Dynamic server name and header in HAProxy - haproxy

Question

I’m looking for the equivalent of this backend code block below for requests to www.example.com and example.com.
http-response set-header X-Target example.com
server web-servers site.example.com:80 check
I take all the requests to www.example.com but I want to serve them to site.example.com using haproxy. There are several variations of example.com so I would like to have a list of allowed domains and then if they're allowed I would like to have a backend code block like below where I could use %[req.hdr(Host)] as the value in the http-response X-Target statement.
http-response set-header X-Target %[req.hdr(Host)]
server web-servers site.%[req.hdr(Host),regsub(^www.,,)]:80 check
HA-Proxy version 2.1.4-273103-54 2020/05/07 - https://haproxy.org/
I’m getting this error when I try haproxy -c -f haproxy.test
[root#pm-prod-haproxy05 haproxy]# haproxy -c -f haproxy.test
[ALERT] 259/180932 (16116) : parsing [haproxy.test:40]: ‘http-response set-header’: sample fetch may not be reliably used here because it needs ‘HTTP request headers’ which is not available here.
[ALERT] 259/180932 (16116) : Error(s) found in configuration file : haproxy.test
[root#pm-prod-haproxy05 haproxy]#
I've also tried this:
http-request set-header X-Target %[req.hdr(Host)]
http-request set-header X-Hostname %[req.hdr(Host),regsub(^www.,site.,)]
http-request web-server do-lookup(hdr(X-Hostname))
server web-servers web-server:80 check
This is my full configuration.
global
log 127.0.0.1 local2 debug
chroot /var/lib/haproxy
pidfile /var/run/haproxy.pid
maxconn 4000
daemon
stats socket /var/lib/haproxy/stats
defaults
mode http
option httplog
log global
option dontlognull
option http-server-close
option redispatch
retries 3
timeout http-request 10s
timeout queue 1m
timeout connect 10s
timeout client 1m
timeout server 1m
timeout http-keep-alive 10s
timeout check 10s
maxconn 3000
frontend frontend-http
bind *:80
bind *:443
acl redirect path_beg -i /rd
use_backend backend-tracking if redirect
default_backend backend-default
backend backend-default
option forwardfor
http-response set-header X-Publishing-system website
http-response set-header X-Target %[req.hdr(Host)]
server web-servers site.%[req.hdr(Host),regsub(^www.,,)]:80 check
backend backend-tracking
option forwardfor
http-response set-header X-Publishing-system redirect
http-request set-uri %[url,regsub(^/rd,/,)]
server web-hp www.trackingplatform.com:80 check

Answer 1

About Header manipulation
As the ALERT message say you can't use request header in the response. You should replace the following line.
Wrong line
http-response set-header X-Target %[req.hdr(Host)]
Right Line
http-request set-header X-Target %[req.hdr(Host)]
The Backend-Server should not remove this header. If you not want to send the Backend-Server the 'X-Target' host header then can you use a session variable to save the host header from the request to the response phase.
http-request set-var(txn.my_host) req.hdr(host),lower
http-response set-header X-Target %[var(txn.my_host)]
In the documentation are the set-var and set-header directive quite good explained.
http://cbonte.github.io/haproxy-dconv/1.8/configuration.html#4-http-request
About the server manipulation
This line could not work because haproxy tries to resolve the target server at start time.
server web-servers site.%[req.hdr(Host),regsub(^www.,,)]:80 check
In newer version of haproxy. like 2.1, can you dynamically resolve and set the destination hosts.
http://cbonte.github.io/haproxy-dconv/2.1/configuration.html#4.2-http-request%20do-resolve
I assume you want to change the host header for the target server that the right virtual server is used.
My suggestion to solve your issue is to change the host header and set the server name to a resolvable address.
backend backend-default
option forwardfor
http-response set-header X-Publishing-system website
http-request set-header X-Target %[req.hdr(Host)]
http-request replace-header Host ^www(.*) site.\1
http-request set-header X-NewTarget %[req.hdr(Host),regsub(^www.,,)]
server web-servers site.example.com:80 check
This backend config is only syntax checked.
About dynamic backend server
The server should be resolved dynamically.
For that solution is at least HAProxy 2.0 necessary.
I copy here some parts of the doc http-request do-resolve for this answer.
You will need to add a section resolvers to your config
resolvers mydns
# use here your prefered DNS Servers
nameserver local 127.0.0.53:53
nameserver google 8.8.8.8:53
timeout retry 1s
hold valid 10s
hold nx 3s
hold other 3s
hold obsolete 0s
accepted_payload_size 8192
frontend frontend-http
bind *:80
bind *:443
# define capture buffer for backend
declare capture request len 60
acl redirect path_beg -i /rd
use_backend backend-tracking if redirect
default_backend backend-default
# ... some more backends
backend backend-default
option forwardfor
http-response set-header X-Publishing-system website
http-request set-header X-Target %[req.hdr(Host)]
# replace www with site in host header
http-request replace-header Host ^www(.*) site.\1
# if necessary set X-NewTarget header
http-request set-header X-NewTarget %[req.hdr(Host),regsub(^www.,,)]
# do dynamic host resolving for dynamic
# server destination for
# the replaced Host Header above
http-request do-resolve(txn.myip,mydns,ipv4) hdr(Host),lower
# print the resolved IP in the log
http-request capture var(txn.myip) id 0
# rule to prevent HAProxy from reconnecting to services
# on the local network (forged DNS name used to scan the network)
# add the IP Range for the destination host here
http-request deny if { var(txn.myip) -m ip 127.0.0.0/8 10.0.0.0/8 }
http-request set-dst var(txn.myip)
server clear 0.0.0.0:0
Please take care about the note in the documentation
NOTE: Don't forget to set the "protection" rules to ensure HAProxy won't be used to scan the network or worst won't loop over itself...