Plain vanilla is an adjective describing the simplest version of something, without any optional extras, by analogy with vanillaice cream, the default flavour.

1. Name servers host a domain’s DNS information in a text file called the zone file. They’re are also known as **Servers of
   Authority (SOAs)**
2. On Linux (and probably most Unix), there is no OS-level DNS caching unless nscd is installed and running. Even then, the DNS caching feature of nscd is disabled by default at least in Debian because it's broken. The practical upshot is that your linux system very very probably does not do any OS-level DNS caching.
3. An AAAA record is just like an A record, but for IPv6 IP addresses. A typical AAAA record looks like the following:

参考 https://www.linode.com/docs/networking/dns/dns-records-an-introduction

Memory Analyzer can currently work with three dump types:

1. IBM Portable Heap Dump (PHD): This proprietary IBM format contains
   only the type and size of each Java object in the process, and the
   relationships among the objects. This dump-file format is
   significantly smaller than the other formats and contains the least
   information. The data is usually sufficient, though, for diagnosing
   memory leaks and getting a basic understanding of the application's
   architecture and footprint.
2. HPROF binary dump: The HPROF binary format contains all the data
   present in the IBM PHD format as well as the primitive data held
   inside the Java objects, and the thread details. You can look at the
   values held in fields inside the objects and see which methods were
   being executed at the time the dump was taken. The additional
   primitive data makes HPROF dumps significantly larger than
   PHD-format dumps; they are approximately the same size as the used
   Java heap.
3. IBM system dumps: When the IBM Java runtime is being used, the
   native operating-system dump file — a core file on AIX® or Linux, a
   minidump on Windows®, or a SVC dump on z/OS®— can be loaded into
   Memory Analyzer. These dumps contain the entire memory image of the
   running application — all the information and data in the HPROF
   format, as well as all of the native-memory and thread information.
   This is the largest and most comprehensive dump-file format.

Both IBM dump types are available only with the Diagnostic Tool Framework for Java (DTFJ) plug-in installed

节选自: https://www.ibm.com/developerworks/library/j-memoryanalyzer/#table1

nc (netcat) - arbitrary TCP and UDP connections and listens
-words [-46DdhklnrStUuvz] [-i interval] [-p source_port] [-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_protocol] [Xo -x proxy_address[: port]] [hostname] [port[s]]
nc -wz baidu.com

mtr - a network diagnostic tool

 mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.

mtr -rw example.com

iostat reports CPU, disk I/O, and NFS statistics.
vmstat reports virtual memory statistics.
mpstat reports processors statictics.

vmstat: Virtual Memory statistic. 实时收集并报告主内存,交换分区(SWAP)以及 CPU 的使用情况. 用来查找和定位内存的性能问题.

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0  90175    239   1434    0    0     0     1    3    2  0  0 100  0  0
 0  0      0  90175    239   1434    0    0     0     0  153  176  0  0 100  0  0
 1  0      0  90151    239   1434    0    0     0     4 3068 1961  1  2 97  0  0

r: process running
b: process blocked
swpd: swapped to swap
free: memory free
buff: memory committed
cache: declared or can be swapped to swap
si: swap in
so: swap out
bi: block in (read disk unit is block, memory unit is page)
bo: block out
in: interrupt
cs: context switch
us: user, sd: system/kernel, id: idle, ws: waste

https://www.linode.com/docs/uptime/monitoring/use-vmstat-to-monitor-system-performance

占位
http://www.thegeekstuff.com/2011/07/iostat-vmstat-mpstat-examples/