Add hostname support in Sentinel. (redis#8282)

This is both a bugfix and an enhancement.

Internally, Sentinel relies entirely on IP addresses to identify
instances. When configured with a new master, it also requires users to
specify and IP and not hostname.

However, replicas may use the replica-announce-ip configuration to
announce a hostname. When that happens, Sentinel fails to match the
announced hostname with the expected IP and considers that a different
instance, triggering reconfiguration, etc.

Another use case is where TLS is used and clients are expected to match
the hostname to connect to with the certificate's SAN attribute. To
properly implement this configuration, it is necessary for Sentinel to
redirect clients to a hostname rather than an IP address.

The new 'resolve-hostnames' configuration parameter determines if
Sentinel is willing to accept hostnames. It is set by default to no,
which maintains backwards compatibility and avoids unexpected DNS
resolution delays on systems with DNS configuration issues.

Internally, Sentinel continues to identify instances by their resolved
IP address and will also report the IP by default. The new
'announce-hostnames' parameter determines if Sentinel should prefer to
announce a hostname, when available, rather than an IP address. This
applies to addresses returned to clients, as well as their
representation in the configuration file, REPLICAOF configuration
commands, etc.

This commit also introduces SENTINEL CONFIG GET and SENTINEL CONFIG SET
which can be used to introspect or configure global Sentinel
configuration that was previously was only possible by directly
accessing the configuration file and possibly restarting the instance.

Co-authored-by: myl1024 <[email protected]>
Co-authored-by: sundb <[email protected]>

sentinel.conf

@@ -321,3 +321,21 @@ sentinel deny-scripts-reconfig yes
 # is possible to just rename a command to itself:
 #
 # SENTINEL rename-command mymaster CONFIG CONFIG
+
+# HOSTNAMES SUPPORT
+#
+# Normally Sentinel uses only IP addresses and requires SENTINEL MONITOR
+# to specify an IP address. Also, it requires the Redis replica-announce-ip
+# keyword to specify only IP addresses.
+#
+# You may enable hostnames support by enabling resolve-hostnames. Note
+# that you must make sure your DNS is configured properly and that DNS
+# resolution does not introduce very long delays.
+#
+SENTINEL resolve-hostnames no
+
+# When resolve-hostnames is enabled, Sentinel still uses IP addresses
+# when exposing instances to users, configuration files, etc. If you want
+# to retain the hostnames when announced, enable announce-hostnames below.
+#
+SENTINEL announce-hostnames no

src/anet.c

@@ -235,14 +235,13 @@ int anetRecvTimeout(char *err, int fd, long long ms) {
     return ANET_OK;
 }
 
-/* anetGenericResolve() is called by anetResolve() and anetResolveIP() to
- * do the actual work. It resolves the hostname "host" and set the string
- * representation of the IP address into the buffer pointed by "ipbuf".
+/* Resolve the hostname "host" and set the string representation of the
+ * IP address into the buffer pointed by "ipbuf".
  *
  * If flags is set to ANET_IP_ONLY the function only resolves hostnames
  * that are actually already IPv4 or IPv6 addresses. This turns the function
  * into a validating / normalizing function. */
-int anetGenericResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len,
+int anetResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len,
                        int flags)
 {
     struct addrinfo hints, *info;
@@ -269,14 +268,6 @@ int anetGenericResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len,
     return ANET_OK;
 }
 
-int anetResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len) {
-    return anetGenericResolve(err,host,ipbuf,ipbuf_len,ANET_NONE);
-}
-
-int anetResolveIP(char *err, char *host, char *ipbuf, size_t ipbuf_len) {
-    return anetGenericResolve(err,host,ipbuf,ipbuf_len,ANET_IP_ONLY);
-}
-
 static int anetSetReuseAddr(char *err, int fd) {
     int yes = 1;
     /* Make sure connection-intensive things like the redis benchmark

src/anet.h

@@ -60,8 +60,7 @@ int anetTcpNonBlockBestEffortBindConnect(char *err, const char *addr, int port,
 int anetUnixConnect(char *err, const char *path);
 int anetUnixNonBlockConnect(char *err, const char *path);
 int anetRead(int fd, char *buf, int count);
-int anetResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len);
-int anetResolveIP(char *err, char *host, char *ipbuf, size_t ipbuf_len);
+int anetResolve(char *err, char *host, char *ipbuf, size_t ipbuf_len, int flags);
 int anetTcpServer(char *err, int port, char *bindaddr, int backlog);
 int anetTcp6Server(char *err, int port, char *bindaddr, int backlog);
 int anetUnixServer(char *err, char *path, mode_t perm, int backlog);