diff --git a/putty.h b/putty.h
index b08efcfd..5e7632e6 100644
--- a/putty.h
+++ b/putty.h
@@ -521,6 +521,7 @@ struct BackendVtable {
 
     int protocol;
     int default_port;
+    unsigned flags;
 };
 
 static inline const char *backend_init(
diff --git a/raw.c b/raw.c
index faf37794..92f28f94 100644
--- a/raw.c
+++ b/raw.c
@@ -326,5 +326,6 @@ const struct BackendVtable raw_backend = {
     NULL /* test_for_upstream */,
     "raw", "Raw",
     PROT_RAW,
+    0,
     0
 };
diff --git a/rlogin.c b/rlogin.c
index dcd6c599..99fed39d 100644
--- a/rlogin.c
+++ b/rlogin.c
@@ -424,5 +424,6 @@ const struct BackendVtable rlogin_backend = {
     NULL /* test_for_upstream */,
     "rlogin", "Rlogin",
     PROT_RLOGIN,
-    513
+    513,
+    0
 };
diff --git a/ssh.c b/ssh.c
index 7d74ccdc..eafae7e8 100644
--- a/ssh.c
+++ b/ssh.c
@@ -1199,7 +1199,8 @@ const struct BackendVtable ssh_backend = {
     ssh_test_for_upstream,
     "ssh", "SSH",
     PROT_SSH,
-    22
+    22,
+    0
 };
 
 const struct BackendVtable sshconn_backend = {
diff --git a/telnet.c b/telnet.c
index a69fe569..374b2684 100644
--- a/telnet.c
+++ b/telnet.c
@@ -1066,5 +1066,6 @@ const struct BackendVtable telnet_backend = {
     NULL /* test_for_upstream */,
     "telnet", "Telnet",
     PROT_TELNET,
-    23
+    23,
+    0
 };
diff --git a/testback.c b/testback.c
index 276f5464..5b733a47 100644
--- a/testback.c
+++ b/testback.c
@@ -59,14 +59,14 @@ const struct BackendVtable null_backend = {
     null_init, null_free, null_reconfig, null_send, null_sendbuffer, null_size,
     null_special, null_get_specials, null_connected, null_exitcode, null_sendok,
     null_ldisc, null_provide_ldisc, null_unthrottle,
-    null_cfg_info, NULL /* test_for_upstream */, "null", "null", -1, 0
+    null_cfg_info, NULL /* test_for_upstream */, "null", "null", -1, 0, 0
 };
 
 const struct BackendVtable loop_backend = {
     loop_init, loop_free, null_reconfig, loop_send, null_sendbuffer, null_size,
     null_special, null_get_specials, null_connected, null_exitcode, null_sendok,
     null_ldisc, null_provide_ldisc, null_unthrottle,
-    null_cfg_info, NULL /* test_for_upstream */, "loop", "loop", -1, 0
+    null_cfg_info, NULL /* test_for_upstream */, "loop", "loop", -1, 0, 0
 };
 
 struct loop_state {