8.8. Capturing HTTP headers



Header captures are useful to track unique request identifiers set by an upper proxy, virtual host names, user-agents, POST content-length, referrers, etc. In the response, one can search for information about the response length, how the server asked the cache to behave, or an object location during a redirection.

Header captures are performed using the “capture request header” and “capture response header” statements in the frontend. Please consult their definition in section 4.2 for more details.

It is possible to include both request headers and response headers at the same time. Non-existent headers are logged as empty strings, and if one header appears more than once, only its last occurrence will be logged.

Request headers are grouped within braces ‘{‘ and ‘}’ in the same order as they were declared, and delimited with a vertical bar ‘|’ without any space.

Response headers follow the same representation, but are displayed after a space following the request headers block. These blocks are displayed just before the HTTP request in the logs.

As a special case, it is possible to specify an HTTP header capture in a TCP frontend. The purpose is to enable logging of headers which will be parsed in an HTTP backend if the request is then switched to this HTTP backend.

Example :

        # This instance chains to the outgoing proxy
        listen proxy-out
            mode http
            option httplog
            option logasap
            log global
            server cache1 192.168.1.1:3128

            # log the name of the virtual server
            capture request  header Host len 20

            # log the amount of data uploaded during a POST
            capture request  header Content-Length len 10

            # log the beginning of the referrer
            capture request  header Referer len 20

            # server name (useful for outgoing proxies only)
            capture response header Server len 20

            # logging the content-length is useful with "option logasap"
            capture response header Content-Length len 10

            # log the expected cache behaviour on the response
            capture response header Cache-Control len 8

            # the Via header will report the next proxy's name
            capture response header Via len 20

            # log the URL location during a redirection
            capture response header Location len 20
    >>> Aug  9 20:26:09 localhost haproxy[2022]: 127.0.0.1:34014 [09/Aug/2004:20:26:09] proxy-out \
          proxy-out/cache1 0/0/0/162/+162 200 +350 - - ---- 0/0/0/0/0 0/0 {fr.adserver.yahoo.co||http://fr.f416.mail.} \ 
          {|864|private||} "GET http://fr.adserver.yahoo.com/"

    >>> Aug  9 20:30:46 localhost haproxy[2022]: 127.0.0.1:34020 [09/Aug/2004:20:30:46] proxy-out \
          proxy-out/cache1 0/0/0/182/+182 200 +279 - - ---- 0/0/0/0/0 0/0 {w.ods.org||} \
          {Formilux/0.1.8|3495|||} "GET http://trafic.1wt.eu/ HTTP/1.1"

    >>> Aug  9 20:30:46 localhost haproxy[2022]: 127.0.0.1:34028 [09/Aug/2004:20:30:46] proxy-out \
          proxy-out/cache1 0/0/2/126/+128 301 +223 - - ---- 0/0/0/0/0 0/0 {www.sytadin.equipement.gouv.fr||http://trafic.1wt.eu/} \
          {Apache|230|||http://www.sytadin.} "GET http://www.sytadin.equipement.gouv.fr/ HTTP/1.1"
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