net: recognize CJDNS addresses as such

In some cases addresses come from an external source as a string or as a
`struct sockaddr_in6`, without a tag to tell whether it is a private
IPv6 or a CJDNS address. In those cases interpret the address as a CJDNS
address instead of an IPv6 address if `-cjdnsreachable` is set and the
seemingly-IPv6-address belongs to `fc00::/8`. Those external sources are:

* `-externalip=`
* `-bind=`
* UPnP
* `getifaddrs(3)` (called through `-discover`)
* `addnode`
* `connect`
* incoming connections (returned by `accept(2)`)

2 files changed, 29 insertions, 4 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 8cf59f0b0d..82e55d4189 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -230,9 +230,27 @@ std::optional<CAddress> GetLocalAddrForPeer(CNode *pnode)
     return std::nullopt;
 }
 
+/**
+ * If an IPv6 address belongs to the address range used by the CJDNS network and
+ * the CJDNS network is reachable (-cjdnsreachable config is set), then change
+ * the type from NET_IPV6 to NET_CJDNS.
+ * @param[in] service Address to potentially convert.
+ * @return a copy of `service` either unmodified or changed to CJDNS.
+ */
+CService MaybeFlipIPv6toCJDNS(const CService& service)
+{
+    CService ret{service};
+    if (ret.m_net == NET_IPV6 && ret.m_addr[0] == 0xfc && IsReachable(NET_CJDNS)) {
+        ret.m_net = NET_CJDNS;
+    }
+    return ret;
+}
+
 // learn a new local address
-bool AddLocal(const CService& addr, int nScore)
+bool AddLocal(const CService& addr_, int nScore)
 {
+    CService addr{MaybeFlipIPv6toCJDNS(addr_)};
+
     if (!addr.IsRoutable())
         return false;
 
@@ -409,7 +427,8 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
     if (pszDest) {
         std::vector<CService> resolved;
         if (Lookup(pszDest, resolved,  default_port, fNameLookup && !HaveNameProxy(), 256) && !resolved.empty()) {
-            addrConnect = CAddress(resolved[GetRand(resolved.size())], NODE_NONE);
+            const CService rnd{resolved[GetRand(resolved.size())]};
+            addrConnect = CAddress{MaybeFlipIPv6toCJDNS(rnd), NODE_NONE};
             if (!addrConnect.IsValid()) {
                 LogPrint(BCLog::NET, "Resolver returned invalid address %s for %s\n", addrConnect.ToString(), pszDest);
                 return nullptr;
@@ -1092,9 +1111,11 @@ void CConnman::AcceptConnection(const ListenSocket& hListenSocket) {
 
     if (!addr.SetSockAddr((const struct sockaddr*)&sockaddr)) {
         LogPrintf("Warning: Unknown socket family\n");
+    } else {
+        addr = CAddress{MaybeFlipIPv6toCJDNS(addr), NODE_NONE};
     }
 
-    const CAddress addr_bind = GetBindAddress(hSocket);
+    const CAddress addr_bind{MaybeFlipIPv6toCJDNS(GetBindAddress(hSocket)), NODE_NONE};
 
     NetPermissionFlags permissionFlags = NetPermissionFlags::None;
     hListenSocket.AddSocketPermissionFlags(permissionFlags);
@@ -2460,7 +2481,10 @@ NodeId CConnman::GetNewNodeId()
 }
 
 
-bool CConnman::Bind(const CService &addr, unsigned int flags, NetPermissionFlags permissions) {
+bool CConnman::Bind(const CService& addr_, unsigned int flags, NetPermissionFlags permissions)
+{
+    const CService addr{MaybeFlipIPv6toCJDNS(addr_)};
+
     if (!(flags & BF_EXPLICIT) && !IsReachable(addr)) {
         return false;
     }
diff --git a/src/netaddress.h b/src/netaddress.h
index 57eb8bc72f..f074c1d3ec 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -550,6 +550,7 @@ public:
     }
 
     friend class CServiceHash;
+    friend CService MaybeFlipIPv6toCJDNS(const CService& service);
 };
 
 class CServiceHash