Scalable architecture

In order to make your architecture scalable, you may often want to use a load-balancer or an application delivery controller.
When using one of them (or a reverse-proxy), the client information is almost all the time hidden. Or if you want to get them, it requires huge modifications in the architecture.

Unfortunately, for troubleshooting and security purpose it would be interesting to know the client information (mainly source IP address)…
That’s where the proxy protocol comes in.

The proxy protocol???

As explained in a previous article“preserve source ip address despite reverse proxies”, the proxy protocol was developped to maintain client information when chaining proxies and reverse-proxies.
Two main advantages when using it:

  • you can provide a downstream proxy or server (aka next hop) the client information (for now, mainly IP and port source)
  • you can use servers in multiple datacenter without a complex network architecture (just need to provide routing for a TCP connection)

Why simply not using TPROXY (transparent proxy) mode?

TPROXY allows a load-balancer or reverse-proxy to open the TCP connection to the server using the client IP address.
One of the drawback of TPROXY is that the default gateway of the application servers must be the load-balancer.
Or you must do policy based routing on your network which could be painful.

Why Postfix and HAProxy?

HAProxy was the first software to implement the proxy protocol.
Note that you’ll have to use HAProxy 1.5 branch or patched HAProxy 1.4.
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An other solution would to use Aloha load-balancer which does everything for you in a box (from the OS to HAPrxoxy) with all the nice features you could expect. 😉
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Lately, Postfix implemented it. It is available in Postfix 2.10.
It is the first application server first application server to ship with it: THANKS and CONGRATULATION!!!!
Hopefully other MTAs will implement it soon. It is simple and brings so many improvements to an architecture.

SMTP, spam and security

In SMTP, it is really important to know the client IP, since we use it most of the time through RBL to fight spam.
For security purpose as well: we may want to allow only some hosts to use our SMTP relays and block any other clients.
Without the proxy protocol, the load-balancer will hide the client IP with its own IP. You would have to maintain whitelists into the load-balancer (which is doable). Thanks to proxy protocol, Postscreen would be aware of the client IP, it means you could maintain lists directly into the MTA.

HAProxy and Postfix connection flow

The diagram below shows the protocols and the process in place in this kind of architecture:

           smtp              proxy-protocol
                             + smtp
(INTERNET) ---> 25 (HAPROXY)      --->      srv1:10024 (Postscreen
                                                       / smtpd)
                                  --->      srv2:10024 (Postscreen
                                                       / smtpd)

Note that the default gateway of the MTA servers is not anymore the load-balancer.
Both servers migt be in the same LAN or datacenter. Any type of architecture is now doable.



frontend ft_smtp
  mode tcp
  timeout client 1m
  log global
  option tcplog
  default_backend bk_postfix

backend bk_postfix
  mode tcp
  log global
  option tcplog
  timeout server 1m
  timeout connect 5s
  server postfix send-proxy


Note: I installed postfix in /opt/postfix directory

queue_directory = /opt/postfix/var/spool/postfix
command_directory = /opt/postfix/usr/sbin
daemon_directory = /opt/postfix/usr/libexec/postfix
data_directory = /opt/postfix/var/lib/postfix
mail_owner = postfix
unknown_local_recipient_reject_code = 550
inet_interfaces = localhost
sendmail_path = /opt/postfix/usr/sbin/sendmail
newaliases_path = /opt/postfix/usr/bin/newaliases
mailq_path = /opt/postfix/usr/bin/mailq
setgid_group = postdrop
html_directory = no
manpage_directory = /opt/postfix/usr/local/man
sample_directory = /opt/postfix/etc/postfix
readme_directory = no
inet_protocols = ipv4
postscreen_upstream_proxy_protocol = haproxy

10024     inet  n       -       n       -       1       postscreen
smtpd     pass  -       -       n       -       -       smtpd

See the results in Postfix logs

No proxy protocol

Jun 30 01:18:14 sd-33932 postfix/postscreen[2375]: 
       CONNECT from []:52841 to []:10024
Jun 30 01:18:22 sd-33932 postfix/smtpd[2376]: 
       disconnect from localhost[]

With proxy protocol

Jun 29 09:13:41 sd-33932 postfix/postscreen[30505]: 
       CONNECT from [<client public IP>]:59338 to [<server IP>]:25
Jun 29 09:13:52 sd-33932 postfix/postscreen[30505]: 
       DISCONNECT [<client public IP>]:59338

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