diff --git a/src/client.cpp b/src/client.cpp
index ef34b4d..a79f0e6 100644
--- a/src/client.cpp
+++ b/src/client.cpp
@@ -468,8 +468,7 @@ Value buildCAMethod()
 }
 
 ContextImpl::ContextImpl(const Config& conf, const evbase& tcp_loop)
-    :canIPv6(evsocket::canIPv6())
-    ,ifmap(IfaceMap::instance())
+    :ifmap(IfaceMap::instance())
     ,effective(conf)
     ,caMethod(buildCAMethod())
     ,searchTx4(AF_INET, SOCK_DGRAM, 0)
diff --git a/src/clientimpl.h b/src/clientimpl.h
index d8b6476..63a7e09 100644
--- a/src/clientimpl.h
+++ b/src/clientimpl.h
@@ -226,7 +226,6 @@ private:
 struct ContextImpl : public std::enable_shared_from_this<ContextImpl>
 {
     SockAttach attach;
-    const bool canIPv6;
     IfaceMap& ifmap;
 
     // "const" after ctor
diff --git a/src/evhelper.cpp b/src/evhelper.cpp
index d319d46..b914f85 100644
--- a/src/evhelper.cpp
+++ b/src/evhelper.cpp
@@ -365,6 +365,8 @@ bool evbase::assertInRunningLoop() const
     throw std::logic_error("Not in running evbase worker");
 }
 
+bool evsocket::canIPv6;
+
 evsocket::evsocket(int af, evutil_socket_t sock)
     :sock(sock)
     ,af(af)
@@ -636,7 +638,7 @@ std::vector<SockAddr> evsocket::broadcasts(const SockAddr* match) const
 #  define EAFNOSUPPORT WSAESOCKTNOSUPPORT
 #endif
 
-bool evsocket::canIPv6()
+bool evsocket::init_canIPv6()
 {
     try {
         evsocket sock(AF_INET6, SOCK_DGRAM, 0);
diff --git a/src/evhelper.h b/src/evhelper.h
index 858a399..5f463df 100644
--- a/src/evhelper.h
+++ b/src/evhelper.h
@@ -248,7 +248,9 @@ struct PVXS_API evsocket
     std::vector<SockAddr> broadcasts(const SockAddr* match=nullptr) const;
 
     static
-    bool canIPv6();
+    bool canIPv6;
+
+    static bool init_canIPv6();
 };
 
 struct PVXS_API IfaceMap {
diff --git a/src/os/WIN32/osdSockExt.cpp b/src/os/WIN32/osdSockExt.cpp
index e0d59a5..4c120be 100644
--- a/src/os/WIN32/osdSockExt.cpp
+++ b/src/os/WIN32/osdSockExt.cpp
@@ -46,6 +46,8 @@ void oseDoOnce(void*)
     }
     if(!WSARecvMsg)
         cantProceed("Unable to get &WSARecvMsg!!");
+
+    evsocket::canIPv6 = evsocket::init_canIPv6();
 }
 
 void osiSockAttachExt()
diff --git a/src/os/default/osdSockExt.cpp b/src/os/default/osdSockExt.cpp
index d2c1216..8a68435 100644
--- a/src/os/default/osdSockExt.cpp
+++ b/src/os/default/osdSockExt.cpp
@@ -37,7 +37,19 @@ namespace pvxs {
 DEFINE_LOGGER(log, "pvxs.util");
 DEFINE_LOGGER(logiface, "pvxs.iface");
 
-void osiSockAttachExt() {}
+static
+epicsThreadOnceId oseOnce = EPICS_THREAD_ONCE_INIT;
+
+static
+void oseDoOnce(void*)
+{
+    evsocket::canIPv6 = evsocket::init_canIPv6();
+}
+
+void osiSockAttachExt() {
+    osiSockAttach();
+    epicsThreadOnce(&oseOnce, &oseDoOnce, nullptr);
+}
 
 void evsocket::enable_SO_RXQ_OVFL() const
 {
diff --git a/src/server.cpp b/src/server.cpp
index 4352b8a..894ba57 100644
--- a/src/server.cpp
+++ b/src/server.cpp
@@ -375,8 +375,7 @@ std::ostream& operator<<(std::ostream& strm, const Server& serv)
 }
 
 Server::Pvt::Pvt(const Config &conf)
-    :canIPv6(evsocket::canIPv6())
-    ,effective(conf)
+    :effective(conf)
     ,beaconMsg(128)
     ,acceptor_loop("PVXTCP", epicsThreadPriorityCAServerLow-2)
     ,beaconSender4(AF_INET, SOCK_DGRAM, 0)
diff --git a/src/serverconn.cpp b/src/serverconn.cpp
index 6820d39..1de3180 100644
--- a/src/serverconn.cpp
+++ b/src/serverconn.cpp
@@ -387,7 +387,7 @@ ServIface::ServIface(const SockAddr &addr, server::Server::Pvt *server, bool fal
     auto orig_port = bind_addr.port();
 
 #ifdef __linux__
-    if(server->canIPv6 && bind_addr.family()==AF_INET && bind_addr.isAny()) {
+    if(evsocket::canIPv6 && bind_addr.family()==AF_INET && bind_addr.isAny()) {
         // Linux IP stack disallows binding both 0.0.0.0 and [::] for the same port.
         // so promote to IPv6 when possible
         bind_addr = SockAddr::any(AF_INET6, bind_addr.port());
diff --git a/src/serverconn.h b/src/serverconn.h
index 0895792..6f668ae 100644
--- a/src/serverconn.h
+++ b/src/serverconn.h
@@ -200,7 +200,6 @@ using namespace impl;
 struct Server::Pvt
 {
     SockAttach attach;
-    const bool canIPv6;
 
     std::weak_ptr<Server::Pvt> internal_self;
 
diff --git a/test/testget.cpp b/test/testget.cpp
index 8f43a8d..83af28e 100644
--- a/test/testget.cpp
+++ b/test/testget.cpp
@@ -502,7 +502,7 @@ MAIN(testget)
     testPlan(59);
     testSetup();
     logger_config_env();
-    bool canIPv6 = pvxs::impl::evsocket::canIPv6();
+    const bool canIPv6 = pvxs::impl::evsocket::canIPv6;
     Tester().testConnector();
     Tester().testWaiter();
     Tester(AF_INET).loopback();
diff --git a/test/testsock.cpp b/test/testsock.cpp
index 63e2e8e..28f2f9b 100644
--- a/test/testsock.cpp
+++ b/test/testsock.cpp
@@ -398,7 +398,7 @@ MAIN(testsock)
     testSetup();
     // check for behavior when binding ipv4 and ipv6 to the same socket
     // as a function of socket type and order.
-    if(evsocket::canIPv6()) {
+    if(evsocket::canIPv6) {
         // IPv4 and v6 loopback addresses are entirely distinct,
         // so no problem binding to both w/ or w/o IPV6_V6ONLY
         test_bind46("127.0.0.1" , "::1"       , SOCK_DGRAM  , 0);