commit 1da19b3c9eb6a670f6a6845cdaa8564c21ae7399
Author: Dan Radez <dradez@redhat.com>
Date:   Wed Sep 30 11:00:07 2020 -0400

    Default dnsmasq --conf-file to /dev/null
    
    Passing --conf-file= with no value has no effect on the dnsmasq
    process. Intended effect here is for the default system dnsmasq.conf
    file not to be read and included in configuring the process. For
    that to happen some value has to be passed to --conf-file. Passing
    /dev/null will invoke the desired outcome to skip the system
    default conf file.
    
    Closes-Bug: #1896945
    Change-Id: I22570a44f84d14a792633747c04d7426ab231009
    (cherry picked from commit 704576e54e041340ed9c2964b110815e074239a6)

diff --git a/neutron/agent/linux/dhcp.py b/neutron/agent/linux/dhcp.py
index c55dc96..50ec1be 100644
--- a/neutron/agent/linux/dhcp.py
+++ b/neutron/agent/linux/dhcp.py
@@ -462,7 +462,8 @@ class Dnsmasq(DhcpLocalProcess):
             cmd.append('--dhcp-option-force=option:T2,%ds' %
                        self.conf.dhcp_rebinding_time)
 
-        cmd.append('--conf-file=%s' % self.conf.dnsmasq_config_file)
+        cmd.append('--conf-file=%s' %
+                   (self.conf.dnsmasq_config_file.strip() or '/dev/null'))
         for server in self.conf.dnsmasq_dns_servers:
             cmd.append('--server=%s' % server)
 
diff --git a/neutron/tests/unit/agent/linux/test_dhcp.py b/neutron/tests/unit/agent/linux/test_dhcp.py
index 6104e48..f88b059 100644
--- a/neutron/tests/unit/agent/linux/test_dhcp.py
+++ b/neutron/tests/unit/agent/linux/test_dhcp.py
@@ -1287,6 +1287,21 @@ class TestDnsmasq(TestBase):
         def mock_get_conf_file_name(kind):
             return '/dhcp/%s/%s' % (network.id, kind)
 
+        # Empty string passed to --conf-file in dnsmasq is invalid
+        # we must force '' to '/dev/null' because the dhcp agent
+        # does the same. Therefore we allow empty string to
+        # be passed to neutron but not to dnsmasq.
+        def check_conf_file_empty(cmd_list):
+            for i in cmd_list:
+                conf_file = ''
+                value = ''
+                if i.startswith('--conf-file='):
+                    conf_file = i
+                    value = i[12:].strip()
+                    if not value:
+                        idx = cmd_list.index(conf_file)
+                        cmd_list[idx] = '--conf-file=/dev/null'
+
         # if you need to change this path here, think twice,
         # that means pid files will move around, breaking upgrades
         # or backwards-compatibility
@@ -1348,7 +1363,9 @@ class TestDnsmasq(TestBase):
             expected.append('--dhcp-option-force=option:T1,%ds' % dhcp_t1)
         if dhcp_t2:
             expected.append('--dhcp-option-force=option:T2,%ds' % dhcp_t2)
+
         expected.extend(extra_options)
+        check_conf_file_empty(expected)
 
         self.execute.return_value = ('', '')