broker: add FLUX_IPADDR_INTERFACE to select broker network interface

doc/man7/flux-environment.rst

@@ -229,6 +229,11 @@ affect the broker's PMI client.
    version 4 addresses.  Setting this variable to any value in the broker's
    environment causes it to prefer version 6 addresses.
 
+.. envvar:: FLUX_IPADDR_HOSTNAME
+
+   Force PMI bootstrap to assign the broker an address associated with a
+   particular network interface, like ``eth0``.
+
 
 CUSTOM OUTPUT FORMATS
 =====================

src/broker/boot_pmi.c

@@ -151,7 +151,7 @@ static int format_bind_uri (char *buf, int bufsz, attr_t *attrs, int rank)
 
         if (ipaddr_getprimary (ipaddr, sizeof (ipaddr),
                                error, sizeof (error)) < 0) {
-            log_err ("%s", error);
+            log_msg ("%s", error);
             return -1;
         }
         if (snprintf (buf, bufsz, "tcp://%s:*", ipaddr) >= bufsz)

src/common/libutil/ipaddr.c

@@ -100,16 +100,17 @@ static struct ifaddrs *find_ifaddr (struct ifaddrs *ifaddr,
     return ifa;
 }
 
-static int getprimary_ifaddr (char *buf, int len, int prefer_family,
-                              char *errstr, int errstrsz)
+static int getnamed_ifaddr (char *buf,
+                            int len,
+                            const char *name,
+                            int prefer_family,
+                            char *errstr,
+                            int errstrsz)
 {
     struct ifaddrs *ifaddr;
     struct ifaddrs *ifa;
-    char name[64];
     int error;
 
-    if (getprimary_iface4 (name, sizeof (name), errstr, errstrsz) < 0)
-        return -1;
     if (getifaddrs (&ifaddr) < 0) {
         esprintf (errstr, errstrsz, "getifaddrs: %s", strerror (errno));
         return -1;
@@ -141,6 +142,18 @@ static int getprimary_ifaddr (char *buf, int len, int prefer_family,
     return 0;
 }
 
+static int getprimary_ifaddr (char *buf,
+                              int len,
+                              int prefer_family,
+                              char *errstr,
+                              int errstrsz)
+{
+    char name[64];
+    if (getprimary_iface4 (name, sizeof (name), errstr, errstrsz) < 0)
+        return -1;
+    return getnamed_ifaddr (buf, len, name, prefer_family, errstr, errstrsz);
+}
+
 static struct addrinfo *find_addrinfo (struct addrinfo *addrinfo, int family)
 {
     struct addrinfo *ai;
@@ -205,12 +218,28 @@ int ipaddr_getprimary (char *buf, int len,
                        char *errstr, int errstrsz)
 {
     int prefer_family = getenv ("FLUX_IPADDR_V6") ? AF_INET6 : AF_INET;
+    const char *iface_name;
     int rc = -1;
 
-    if (getenv ("FLUX_IPADDR_HOSTNAME") == NULL)
-        rc = getprimary_ifaddr (buf, len, prefer_family, errstr, errstrsz);
-    if (rc < 0)
-        rc = getprimary_hostaddr (buf, len, prefer_family, errstr, errstrsz);
+    if ((iface_name = getenv ("FLUX_IPADDR_INTERFACE"))) {
+        rc = getnamed_ifaddr (buf,
+                              len,
+                              iface_name,
+                              prefer_family,
+                              errstr,
+                              errstrsz);
+    }
+    else {
+        if (getenv ("FLUX_IPADDR_HOSTNAME") == NULL)
+            rc = getprimary_ifaddr (buf, len, prefer_family, errstr, errstrsz);
+        if (rc < 0) {
+            rc = getprimary_hostaddr (buf,
+                                      len,
+                                      prefer_family,
+                                      errstr,
+                                      errstrsz);
+        }
+    }
     return rc;
 }
 

src/common/libutil/ipaddr.h

@@ -18,6 +18,7 @@
  * 1. Find the interface associated with the default route, then
  *    look up address of that interface.
  * 2. Look up address associated with the hostname
+ * 3. Look up address associated with a specific interface.
  *
  * Main use case: determine bind address for a PMI-bootstrapped flux broker.
  *
@@ -29,6 +30,8 @@
  * FLUX_IPADDR_HOSTNAME
  *   if set, only method 2 is tried above
  *   if unset, first method 1 is tried, then if that fails, method 2 is tried
+ * FLUX_IPADDR_INTERFACE
+ *   if set, only method 3 is tried above
  *
  * Return address as a string in buf (up to len bytes, always null terminated)
  * Return 0 on success, -1 on error with error message written to errstr

t/t0001-basic.t

@@ -429,6 +429,17 @@ test_expect_success 'tbon.endpoint uses tcp:// if tbon.prefertcp is set' '
 		flux getattr tbon.endpoint >endpoint2.out &&
 	grep "^tcp" endpoint2.out
 '
+test_expect_success 'FLUX_IPADDR_INTERFACE=lo works' '
+	FLUX_IPADDR_INTERFACE=lo flux start \
+		${ARGS} -s2 -o,-Stbon.prefertcp=1 \
+		flux getattr tbon.endpoint >endpoint3.out &&
+	grep "127.0.0.1" endpoint3.out
+'
+test_expect_success 'FLUX_IPADDR_INTERFACE=badiface fails' '
+	(export FLUX_IPADDR_INTERFACE=badiface; \
+		test_expect_code 137 flux start \
+		${ARGS} --test-exit-timeout=1s -s2 -o,-Stbon.prefertcp=1 true)
+'
 test_expect_success 'tbon.endpoint cannot be set' '
 	test_must_fail flux start ${ARGS} -s2 \
 		-o,--setattr=tbon.endpoint=ipc:///tmp/customflux /bin/true