diff --git a/.cirrus.tasks.yml b/.cirrus.tasks.yml
index 72f553e69f..a2388cd503 100644
--- a/.cirrus.tasks.yml
+++ b/.cirrus.tasks.yml
@@ -20,7 +20,7 @@ env:
   MTEST_ARGS: --print-errorlogs --no-rebuild -C build
   PGCTLTIMEOUT: 120 # avoids spurious failures during parallel tests
   TEMP_CONFIG: ${CIRRUS_WORKING_DIR}/src/tools/ci/pg_ci_base.conf
-  PG_TEST_EXTRA: kerberos ldap ssl load_balance
+  PG_TEST_EXTRA: kerberos ldap ssl libpq_encryption load_balance
 
 
 # What files to preserve in case tests fail
diff --git a/src/test/libpq_encryption/Makefile b/src/test/libpq_encryption/Makefile
new file mode 100644
index 0000000000..3ad3da7031
--- /dev/null
+++ b/src/test/libpq_encryption/Makefile
@@ -0,0 +1,25 @@
+#-------------------------------------------------------------------------
+#
+# Makefile for src/test/libpq_encryption
+#
+# Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+# Portions Copyright (c) 1994, Regents of the University of California
+#
+# src/test/libpq_encryption/Makefile
+#
+#-------------------------------------------------------------------------
+
+subdir = src/test/libpq_encryption
+top_builddir = ../../..
+include $(top_builddir)/src/Makefile.global
+
+export with_ssl with_gssapi with_krb_srvnam
+
+check:
+	$(prove_check)
+
+installcheck:
+	$(prove_installcheck)
+
+clean distclean:
+	rm -rf tmp_check
diff --git a/src/test/libpq_encryption/README b/src/test/libpq_encryption/README
new file mode 100644
index 0000000000..8ceb194527
--- /dev/null
+++ b/src/test/libpq_encryption/README
@@ -0,0 +1,31 @@
+src/test/libpq_encryption/README
+
+Tests for negotiating network encryption method
+===============================================
+
+This directory contains a test suite for the libpq options to
+negotiate encryption with the server. This requires reconfiguring a
+test server, enabling/disabling SSL and GSSAPI, and is therefore kept
+separate and not run by default.
+
+CAUTION: The test server run by this test is configured to listen for TCP
+connections on localhost. Any user on the same host is able to log in to the
+test server while the tests are running. Do not run this suite on a multi-user
+system where you don't trust all local users! Also, this test suite creates a
+KDC server that listens for TCP/IP connections on localhost without any real
+access control.
+
+Running the tests
+=================
+
+NOTE: You must have given the --enable-tap-tests argument to configure.
+
+Run
+    make check PG_TEST_EXTRA=libpq_encryption
+
+You can use "make installcheck" if you previously did "make install".
+In that case, the code in the installation tree is tested.  With
+"make check", a temporary installation tree is built from the current
+sources and then tested.
+
+See src/test/perl/README for more info about running these tests.
diff --git a/src/test/libpq_encryption/meson.build b/src/test/libpq_encryption/meson.build
new file mode 100644
index 0000000000..ac1db10d74
--- /dev/null
+++ b/src/test/libpq_encryption/meson.build
@@ -0,0 +1,18 @@
+# Copyright (c) 2022-2024, PostgreSQL Global Development Group
+
+tests += {
+  'name': 'libpq_encryption',
+  'sd': meson.current_source_dir(),
+  'bd': meson.current_build_dir(),
+  'tap': {
+    'tests': [
+      't/001_negotiate_encryption.pl',
+    ],
+    'env': {
+      'with_ssl': ssl_library,
+      'OPENSSL': openssl.found() ? openssl.path() : '',
+      'with_gssapi': gssapi.found() ? 'yes' : 'no',
+      'with_krb_srvnam': 'postgres',
+    },
+  },
+}
diff --git a/src/test/libpq_encryption/t/001_negotiate_encryption.pl b/src/test/libpq_encryption/t/001_negotiate_encryption.pl
new file mode 100644
index 0000000000..ed33988aa7
--- /dev/null
+++ b/src/test/libpq_encryption/t/001_negotiate_encryption.pl
@@ -0,0 +1,548 @@
+
+# Copyright (c) 2021-2024, PostgreSQL Global Development Group
+
+# OVERVIEW
+# --------
+#
+# Test negotiation of SSL and GSSAPI encryption
+#
+# We test all combinations of:
+#
+# - all the libpq client options that affect the protocol negotiations
+#   (gssencmode, sslmode)
+# - server accepting or rejecting the authentication due to
+#   pg_hba.conf entries
+# - SSL and GSS enabled/disabled in the server
+#
+# That's a lot of combinations, so we use a table-driven approach.
+# Each combination is represented by a line in a table. The line lists
+# the options specifying the test case, and an expected outcome. The
+# expected outcome includes whether the connection succeeds or fails,
+# and whether it uses SSL, GSS or no encryption.
+#
+# TEST TABLE FORMAT
+# -----------------
+#
+# Example of the test table format:
+#
+# # USER     GSSENCMODE  SSLMODE    OUTCOME
+# testuser   disable     allow      plain
+# .          .           prefer     ssl
+# testuser   require     *          fail
+#
+# USER, GSSENCMODE and SSLMODE fields are the libpq 'user',
+# 'gssencmode' and 'sslmode' options used in the test. As a shorthand,
+# a single dot ('.') can be used in the USER, GSSENCMODE, and SSLMODE
+# fields, to indicate "same as on previous line". A '*' can be used
+# as a wildcard; it is expanded to mean all possible values of that
+# field.
+#
+# The OUTCOME field indicates the expected result of the test:
+#
+#  plain:     an unencrypted connection was established
+#  ssl:       SSL connection was established
+#  gss:       GSSAPI encrypted connection was established
+#  fail:      the connection attempt failed
+#
+# Empty lines are ignored. '#' can be used to mark the rest of the
+# line as a comment.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Utils;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Kerberos;
+use File::Basename;
+use File::Copy;
+use Test::More;
+
+if (!$ENV{PG_TEST_EXTRA} || $ENV{PG_TEST_EXTRA} !~ /\blibpq_encryption\b/)
+{
+	plan skip_all =>
+	  'Potentially unsafe test libpq_encryption not enabled in PG_TEST_EXTRA';
+}
+
+my $ssl_supported = $ENV{with_ssl} eq 'openssl';
+my $gss_supported = $ENV{with_gssapi} eq 'yes';
+
+###
+### Prepare test server for GSSAPI and SSL authentication, with a few
+### different test users and helper functions. We don't actually
+### enable SSL and kerberos in the server yet, we will do that later.
+###
+
+my $host = 'enc-test-localhost.postgresql.example.com';
+my $hostaddr = '127.0.0.1';
+my $servercidr = '127.0.0.1/32';
+
+my $node = PostgreSQL::Test::Cluster->new('node');
+$node->init;
+$node->append_conf(
+	'postgresql.conf', qq{
+listen_addresses = '$hostaddr'
+log_connections = on
+lc_messages = 'C'
+});
+my $pgdata = $node->data_dir;
+
+my $dbname = 'postgres';
+my $username = 'enctest';
+my $application = '001_negotiate_encryption.pl';
+
+my $gssuser_password = 'secret1';
+
+my $krb;
+
+if ($gss_supported != 0)
+{
+	note "setting up Kerberos";
+
+	my $realm = 'EXAMPLE.COM';
+	$krb = PostgreSQL::Test::Kerberos->new($host, $hostaddr, $realm);
+	$node->append_conf('postgresql.conf', "krb_server_keyfile = '$krb->{keytab}'\n");
+}
+
+if ($ssl_supported != 0)
+{
+	my $certdir = dirname(__FILE__) . "/../../ssl/ssl";
+
+	copy "$certdir/server-cn-only.crt", "$pgdata/server.crt"
+	  || die "copying server.crt: $!";
+	copy "$certdir/server-cn-only.key", "$pgdata/server.key"
+	  || die "copying server.key: $!";
+	chmod(0600, "$pgdata/server.key");
+
+	# Start with SSL disabled.
+	$node->append_conf('postgresql.conf', "ssl = off\n");
+}
+
+$node->start;
+
+$node->safe_psql('postgres', 'CREATE USER localuser;');
+$node->safe_psql('postgres', 'CREATE USER testuser;');
+$node->safe_psql('postgres', 'CREATE USER ssluser;');
+$node->safe_psql('postgres', 'CREATE USER nossluser;');
+$node->safe_psql('postgres', 'CREATE USER gssuser;');
+$node->safe_psql('postgres', 'CREATE USER nogssuser;');
+
+my $unixdir = $node->safe_psql('postgres', 'SHOW unix_socket_directories;');
+chomp($unixdir);
+
+# Helper function that returns the encryption method in use in the
+# connection.
+$node->safe_psql('postgres', q{
+CREATE FUNCTION current_enc() RETURNS text LANGUAGE plpgsql AS $$
+DECLARE
+  ssl_in_use bool;
+  gss_in_use bool;
+BEGIN
+  ssl_in_use = (SELECT ssl FROM pg_stat_ssl WHERE pid = pg_backend_pid());
+  gss_in_use = (SELECT encrypted FROM pg_stat_gssapi WHERE pid = pg_backend_pid());
+
+  raise log 'ssl %  gss %', ssl_in_use, gss_in_use;
+
+  IF ssl_in_use AND gss_in_use THEN
+    RETURN 'ssl+gss';   -- shouldn't happen
+  ELSIF ssl_in_use THEN
+    RETURN 'ssl';
+  ELSIF gss_in_use THEN
+    RETURN 'gss';
+  ELSE
+    RETURN 'plain';
+  END IF;
+END;
+$$;
+});
+
+# Only accept SSL connections from $servercidr. Our tests don't depend on this
+# but seems best to keep it as narrow as possible for security reasons.
+open my $hba, '>', "$pgdata/pg_hba.conf";
+print $hba qq{
+# TYPE        DATABASE        USER            ADDRESS                 METHOD             OPTIONS
+local         postgres        localuser                               trust
+host          postgres        testuser        $servercidr             trust
+hostnossl     postgres        nossluser       $servercidr             trust
+hostnogssenc  postgres        nogssuser       $servercidr             trust
+};
+
+print $hba qq{
+hostssl       postgres        ssluser         $servercidr             trust
+} if ($ssl_supported != 0);
+
+print $hba qq{
+hostgssenc    postgres        gssuser         $servercidr             trust
+} if ($gss_supported != 0);
+close $hba;
+$node->reload;
+
+# Ok, all prepared. Run the tests.
+
+my @all_test_users = ('testuser', 'ssluser', 'nossluser', 'gssuser', 'nogssuser');
+my @all_gssencmodes = ('disable', 'prefer', 'require');
+my @all_sslmodes = ('disable', 'allow', 'prefer', 'require');
+
+my $server_config = {
+	server_ssl => 0,
+	server_gss => 0,
+};
+
+###
+### Run tests with GSS and SSL disabled in the server
+###
+my $test_table = q{
+# USER      GSSENCMODE   SSLMODE      OUTCOME
+testuser    disable      disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+.           prefer       disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+
+# All attempts with gssencmode=require fail because no credential
+# cache has been configured in the client (and the server isn't
+# configured for GSS either)
+.           require      *            fail
+};
+note("Running tests with SSL and GSS disabled in the server");
+test_matrix($node, $server_config,
+			['testuser'],
+			\@all_sslmodes, \@all_gssencmodes, parse_table($test_table));
+
+###
+### Run tests with GSS disabled and SSL enabled in the server
+###
+SKIP:
+{
+	skip "SSL not supported by this build" if $ssl_supported == 0;
+
+	$test_table = q{
+# USER      GSSENCMODE   SSLMODE      OUTCOME
+testuser    disable      disable      plain
+.           .            allow        plain
+.           .            prefer       ssl
+.           .            require      ssl
+ssluser     .            disable      fail
+.           .            allow        ssl
+.           .            prefer       ssl
+.           .            require      ssl
+nossluser   .            disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+};
+
+	# Enable SSL in the server
+	$node->adjust_conf('postgresql.conf', 'ssl', 'on');
+	$node->reload;
+	$server_config->{server_ssl} = 1;
+
+	note("Running tests with SSL enabled in server");
+	test_matrix($node, $server_config,
+				['testuser', 'ssluser', 'nossluser'],
+				\@all_sslmodes, ['disable'], parse_table($test_table));
+
+	# Disable SSL again
+	$node->adjust_conf('postgresql.conf', 'ssl', 'off');
+	$node->reload;
+	$server_config->{server_ssl} = 0;
+}
+
+###
+### Run tests with GSS enabled, SSL disabled in the server
+###
+SKIP:
+{
+	skip "GSSAPI/Kerberos not supported by this build" if $gss_supported == 0;
+	$test_table = q{
+# USER      GSSENCMODE   SSLMODE      OUTCOME
+testuser    disable      disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+.           require      *            gss
+.           prefer       *            gss
+
+gssuser     disable      disable      fail
+.           .            allow        fail
+.           .            prefer       fail
+.           .            require      fail
+.           prefer       *            gss
+.           require      *            gss
+
+nogssuser   disable      disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+.           prefer       disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+.           require      *            fail
+};
+
+	# Sanity check that the connection fails when no kerberos ticket
+	# is present in the client
+	connect_test($node, 'user=testuser gssencmode=require sslmode=disable', 'fail');
+
+	$krb->create_principal('gssuser', $gssuser_password);
+	$krb->create_ticket('gssuser', $gssuser_password);
+	$server_config->{server_gss} = 1;
+
+	note("Running tests with GSS enabled in server");
+	test_matrix($node, $server_config,
+				['testuser', 'gssuser', 'nogssuser'],
+				\@all_sslmodes, \@all_gssencmodes, parse_table($test_table));
+
+	# Check that logs match the expected 'no pg_hba.conf entry' line, too, as
+	# that is not tested by test_matrix.
+	connect_test($node, 'user=nogssuser gssencmode=require sslmode=prefer', 'fail',
+				 'no pg_hba.conf entry for host "127.0.0.1", user "nogssuser", database "postgres", GSS encryption');
+
+	# With 'gssencmode=prefer', libpq will first negotiate GSSAPI
+	# encryption, but the connection will fail because pg_hba.conf
+	# forbids GSSAPI encryption for this user. It will then reconnect
+	# with SSL, but the server doesn't support it, so it will continue
+	# with no encryption.
+	connect_test($node, 'user=nogssuser gssencmode=prefer sslmode=prefer', 'plain',
+				 'no pg_hba.conf entry for host "127.0.0.1", user "nogssuser", database "postgres", GSS encryption');
+}
+
+###
+### Tests with both GSS and SSL enabled in the server
+###
+SKIP:
+{
+	skip "GSSAPI/Kerberos or SSL not supported by this build" unless ($ssl_supported && $gss_supported);
+
+	$test_table = q{
+# USER      GSSENCMODE   SSLMODE      OUTCOME
+testuser    disable      disable      plain
+.           .            allow        plain
+.           .            prefer       ssl
+.           .            require      ssl
+.           prefer       disable      gss
+.           .            allow        gss
+.           .            prefer       gss
+.           .            require      gss         # If both GSS and SSL is possible, GSS is chosen over SSL, even if sslmode=require
+.           require      disable      gss
+.           .            allow        gss
+.           .            prefer       gss
+.           .            require      gss         # If both GSS and SSL is possible, GSS is chosen over SSL, even if sslmode=require
+
+gssuser     disable      *            fail
+.           prefer       *            gss
+.           require      *            gss
+
+ssluser     disable      disable      fail
+.           .            allow        ssl
+.           .            prefer       ssl
+.           .            require      ssl
+.           prefer       disable      fail
+.           .            allow        ssl
+.           .            prefer       ssl
+.           .            require      ssl
+.           require      disable      fail
+.           .            allow        fail
+.           .            prefer       fail
+.           .            require      fail         # If both GSS and SSL are required, the sslmode=require is effectively ignored and GSS is required
+
+nogssuser   disable      disable      plain
+.           .            allow        plain
+.           .            prefer       ssl
+.           .            require      ssl
+.           prefer       disable      plain
+.           .            allow        plain
+.           .            prefer       ssl
+.           .            require      ssl
+.           require      *            fail
+
+nossluser   disable      disable      plain
+.           .            allow        plain
+.           .            prefer       plain
+.           .            require      fail
+.           prefer       *            gss
+.           require      *            gss
+};
+
+	# Sanity check that GSSAPI is still enabled from previous test.
+	connect_test($node, 'user=testuser gssencmode=prefer sslmode=prefer', 'gss');
+
+	# Enable SSL
+	$node->adjust_conf('postgresql.conf', 'ssl', 'on');
+	$node->reload;
+	$server_config->{server_ssl} = 1;
+
+	note("Running tests with both GSS and SSL enabled in server");
+	test_matrix($node, $server_config,
+				['testuser', 'gssuser', 'ssluser', 'nogssuser', 'nossluser'],
+				\@all_sslmodes, \@all_gssencmodes, parse_table($test_table));
+
+	# Test case that server supports GSSAPI, but it's not allowed for
+	# this user. Special cased because we check output
+	connect_test($node, 'user=nogssuser gssencmode=require sslmode=prefer', 'fail',
+				 'no pg_hba.conf entry for host "127.0.0.1", user "nogssuser", database "postgres", GSS encryption');
+
+	# with 'gssencmode=prefer', libpq will first negotiate GSSAPI
+	# encryption, but the connection will fail because pg_hba.conf
+	# forbids GSSAPI encryption for this user. It will then reconnect
+	# with SSL.
+	connect_test($node, 'user=nogssuser gssencmode=prefer sslmode=prefer', 'ssl',
+				 'no pg_hba.conf entry for host "127.0.0.1", user "nogssuser", database "postgres", GSS encryption');
+
+	# Setting both gssencmode=require and sslmode=require fails if
+	# GSSAPI is not available.
+	connect_test($node, 'user=nogssuser gssencmode=require sslmode=require ', 'fail');
+}
+
+###
+### Test negotiation over unix domain sockets.
+###
+SKIP:
+{
+	skip "Unix domain sockets not supported" unless ($unixdir ne "");
+
+	connect_test($node, "user=localuser gssencmode=prefer sslmode=require host=$unixdir", 'plain');
+	connect_test($node, "user=localuser gssencmode=require sslmode=prefer host=$unixdir", 'fail');
+}
+
+done_testing();
+
+
+### Helper functions
+
+# Test the cube of parameters: user, sslmode, and gssencmode
+sub test_matrix
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($pg_node, $node_conf,
+		$test_users, $ssl_modes, $gss_modes, %expected) = @_;
+
+	foreach my $test_user (@{$test_users})
+	{
+		foreach my $gssencmode (@{$gss_modes})
+		{
+			foreach my $client_mode (@{$ssl_modes})
+			{
+				my %params = (
+					server_ssl=>$node_conf->{server_ssl},
+					server_gss=>$node_conf->{server_gss},
+					user=>$test_user,
+					gssencmode=>$gssencmode,
+					sslmode=>$client_mode,
+				);
+				my $key = "$test_user $gssencmode $client_mode";
+				my $res = $expected{$key};
+				if (!defined $res) {
+					$res = "<expected result missing>";
+				}
+				connect_test($pg_node, "user=$test_user gssencmode=$gssencmode sslmode=$client_mode", $res);
+			}
+		}
+	}
+}
+
+sub connect_test
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($node, $connstr, $expected_outcome, @expect_log_msgs)
+	  = @_;
+
+	my $test_name = " '$connstr' -> $expected_outcome";
+
+	my $connstr_full = "";
+	$connstr_full .= "dbname=postgres " unless $connstr =~ m/dbname=/;
+	$connstr_full .= "host=$host hostaddr=$hostaddr " unless $connstr =~ m/host=/;
+	$connstr_full .= $connstr;
+
+	my $log_location = -s $node->logfile;
+
+	# XXX: Pass command with -c, because I saw intermittent test
+	# failures like this:
+	#
+	# ack Broken pipe: write( 13, 'SELECT current_enc()' ) at /usr/local/lib/perl5/site_perl/IPC/Run/IO.pm line 550.
+	#
+	# I think that happens if the connection fails before we write the
+	# query to its stdin. This test gets a lot of connection failures
+	# on purpose.
+	my ($ret, $stdout, $stderr) = $node->psql(
+		'postgres',
+		'',
+		extra_params => ['-w', '-c', 'SELECT current_enc()'],
+		connstr => "$connstr_full",
+		on_error_stop => 0);
+
+	my $outcome = $ret == 0 ? $stdout : 'fail';
+
+	is($outcome, $expected_outcome, $test_name) or diag("$stderr");
+
+	if (@expect_log_msgs)
+	{
+		# Match every message literally.
+		my @regexes = map { qr/\Q$_\E/ } @expect_log_msgs;
+		my %params = ();
+		$params{log_like} = \@regexes;
+		$node->log_check($test_name, $log_location, %params);
+	}
+}
+
+sub parse_table
+{
+	my ($table) = @_;
+	my @lines = split /\n/, $table;
+
+	my %expected;
+
+	my ($user, $gssencmode, $sslmode);
+	foreach my $line (@lines) {
+
+		# Trim comments
+		$line =~ s/#.*$//;
+
+		# Trim whitespace at beginning and end
+		$line =~ s/^\s+//;
+		$line =~ s/\s+$//;
+
+		# Ignore empty lines (includes comment-only lines)
+		next if $line eq '';
+
+		my @cols = split /\s+/, $line;
+		die "test table line \"$line\" has incorrect number of columns\n" if scalar(@cols) != 4 ;
+
+		$user = $cols[0] unless $cols[0] eq ".";
+		$gssencmode = $cols[1] unless $cols[1] eq ".";
+		$sslmode = $cols[2] unless $cols[2] eq ".";
+		my $outcome = $cols[3];
+
+		my %expanded = expand_expected_line($user, $gssencmode, $sslmode, $outcome);
+		%expected = (%expected, %expanded);
+	}
+	return %expected;
+}
+
+# Expand wildcards on a test table line
+sub expand_expected_line
+{
+	my ($user, $gssencmode, $sslmode, $expected) = @_;
+
+	my %result;
+	if ($user eq '*') {
+		foreach my $x (@all_test_users) {
+			%result = (%result, expand_expected_line($x, $gssencmode, $sslmode, $expected));
+		}
+	} elsif ($gssencmode eq '*') {
+		foreach my $x (@all_gssencmodes) {
+			%result = (%result, expand_expected_line($user, $x, $sslmode, $expected));
+		}
+	} elsif ($sslmode eq '*') {
+		foreach my $x (@all_sslmodes) {
+			%result = (%result, expand_expected_line($user, $gssencmode, $x, $expected));
+		}
+	} else {
+		$result{"$user $gssencmode $sslmode"} = $expected;
+	}
+	return %result;
+}
diff --git a/src/test/meson.build b/src/test/meson.build
index c3d0dfedf1..702213bc6f 100644
--- a/src/test/meson.build
+++ b/src/test/meson.build
@@ -4,6 +4,7 @@ subdir('regress')
 subdir('isolation')
 
 subdir('authentication')
+subdir('libpq_encryption')
 subdir('recovery')
 subdir('subscription')
 subdir('modules')