Build a dynamic SNI value to use in a HAProxy backend connection over SSL

haproxy.cfg

frontend foo
  bind :443 ssl crt /path/to/certs

  # Ensure we have a clean state to start with
  http-request del-header X-SERVER-SNI

  # Set the concatenated value of the SNI value to a temporary header
  http-request set-header X-SERVER-SNI haproxy.%[ssl_fc_sni] if { ssl_fc_sni -m found }

  # Set the value of the header to a transaction-level variable
  http-request set-var txn.fc_sni hdr(X-SERVER-SNI) if { hdr(X-SERVER-SNI) -m found }

  # Delete the header again to cleanup after us
  http-request del-header X-SERVER-SNI

  use_backend servers

backend servers
  # Use the variable's value in the backend connection
  server foo ssl sni var(txn.fc_sni)

  # TODO: figure out what happens if the client does not send an SNI header at
  # all or if it sends an invalid one. You should do something sensible on both
  # cases :)

In v2 (at least) it should apparently be
http-request set-var(txn.fc_sni) hdr(X-SERVER-SNI) if { hdr(X-SERVER-SNI) -m found }