5.1.1: Plugin installation behind Proxy

Elastic Stack Logstash
6

Same problem here. I'm using Logstash 5.1.1 and have set the proxy according to: https://www.elastic.co/guide/en/logstash/current/working-with-plugins.html#proxy-plugins:

root@server:/usr/share/logstash/bin# echo $http_proxy
http://proxy.host:3128
root@server:/usr/share/logstash/bin# echo $https_proxy
http://proxy.host:3128
root@server:/usr/share/logstash/bin# echo $HTTP_PROXY
http://proxy.host:3128
root@server:/usr/share/logstash/bin# echo $HTTPS_PROXY
http://proxy.host:3128

Trying to install the "multiline" plugin and I get a Connection refused:

root@server:/usr/share/logstash/bin# DEBUG=1 ./logstash-plugin install logstash-filter-multiline
DEBUG: exec /usr/share/logstash/vendor/jruby/bin/jruby --1.9 /usr/share/logstash/lib/pluginmanager/main.rb install logstash-filter-multiline
Looking if package named: logstash-filter-multiline exists at https://artifacts.elastic.co/downloads/logstash-plugins/logstash-filter-multiline/logstash-filter-multiline-5.1.1.zip
Errno::ECONNREFUSED: Connection refused - Connection refused
          initialize at org/jruby/ext/socket/RubyTCPSocket.java:126
                open at org/jruby/RubyIO.java:1197
             connect at /usr/share/logstash/vendor/jruby/lib/ruby/1.9/net/http.rb:763
             timeout at org/jruby/ext/timeout/Timeout.java:98
             connect at /usr/share/logstash/vendor/jruby/lib/ruby/1.9/net/http.rb:763
            do_start at /usr/share/logstash/vendor/jruby/lib/ruby/1.9/net/http.rb:756
               start at /usr/share/logstash/vendor/jruby/lib/ruby/1.9/net/http.rb:745
               start at /usr/share/logstash/vendor/jruby/lib/ruby/1.9/net/http.rb:557
               start at /usr/share/logstash/lib/pluginmanager/utils/http_client.rb:13
  remote_file_exist? at /usr/share/logstash/lib/pluginmanager/utils/http_client.rb:23
   get_installer_for at /usr/share/logstash/lib/pluginmanager/pack_fetch_strategy/repository.rb:27
              create at /usr/share/logstash/lib/pluginmanager/install_strategy_factory.rb:15
                each at org/jruby/RubyArray.java:1613
              create at /usr/share/logstash/lib/pluginmanager/install_strategy_factory.rb:14
             execute at /usr/share/logstash/lib/pluginmanager/install.rb:27
                 run at /usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
             execute at /usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/subcommand/execution.rb:11
                 run at /usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
                 run at /usr/share/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132
              (root) at /usr/share/logstash/lib/pluginmanager/main.rb:46
root@server:/usr/share/logstash/bin#

It seems that logstash-plugin is simply ignoring the proxy.