17:00

The following should be an equivalent to RequestHeader setIfEmpty in Apache versions <=2.4.6 (where setIfEmpty action is not supported):

RequestHeader set MY_HEADER "value" "expr=-z %{req:MY_HEADER}"

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