【转】为什么只有偶数个源端口被指定为短暂的TCP源端口？

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#rhel #7.3 #even #number #port

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自RHEL 7.3起，系统为优化连接调用和绑定调用的效率，引入了源端口分配策略：偶数端口优先分配给connect()调用，而奇数端口则保留给bind()调用，以减少忙碌系统中寻找可用端口的时间。

https://access.redhat.com/solutions/3307511

Why only even number source ports are assigned as ephemeral TCP source port?

SOLUTION 已验证 - 已更新 2018年二月15日17:39 -

English

环境

Red Hat Enterprise Linux 7.3 and above

问题

RHEL host is not assigning random source ports for the new connection.
Source port randomization doesn't work.
RHEL 7.3 system is assigning even source ports for the connect call.

决议

As a part of enhancement, this is an accepted behaviour from RHEL7.3 and above systems as a result of commit happened in RHEL7.3 .
This commit segregates the allocation of ports from available local port range for connect() and bind() call and will try to favour even ports to connect() call leaving odd ports for bind() calls.

Raw

- bind() call associates the socket with its local address, whereas
- connect() is used to connect to a remote [server] address (source port/system for a new connection)

根源

Before RHEL7.3 there was no segregation of ports between bind() and connect() call, where in order to create new connection system used to scan through all available port range /proc/sys/net/ipv4/ip_local_port_range to find a random port and this whole process consumes a considerable amount of time on a busy system.
On a busy host, when the system makes a connect() call, in order to save a significant amount of time to scan for the available source port , the port range has been by-fragmented in 2 parts :
Even port range will favour the connect() call or will provide the priority to connect() call.
Odd port range will favour the bind() call or will provide the priority to bind() call.