openjdk 中的 networkaddress.cache.ttl null

Question

当我执行：

System.getProperty("networkaddress.cache.ttl");
Security.getProperty("networkaddress.cache.ttl");

结果为空。

我正在使用高山，openJdk8。我做了一些测试，发现我的资源 dns 正在发生变化，这是我想要的行为，解析 dns，而不是永远缓存。

我读到如果安装了 SecurityManager，默认值为：-1，这意味着“永远缓存 dns”

我没有安装 SecurityManager。

这种情况下的正确行为是什么？未安装 SecurityManager 且 networkaddress.cache.ttl 为空时？Dns 缓存是否会刷新？

Answer 1

这些设置实际上在配置文件中。

OpenJDK 8

使用 Docker 镜像openjdk:8，当没有安全管理器时，实现使用 30 秒。

/usr/local/openjdk-8/jre/lib/security/java.security ：

#
# The Java-level namelookup cache policy for successful lookups:
#
# any negative value: caching forever
# any positive value: the number of seconds to cache an address for
# zero: do not cache
#
# default value is forever (FOREVER). For security reasons, this
# caching is made forever when a security manager is set. When a security
# manager is not set, the default behavior in this implementation
# is to cache for 30 seconds.
#
# NOTE: setting this to anything other than the default value can have
#       serious security implications. Do not set it unless
#       you are sure you are not exposed to DNS spoofing attack.
#
#networkaddress.cache.ttl=-1

OpenJDK 11

使用Docker镜像openjdk:11，实现同上。

/usr/local/openjdk-11/conf/security/java.security ：

#
# The Java-level namelookup cache policy for successful lookups:
#
# any negative value: caching forever
# any positive value: the number of seconds to cache an address for
# zero: do not cache
#
# default value is forever (FOREVER). For security reasons, this
# caching is made forever when a security manager is set. When a security
# manager is not set, the default behavior in this implementation
# is to cache for 30 seconds.
#
# NOTE: setting this to anything other than the default value can have
#       serious security implications. Do not set it unless
#       you are sure you are not exposed to DNS spoofing attack.
#
#networkaddress.cache.ttl=-1

在其他一些版本中，它可以在 /etc 下，例如 /etc/java-11-openjdk/security/java.security

使用 AdoptOpenJDK 11，您可以看到当没有找到安全管理器时，TTL 设置为 30 秒。