{"id":4734,"date":"2017-05-16T08:55:24","date_gmt":"2017-05-16T13:55:24","guid":{"rendered":"http:\/\/appcrawler.com\/wordpress\/?p=4734"},"modified":"2017-05-16T08:55:24","modified_gmt":"2017-05-16T13:55:24","slug":"dns-server-address-cached-in-java","status":"publish","type":"post","link":"http:\/\/appcrawler.com\/wordpress\/2017\/05\/16\/dns-server-address-cached-in-java\/","title":{"rendered":"DNS server address cached in java?"},"content":{"rendered":"

We found that an old DNS address was used until we restarted the JVM. I need to review the class used to perform this activity.<\/a>.<\/p>\n

Test this with a simple class.<\/p>\n

\r\nimport java.net.*;\r\n\r\npublic class test {\r\n  static {\r\n    java.security.Security.setProperty (\"networkaddress.cache.ttl\" , \"10\");\r\n  }\r\n  public static void main (String args[]) throws Exception {\r\n    \/\/System.setProperty(\"sun.net.spi.nameservice.nameservers\", \"192.168.56.50\");\r\n    \/\/System.setProperty(\"sun.net.spi.nameservice.provider.1\", \"dns,sun\");\r\n    System.out.println(\"DEFAULT DNS TTL: \"+sun.net.InetAddressCachePolicy.get());\r\n\r\n    while (true) {\r\n      try {\r\n        System.out.println(InetAddress.getByName(args[0]).getHostName());\r\n      }\r\n      catch (Exception e) {\r\n        e.printStackTrace();\r\n      }\r\n      Thread.sleep(10000);\r\n    }\r\n  }\r\n}\r\n<\/pre>\n
This is done mainly for security.  If someone hacks the name resolution configuration, your application can’t be erroneously pointed to malicious hostnames.  Of course, this is only relevant if you check the name resolution configuration before a restart.<\/p>\n
It should be noted that as shown below, setting the cache timeout on the JVM also won’t impact the DNS servers used for resolution of names to IP addresses.  While this test ran, we changed the name server in \/etc\/resolv.conf from 192.168.56.50 to something else, which had no impact on our running class.  Once started, the DNS server used by the JVM remains the same…<\/p>\n
\r\n[root@hdp ~]# strace -f -e trace=connect java test hdp\r\nProcess 18123 attached\r\n[pid 18123] connect(3, {sa_family=AF_FILE, path=\"\/var\/run\/nscd\/socket\"}, 110) = -1 ENOENT (No such file or directory)\r\n[pid 18123] connect(3, {sa_family=AF_FILE, path=\"\/var\/run\/nscd\/socket\"}, 110) = -1 ENOENT (No such file or directory)\r\nProcess 18124 attached\r\nProcess 18125 attached\r\nProcess 18126 attached\r\nProcess 18129 attached\r\nProcess 18130 attached\r\nProcess 18131 attached\r\nProcess 18132 attached\r\nProcess 18133 attached\r\nDEFAULT DNS TTL: 10\r\n[pid 18123] connect(4, {sa_family=AF_FILE, path=\"\/var\/run\/nscd\/socket\"}, 110) = -1 ENOENT (No such file or directory)\r\n[pid 18123] connect(4, {sa_family=AF_FILE, path=\"\/var\/run\/nscd\/socket\"}, 110) = -1 ENOENT (No such file or directory)\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\nhdp\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\nhdp\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\nhdp\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\nhdp\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\nhdp\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n[pid 18123] connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(\"192.168.56.50\")}, 16) = 0\r\n<\/pre>\n
Regardless, beware of this unlikely issue.<\/p>\n","protected":false},"excerpt":{"rendered":"
We found that an old DNS address was used until we restarted the JVM. I need to review the class used to perform this activity.. Test this with a simple class. import java.net.*; public class test { static { java.security.Security.setProperty…<\/p>\n
Read more →<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_mi_skip_tracking":false,"footnotes":""},"categories":[24,25,56],"tags":[],"_links":{"self":[{"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/posts\/4734"}],"collection":[{"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/comments?post=4734"}],"version-history":[{"count":12,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/posts\/4734\/revisions"}],"predecessor-version":[{"id":4746,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/posts\/4734\/revisions\/4746"}],"wp:attachment":[{"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/media?parent=4734"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/categories?post=4734"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/appcrawler.com\/wordpress\/wp-json\/wp\/v2\/tags?post=4734"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}