Make outbound wildcard default to inbound IP

outbound=:1234
    outbound=0.0.0.0:1234
    outbound=
    outbound=0.0.0.0

now all default to use the inbound= IP.  (If multiple inbound= IPs are
given, we raise an exception to abort startup).

Only applies to routers (since clients don't have inbound IPs), and
eliminates potential weird edge cases with local system IP and routing
shenanigans.

llarp/config/config.cpp

@@ -936,7 +936,8 @@ namespace llarp
             "IP and/or port to listen on for incoming connections.",
             "",
             "If IP is omitted then lokinet will search for a local network interface with a",
-            "public IP address and use that IP. If port is omitted then lokinet defaults to 1090.",
+            "public IP address and use that IP (and will exit with an error if no such IP is found",
+            "on the system).  If port is omitted then lokinet defaults to 1090.",
             "",
             "Examples:",
             "    inbound=15.5.29.5:443",
@@ -957,19 +958,36 @@ namespace llarp
         "bind",
         "outbound",
         MultiValue,
-        Comment{
+        params.isRelay ? Comment{
+            "IP and/or port to use for outbound socket connections to other lokinet routers.",
+            "",
+            "If no outbound bind IP is configured, or the 0.0.0.0 wildcard IP is given, then",
+            "lokinet will bind to the same IP being used for inbound connections (either an",
+            "explicit inbound= provided IP, or the default).  If no port is given, or port is",
+            "given as 0, then a random high port will be used.",
+            "",
+            "If using multiple inbound= addresses then you *must* provide an explicit oubound= IP.",
+            "",
+            "Examples:",
+            "    outbound=1.2.3.4:5678",
+            "    outbound=:9000",
+            "    outbound=8.9.10.11",
+            "",
+            "The second example binds on the default incoming IP using port 9000; the third",
+            "example binds on the given IP address using a random high port.",
+        } : Comment{
             "IP and/or port to use for outbound socket connections to lokinet routers.",
             "",
-            "If no outbound bind IP is configured then lokinet will use a wildcard address to use",
-            "any available local IP.  If no port is given, or port is given as 0, then a random",
-            "high port will be used.",
+            "If no outbound bind IP is configured then lokinet will use a wildcard IP address",
+            "(equivalent to specifying 0.0.0.0).  If no port is given then a random high port",
+            "will be used.",
             "",
             "Examples:",
             "    outbound=1.2.3.4:5678",
             "    outbound=:9000",
             "    outbound=8.9.10.11",
             "",
-            "The second example binds an outbound socket on all interfaces; the third example",
+            "The second example binds on the wildcard address using port 9000; the third example",
             "binds on the given IP address using a random high port.",
         },
         [this, net_ptr, parse_addr_for_link](const std::string& arg) {

llarp/router/router.cpp

@@ -1645,7 +1645,7 @@ namespace llarp
     if (addrs.empty())
       addrs.emplace_back(Net().Wildcard());
 
-    for (auto bind_addr : addrs)
+    for (auto& bind_addr : addrs)
     {
       auto link = iwp::NewOutboundLink(
           m_keyManager,
@@ -1671,13 +1671,19 @@ namespace llarp
 
       const auto& net = Net();
 
-      // try to use a public address if we have one set on our inbound links
-      _linkManager.ForEachInboundLink([&bind_addr, &net](const auto& link) {
-        if (not net.IsBogon(bind_addr))
-          return;
-        if (auto addr = link->LocalSocketAddr(); not net.IsBogon(addr))
-          bind_addr.setIP(addr.getIP());
-      });
+      // If outbound is set to wildcard and we have just one inbound, then bind to the inbound IP;
+      // if you have more than one inbound you have to be explicit about your outbound.
+      if (net.IsWildcardAddress(bind_addr.getIP()))
+      {
+        bool multiple = false;
+        _linkManager.ForEachInboundLink([&bind_addr, &multiple](const auto& link) {
+          if (multiple)
+            throw std::runtime_error{
+                "outbound= IP address must be specified when using multiple inbound= addresses"};
+          multiple = true;
+          bind_addr.setIP(link->LocalSocketAddr().getIP());
+        });
+      }
 
       link->Bind(this, bind_addr);