app.rb

require "sinatra"

post "/foo" do
  redirect "/bar"
end

get "/bar" do
  "Hello, World!"
end

getit.rb

require "rest_client"

RestClient.post "http://localhost:9393/foo", {}

the outcome

# the server
~/c/s/rest> shotgun app.rb
== Shotgun starting Rack::Handler::Mongrel on 0.0.0.0:9393
127.0.0.1 - - [14/Mar/2009 04:31:09] "POST /foo HTTP/1.1" 302 - 0.0008
127.0.0.1 - - [14/Mar/2009 04:31:09] "POST /foo HTTP/1.1" 302 - 0.0010
127.0.0.1 - - [14/Mar/2009 04:31:09] "POST /bar HTTP/1.1" 404 413 0.0008
127.0.0.1 - - [14/Mar/2009 04:31:09] "POST /bar HTTP/1.1" 404 413 0.0010

# the client
~/c/s/rest> ruby -rubygems getit.rb
/Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:142:in `process_result': Resource not found (RestClient::ResourceNotFound)
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:106:in `transmit'
        from /opt/local/lib/ruby/1.8/net/http.rb:543:in `start'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:103:in `transmit'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:36:in `execute_inner'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:28:in `execute'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:31:in `execute'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient/request.rb:12:in `execute'
        from /Users/h/.gems/gems/rest-client-0.9.2/lib/restclient.rb:61:in `post'
        from getit.rb:3