Add TTL support to net module (#1756)

app/modules/net.c

@@ -638,6 +638,27 @@ int net_dns( lua_State *L ) {
   return 0;
 }
 
+// Lua: client/socket:ttl([ttl])
+int net_ttl( lua_State *L ) {
+  lnet_userdata *ud = net_get_udata(L);
+  if (!ud || ud->type == TYPE_TCP_SERVER)
+    return luaL_error(L, "invalid user data");
+  if (!ud->pcb)
+    return luaL_error(L, "socket is not open/bound yet");
+  int ttl = luaL_optinteger(L, 2, -1);
+  // Since `ttl` field is part of IP_PCB macro
+  // (which are at beginning of both udp_pcb/tcp_pcb)
+  // and PCBs declared as `union` there is safe to
+  // access ttl field without checking for type.
+  if (ttl == -1) {
+    ttl = ud->udp_pcb->ttl;
+  } else {
+    ud->udp_pcb->ttl = ttl;
+  }
+  lua_pushinteger(L, ttl);
+  return 1;
+}
+
 // Lua: client:getpeer()
 int net_getpeer( lua_State *L ) {
   lnet_userdata *ud = net_get_udata(L);
@@ -951,6 +972,7 @@ static const LUA_REG_TYPE net_tcpsocket_map[] = {
   { LSTRKEY( "hold" ),    LFUNCVAL( net_hold ) },
   { LSTRKEY( "unhold" ),  LFUNCVAL( net_unhold ) },
   { LSTRKEY( "dns" ),     LFUNCVAL( net_dns ) },
+  { LSTRKEY( "ttl" ),     LFUNCVAL( net_ttl ) },
   { LSTRKEY( "getpeer" ), LFUNCVAL( net_getpeer ) },
   { LSTRKEY( "getaddr" ), LFUNCVAL( net_getaddr ) },
   { LSTRKEY( "__gc" ),    LFUNCVAL( net_delete ) },
@@ -964,6 +986,7 @@ static const LUA_REG_TYPE net_udpsocket_map[] = {
   { LSTRKEY( "on" ),      LFUNCVAL( net_on ) },
   { LSTRKEY( "send" ),    LFUNCVAL( net_send ) },
   { LSTRKEY( "dns" ),     LFUNCVAL( net_dns ) },
+  { LSTRKEY( "ttl" ),     LFUNCVAL( net_ttl ) },
   { LSTRKEY( "getaddr" ), LFUNCVAL( net_getaddr ) },
   { LSTRKEY( "__gc" ),    LFUNCVAL( net_delete ) },
   { LSTRKEY( "__index" ), LROVAL( net_udpsocket_map ) },

docs/en/modules/net.md

@@ -400,6 +400,29 @@ end)
 #### See also
 [`net.socket:on()`](#netsocketon)
 
+## net.socket:ttl()
+
+Changes or retrieves Time-To-Live value on socket.
+
+#### Syntax
+`ttl([ttl])`
+
+#### Parameters
+- `ttl` (optional) new time-to-live value
+
+#### Returns
+current / new ttl value
+
+#### Example
+```lua
+sk = net.createConnection(net.TCP, 0)
+sk:connect(80, '192.168.1.1')
+sk:ttl(1) -- restrict frames to single subnet
+```
+
+#### See also
+[`net.createConnection()`](#netcreateconnection)
+
 ## net.socket:unhold()
 
 Unblock TCP receiving data by revocation of a preceding `hold()`.
@@ -492,6 +515,12 @@ Retrieve local port and ip of socket.
 
 The syntax and functional identical to [`net.socket:getaddr()`](#netsocketgetaddr).
 
+## net.udpsocket:ttl()
+
+Changes or retrieves Time-To-Live value on socket.
+
+The syntax and functional identical to [`net.socket:ttl()`](#netsocketttl).
+
 # net.dns Module
 
 ## net.dns.getdnsserver()