Merge bitcoin#14532: Never bind INADDR_ANY by default, and warn when doing so explicitly

27c44ef rpcbind: Warn about exposing RPC to untrusted networks (Luke Dashjr)
d6a1287 CNetAddr: Add IsBindAny method to check for INADDR_ANY (Luke Dashjr)
3615003 net: Always default rpcbind to localhost, never "all interfaces" (Luke Dashjr)

Pull request description:

  A disturbingly large number of listening nodes appear to be also exposing their RPC server to the public internet. To attempt to mitigate this:

  * Only ever bind localhost by default, even if `rpcallowip` is specified. (A warning is given if `rpcallowip` is specified without `rpcbind`, since it doesn't really make sense to do.)
  * Warn about exposing the RPC server to untrusted networks if the user explicitly binds to any INADDR_ANY address.
  * Include a warning about untrusted networks in the `--help` documentation for `rpcbind`.

Tree-SHA512: 755bbca3db416a31393672eccf6675a5ee4d1eb1812cba73ebb4ff8c6b855ecc5df4c692566e9aa7b0f7d4dce6fedb9c0e9f3c265b9663aca36c4a6ba5efdbd4

Author: laanwj

src/httpserver.cpp

@@ -300,9 +300,12 @@ static bool HTTPBindAddresses(struct evhttp* http)
     std::vector<std::pair<std::string, uint16_t> > endpoints;
 
     // Determine what addresses to bind to
-    if (!gArgs.IsArgSet("-rpcallowip")) { // Default to loopback if not allowing external IPs
+    if (!(gArgs.IsArgSet("-rpcallowip") && gArgs.IsArgSet("-rpcbind"))) { // Default to loopback if not allowing external IPs
         endpoints.push_back(std::make_pair("::1", defaultPort));
         endpoints.push_back(std::make_pair("127.0.0.1", defaultPort));
+        if (gArgs.IsArgSet("-rpcallowip")) {
+            LogPrintf("WARNING: option -rpcallowip was specified without -rpcbind; this doesn't usually make sense\n");
+        }
         if (gArgs.IsArgSet("-rpcbind")) {
             LogPrintf("WARNING: option -rpcbind was ignored because -rpcallowip was not specified, refusing to allow everyone to connect\n");
         }
@@ -313,16 +316,17 @@ static bool HTTPBindAddresses(struct evhttp* http)
             SplitHostPort(strRPCBind, port, host);
             endpoints.push_back(std::make_pair(host, port));
         }
-    } else { // No specific bind address specified, bind to any
-        endpoints.push_back(std::make_pair("::", defaultPort));
-        endpoints.push_back(std::make_pair("0.0.0.0", defaultPort));
     }
 
     // Bind addresses
     for (std::vector<std::pair<std::string, uint16_t> >::iterator i = endpoints.begin(); i != endpoints.end(); ++i) {
         LogPrint(BCLog::HTTP, "Binding RPC on address %s port %i\n", i->first, i->second);
         evhttp_bound_socket *bind_handle = evhttp_bind_socket_with_handle(http, i->first.empty() ? nullptr : i->first.c_str(), i->second);
         if (bind_handle) {
+            CNetAddr addr;
+            if (i->first.empty() || (LookupHost(i->first.c_str(), addr, false) && addr.IsBindAny())) {
+                LogPrintf("WARNING: the RPC server is not safe to expose to untrusted networks such as the public internet\n");
+            }
             boundSockets.push_back(bind_handle);
         } else {
             LogPrintf("Binding RPC on address %s port %i failed.\n", i->first, i->second);

src/init.cpp

@@ -622,7 +622,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageGroup(_("RPC server options:"));
     strUsage += HelpMessageOpt("-server", _("Accept command line and JSON-RPC commands"));
     strUsage += HelpMessageOpt("-rest", strprintf(_("Accept public REST requests (default: %u)"), DEFAULT_REST_ENABLE));
-    strUsage += HelpMessageOpt("-rpcbind=<addr>[:port]", _("Bind to given address to listen for JSON-RPC connections. This option is ignored unless -rpcallowip is also passed. Port is optional and overrides -rpcport. Use [host]:port notation for IPv6. This option can be specified multiple times (default: 127.0.0.1 and ::1 i.e., localhost, or if -rpcallowip has been specified, 0.0.0.0 and :: i.e., all addresses)"));
+    strUsage += HelpMessageOpt("-rpcbind=<addr>[:port]", _("Bind to given address to listen for JSON-RPC connections. Do not expose the RPC server to untrusted networks such as the public internet! This option is ignored unless -rpcallowip is also passed. Port is optional and overrides -rpcport. Use [host]:port notation for IPv6. This option can be specified multiple times (default: 127.0.0.1 and ::1 i.e., localhost, or if -rpcallowip has been specified, 0.0.0.0 and :: i.e., all addresses)"));
     strUsage += HelpMessageOpt("-rpccookiefile=<loc>", _("Location of the auth cookie (default: data dir)"));
     strUsage += HelpMessageOpt("-rpcuser=<user>", _("Username for JSON-RPC connections"));
     strUsage += HelpMessageOpt("-rpcpassword=<pw>", _("Password for JSON-RPC connections"));

src/netaddress.cpp

@@ -86,6 +86,16 @@ unsigned int CNetAddr::GetByte(int n) const
     return ip[15-n];
 }
 
+bool CNetAddr::IsBindAny() const
+{
+    const int cmplen = IsIPv4() ? 4 : 16;
+    for (int i = 0; i < cmplen; ++i) {
+        if (GetByte(i)) return false;
+    }
+
+    return true;
+}
+
 bool CNetAddr::IsIPv4() const
 {
     return (memcmp(ip, pchIPv4, sizeof(pchIPv4)) == 0);

src/netaddress.h

@@ -56,6 +56,7 @@ class CNetAddr
         bool SetInternal(const std::string& name);
 
         bool SetSpecial(const std::string &strName); // for Tor addresses
+        bool IsBindAny() const; // INADDR_ANY equivalent
         bool IsIPv4() const;    // IPv4 mapped address (::FFFF:0:0/96, 0.0.0.0/0)
         bool IsIPv6() const;    // IPv6 address (not mapped IPv4, not Tor)
         bool IsRFC1918() const; // IPv4 private networks (10.0.0.0/8, 192.168.0.0/16, 172.16.0.0/12)