Is it possible to add a custom package to HAproxy when connecting (similar to send-proxy)?

Question

HAproxy

Actually the essence of the issue in the topic.

frontend testfront
  bind 0.0.0.0:1234
  mode tcp
  use_backend testback
backend testback
  mode tcp
  server test destantion:1234 send-proxy

It is necessary for each client immediately after the send-proxy package (or before it) to somehow transfer a custom server identifier to the endpoint, for example, the string "1234", then communicate with the client. Did a search, didn't find anything. If HAproxy does not know how to do this, you can tell me other proxies that can transfer both IP and custom connection packets for TCP.

Answer 1

The issue is resolved, if anyone suddenly needs it, it is done through LUA:
haproxy.cfg:

global
  lua-load /path-to/script.lua
frontend testfront
  bind 0.0.0.0:1234
  mode tcp
  tcp-request content lua.testfunc testvalue
  use_backend testback
backend testback
  mode tcp
  server test destantion:1234 send-proxy

script.lua:

core.register_action("testfunc", { "tcp-req" }, function (txn, varfromconfig)
  txn.req:send("TEST " .. varfromconfig .. "\n")
end, 1)

This passes the string "TEST testvalue" to the backend after send-proxy.
If someone knows a more beautiful version without LUA - share.