First, we need to have installed unixodbc and freetds in our Linux server, in this case Ubuntu 14.04:

$ sudo apt-get install unixodbc unixodbc-dev freetds-dev tdsodbc
And configure freetds:
  $ sudo vim /etc/freetds/freetds.conf

  [SERVER]
    host = server ip address
    port = 1433
    tds version = 7.0
    # client charset = UTF-8 (optional)
Cause we need odbc, we need to configure odbc as follow:
  $ sudo vim /etc/odbcinst.ini
    [FreeTDS]
    Description     = TDS driver (Sybase/MS SQL)
    Driver          = /usr/lib/odbc/libtdsodbc.so
    Setup           = /usr/lib/odbc/libtdsS.so
    CPTimeout       =
    CPReuse         =
    FileUsage       = 1
And then:
  [SERVER] # same server name from freetds.conf
    Driver          = FreeTDS
    Description     = Conexion a Sql  con FreeTDS / ODBC
    Trace           = No
    Servername      = server ip address
    Database        = data base name
Now we can test our connection to the database:
  $ sqsh -S SERVER -U USER -P PASSWORD
  $> use 'database'
  $> select col1, col2, col3 from table where col2='test'
  $> go # to exec the script
Now that we are able to access to MSSQL we have to add two gems to rail app Gemfile in order to acces from our application then reconfigure database.yml:
#Gemfile
  # SQL server
  gem 'activerecord-sqlserver-adapter', '~> 4.2.4'
  gem 'tiny_tds', '~> 0.7.0'

#config/database.yml
  adapter: sqlserver
  database: database
  username: user
  password: password
  host: server ip address
Also if we want to connet to multiple databases the only thing we have to do is modify the database.yml to add our new MSSQL database and create a model to establish the connection:
#config/database.yml
  development_sql:
    adapter: sqlserver
    database: database
    username: user
    password: password
    host: server ip address

#app/models/model_name
  class AccpacModel < ActiveRecord::Base
    establish_connection "#{Rails.env}_sql"

    self.table_name  = 'TABLE_NAME'
    self.primary_key = 'PRIMARY_KEY'
  end