1 files changed, 52 insertions, 0 deletions
diff --git a/lib/gitlab/database/load_balancing/resolver.rb b/lib/gitlab/database/load_balancing/resolver.rb
new file mode 100644
index 00000000000..a291080cc3d
--- /dev/null
+++ b/lib/gitlab/database/load_balancing/resolver.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require 'net/dns'
+require 'resolv'
+
+module Gitlab
+  module Database
+    module LoadBalancing
+      class Resolver
+        UnresolvableNameserverError = Class.new(StandardError)
+
+        def initialize(nameserver)
+          @nameserver = nameserver
+        end
+
+        def resolve
+          address = ip_address || ip_address_from_hosts_file ||
+            ip_address_from_dns
+
+          unless address
+            raise UnresolvableNameserverError,
+              "could not resolve #{@nameserver}"
+          end
+
+          address
+        end
+
+        private
+
+        def ip_address
+          IPAddr.new(@nameserver)
+        rescue IPAddr::InvalidAddressError
+        end
+
+        def ip_address_from_hosts_file
+          ip = Resolv::Hosts.new.getaddress(@nameserver)
+          IPAddr.new(ip)
+        rescue Resolv::ResolvError
+        end
+
+        def ip_address_from_dns
+          answer = Net::DNS::Resolver.start(@nameserver, Net::DNS::A).answer
+          return if answer.empty?
+
+          answer.first.address
+        rescue Net::DNS::Resolver::NoResponseError
+          raise UnresolvableNameserverError, "no response from DNS server(s)"
+        end
+      end
+    end
+  end
+end