Restore original remote_ip algorithm.

Proxy servers add X-Forwarded-For headers, resulting in a list of IPs. We
remove trusted IP values, and then take the last given value, assuming that
it is the most likely to be the correct, unfaked value. See [1] for a very
thorough discussion of why that is the best option we have at the moment.

[1]: http://blog.gingerlime.com/2012/rails-ip-spoofing-vulnerabilities-and-protection/

Fixes #7979

1 files changed, 112 insertions, 53 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/remote_ip.rb b/actionpack/lib/action_dispatch/middleware/remote_ip.rb
index 5abf8f2802..ad5e1993aa 100644
--- a/actionpack/lib/action_dispatch/middleware/remote_ip.rb
+++ b/actionpack/lib/action_dispatch/middleware/remote_ip.rb
@@ -1,23 +1,58 @@
 module ActionDispatch
+  # This middleware calculates the IP address of the remote client that is
+  # making the request. It does this by checking various headers that could
+  # contain the address, and then picking the last-set address that is not
+  # on the list of trusted IPs. This follows the precendent set by e.g.
+  # {the Tomcat server}[https://issues.apache.org/bugzilla/show_bug.cgi?id=50453],
+  # with {reasoning explained at length}[http://blog.gingerlime.com/2012/rails-ip-spoofing-vulnerabilities-and-protection]
+  # by @gingerlime. A more detailed explanation of the algorithm is given
+  # at GetIp#calculate_ip.
+  #
+  # Some Rack servers concatenate repeated headers, like {HTTP RFC 2616}[http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2]
+  # requires. Some Rack servers simply drop preceeding headers, and only report
+  # the value that was {given in the last header}[http://andre.arko.net/2011/12/26/repeated-headers-and-ruby-web-servers].
+  # If you are behind multiple proxy servers (like Nginx to HAProxy to Unicorn)
+  # then you should test your Rack server to make sure your data is good.
+  #
+  # IF YOU DON'T USE A PROXY, THIS MAKES YOU VULNERABLE TO IP SPOOFING.
+  # This middleware assumes that there is at least one proxy sitting around
+  # and setting headers with the client's remote IP address. If you don't use
+  # a proxy, because you are hosted on e.g. Heroku, any client can claim to
+  # have any IP address by setting the X-Forwarded-For header. If you care
+  # about that, please take precautions.
   class RemoteIp
-    class IpSpoofAttackError < StandardError ; end
+    class IpSpoofAttackError < StandardError; end
 
-    # IP addresses that are "trusted proxies" that can be stripped from
-    # the comma-delimited list in the X-Forwarded-For header. See also:
-    # http://en.wikipedia.org/wiki/Private_network#Private_IPv4_address_spaces
-    # http://en.wikipedia.org/wiki/Private_network#Private_IPv6_addresses.
+    # The default trusted IPs list simply includes IP addresses that are
+    # guaranteed by the IP specification to be private addresses. Those will
+    # not be the ultimate client IP in production, and so are discarded. See
+    # http://en.wikipedia.org/wiki/Private_network for details.
     TRUSTED_PROXIES = %r{
-      ^127\.0\.0\.1$                | # localhost
-      ^::1$                         |
-      ^(10                          | # private IP 10.x.x.x
-        172\.(1[6-9]|2[0-9]|3[0-1]) | # private IP in the range 172.16.0.0 .. 172.31.255.255
-        192\.168                    | # private IP 192.168.x.x
-        fc00::                        # private IP fc00
-       )\.
+      ^127\.0\.0\.1$                | # localhost IPv4
+      ^::1$                         | # localhost IPv6
+      ^fc00:                        | # private IPv6 range fc00
+      ^10\.                         | # private IPv4 range 10.x.x.x
+      ^172\.(1[6-9]|2[0-9]|3[0-1])\.| # private IPv4 range 172.16.0.0 .. 172.31.255.255
+      ^192\.168\.                     # private IPv4 range 192.168.x.x
     }x
 
     attr_reader :check_ip, :proxies
 
+    # Create a new +RemoteIp+ middleware instance.
+    #
+    # The +check_ip_spoofing+ option is on by default. When on, an exception
+    # is raised if it looks like the client is trying to lie about its own IP
+    # address. It makes sense to turn off this check on sites aimed at non-IP
+    # clients (like WAP devices), or behind proxies that set headers in an
+    # incorrect or confusing way (like AWS ELB).
+    #
+    # The +custom_trusted+ argument can take a regex, which will be used
+    # instead of +TRUSTED_PROXIES+, or a string, which will be used in addition
+    # to +TRUSTED_PROXIES+. Any proxy setup will put the value you want in the
+    # middle (or at the beginning) of the X-Forwarded-For list, with your proxy
+    # servers after it. If your proxies aren't removed, pass them in via the
+    # +custom_trusted+ parameter. That way, the middleware will ignore those
+    # IP addresses, and return the one that you want.
     def initialize(app, check_ip_spoofing = true, custom_proxies = nil)
       @app = app
       @check_ip = check_ip_spoofing
@@ -31,15 +66,23 @@ module ActionDispatch
         end
     end
 
+    # Since the IP address may not be needed, we store the object here
+    # without calculating the IP to keep from slowing down the majority of
+    # requests. For those requests that do need to know the IP, the
+    # GetIp#calculate_ip method will calculate the memoized client IP address.
     def call(env)
       env["action_dispatch.remote_ip"] = GetIp.new(env, self)
       @app.call(env)
     end
 
+    # The GetIp class exists as a way to defer processing of the request data
+    # into an actual IP address. If the ActionDispatch::Request#remote_ip method
+    # is called, this class will calculate the value and then memoize it.
     class GetIp
 
-      # IP v4 and v6 (with compression) validation regexp
-      # https://gist.github.com/1289635
+      # This constant contains a regular expression that validates every known
+      # form of IP v4 and v6 address, with or without abbreviations, adapted
+      # from {this gist}[https://gist.github.com/1289635].
       VALID_IP = %r{
         (^(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[0-9]{1,2})(\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[0-9]{1,2})){3}$)                                                        | # ip v4
         (^(
@@ -63,62 +106,78 @@ module ActionDispatch
       }x
 
       def initialize(env, middleware)
-        @env        = env
-        @middleware = middleware
-        @ip         = nil
+        @env      = env
+        @check_ip = middleware.check_ip
+        @proxies  = middleware.proxies
       end
 
-      # Determines originating IP address. REMOTE_ADDR is the standard
-      # but will be wrong if the user is behind a proxy. Proxies will set
-      # HTTP_CLIENT_IP and/or HTTP_X_FORWARDED_FOR, so we prioritize those.
-      # HTTP_X_FORWARDED_FOR may be a comma-delimited list in the case of
-      # multiple chained proxies. The first address which is in this list
-      # if it's not a known proxy will be the originating IP.
-      # Format of HTTP_X_FORWARDED_FOR:
-      # client_ip, proxy_ip1, proxy_ip2...
-      # http://en.wikipedia.org/wiki/X-Forwarded-For
+      # Sort through the various IP address headers, looking for the IP most
+      # likely to be the address of the actual remote client making this
+      # request.
+      #
+      # REMOTE_ADDR will be correct if the request is made directly against the
+      # Ruby process, on e.g. Heroku. When the request is proxied by another
+      # server like HAProxy or Nginx, the IP address that made the original
+      # request will be put in an X-Forwarded-For header. If there are multiple
+      # proxies, that header may contain a list of IPs. Other proxy services
+      # set the Client-Ip header instead, so we check that too.
+      #
+      # As discussed in {this post about Rails IP Spoofing}[http://blog.gingerlime.com/2012/rails-ip-spoofing-vulnerabilities-and-protection/],
+      # while the first IP in the list is likely to be the "originating" IP,
+      # it could also have been set by the client maliciously.
+      #
+      # In order to find the first address that is (probably) accurate, we
+      # take the list of IPs, remove known and trusted proxies, and then take
+      # the last address left, which was presumably set by one of those proxies.
       def calculate_ip
-        client_ip    = @env['HTTP_CLIENT_IP']
-        forwarded_ip = ips_from('HTTP_X_FORWARDED_FOR').first
-        remote_addrs = ips_from('REMOTE_ADDR')
-
-        check_ip = client_ip && @middleware.check_ip
-        if check_ip && forwarded_ip != client_ip
+        # Set by the Rack web server, this is a single value.
+        remote_addr = ips_from('REMOTE_ADDR').last
+
+        # Could be a CSV list and/or repeated headers that were concatenated.
+        client_ips    = ips_from('HTTP_CLIENT_IP').reverse
+        forwarded_ips = ips_from('HTTP_X_FORWARDED_FOR').reverse
+
+        # +Client-Ip+ and +X-Forwarded-For+ should not, generally, both be set.
+        # If they are both set, it means that this request passed through two
+        # proxies with incompatible IP header conventions, and there is no way
+        # for us to determine which header is the right one after the fact.
+        # Since we have no idea, we give up and explode.
+        should_check_ip = @check_ip && client_ips.last
+        if should_check_ip && !forwarded_ips.include?(client_ips.last)
           # We don't know which came from the proxy, and which from the user
-          raise IpSpoofAttackError, "IP spoofing attack?!" \
-            "HTTP_CLIENT_IP=#{@env['HTTP_CLIENT_IP'].inspect}" \
+          raise IpSpoofAttackError, "IP spoofing attack?! " +
+            "HTTP_CLIENT_IP=#{@env['HTTP_CLIENT_IP'].inspect} " +
             "HTTP_X_FORWARDED_FOR=#{@env['HTTP_X_FORWARDED_FOR'].inspect}"
         end
 
-        client_ips = remove_proxies [client_ip, forwarded_ip, remote_addrs].flatten
-        if client_ips.present?
-          client_ips.first
-        else
-          # If there is no client ip we can return first valid proxy ip from REMOTE_ADDR
-          remote_addrs.find { |ip| valid_ip? ip }
-        end
+        # We assume these things about the IP headers:
+        #
+        #   - X-Forwarded-For will be a list of IPs, one per proxy, or blank
+        #   - Client-Ip is propagated from the outermost proxy, or is blank
+        #   - REMOTE_ADDR will be the IP that made the request to Rack
+        ips = [forwarded_ips, client_ips, remote_addr].flatten.compact
+
+        # If every single IP option is in the trusted list, just return REMOTE_ADDR
+        filter_proxies(ips).first || remote_addr
       end
 
+      # Memoizes the value returned by #calculate_ip and returns it for
+      # ActionDispatch::Request to use.
       def to_s
         @ip ||= calculate_ip
       end
 
-      private
+    protected
 
       def ips_from(header)
-        @env[header] ? @env[header].strip.split(/[,\s]+/) : []
-      end
-
-      def valid_ip?(ip)
-        ip =~ VALID_IP
-      end
-
-      def not_a_proxy?(ip)
-        ip !~ @middleware.proxies
+        # Split the comma-separated list into an array of strings
+        ips = @env[header] ? @env[header].strip.split(/[,\s]+/) : []
+        # Only return IPs that are valid according to the regex
+        ips.select{ |ip| ip =~ VALID_IP }
       end
 
-      def remove_proxies(ips)
-        ips.select { |ip| valid_ip?(ip) && not_a_proxy?(ip) }
+      def filter_proxies(ips)
+        ips.reject { |ip| ip =~ @proxies }
       end
 
     end