diff --git a/keyring.c b/keyring.c
index cdd64f5d..a116fe73 100644
--- a/keyring.c
+++ b/keyring.c
@@ -1502,10 +1502,11 @@ static keyring_identity *keyring_new_identity()
 
 keyring_identity *keyring_inmemory_identity(){
   keyring_identity *id = keyring_new_identity();
-  keyring_finalise_identity(NULL, id);
-  if (id)
+  if (id) {
+    keyring_finalise_identity(NULL, id);
     add_subscriber(id);
-  INFOF("created in-memory identity SID=%s", alloca_tohex_sid_t(id->subscriber->sid));
+    INFOF("created in-memory identity SID=%s", alloca_tohex_sid_t(id->subscriber->sid));
+  }
   return id;
 }
 
diff --git a/overlay_address.c b/overlay_address.c
index abfc638b..908fa728 100644
--- a/overlay_address.c
+++ b/overlay_address.c
@@ -1,5 +1,6 @@
 /*
 Serval DNA MDP addressing
+Copyright (C) 2016-2018 Flinders University
 Copyright (C) 2012-2015 Serval Project Inc.
 Copyright (C) 2012 Paul Gardner-Stephen
  
@@ -28,6 +29,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
 #include <assert.h>
 #include <arpa/inet.h>
+#include "lang.h" // for bool_t
 #include "serval.h"
 #include "conf.h"
 #include "keyring.h"
@@ -54,37 +56,64 @@ static struct broadcast bpilist[MAX_BPIS];
 
 static __thread struct tree_root root={.index_size_bytes=SID_SIZE};
 
-static __thread struct subscriber *my_subscriber=NULL;
+static __thread bool_t primary_sid_written = 0;
+static __thread struct subscriber *primary_sid = NULL;
+static __thread struct subscriber *my_subscriber = NULL;
 
-struct subscriber *get_my_subscriber(bool_t create){
+struct subscriber *get_my_subscriber(bool_t create)
+{
   if (!serverMode)
     return NULL;
-  assert(keyring != NULL);
   if (my_subscriber && my_subscriber->reachable != REACHABLE_SELF)
     my_subscriber = NULL;
-  if (!my_subscriber){
-    keyring_identity *id = keyring->identities;
-    while(id){
-      if (id->subscriber->reachable == REACHABLE_SELF){
+  // Look for a reachable self-identity in the keyring.
+  if (!my_subscriber) {
+    assert(keyring != NULL);
+    keyring_identity *id;
+    for (id = keyring->identities; id; id = id->next)
+      if (id->subscriber->reachable == REACHABLE_SELF) {
 	my_subscriber = id->subscriber;
-	return my_subscriber;
+	break;
       }
-      id = id->next;
-    }
-    // If there is no reachable self-identity in the keyring, then roll one in-memory, which will
-    // persist until the server terminates.
-    if (create){
-      id = keyring_inmemory_identity();
+  }
+  // If there is no reachable self-identity in the keyring, then roll one in-memory, which will
+  // persist until the server terminates.
+  if (!my_subscriber && create) {
+    keyring_identity *id = keyring_inmemory_identity();
+    if (id)
       my_subscriber = id->subscriber;
+  }
+  // Normally, the server creates files in proc/ before it creates its pidfile, and does not modify
+  // those files while running.  This avoids any race conditions with other processes that read the
+  // files.  In this case, the proc/primary_sid file is written _after_ the pidfile, and could
+  // potentially be re-written while running (eg, if the primary identity is deleted, causing the
+  // server to choose another).  Potential race conditions are avoided because (1) the size of the
+  // file never alters, and (2) the server_write_proc_state() function overwrites any existing
+  // content using a single, indivisible write(2) system call.
+  if (!primary_sid_written || primary_sid != my_subscriber) {
+    primary_sid = my_subscriber;
+    if (my_subscriber) {
+      const char *sidhex = alloca_tohex_sid_t(primary_sid->sid);
+      server_write_proc_state("primary_sid", "%s", sidhex);
+      INFOF("PRIMARY IDENTITY sid=%s", sidhex);
     }
+    else {
+      server_unlink_proc_state("primary_sid");
+      INFOF("NO PRIMARY IDENTITY");
+    }
+    primary_sid_written = 1;
   }
   return my_subscriber;
 }
 
-void release_my_subscriber(){
+void release_my_subscriber()
+{
   if (my_subscriber && my_subscriber->identity->slot==0)
     keyring_free_identity(my_subscriber->identity);
+  server_unlink_proc_state("primary_sid");
   my_subscriber = NULL;
+  primary_sid = NULL;
+  primary_sid_written = 0;
 }
 
 static int free_node(void **record, void *UNUSED(context))
@@ -98,6 +127,8 @@ static int free_node(void **record, void *UNUSED(context))
       alloca_tohex_sid_t(subscriber->sid), subscriber->sync_state);
   if (subscriber->identity)
     FATAL("Can't free a subscriber that is unlocked in the keyring");
+  if (subscriber == my_subscriber)
+    FATAL("Can't free a subscriber that is the primary identity");
   free(subscriber);
   *record=NULL;
   return 0;
diff --git a/server.c b/server.c
index 9311f9eb..b0d9e49b 100644
--- a/server.c
+++ b/server.c
@@ -77,7 +77,6 @@ static void serverCleanUp();
 static const char *_server_pidfile_path(struct __sourceloc __whence);
 #define server_pidfile_path() (_server_pidfile_path(__WHENCE__))
 
-static int server_write_proc_state(const char *path, const char *fmt, ...);
 static int server_get_proc_state(const char *path, char *buff, size_t buff_len);
 static void server_stop_alarms();
 
@@ -343,6 +342,9 @@ static int server_write_pid()
   server_write_proc_state("http_port", "%d", httpd_server_port);
   server_write_proc_state("mdp_inet_port", "%d", mdp_loopback_port);
 
+  // Create or unlink the "primary_sid" proc state file.
+  get_my_subscriber(0);
+
   // Create a locked pidfile to advertise that the server is now running.
   const char *pidfile_path = server_pidfile_path();
   if (pidfile_path == NULL)
@@ -452,29 +454,46 @@ static int get_proc_path(const char *path, char *buf, size_t bufsiz)
   return 0;
 }
 
-static int server_write_proc_state(const char *path, const char *fmt, ...)
+int server_write_proc_state(const char *path, const char *fmt, ...)
 {
   char path_buf[400];
   if (get_proc_path(path, path_buf, sizeof path_buf)==-1)
     return -1;
     
+  // Create the directory that contains the path, if it does not already exist.
   size_t dirsiz = strlen(path_buf) + 1;
   char dir_buf[dirsiz];
   strcpy(dir_buf, path_buf);
   const char *dir = dirname(dir_buf); // modifies dir_buf[]
-  if (mkdirs_info(dir, 0700) == -1)
-    return WHY_perror("mkdirs()");
+  if (emkdirs_info(dir, 0700) == -1)
+    return -1;
+
+  // Format the file's new content in a local buffer on the stack.
+  strbuf sb;
+  STRBUF_ALLOCA_FIT(sb, 1024, strbuf_va_printf(sb, fmt));
   
-  FILE *f = fopen(path_buf, "w");
-  if (!f)
-    return WHY_perror("fopen()");
-  
-  va_list ap;
-  va_start(ap, fmt);
-  vfprintf(f, fmt, ap);
-  va_end(ap);
-  
-  fclose(f);
+  // Overwrite the file, creating it if necessary, using a single write(2) system call, followed by
+  // a ftruncate(2) system call (in case the file already existed and was longer than its new
+  // content).  This allows potential race conditions to be avoided for files that are overwritten
+  // while the server runs, as long as the written contents are always the same size, or always
+  // contain a terminating sequence (eg, newline).
+  int fd = open(path_buf, O_CREAT | O_WRONLY, 0700);
+  if (fd == -1)
+    return WHYF_perror("open(%s, O_CREAT|O_WRONLY, 0700)", alloca_str_toprint(path_buf));
+  int ret = write_all(fd, strbuf_str(sb), strbuf_len(sb));
+  if (ret != -1 && (ret = ftruncate(fd, strbuf_len(sb)) == -1))
+    ret = WHYF_perror("ftruncate(%s, %zu)", alloca_str_toprint(path_buf), strbuf_len(sb));
+  close(fd);
+  return ret;
+}
+
+int server_unlink_proc_state(const char *path)
+{
+  char path_buf[400];
+  if (get_proc_path(path, path_buf, sizeof path_buf)==-1)
+    return -1;
+  if (unlink(path) == -1 && errno != ENOENT)
+    return WHYF_perror("unlink(%s)", alloca_str_toprint(path));
   return 0;
 }
 
@@ -483,16 +502,20 @@ static int server_get_proc_state(const char *path, char *buff, size_t buff_len)
   char path_buf[400];
   if (get_proc_path(path, path_buf, sizeof path_buf)==-1)
     return -1;
-  
   FILE *f = fopen(path_buf, "r");
-  if (!f)
-    return -1;
-  
-  int ret=0;
-  
-  if (!fgets(buff, buff_len, f))
-    ret = WHY_perror("fgets");
-  
+  if (!f) {
+    if (errno != ENOENT)
+      return WHYF_perror("fopen(%s)", alloca_str_toprint(path_buf));
+    return 1;
+  }
+  int ret = 0;
+  errno = 0; // fgets() does not set errno on end-of-file
+  if (!fgets(buff, buff_len, f)) {
+    if (errno)
+      ret = WHYF_perror("fgets from %s", alloca_str_toprint(path_buf));
+    else
+      ret = 1;
+  }
   fclose(f);
   return ret;
 }
@@ -788,11 +811,15 @@ static void cli_server_details(struct cli_context *context, const struct pid_tid
       cli_put_long(context, id->tid, "\n");
     }
     char buff[256];
-    if (server_get_proc_state("http_port", buff, sizeof buff)!=-1){
+    if (server_get_proc_state("primary_sid", buff, sizeof buff) == 0){
+      cli_field_name(context, "primary_sid", ":");
+      cli_put_string(context, buff, "\n");
+    }
+    if (server_get_proc_state("http_port", buff, sizeof buff) == 0){
       cli_field_name(context, "http_port", ":");
       cli_put_string(context, buff, "\n");
     }
-    if (server_get_proc_state("mdp_inet_port", buff, sizeof buff)!=-1){
+    if (server_get_proc_state("mdp_inet_port", buff, sizeof buff) == 0){
       cli_field_name(context, "mdp_inet_port", ":");
       cli_put_string(context, buff, "\n");
     }
diff --git a/server.h b/server.h
index ea5bfa87..9cb085c1 100644
--- a/server.h
+++ b/server.h
@@ -49,6 +49,9 @@ void server_close();
 
 void server_loop(time_ms_t (*waiting)(time_ms_t, time_ms_t, time_ms_t), void (*wokeup)());
 
+int server_write_proc_state(const char *path, const char *fmt, ...);
+int server_unlink_proc_state(const char *path);
+
 void server_rhizome_add_bundle(uint64_t rowid);
 
 DECLARE_TRIGGER(shutdown);
diff --git a/testdefs.sh b/testdefs.sh
index 4d8d9bde..55223e2b 100644
--- a/testdefs.sh
+++ b/testdefs.sh
@@ -421,6 +421,22 @@ servald_start() {
    SERVALD_SERVER_CHDIR="$instance_dir" SERVALD_LOG_FILE="$instance_servald_log" $servald start "$@"
 }
 
+# Utility function:
+#  - fetch the daemon's primary SID
+get_servald_primary_sid() {
+   local _instance="$2"
+   [ -z "$_instance" ] || push_and_set_instance $_instance || return $?
+   local _var="$1"
+   local _sid=$(<"$SERVALINSTANCE_PATH/proc/primary_sid")
+   assert --message="instance $instance_name primary SID is known" [ -n "$_sid" ]
+   if [ -n "$_var" ]; then
+      eval "$_var=\$_sid"
+      tfw_log "$_var=$_sid"
+   fi
+   [ -z "$_instance" ] || pop_instance
+   return 0
+}
+
 # Utility function:
 #  - test whether the daemon's HTTP server has started
 servald_http_server_started() {