Enhanced SSL Load Balancing with Server Name Indication (SNI) TLS Extension

Baptiste Assmann Baptiste Assmann | Apr 13, 2012 | SSL, TECH | 8 comments

Synopsis

Some time ago, we wrote an article which explained how to load-balance SSL services, maintaining affinity using the SSLID.
The main limitation of this kind of architecture is that you must dedicate a public IP address and port per service.
If you’re hosting web or mail services, you could run out of public IP address quickly.

TLS protocol has been extended in 2003, RFC 3546, by an extension called SNI: Server Name Indication, which allows a client to announce in clear the server name it is contacting.

NOTE: two RFC have obsoleted the one above, the latest one is RFC 6066

The ALOHA Load-balancer can use this information to choose a backend or a server.
This allows people to share a single VIP for several services.

Of course, we can use SNI switching with SSLID affinity to build a smart and reliable SSL load-balanced platform.

NOTE: Server Name information is sent with each SSL Handshake, whether you’re establishing a new session or you’re resuming an old one.

SNI is independent from the protocol used at layer 7. So basically, it will work with IMAP, HTTP, SMTP, POP, etc…

Limitation

Bear in mind, that in 2012, not all clients are compatible with SNI.
Concerning web browsers, a few of used in 2012 them are still not compatible with this TLS protocol extension.

We strongly recommend you to read the Wikipedia Server Name Indication page which lists all the limitation of this extension.

Only HAProxy nightly snapshots from 8th of April are compatible (with no bug knows) with it.
Concerning ALOHA, it will be available by ALOHA Load-balancer firmware 5.0.2.

Diagram

The picture below shows a platform with a single VIP which host services for 2 applications:

We can use SNI information to choose a backend, then, inside a backend, we can use SSLID affinity.

Configuration

Choose a backend using SNI TLS extension

The configuration below matches names provided by the SNI extention and choose a farm based on it.
In the farm, it provides SSLID affinity.
If no SNI extention is sent, then we redirect the user to a server farm which can be used to tell the user to upgrade its software.

# Adjust the timeout to your needs
defaults
  timeout client 30s
  timeout server 30s
  timeout connect 5s

# Single VIP with sni content switching
frontend ft_ssl_vip
  bind 10.0.0.10:443
  mode tcp

  tcp-request inspect-delay 5s
  tcp-request content accept if { req_ssl_hello_type 1 }
  
  acl application_1 req_ssl_sni -i application1.domain.com
  acl application_2 req_ssl_sni -i application2.domain.com

  use_backend bk_ssl_application_1 if application_1
  use_backend bk_ssl_application_2 if application_2

  default_backend bk_ssl_default

# Application 1 farm description
backend bk_ssl_application_1
  mode tcp
  balance roundrobin

  # maximum SSL session ID length is 32 bytes.
  stick-table type binary len 32 size 30k expire 30m

  acl clienthello req_ssl_hello_type 1
  acl serverhello rep_ssl_hello_type 2

  # use tcp content accepts to detects ssl client and server hello.
  tcp-request inspect-delay 5s
  tcp-request content accept if clienthello

  # no timeout on response inspect delay by default.
  tcp-response content accept if serverhello

  stick on payload_lv(43,1) if clienthello

  # Learn on response if server hello.
  stick store-response payload_lv(43,1) if serverhello

  option ssl-hello-chk
  server server1 192.168.1.1:443 check
  server server2 192.168.1.2:443 check

# Application 2 farm description
backend bk_ssl_application_2
  mode tcp
  balance roundrobin

  # maximum SSL session ID length is 32 bytes.
  stick-table type binary len 32 size 30k expire 30m

  acl clienthello req_ssl_hello_type 1
  acl serverhello rep_ssl_hello_type 2

  # use tcp content accepts to detects ssl client and server hello.
  tcp-request inspect-delay 5s
  tcp-request content accept if clienthello

  # no timeout on response inspect delay by default.
  tcp-response content accept if serverhello

  stick on payload_lv(43,1) if clienthello

  # Learn on response if server hello.
  stick store-response payload_lv(43,1) if serverhello

  option ssl-hello-chk
  server server1 192.168.2.1:443 check
  server server2 192.168.2.2:443 check

# Sorry backend which should invite the user to update its client
backend bk_ssl_default
  mode tcp
  balance roundrobin
  
  # maximum SSL session ID length is 32 bytes.
  stick-table type binary len 32 size 30k expire 30m

  acl clienthello req_ssl_hello_type 1
  acl serverhello rep_ssl_hello_type 2

  # use tcp content accepts to detects ssl client and server hello.
  tcp-request inspect-delay 5s
  tcp-request content accept if clienthello

  # no timeout on response inspect delay by default.
  tcp-response content accept if serverhello

  stick on payload_lv(43,1) if clienthello

  # Learn on response if server hello.
  stick store-response payload_lv(43,1) if serverhello

  option ssl-hello-chk
  server server1 10.0.0.11:443 check
  server server2 10.0.0.12:443 check

Choose a server using SNI: aka SSL routing

The configuration below matches names provided by the SNI extention and choose a server based on it.
If no SNI is provided or we can’t find the expected name, then the traffic is forwarded to server3 which can be used to tell the user to upgrade its software.

# Adjust the timeout to your needs
defaults
  timeout client 30s
  timeout server 30s
  timeout connect 5s

# Single VIP 
frontend ft_ssl_vip
  bind 10.0.0.10:443
  mode tcp

  tcp-request inspect-delay 5s
  tcp-request content accept if { req_ssl_hello_type 1 }

  default_backend bk_ssl_default

# Using SNI to take routing decision
backend bk_ssl_default
  mode tcp

  acl application_1 req_ssl_sni -i application1.domain.com
  acl application_2 req_ssl_sni -i application2.domain.com

  use-server server1 if application_1
  use-server server2 if application_2
  use-server server3 if !application_1 !application_2

  option ssl-hello-chk
  server server1 10.0.0.11:443 check
  server server2 10.0.0.12:443 check
  server server3 10.0.0.13:443 check

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Tags: IMAP, SMTP, SNI, SSL, SSLID, TLS

8 Comments

Jordan on February 21, 2013 at 12:55 am

Hi,

Thanks for this, very usefull 🙂
Is it possible to specifiy multiple ssl certs ? For example one for application 1 and another for application 2 ?

In pound in can set à list of certificates to use, how can i do the same ?
Thanks
- Baptiste Assmann on August 12, 2013 at 10:35 am
  
  Hi,
  
  Of course you can precise a list of certificates to use and HAProxy will pick the right one based on SNI. Just specify several crt directive on the bind line.
  You can also make crt points to a directory, then HAProxy will load all the certs from this directory.
  
  Baptiste
  - Jeremy on December 19, 2013 at 4:24 pm
    
    Hello Baptiste,
    
    When using a directory, will HAProxy select the correct crt for the correct domain?
    
    We are trying to send many domains at our head IP and some will have SSL and some won’t.
    
    Our tests have shown that if we try to go to a domain that has NO cert, HAProxy tries to give one up anyways resulting in a browser warning saying this certificate is bad instead of the SSL Connection Error.
    
    Any thoughts?
    - Baptiste Assmann on December 19, 2013 at 10:52 pm
      
      Hi,
      
      HAProxy select the right certificate based on SNI sent by the client.
      Unfortunately, not all clients send it (Cf win XP and some mobile application).
      Then HAProxy delivers the first certificate it found in the directory.
      
      Baptiste
Fred on November 7, 2013 at 9:52 pm

Hey,

thanks a lot for this example. It helps me to solve my problem with different https servers using TCP Mode.

Fred
Mennat Mokhtar on February 16, 2015 at 5:52 pm

Thank you for the example!! Saved me hours.
Tarikur Rahamamn on June 14, 2017 at 10:12 am

This is very helpfull. But how can i use host base routing for same host for both 443 and 80 like app1.domain.com

frontend http
bind *:80
mode tcp
tcp-request inspect-delay 5s
acl mytonicnonssl req_ssl_sni -i staging.mytonic.com
use_backend mytonic-ssl if mytonicnonssl

backend mytonic-nonssl
mode tcp
balance roundrobin
server server1 10.10.17.222:8080 check

above configuration is not working for me. but ssl is working fine as described in this blog.
- wtarreau on June 14, 2017 at 4:26 pm
  
  you need a “tcp-request content” rule to wait for the SNI to be presented (in the client hello). Try with this after your tcp-request rule :
  
  tcp-request content accept if { req_ssl_hello_type 1 }
  
  Also in general you need to keep in mind that it’s not a good idea to use the same port for both SSL and non-SSL. If you want to do the normal stuff consisting in having both 80 and 443 on the same frontend, it’s as simple as :
  
  bind :80
  bind :443 ssl crt foo.pem
  
  Traffic on port 80 will be processed as-is, traffic on port 443 will be decrypted before being processed. In both cases you have the Host header field which is the only valid authority to decide what backend to use.