预留端口避免占用ip_local_reserved_ports

问题描述：
业务遇到这个情况，在重启服务时，出现1986端口被占用而无法启动，非得等该端口释放后才启动成功。

问题分析：
1986端口被该服务器上的客户端随机选取源端口给占用掉了。文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

解决方案：
使用net.ipv4.ip_local_port_range参数，规划出一段端口段预留作为服务的端口，这种方法是可以解决当前问题，但是会有个问题，端口使用量减少了，当服务器需要消耗大量的端口号的话，比如反代服务器，就存在瓶颈了。
最好的做法是将服务监听的端口以逗号分隔全部添加到ip_local_reserved_ports中，TCP/IP协议栈从ip_local_port_range中随机选取源端口时，会排除ip_local_reserved_ports中定义的端口，因此就不会出现端口被占用了服务无法启动。文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

ip_local_reserved_ports解释如下：
ip_local_reserved_ports - list of comma separated ranges
Specify the ports which are reserved for known third-party
applications. These ports will not be used by automatic port
assignments (e.g. when calling connect() or bind() with port
number 0). Explicit port allocation behavior is unchanged.文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

The format used for both input and output is a comma separated
list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
10). Writing to the file will clear all previously reserved
ports and update the current list with the one given in the
input.文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

Note that ip_local_port_range and ip_local_reserved_ports
settings are independent and both are considered by the kernel
when determining which ports are available for automatic port
assignments.文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

You can reserve ports which are not in the current
ip_local_port_range, e.g.:文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

$ cat /proc/sys/net/ipv4/ip_local_port_range
32000 61000
$ cat /proc/sys/net/ipv4/ip_local_reserved_ports
8080,9148文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

although this is redundant. However such a setting is useful
if later the port range is changed to a value that will
include the reserved ports.文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

Default: Empty
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

# vim /etc/sysctl.conf
net.ipv4.ip_local_reserved_ports = 1986, 11211-11220
# sysctl -p

[warning]注意：内核版本要大于2.6.18-164，否则不支持该参数。[/warning]文章源自运维生存时间-https://www.ttlsa.com/linux/reserved-port-to-avoid-occupying-ip_local_reserved_ports/

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