diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1bc168b..c834d01 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,13 +13,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: [2.7, "3.0", 3.1, 3.2]
+        ruby-version: [2.6, 2.7, "3.0", 3.1, 3.2]
         bundler-version: [2.4.19, 2.4.22, 2.5.2]
         exclude:
+          - ruby-version: 2.6
+            bundler-version: 2.5.2
           - ruby-version: 2.7
             bundler-version: 2.5.2
     env:
       BUNDLER_VERSION: ${{ matrix.bundler-version }}
+      BUNDLE_LOCKFILE: active
     steps:
       - uses: actions/checkout@v3
       - name: Set up Ruby
diff --git a/.gitignore b/.gitignore
index b3974b5..7aa235e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /*.gem
-/pkg
+/.bundle/
+/pkg/
diff --git a/.rubocop.yml b/.rubocop.yml
index 3e9e3af..2cd88ee 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -7,7 +7,10 @@ require:
   - rubocop-rake
 
 AllCops:
-  TargetRubyVersion: 2.7
+  TargetRubyVersion: 2.6
+
+Bundler/DuplicatedGem:
+  Enabled: false
 
 Naming/FileName:
   Exclude:
diff --git a/Gemfile b/Gemfile
index 468ec28..ce42cac 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,3 +6,24 @@ source "https://rubygems.org"
 # Bundler will treat runtime dependencies like base dependencies, and
 # development dependencies will be added by default to the :development group.
 gemspec
+
+plugin "bundler-multilock", path: "."
+return unless Plugin.installed?("bundler-multilock")
+
+Plugin.send(:load_plugin, "bundler-multilock")
+
+lockfile active: RUBY_VERSION >= "2.7" do
+  gem "debug", "~> 1.9", require: false
+  gem "irb", "~> 1.11", require: false
+  gem "rubocop-inst", "~> 1.0", require: false
+  gem "rubocop-rake", "~> 0.6", require: false
+  gem "rubocop-rspec", "~> 2.24", require: false
+  gem "stringio", "~> 3.1", require: false
+end
+
+lockfile "ruby-2.6", active: RUBY_VERSION < "2.7" do
+  # newer versions of these gems are not compatible with Ruby > 2.6
+  gem "debug", "1.8.0", require: false
+  gem "irb", "1.6.3", require: false
+  gem "stringio", "3.0.6", require: false
+end
diff --git a/Gemfile.lock b/Gemfile.lock
index e477ee9..f64c5fb 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -9,12 +9,12 @@ GEM
   specs:
     ast (2.4.2)
     base64 (0.1.1)
-    debug (1.8.0)
-      irb (>= 1.5.0)
-      reline (>= 0.3.1)
+    debug (1.9.0)
+      irb (~> 1.10)
+      reline (>= 0.3.8)
     diff-lcs (1.5.0)
-    io-console (0.6.0)
-    irb (1.8.1)
+    io-console (0.7.1)
+    irb (1.11.0)
       rdoc
       reline (>= 0.3.8)
     json (2.6.3)
@@ -23,15 +23,15 @@ GEM
     parser (3.2.2.3)
       ast (~> 2.4.1)
       racc
-    psych (5.1.0)
+    psych (5.1.2)
       stringio
     racc (1.7.1)
     rainbow (3.1.1)
     rake (13.0.6)
-    rdoc (6.5.0)
+    rdoc (6.6.2)
       psych (>= 4.0.0)
     regexp_parser (2.8.1)
-    reline (0.3.8)
+    reline (0.4.1)
       io-console (~> 0.5)
     rexml (3.2.6)
     rspec (3.12.0)
@@ -78,7 +78,7 @@ GEM
       rubocop-capybara (~> 2.17)
       rubocop-factory_bot (~> 2.22)
     ruby-progressbar (1.13.0)
-    stringio (3.0.8)
+    stringio (3.1.0)
     unicode-display_width (2.4.2)
 
 PLATFORMS
@@ -90,12 +90,14 @@ PLATFORMS
 
 DEPENDENCIES
   bundler-multilock!
-  debug (~> 1.8)
+  debug (~> 1.9)
+  irb (~> 1.11)
   rake (~> 13.0)
   rspec (~> 3.12)
   rubocop-inst (~> 1.0)
   rubocop-rake (~> 0.6)
   rubocop-rspec (~> 2.24)
+  stringio (~> 3.1)
 
 BUNDLED WITH
    2.5.2
diff --git a/Gemfile.ruby-2.6.lock b/Gemfile.ruby-2.6.lock
new file mode 100644
index 0000000..7ad6156
--- /dev/null
+++ b/Gemfile.ruby-2.6.lock
@@ -0,0 +1,51 @@
+PATH
+  remote: .
+  specs:
+    bundler-multilock (1.2.1)
+      bundler (>= 2.4.19, < 2.6)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    debug (1.8.0)
+      irb (>= 1.5.0)
+      reline (>= 0.3.1)
+    diff-lcs (1.5.0)
+    io-console (0.7.1)
+    irb (1.6.3)
+      reline (>= 0.3.0)
+    rake (13.0.6)
+    reline (0.4.1)
+      io-console (~> 0.5)
+    rspec (3.12.0)
+      rspec-core (~> 3.12.0)
+      rspec-expectations (~> 3.12.0)
+      rspec-mocks (~> 3.12.0)
+    rspec-core (3.12.2)
+      rspec-support (~> 3.12.0)
+    rspec-expectations (3.12.3)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.12.0)
+    rspec-mocks (3.12.6)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.12.0)
+    rspec-support (3.12.1)
+    stringio (3.0.6)
+
+PLATFORMS
+  aarch64-linux
+  arm64-darwin
+  ruby
+  x86_64-darwin
+  x86_64-linux
+
+DEPENDENCIES
+  bundler-multilock!
+  debug (= 1.8.0)
+  irb (= 1.6.3)
+  rake (~> 13.0)
+  rspec (~> 3.12)
+  stringio (= 3.0.6)
+
+BUNDLED WITH
+   2.5.2
diff --git a/bundler-multilock.gemspec b/bundler-multilock.gemspec
index cdf70db..9016c2a 100644
--- a/bundler-multilock.gemspec
+++ b/bundler-multilock.gemspec
@@ -14,14 +14,10 @@ Gem::Specification.new do |spec|
   spec.files         = Dir.glob("lib/**/*") + %w[plugins.rb]
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = ">= 2.7"
+  spec.required_ruby_version = ">= 2.6"
 
   spec.add_dependency "bundler", ">= 2.4.19", "< 2.6"
 
-  spec.add_development_dependency "debug", "~> 1.8"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.12"
-  spec.add_development_dependency "rubocop-inst", "~> 1.0"
-  spec.add_development_dependency "rubocop-rake", "~> 0.6"
-  spec.add_development_dependency "rubocop-rspec", "~> 2.24"
 end
diff --git a/lib/bundler/multilock/check.rb b/lib/bundler/multilock/check.rb
index 662f381..2c73d36 100644
--- a/lib/bundler/multilock/check.rb
+++ b/lib/bundler/multilock/check.rb
@@ -190,7 +190,8 @@ def check(lockfile_definition)
       private
 
       def cache_reverse_dependencies(lockfile)
-        reverse_dependencies = Hash.new { |h, k| h[k] = Gem::Requirement.default_prerelease }
+        # can use Gem::Requirement.default_prelease when Ruby 2.6 support is dropped
+        reverse_dependencies = Hash.new { |h, k| h[k] = Gem::Requirement.new(">= 0.a") }
 
         lockfile.dependencies.each_value do |spec|
           reverse_dependencies[spec.name].requirements.concat(spec.requirement.requirements)
diff --git a/lib/bundler/multilock/ext/plugin/dsl.rb b/lib/bundler/multilock/ext/plugin/dsl.rb
index 36a2afd..4c8eecb 100644
--- a/lib/bundler/multilock/ext/plugin/dsl.rb
+++ b/lib/bundler/multilock/ext/plugin/dsl.rb
@@ -7,7 +7,7 @@ module PluginExt
         module DSL
           ::Bundler::Plugin::DSL.include(self)
 
-          def lockfile(...)
+          def lockfile(*, **)
             # pass
           end
         end
diff --git a/lib/bundler/multilock/ext/source.rb b/lib/bundler/multilock/ext/source.rb
index 845b30b..2df72fa 100644
--- a/lib/bundler/multilock/ext/source.rb
+++ b/lib/bundler/multilock/ext/source.rb
@@ -6,7 +6,7 @@ module Ext
       module Source
         ::Bundler::Source.prepend(self)
 
-        def print_using_message(...)
+        def print_using_message(*)
           return if Bundler.settings[:suppress_install_using_messages]
 
           super
diff --git a/spec/bundler/multilock_spec.rb b/spec/bundler/multilock_spec.rb
index 32966ed..e5ff5ad 100644
--- a/spec/bundler/multilock_spec.rb
+++ b/spec/bundler/multilock_spec.rb
@@ -303,28 +303,28 @@
   it "syncs from a parent lockfile" do
     with_gemfile(<<~RUBY) do
       lockfile do
-        gem "activesupport", "~> 7.0.0"
+        gem "activesupport", "~> 6.1.0"
       end
 
-      lockfile("6.1") do
-        gem "activesupport", "~> 6.1.0"
+      lockfile("6.0") do
+        gem "activesupport", "~> 6.0.0"
       end
 
-      lockfile("6.1-alt", parent: "6.1") do
-        gem "activesupport", "> 6.0", "< 7.2"
+      lockfile("6.0-alt", parent: "6.0") do
+        gem "activesupport", "> 5.2", "< 7.2"
       end
     RUBY
       invoke_bundler("install")
 
-      default = invoke_bundler("info activesupport").split("\n").first
-      six_one = invoke_bundler("info activesupport", env: { "BUNDLE_LOCKFILE" => "6.1" }).split("\n").first
-      alt = invoke_bundler("info activesupport", env: { "BUNDLE_LOCKFILE" => "6.1-alt" }).split("\n").first
+      default = invoke_bundler("info activesupport")
+      six_oh = invoke_bundler("info activesupport 2> /dev/null", env: { "BUNDLE_LOCKFILE" => "6.0" })
+      alt = invoke_bundler("info activesupport 2> /dev/null", env: { "BUNDLE_LOCKFILE" => "6.0-alt" })
 
-      expect(default).to include("7.0")
-      expect(default).not_to eq six_one
-      expect(six_one).to include("6.1")
-      # alt is the same as 6.1, even though it should allow 7.1
-      expect(alt).to eq six_one
+      expect(default).to include("6.1")
+      expect(default).not_to eq six_oh
+      expect(six_oh).to include("6.0")
+      # alt is the same as 6.0, even though it should allow 6.1
+      expect(alt).to eq six_oh
     end
   end
 
@@ -359,23 +359,23 @@
     it "notifies about mismatched versions between different lockfiles" do
       with_gemfile(<<~RUBY) do
         lockfile do
-          gem "activesupport", ">= 6.1", "< 7.2"
+          gem "activesupport", ">= 6.0", "< 7.0"
         end
 
         lockfile("full") do
-          gem "activesupport", "6.1.7.6"
+          gem "activesupport", "6.0.6.1"
         end
       RUBY
         expect do
           invoke_bundler("install")
-        end.to raise_error(Regexp.new("activesupport \\(6.1.7.6\\) in Gemfile.full.lock " \
+        end.to raise_error(Regexp.new("activesupport \\(6.0.6.1\\) in Gemfile.full.lock " \
                                       "does not match the parent lockfile's version"))
       end
     end
 
     it "notifies about mismatched versions between different lockfiles for sub-dependencies" do
       with_gemfile(<<~RUBY) do
-        gem "activesupport", "7.0.4.3" # depends on tzinfo ~> 2.0, so will get >= 2.0.6
+        gem "activesupport", "6.1.7.6" # depends on tzinfo ~> 2.0, so will get >= 2.0.6
 
         lockfile("full") do
           gem "tzinfo", "2.0.5"
@@ -392,18 +392,18 @@
   it "allows mismatched explicit dependencies by default" do
     with_gemfile(<<~RUBY) do
       lockfile do
-        gem "activesupport", "~> 6.1.0"
+        gem "activesupport", "~> 6.0.0"
       end
 
       lockfile("new") do
-        gem "activesupport", "7.0.4.3"
+        gem "activesupport", "6.1.7.6"
       end
     RUBY
       invoke_bundler("install") # no error
-      expect(File.read("Gemfile.lock")).to include("6.1.")
-      expect(File.read("Gemfile.lock")).not_to include("7.0.4.3")
-      expect(File.read("Gemfile.new.lock")).not_to include("6.1.")
-      expect(File.read("Gemfile.new.lock")).to include("7.0.4.3")
+      expect(File.read("Gemfile.lock")).to include("6.0.")
+      expect(File.read("Gemfile.lock")).not_to include("6.1.7.6")
+      expect(File.read("Gemfile.new.lock")).not_to include("6.0.")
+      expect(File.read("Gemfile.new.lock")).to include("6.1.7.6")
     end
   end
 
@@ -686,8 +686,8 @@
     with_gemfile(<<~RUBY) do
       gemspec
 
-      lockfile("rails-7.0", default: true) do
-        gem "activesupport", "~> 7.0.0"
+      lockfile("rails-6.1", default: true) do
+        gem "activesupport", "~> 6.1.0"
       end
     RUBY
       create_local_gem("test", subdirectory: false)
@@ -701,11 +701,11 @@
   it "does not re-sync lockfiles that have conflicting sub-dependencies" do
     with_gemfile(<<~RUBY) do
       lockfile do
-        gem "activemodel", "~> 7.1.0"
+        gem "activemodel", "~> 6.1.0"
       end
 
-      lockfile("rails-7.0") do
-        gem "activemodel", "~> 7.0.0"
+      lockfile("rails-6.0") do
+        gem "activemodel", "~> 6.0.0"
       end
     RUBY
       output = invoke_bundler("install")
@@ -718,7 +718,7 @@
 
   it "removes now-missing explicit dependencies from secondary lockfiles" do
     with_gemfile(<<~RUBY) do
-      gem "inst-jobs", "3.1.13"
+      gem "inst-jobs", "3.1.6"
       gem "activerecord-pg-extensions"
 
       lockfile("alt") {}
@@ -726,7 +726,7 @@
       invoke_bundler("install")
 
       write_gemfile(<<~RUBY)
-        gem "inst-jobs", "3.1.13"
+        gem "inst-jobs", "3.1.6"
 
         lockfile("alt") {}
       RUBY
@@ -739,7 +739,7 @@
   it "does not evaluate the default lockfile at all if an alternate is active, " \
      "without specifying that lockfile explicitly" do
     with_gemfile(<<~RUBY) do
-      gem "inst-jobs", "3.1.13"
+      gem "inst-jobs", "3.1.6"
 
       lockfile active: ENV["ALTERNATE"] != "1" do
         raise "evaluated!" if ENV["ALTERNATE"] == "1"