diff --git a/documentation/RELEASE_NOTES.html b/documentation/RELEASE_NOTES.html
index d2d921b35..7aa50e250 100644
--- a/documentation/RELEASE_NOTES.html
+++ b/documentation/RELEASE_NOTES.html
@@ -15,6 +15,16 @@ EPICS Base 3.15.0.x releases are not intended for use in production systems.</p>
 <h2 align="center">Changes between 3.14.x and 3.15.0.x</h2>
 <!-- Insert new items immediately below here ... -->
 
+<h3>
+Added support for iocLogPrefix</h3>
+
+<p>
+Added a <code>iocLogPrefix</code> command to <code>iocsh</code>. This adds a
+prefix to all messages from this IOC (or other log client) as they get sent to the
+iocLogServer. This lets sites use the "fac=&lt;<i>facility</i>&gt;" syntax for
+displaying the facility, process name etc. in log viewers like the
+<code>cmlogviewer<code>.</p>
+
 <h3>
 Reworked the epicsEvent C &amp; C++ APIs</h3>
 
diff --git a/src/libCom/iocsh/libComRegister.c b/src/libCom/iocsh/libComRegister.c
index 6cd79bb66..f6ce30284 100644
--- a/src/libCom/iocsh/libComRegister.c
+++ b/src/libCom/iocsh/libComRegister.c
@@ -191,6 +191,15 @@ static void errlogCallFunc(const iocshArgBuf *args)
     errlogPrintfNoConsole("%s\n", args[0].sval);
 }
 
+/* iocLogPrefix */
+static const iocshArg iocLogPrefixArg0 = { "prefix",iocshArgString};
+static const iocshArg * const iocLogPrefixArgs[1] = {&iocLogPrefixArg0};
+static const iocshFuncDef iocLogPrefixFuncDef = {"iocLogPrefix",1,iocLogPrefixArgs};
+static void iocLogPrefixCallFunc(const iocshArgBuf *args)
+{
+    iocLogPrefix(args[0].sval);
+}
+
 /* epicsThreadShowAll */
 static const iocshArg epicsThreadShowAllArg0 = { "level",iocshArgInt};
 static const iocshArg * const epicsThreadShowAllArgs[1] = {&epicsThreadShowAllArg0};
@@ -355,6 +364,7 @@ void epicsShareAPI libComRegister(void)
     iocshRegister(&errlogInitFuncDef,errlogInitCallFunc);
     iocshRegister(&errlogInit2FuncDef,errlogInit2CallFunc);
     iocshRegister(&errlogFuncDef, errlogCallFunc);
+    iocshRegister(&iocLogPrefixFuncDef, iocLogPrefixCallFunc);
 
     iocshRegister(&epicsThreadShowAllFuncDef,epicsThreadShowAllCallFunc);
     iocshRegister(&threadFuncDef, threadCallFunc);
diff --git a/src/libCom/log/logClient.c b/src/libCom/log/logClient.c
index adaaa7f95..6aa223b5d 100644
--- a/src/libCom/log/logClient.c
+++ b/src/libCom/log/logClient.c
@@ -57,6 +57,11 @@ static const double      LOG_RESTART_DELAY = 5.0; /* sec */
 static const double      LOG_SERVER_CREATE_CONNECT_SYNC_TIMEOUT = 5.0; /* sec */
 static const double      LOG_SERVER_SHUTDOWN_TIMEOUT = 30.0; /* sec */
 
+/*
+ * If set using iocLogPrefix() this string is prepended to all log messages:
+ */
+static char* logClientPrefix = NULL;
+
 /*
  * logClientClose ()
  */
@@ -159,22 +164,13 @@ static void logClientDestroy (logClientId id)
     free ( pClient );
 }
 
-/* 
- * logClientSend ()
+/*
+ * This method requires the pClient->mutex be owned already.
  */
-void epicsShareAPI logClientSend ( logClientId id, const char * message )
-{
-    logClient * pClient = ( logClient * ) id;
+static void sendMessageChunk(logClient * pClient, const char * message) {
     unsigned strSize;
 
-    if ( ! pClient || ! message ) {
-        return;
-    }
-
     strSize = strlen ( message );
-
-    epicsMutexMustLock ( pClient->mutex );
-
     while ( strSize ) {
         unsigned msgBufBytesLeft = 
             sizeof ( pClient->msgBuf ) - pClient->nextMsgIndex;
@@ -231,10 +227,31 @@ void epicsShareAPI logClientSend ( logClientId id, const char * message )
             break;
         }
     }
-    
+}
+
+
+/* 
+ * logClientSend ()
+ */
+void epicsShareAPI logClientSend ( logClientId id, const char * message )
+{
+    logClient * pClient = ( logClient * ) id;
+
+    if ( ! pClient || ! message ) {
+        return;
+    }
+
+    epicsMutexMustLock ( pClient->mutex );
+
+    if (logClientPrefix) {
+        sendMessageChunk(pClient, logClientPrefix);
+    }
+    sendMessageChunk(pClient, message);
+
     epicsMutexUnlock (pClient->mutex);
 }
 
+
 void epicsShareAPI logClientFlush ( logClientId id )
 {
     logClient * pClient = ( logClient * ) id;
@@ -553,5 +570,36 @@ void epicsShareAPI logClientShow (logClientId id, unsigned level)
             pClient->sock==INVALID_SOCKET?"INVALID":"OK",
             pClient->connectCount);
     }
+
+    if (logClientPrefix) {
+        printf ("log client: prefix is \"%s\"\n", logClientPrefix);
+    }
 }
 
+/*
+ * iocLogPrefix()
+ */
+void epicsShareAPI iocLogPrefix(const char * prefix)
+{
+
+    /* If we have already established a log prefix, don't let the user change
+     * it.  The iocLogPrefix command is expected to be run from the IOC startup
+     * script during initialization; the prefix can't be changed once it has
+     * been set.
+     */
+
+    if (logClientPrefix) {
+        printf ("iocLogPrefix: The prefix was already set to \"%s\" "
+            "and can't be changed.\n", logClientPrefix);
+        return;
+    }
+
+    if (prefix) {
+        unsigned prefixLen = strlen(prefix);
+        if (prefixLen > 0) {
+            char * localCopy = malloc(prefixLen+1);
+            strcpy(localCopy, prefix);
+            logClientPrefix = localCopy;
+        }
+    }
+}
diff --git a/src/libCom/log/logClient.h b/src/libCom/log/logClient.h
index 8f1ec09d5..1797bbb20 100644
--- a/src/libCom/log/logClient.h
+++ b/src/libCom/log/logClient.h
@@ -33,6 +33,7 @@ epicsShareFunc logClientId epicsShareAPI logClientCreate (
 epicsShareFunc void epicsShareAPI logClientSend (logClientId id, const char *message);
 epicsShareFunc void epicsShareAPI logClientShow (logClientId id, unsigned level);
 epicsShareFunc void epicsShareAPI logClientFlush (logClientId id);
+epicsShareFunc void epicsShareAPI iocLogPrefix(const char* prefix);
 
 /* deprecated interface; retained for backward compatibility */
 /* note: implementations are in iocLog.c, not logClient.c */
diff --git a/src/libCom/test/epicsErrlogTest.c b/src/libCom/test/epicsErrlogTest.c
index 08389e438..e1729104e 100644
--- a/src/libCom/test/epicsErrlogTest.c
+++ b/src/libCom/test/epicsErrlogTest.c
@@ -22,6 +22,11 @@
 #include "errlog.h"
 #include "epicsUnitTest.h"
 #include "testMain.h"
+#include "iocLog.h"
+#include "logClient.h"
+#include "envDefs.h"
+#include "osiSock.h"
+#include "fdmgr.h"
 
 #define LOGBUFSIZE 2048
 
@@ -75,6 +80,28 @@ typedef struct {
     int jam;
 } clientPvt;
 
+static void testLogPrefix(void);
+static void acceptNewClient( void *pParam );
+static void readFromClient( void *pParam );
+static void testPrefixLogandCompare( const char* logmessage);
+
+static void *pfdctx;
+static SOCKET sock;
+static SOCKET insock;
+
+static const char* prefixactualmsg[]= {
+                     "A message without prefix",
+                     "A message with prefix",
+                     "DONE"
+                     };
+static const char *prefixstring = "fac=LI21 ";
+static const char prefixexpectedmsg[] = "A message without prefix"
+                     "fac=LI21 A message with prefix"
+                     "fac=LI21 DONE"
+                     ;
+static char prefixmsgbuffer[1024];
+
+
 static
 void logClient(void* raw, const char* msg)
 {
@@ -115,7 +142,7 @@ MAIN(epicsErrlogTest)
     char msg[256];
     clientPvt pvt, pvt2;
 
-    testPlan(25);
+    testPlan(29);
 
     strcpy(msg, truncmsg);
 
@@ -143,7 +170,7 @@ MAIN(epicsErrlogTest)
     pvt.expect = "Testing";
     pvt.checkLen = strlen(pvt.expect);
 
-    errlogPrintfNoConsole(pvt.expect);
+    errlogPrintfNoConsole("%s", pvt.expect);
     errlogFlush();
 
     testOk1(pvt.count == 1);
@@ -153,7 +180,7 @@ MAIN(epicsErrlogTest)
     pvt2.expect = pvt.expect = "Testing2";
     pvt2.checkLen = pvt.checkLen = strlen(pvt.expect);
 
-    errlogPrintfNoConsole(pvt.expect);
+    errlogPrintfNoConsole("%s", pvt.expect);
     errlogFlush();
 
     testOk1(pvt.count == 2);
@@ -165,7 +192,7 @@ MAIN(epicsErrlogTest)
     pvt2.expect = "Testing3";
     pvt2.checkLen = strlen(pvt2.expect);
 
-    errlogPrintfNoConsole(pvt2.expect);
+    errlogPrintfNoConsole("%s", pvt2.expect);
     errlogFlush();
 
     testOk1(pvt.count == 2);
@@ -227,7 +254,7 @@ MAIN(epicsErrlogTest)
         pvt.jam = 1;
 
         for (i = 0; i < N; i++) {
-            errlogPrintfNoConsole(msg);
+            errlogPrintfNoConsole("%s", msg);
         }
 
         epicsEventSignal(pvt.jammer);
@@ -258,7 +285,7 @@ MAIN(epicsErrlogTest)
     testDiag("Filling with %d messages of size %d", (int) N, (int) mlen);
 
     for (i = 0; i < N; i++) {
-        errlogPrintfNoConsole(msg);
+        errlogPrintfNoConsole("%s", msg);
     }
     epicsThreadSleep(0.1); /* should really be a second Event */
 
@@ -273,10 +300,10 @@ MAIN(epicsErrlogTest)
     testOk1(pvt.count == 2);
 
     /* The buffer has space for 1 more message: sizeof(msgNode) + 256 bytes */
-    errlogPrintfNoConsole(msg); /* Use up that space */
+    errlogPrintfNoConsole("%s", msg); /* Use up that space */
 
     testDiag("Overflow the buffer");
-    errlogPrintfNoConsole(msg);
+    errlogPrintfNoConsole("%s", msg);
 
     testOk1(pvt.count == 2);
 
@@ -289,5 +316,143 @@ MAIN(epicsErrlogTest)
     /* Clean up */
     errlogRemoveListener(&logClient);
 
+    testLogPrefix();
+
     return testDone();
 }
+/*
+ * Tests the log prefix code
+ * The prefix is only applied to log messages as they go out to the socket,
+ * so we need to create a server listening on a port, accept connections etc.
+ * This code is a reduced version of the code in iocLogServer.
+ */
+static void testLogPrefix(void) {
+    struct sockaddr_in serverAddr;
+    int status;
+    struct timeval timeout;
+    struct sockaddr_in actualServerAddr;
+    osiSocklen_t actualServerAddrSize;
+    char portstring[16];
+
+
+    testDiag("Testing iocLogPrefix");
+
+    timeout.tv_sec = 5; /* in seconds */
+    timeout.tv_usec = 0;
+
+    memset((void*)prefixmsgbuffer, 0, sizeof prefixmsgbuffer);
+
+    /* Clear "errlog: <n> messages were discarded" status */
+    errlogPrintfNoConsole(".");
+    errlogFlush();
+
+    sock = epicsSocketCreate(AF_INET, SOCK_STREAM, 0);
+    if (sock == INVALID_SOCKET) {
+        testAbort("epicsSocketCreate failed.");
+    }
+
+    /* We listen on a an available port. */
+    memset((void *)&serverAddr, 0, sizeof serverAddr);
+    serverAddr.sin_family = AF_INET;
+    serverAddr.sin_port = htons(0);
+
+    status = bind (sock, (struct sockaddr *)&serverAddr,
+                   sizeof (serverAddr) );
+    if (status < 0) {
+        testAbort("bind failed; all ports in use?");
+    }
+
+    status = listen(sock, 10);
+    if (status < 0) {
+        testAbort("listen failed!");
+    }
+
+    /* Determine the port that the OS chose */
+    actualServerAddrSize = sizeof actualServerAddr;
+    memset((void *)&actualServerAddr, 0, sizeof serverAddr);
+    status = getsockname(sock, (struct sockaddr *) &actualServerAddr,
+         &actualServerAddrSize);
+    if (status < 0) {
+        testAbort("Can't find port number!");
+    }
+
+    sprintf(portstring, "%d", ntohs(actualServerAddr.sin_port));
+    testDiag("Listening on port %s", portstring);
+
+    /* Set the EPICS environment variables for logging. */
+    epicsEnvSet ( "EPICS_IOC_LOG_INET", "localhost" );
+    epicsEnvSet ( "EPICS_IOC_LOG_PORT", portstring );
+
+    pfdctx = (void *) fdmgr_init();
+    if (status < 0) {
+        testAbort("fdmgr_init failed!");
+    }
+
+    status = fdmgr_add_callback(pfdctx, sock, fdi_read,
+        acceptNewClient, &serverAddr);
+
+    if (status < 0) {
+        testAbort("fdmgr_add_callback failed!");
+    }
+
+    testOk1(iocLogInit() == 0);
+    fdmgr_pend_event(pfdctx, &timeout);
+
+    testPrefixLogandCompare(prefixactualmsg[0]);
+
+    iocLogPrefix(prefixstring);
+    testPrefixLogandCompare(prefixactualmsg[1]);
+    testPrefixLogandCompare(prefixactualmsg[2]);
+    close(sock);
+}
+
+static void testPrefixLogandCompare( const char* logmessage ) {
+    struct timeval timeout;
+    timeout.tv_sec = 5; /* in seconds */
+    timeout.tv_usec = 0;
+
+    errlogPrintfNoConsole("%s", logmessage);
+    errlogFlush();
+    iocLogFlush();
+    fdmgr_pend_event(pfdctx, &timeout);
+}
+
+static void acceptNewClient ( void *pParam )
+{
+    osiSocklen_t addrSize;
+    struct sockaddr_in addr;
+    int status;
+
+    addrSize = sizeof ( addr );
+    insock = epicsSocketAccept ( sock, (struct sockaddr *)&addr, &addrSize );
+    testOk(insock != INVALID_SOCKET && addrSize >= sizeof (addr),
+        "Accepted new client");
+
+    status = fdmgr_add_callback(pfdctx, insock, fdi_read,
+        readFromClient,  NULL);
+
+    testOk(status >= 0, "Client read configured");
+}
+
+static void readFromClient(void *pParam)
+{
+    char recvbuf[1024];
+    int recvLength;
+
+    memset(&recvbuf, 0, 1024);
+    recvLength = recv(insock, &recvbuf, 1024, 0);
+    if (recvLength > 0) {
+        strcat(prefixmsgbuffer, recvbuf);
+
+        /* If we have received all of the messages. */
+        if (strstr(prefixmsgbuffer, "DONE") != NULL) {
+            size_t msglen = strlen(prefixexpectedmsg);
+            int prefixcmp = strncmp(prefixexpectedmsg, prefixmsgbuffer, msglen);
+
+            if (!testOk(prefixcmp == 0, "prefix matches")) {
+                testDiag("Expected '%s'\n", prefixexpectedmsg);
+                testDiag("Obtained '%s'\n", prefixmsgbuffer);
+            }
+        }
+    }
+}