Depends on what you want to get and use the header data for. You could use an iRule to capture the headers from the server response, for example:
when HTTP_RESPONSE {
foreach hdr [HTTP::header names] {
log local0. "HTTP Header : $hdr = [HTTP::header values $hdr]"
}
}
will produce log lines in /var/log/ltm, similar like this:
Mar 25 22:18:19 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Content-Length = 1293
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Content-Type = text/html
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Content-Encoding = gzip
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Last-Modified = {Fri, 10 Feb 2012 06:48:40 GMT}
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Accept-Ranges = bytes
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : ETag = {"094905c0e7cc1:0"}
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Vary = Accept-Encoding
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Server = Microsoft-IIS/7.0
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : X-Powered-By = ASP.NET
Mar 25 22:18:40 apmlab1 info tmm1[11675]: Rule /timo/test_hdr_iRule : HTTP Header : Date = {Wed, 25 Mar 2015 20:18:40 GMT}
This is just for reference, so modify to suit your needs and attach your iRule to your virtual server as a resource in addition to the server pool.
Note that the foreach -loop will produce multiple occurrences of same log line if there are multiple occurrences of the same header name. HTTP::header values -command will list all values of the same header name in this case.