在 redis-3.2.1 版本中，注释掉 bind，启动发现日志报 Creating Server TCP listening socket *:6379: unable to bind socket，但在 redis-5.0.0 中，同样的配置却能启动起来，本着刨根问底的精神，下载了两个版本的源码，对比下发现了原因。
先看 redis-3.2.1
C
/* Initialize a set of file descriptors to listen to the specified 'port'
* binding the addresses specified in the Redis server configuration.
*
* The listening file descriptors are stored in the integer array 'fds'
* and their number is set in '*count'.
*
* The addresses to bind are specified in the global server.bindaddr array
* and their number is server.bindaddr_count. If the server configuration
* contains no specific addresses to bind, this function will try to
* bind * (all addresses) for both the IPv4 and IPv6 protocols.
*
* On success the function returns C_OK.
*
* On error the function returns C_ERR. For the function to be on
* error, at least one of the server.bindaddr addresses was
* impossible to bind, or no bind addresses were specified in the server
* configuration but the function is not able to bind * for at least
* one of the IPv4 or IPv6 protocols. */
int listenToPort(int port, int *fds, int *count) {
int j;
/* Force binding of 0.0.0.0 if no bind address is specified, always
* entering the loop if j == 0. */
if (server.bindaddr_count == 0) server.bindaddr[0] = NULL;
for (j = 0; j < server.bindaddr_count || j == 0; j++) {
if (server.bindaddr[j] == NULL) {
/* Bind * for both IPv6 and IPv4, we enter here only if
* server.bindaddr_count == 0. */
fds[*count] = anetTcp6Server(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
/* Bind the IPv4 address as well. */
fds[*count] = anetTcpServer(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
}
}
/* Exit the loop if we were able to bind * on IPv4 and IPv6,
* otherwise fds[*count] will be ANET_ERR and we'll print an
* error and return to the caller with an error. */
if (*count == 2) break;
} else if (strchr(server.bindaddr[j],':')) {
/* Bind IPv6 address. */
fds[*count] = anetTcp6Server(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
} else {
/* Bind IPv4 address. */
fds[*count] = anetTcpServer(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
}
if (fds[*count] == ANET_ERR) {
serverLog(LL_WARNING,
"Creating Server TCP listening socket %s:%d: %s",
server.bindaddr[j] ? server.bindaddr[j] : "*",
port, server.neterr);
return C_ERR;
}
anetNonBlock(NULL,fds[*count]);
(*count)++;
}
return C_OK;
}由于 bind 都被注释了，因此 if (server.bindaddr_count == 0) server.bindaddr[0] = NULL; 被执行了。
同时呢，代码注释中也写的很清楚，如果没有打开 bind，那么默认会连到 0.0.0.0，那么接下来走到 for 循环的 if 判断中，仔细看 fds[*count] = anetTcp6Server(server.neterr,port,NULL, server.tcp_backlog); 这段代码，会去连接 ipv6,如果本机没有开启 ipv6，那么便不会连接 ipv4，而是走到 if (fds[*count] == ANET_ERR) { 中打印日志便退出。
再来看看 redis-5.0.0
c
/* Initialize a set of file descriptors to listen to the specified 'port'
* binding the addresses specified in the Redis server configuration.
*
* The listening file descriptors are stored in the integer array 'fds'
* and their number is set in '*count'.
*
* The addresses to bind are specified in the global server.bindaddr array
* and their number is server.bindaddr_count. If the server configuration
* contains no specific addresses to bind, this function will try to
* bind * (all addresses) for both the IPv4 and IPv6 protocols.
*
* On success the function returns C_OK.
*
* On error the function returns C_ERR. For the function to be on
* error, at least one of the server.bindaddr addresses was
* impossible to bind, or no bind addresses were specified in the server
* configuration but the function is not able to bind * for at least
* one of the IPv4 or IPv6 protocols. */
int listenToPort(int port, int *fds, int *count) {
int j;
/* Force binding of 0.0.0.0 if no bind address is specified, always
* entering the loop if j == 0. */
if (server.bindaddr_count == 0) server.bindaddr[0] = NULL;
for (j = 0; j < server.bindaddr_count || j == 0; j++) {
if (server.bindaddr[j] == NULL) {
int unsupported = 0;
/* Bind * for both IPv6 and IPv4, we enter here only if
* server.bindaddr_count == 0. */
fds[*count] = anetTcp6Server(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
} else if (errno == EAFNOSUPPORT) {
unsupported++;
serverLog(LL_WARNING,"Not listening to IPv6: unsupproted");
}
if (*count == 1 || unsupported) {
/* Bind the IPv4 address as well. */
fds[*count] = anetTcpServer(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
} else if (errno == EAFNOSUPPORT) {
unsupported++;
serverLog(LL_WARNING,"Not listening to IPv4: unsupproted");
}
}
/* Exit the loop if we were able to bind * on IPv4 and IPv6,
* otherwise fds[*count] will be ANET_ERR and we'll print an
* error and return to the caller with an error. */
if (*count + unsupported == 2) break;
} else if (strchr(server.bindaddr[j],':')) {
/* Bind IPv6 address. */
fds[*count] = anetTcp6Server(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
} else {
/* Bind IPv4 address. */
fds[*count] = anetTcpServer(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
}
if (fds[*count] == ANET_ERR) {
serverLog(LL_WARNING,
"Creating Server TCP listening socket %s:%d: %s",
server.bindaddr[j] ? server.bindaddr[j] : "*",
port, server.neterr);
return C_ERR;
}
anetNonBlock(NULL,fds[*count]);
(*count)++;
}
return C_OK;
}看了下代码才发现，更改了连接的判断，先尝试连接 ipv6，再尝试连接 ipv4，这样即使本机没有开启 ipv6 也能成功。
总结下：
1、redis 3 那个版本，如果 bind 没开启，先绑定 ipv6，ipv6 绑定失败就报错了（开发机不支持或关闭了），绑定成功了，再绑定 ipv4；
2、redis 5 ,如果 bind 没开启，则先尝试绑定 ipv6，ipv6 绑定失败就打印个日志，再去尝试绑定 ipv4，成功了就继续往下走。