diff --git a/CHANGELOG.md b/CHANGELOG.md
index baa8322..03bae10 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 ## [Unreleased]
 
+## Added
+
+- Support using alternative documents and parameters with
+  `SsmPortForwardingSession`
+  ([#22](https://github.com/ackama/aws_ec2_environment/pull/22))
+
 ## [0.1.0] - 2022-08-17
 
 - Initial release
diff --git a/README.md b/README.md
index 097d327..2391d17 100644
--- a/README.md
+++ b/README.md
@@ -199,6 +199,44 @@ task :forward_port, %i[instance_id remote_port local_port] => :environment do |_
 end
 ```
 
+You can also use specific documents, and pass in extra parameters, which can be
+useful for using tunnels to access other private resources like database
+instances:
+
+```ruby
+require "aws_ec2_environment"
+
+desc "Dumps a copy of the postgres database using AWS and PG environment variables"
+task :dump_pg_database, %i[instance_id dump_file] => :environment do |_, args|
+  instance_id = args.fetch(:instance_id)
+  dump_file = args.fetch(:dump_file)
+
+  remote_host = ENV.fetch("PGHOST")
+  remote_port = ENV.fetch("PGPORT", 5432)
+
+  session = AwsEc2Environment::SsmPortForwardingSession.new(
+    instance_id,
+    remote_port,
+    document: "AWS-StartPortForwardingSessionToRemoteHost",
+    extra_params: { "host" => [remote_host] }
+  )
+
+  at_exit { session.close }
+
+  local_port = session.wait_for_local_port
+
+  system([
+    "pg_dump",
+    "--format=c",
+    "--no-owner",
+    "--no-privileges",
+    "--host=localhost",
+    "--port=#{local_port}",
+    "--file=#{dump_file}",
+  ].join(" "))
+end
+```
+
 ### AWS Authentication and Permissions
 
 Since this gem interacts with AWS, it must be configured with credentials - see
diff --git a/lib/aws_ec2_environment/ssm_port_forwarding_session.rb b/lib/aws_ec2_environment/ssm_port_forwarding_session.rb
index 65cb1ab..02898f6 100644
--- a/lib/aws_ec2_environment/ssm_port_forwarding_session.rb
+++ b/lib/aws_ec2_environment/ssm_port_forwarding_session.rb
@@ -22,8 +22,9 @@ class SessionProcessError < Error; end
     # rubocop:disable Metrics/ParameterLists
     def initialize(
       instance_id, remote_port,
+      document: "AWS-StartPortForwardingSession",
       local_port: nil, logger: Logger.new($stdout),
-      timeout: 15, reason: nil
+      timeout: 15, reason: nil, extra_params: {}
     )
       # rubocop:enable Metrics/ParameterLists
       @logger = logger
@@ -32,7 +33,7 @@ def initialize(
       @local_port = nil
       @timeout = timeout
 
-      @reader, @writer, @pid = PTY.spawn(ssm_port_forward_cmd(local_port, reason))
+      @reader, @writer, @pid = PTY.spawn(ssm_port_forward_cmd(local_port, reason, document, extra_params))
 
       @cmd_output = ""
       @session_id = wait_for_session_id
@@ -64,9 +65,8 @@ def wait_for_local_port
 
     private
 
-    def ssm_port_forward_cmd(local_port, reason)
-      document_name = "AWS-StartPortForwardingSession"
-      parameters = { "portNumber" => [remote_port.to_s] }
+    def ssm_port_forward_cmd(local_port, reason, document_name, extra_parameters)
+      parameters = extra_parameters.merge({ "portNumber" => [remote_port.to_s] })
       parameters["localPortNumber"] = [local_port.to_s] unless local_port.nil?
       flags = [
         ["--target", instance_id],
diff --git a/sig/aws_ec2_environment/ssm_port_forwarding_session.rbs b/sig/aws_ec2_environment/ssm_port_forwarding_session.rbs
index 127843d..8dd1310 100644
--- a/sig/aws_ec2_environment/ssm_port_forwarding_session.rbs
+++ b/sig/aws_ec2_environment/ssm_port_forwarding_session.rbs
@@ -16,10 +16,12 @@ class AwsEc2Environment
     def initialize: (
         String instance_id,
         Integer remote_port,
+        ?document: String,
         ?local_port: Integer | nil,
         ?logger: Logger,
         ?timeout: Numeric,
-        ?reason: String | nil
+        ?reason: String | nil,
+        ?extra_params: Hash[String, untyped]
       ) -> void
 
     def close: () -> void
@@ -38,7 +40,7 @@ class AwsEc2Environment
     @writer: IO
     @cmd_output: String
 
-    def ssm_port_forward_cmd: (Integer | nil local_port, String | nil reason) -> String
+    def ssm_port_forward_cmd: (Integer | nil local_port, String | nil reason, String document_name, Hash[String, untyped] extra_params) -> String
 
     # Checks the cmd process output until either the given +pattern+ matches or the +timeout+ is over.
     #
diff --git a/spec/aws_ec2_environment/ssm_port_forwarding_session_spec.rb b/spec/aws_ec2_environment/ssm_port_forwarding_session_spec.rb
index 20d937e..86260ba 100644
--- a/spec/aws_ec2_environment/ssm_port_forwarding_session_spec.rb
+++ b/spec/aws_ec2_environment/ssm_port_forwarding_session_spec.rb
@@ -175,6 +175,98 @@ def close; end
         )
       end
     end
+
+    context "when a specific document is provided" do
+      subject(:session) do
+        described_class.new(
+          "i-0d9c4bg3f26157a8e",
+          22,
+          document: "AWS-StartPortForwardingSessionToRemoteHost",
+          logger: Logger.new(StringIO.new(log)),
+          # we can use a really low timeout to make the tests a lot faster,
+          # since we're not actually going to be writing asynchronously
+          timeout: 0.00001
+        )
+      end
+
+      it "uses the document" do
+        expect { session }.not_to raise_error
+
+        parameters = { "portNumber" => ["22"] }
+        parameters_escaped = Shellwords.escape(parameters.to_json)
+
+        expect(PTY).to have_received(:spawn).with(
+          %w[
+            aws ssm start-session
+            --target i-0d9c4bg3f26157a8e
+            --document-name AWS-StartPortForwardingSessionToRemoteHost
+            --parameters
+          ].join(" ") + " #{parameters_escaped}"
+        )
+      end
+    end
+
+    context "when extra parameters are provided" do
+      it "merges them" do
+        expect do
+          described_class.new(
+            "i-0d9c4bg3f26157a8e",
+            22,
+            extra_params: { "host" => ["my-database.abc.ap-southeast-2.rds.amazonaws.com"] },
+            logger: Logger.new(StringIO.new(log)),
+            # we can use a really low timeout to make the tests a lot faster,
+            # since we're not actually going to be writing asynchronously
+            timeout: 0.00001
+          )
+        end.not_to raise_error
+
+        parameters = { "host" => ["my-database.abc.ap-southeast-2.rds.amazonaws.com"], "portNumber" => ["22"] }
+        parameters_escaped = Shellwords.escape(parameters.to_json)
+
+        expect(PTY).to have_received(:spawn).with(
+          %w[
+            aws ssm start-session
+            --target i-0d9c4bg3f26157a8e
+            --document-name AWS-StartPortForwardingSession
+            --parameters
+          ].join(" ") + " #{parameters_escaped}"
+        )
+      end
+
+      it "overrides them with specific parameters" do
+        expect do
+          described_class.new(
+            "i-0d9c4bg3f26157a8e",
+            22,
+            extra_params: {
+              "host" => ["my-database.abc.ap-southeast-2.rds.amazonaws.com"],
+              "localPortNumber" => [1234]
+            },
+            local_port: 5432,
+            logger: Logger.new(StringIO.new(log)),
+            # we can use a really low timeout to make the tests a lot faster,
+            # since we're not actually going to be writing asynchronously
+            timeout: 0.00001
+          )
+        end.not_to raise_error
+
+        parameters = {
+          "host" => ["my-database.abc.ap-southeast-2.rds.amazonaws.com"],
+          "localPortNumber" => ["5432"],
+          "portNumber" => ["22"]
+        }
+        parameters_escaped = Shellwords.escape(parameters.to_json)
+
+        expect(PTY).to have_received(:spawn).with(
+          %w[
+            aws ssm start-session
+            --target i-0d9c4bg3f26157a8e
+            --document-name AWS-StartPortForwardingSession
+            --parameters
+          ].join(" ") + " #{parameters_escaped}"
+        )
+      end
+    end
   end
 
   describe "#instance_id" do