Only write /etc/hosts when necessary.

Without this patch, sshuttle 'restores' /etc/hosts even if it didn't
make any modifications to it. This can be confirmed by running without
--auto-hosts and confirming that the modification time of /etc/hosts
is unchanged while sshuttle is running, but is updated when sshuttle
exits (and a debug2() message is printed indicating the file is
written).

I'm not aware of the previous behavior causing problems. However,
writing an important file unnecessarily as root should be avoided.

sshuttle/firewall.py

@@ -47,7 +47,10 @@ def rewrite_etc_hosts(hostmap, port):
     os.rename(tmpname, HOSTSFILE)
 
 
-def restore_etc_hosts(port):
+def restore_etc_hosts(hostmap, port):
+    # Only restore if we added hosts to /etc/hosts previously.
+    if len(hostmap) > 0:
+        debug2('firewall manager: undoing /etc/hosts changes.\n')
         rewrite_etc_hosts({}, port)
 
 
@@ -275,8 +278,8 @@ def main(method_name, syslog):
                 debug2('An error occurred, ignoring it.')
 
         try:
-            debug2('firewall manager: undoing /etc/hosts changes.\n')
-            restore_etc_hosts(port_v6 or port_v4)
+            # debug2() message printed in restore_etc_hosts() function.
+            restore_etc_hosts(hostmap, port_v6 or port_v4)
         except BaseException:
             try:
                 debug1("firewall manager: "

tests/client/test_firewall.py

@@ -55,7 +55,7 @@ def test_rewrite_etc_hosts(tmpdir):
         assert line == ""
 
     with patch('sshuttle.firewall.HOSTSFILE', new=str(new_hosts)):
-        sshuttle.firewall.restore_etc_hosts(10)
+        sshuttle.firewall.restore_etc_hosts(hostmap, 10)
     assert orig_hosts.computehash() == new_hosts.computehash()