Haraka natively supports the PROXY protocol [1].
This allows an upstream proxy to pass IP address and port of the client which Haraka will use instead of the socket IP address (which is of the proxy). This allows DNSBLs and access control lists to operate on the proxied address.
Support is disabled by default and if HAProxy or other attempts to send a PROXY command then Haraka will return a DENYSOFTDISCONNECT error. DENYSOFT is used to prevent configuration errors from rejecting valid mail.
To enable support for PROXY you must create a haproxy_hosts configuration
file which should contain a list of IP addresses of the HAProxy hosts that
should be allowed to send the PROXY command.
When a host connects to Haraka that matches an IP address present in the
haproxy_hosts file - a banner is not sent, instead Haraka waits for the
PROXY command to be sent before proceeding. The connection will timeout
with 421 PROXY timed out if the command is not sent within 30 seconds.
NOTE: because Haraka does not send a banner when a listed HAProxy host
connects you cannot use the HAProxy option smtpchk to test the host,
you must just use the basic TCP check that HAProxy uses by default.
[1] http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
HAProxy supports the PROXY protocol in version 1.5 or later however there are patches available to add support for 1.4.
Here is an example listener section for haproxy.cfg:
listen smtp :25
mode tcp
option tcplog
balance roundrobin
server smtp1 ip.of.haraka.server1:25 check inter 10s send-proxy
server smtp2 ip.of.haraka.server2:25 check inter 10s send-proxy
server smtp3 ip.of.haraka.server3:25 check inter 10s send-proxy
server smtp4 ip.of.haraka.server4:25 check inter 10s send-proxy
server smtp5 ip.of.haraka.server5:25 check inter 10s send-proxy
The important part is send-proxy which causes HAProxy to send the PROXY
extension on connection.