Setting a unique http request id with haproxy's http-request set-header - haproxy

Question

So I have some existing code that sets a unique request ID in our front-end load balancer:
unique-id-format %{+X}o\ %Ts_%ci_%cp_%fi_%fp_%rt_%pid
unique-id-header X-Request-Id
log-format %ci\ %r\ %ST\ %B\ %Tr\ %Tt\ %s\ %ID\ %hr
This works as expected -- X-Request-Id is created as expected, logged and passed to the backend. No problems. However, I'd like to make this request ID generation conditional. No problem -- I should just be able to use http-request set-header instead of unique-id-header:
unique-id-format %{+X}o\ %Ts_%ci_%cp_%fi_%fp_%rt_%pid
http-request set-header X-Request-Id %ID
log-format %ci\ %r\ %ST\ %B\ %Tr\ %Tt\ %s\ %ID\ %hr
(These are all in a front_end section). Maddeningly, however %ID seems to evaluate to empty when used this way. I can use a hardcoded value instead of using %ID and it works. I can also use another log field (like %Ts) and it works. It does not, however, work with %ID. Any clues would be helpful -- thanks in advance.
EDIT: Version is 1.6.11

Answer 1

I had the exact same issue in that I wanted to conditionally set the header if not present and %ID wasn't working as you'd expect. I found solutions suggesting to use %[unique-id] but it turns out that's only in version 1.7+. I have subsequently upgraded to 1.7 and now it works perfectly.
unique-id-format %{+X}o\ %pid%ci%cp%fi%fp%Ts%ms%rt
acl cid_exists req.hdr(X-Correlation-ID) -m found
http-request set-header X-Correlation-ID %[unique-id] unless cid_exists
http-request capture hdr(X-Correlation-ID) len 64
log-format "%ci:%cp [%tr] %ft %b/%s %Th/%Ti/%TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %{+Q}r %[capture.req.hdr(0)]"
The captured X-Correlation-ID header contains either a preexisting CID or one that this loadbalancer has created itself if missing.
The unique-id HTTP sample is referenced here.