diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec5904c7..559d36c9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -136,6 +136,12 @@ https://github.com/capistrano/capistrano/compare/v3.2.1...v3.3.3
 
     This allows roles to specify properties common to all servers and
     then for individual servers to modify them, keeping things DRY
+  
+* Enhancements (@Kriechi)
+  * Added validate method to DSL to allow validation of certain values
+    - validate values before assignment inside of `set(:key, value)`
+    - should raise a `Capistrano::ValidationError` if invalid
+  * Added default validation for Capistrano-specific variables
 
 Breaking Changes:
   * By using Ruby's noecho method introduced in Ruby version 1.9.3, we dropped support for Ruby versions prior to 1.9.3. See [issue #878](https://github.com/capistrano/capistrano/issues/878) and [PR #1112](https://github.com/capistrano/capistrano/pull/1112) for more information. (@kaikuchn)
diff --git a/README.md b/README.md
index 9b388ee8..832a66dc 100644
--- a/README.md
+++ b/README.md
@@ -85,6 +85,24 @@ $ bundle exec cap production deploy --prereqs
 $ bundle exec cap production deploy --trace
 ```
 
+## Validation of variables
+
+To validate a variable, each time before it is set, define a validation:
+
+```ruby
+validate :some_key do |key, value|
+  if value.length < 5
+    raise Capistrano::ValidationError, "Length of #{key} is too short!"
+  end
+end
+```
+
+Multiple validation can be assigned to a single key. Validations will be executed
+in the order of registration.
+
+Validations can be used to ensure certain properties of user-supplied values,
+e.g. from `ask` or `ENV`. ``
+
 ## Testing
 
 Capistrano has two test suites: an RSpec suite and a Cucumber suite. The
diff --git a/lib/capistrano/configuration.rb b/lib/capistrano/configuration.rb
index c56bf567..cbff7815 100644
--- a/lib/capistrano/configuration.rb
+++ b/lib/capistrano/configuration.rb
@@ -4,6 +4,8 @@ require_relative 'configuration/server'
 require_relative 'configuration/servers'
 
 module Capistrano
+  class ValidationError < Exception; end
+
   class Configuration
 
     def initialize(config = nil)
@@ -24,11 +26,15 @@ module Capistrano
     end
 
     def set(key, value)
+      invoke_validations key, value
       config[key] = value
     end
 
     def set_if_empty(key, value)
-      config[key] = value unless config.has_key? key
+      unless config.has_key? key
+        invoke_validations key, value
+        config[key] = value
+      end
     end
 
     def delete(key)
@@ -43,6 +49,12 @@ module Capistrano
       return value
     end
 
+    def validate(key, &validator)
+      vs = (validators[key] || [])
+      vs << validator
+      validators[key] = vs
+    end
+
     def keys
       config.keys
     end
@@ -129,6 +141,10 @@ module Capistrano
       @config ||= Hash.new
     end
 
+    def validators
+      @validators ||= Hash.new
+    end
+
     def fetch_for(key, default, &block)
       if block_given?
         config.fetch(key, &block)
@@ -140,5 +156,13 @@ module Capistrano
     def callable_without_parameters?(x)
       x.respond_to?(:call) && ( !x.respond_to?(:arity) || x.arity == 0)
     end
+
+    def invoke_validations(key, value)
+      return unless validators.has_key? key
+
+      validators[key].each do |validator|
+        validator.call(key, value)
+      end
+    end
   end
 end
diff --git a/lib/capistrano/defaults.rb b/lib/capistrano/defaults.rb
index 92dff7cb..5eb82d83 100644
--- a/lib/capistrano/defaults.rb
+++ b/lib/capistrano/defaults.rb
@@ -1,3 +1,12 @@
+validate :application do |key, value|
+  changed_value = value.gsub(/[^[[:alnum:]]]/, '_')
+  if value != changed_value
+    warn "Invalid value for :application detected! Try using this: "
+    warn "  set :application, '#{changed_value}'"
+    raise Capistrano::ValidationError
+  end
+end
+
 set_if_empty :scm, :git
 set_if_empty :branch, :master
 set_if_empty :deploy_to, -> { "/var/www/#{fetch(:application)}" }
diff --git a/spec/lib/capistrano/configuration_spec.rb b/spec/lib/capistrano/configuration_spec.rb
index ea2fa52e..880dba03 100644
--- a/spec/lib/capistrano/configuration_spec.rb
+++ b/spec/lib/capistrano/configuration_spec.rb
@@ -130,6 +130,22 @@ module Capistrano
           expect { subject }.to raise_error
         end
       end
+
+      context 'validations' do
+        before do
+          config.validate :key do |_, value|
+            raise Capistrano::ValidationError unless value.length > 3
+          end
+        end
+
+        it 'validates without error' do
+          expect{ config.set(:key, 'longer_value') }.not_to raise_error
+        end
+
+        it 'raises an exception' do
+          expect{ config.set(:key, 'sho') }.to raise_error(Capistrano::ValidationError)
+        end
+      end
     end
 
     describe 'keys' do