問題現象：
使用 docker pull centos拉取鏡像創建容器後，無法在容器內使用netstat 命令
原因分析：
pull下來的centos系統中並未包含netstat命令，需要手動安裝。
解決辦法：
方法1. 安裝net-tools
執行以下命令完成安裝：
yum install net-tools
方法2. 使用ss
centos7開始，系統就廢棄了netstat命令，改爲ss命令代替。
命令使用方法如下：
Usage: ss [ OPTIONS ]
ss [ OPTIONS ] [ FILTER ]
-h, --help this message
-V, --version output version information
-n, --numeric don't resolve service names
-r, --resolve resolve host names
-a, --all display all sockets
-l, --listening display listening sockets
-o, --options show timer information
-e, --extended show detailed socket information
-m, --memory show socket memory usage
-p, --processes show process using socket
-i, --info show internal TCP information
--tipcinfo show internal tipc socket information
-s, --summary show socket usage summary
-b, --bpf show bpf filter socket information
-E, --events continually display sockets as they are destroyed
-Z, --context display process SELinux security contexts
-z, --contexts display process and socket SELinux security contexts
-N, --net switch to the specified network namespace name
-4, --ipv4 display only IP version 4 sockets
-6, --ipv6 display only IP version 6 sockets
-0, --packet display PACKET sockets
-t, --tcp display only TCP sockets
-S, --sctp display only SCTP sockets
-u, --udp display only UDP sockets
-d, --dccp display only DCCP sockets
-w, --raw display only RAW sockets
-x, --unix display only Unix domain sockets
--tipc display only TIPC sockets
--vsock display only vsock sockets
-f, --family=FAMILY display sockets of type FAMILY
FAMILY := {inet|inet6|link|unix|netlink|vsock|tipc|help}
-K, --kill forcibly close sockets, display what was closed
-H, --no-header Suppress header line
-A, --query=QUERY, --socket=QUERY
QUERY := {all|inet|tcp|udp|raw|unix|unix_dgram|unix_stream|unix_seqpacket|packet|netlink|vsock_stream|vsock_dgram|tipc}[,QUERY]
-D, --diag=FILE Dump raw information about TCP sockets to FILE
-F, --filter=FILE read filter information from FILE
FILTER := [ state STATE-FILTER ] [ EXPRESSION ]
STATE-FILTER := {all|connected|synchronized|bucket|big|TCP-STATES}
TCP-STATES := {established|syn-sent|syn-recv|fin-wait-{1,2}|time-wait|closed|close-wait|last-ack|listening|closing}
connected := {established|syn-sent|syn-recv|fin-wait-{1,2}|time-wait|close-wait|last-ack|closing}
synchronized := {established|syn-recv|fin-wait-{1,2}|time-wait|close-wait|last-ack|closing}
bucket := {syn-recv|time-wait}
big := {established|syn-sent|fin-wait-{1,2}|closed|close-wait|last-ack|listening|closing}
附錄：
netstat的使用方法：
usage: netstat [-vWeenNcCF] [<Af>] -r netstat {-V|--version|-h|--help}
netstat [-vWnNcaeol] [<Socket> ...]
netstat { [-vWeenNac] -I[<Iface>] | [-veenNac] -i | [-cnNe] -M | -s [-6tuw] } [delay]
-r, --route display routing table
-I, --interfaces=<Iface> display interface table for <Iface>
-i, --interfaces display interface table
-g, --groups display multicast group memberships
-s, --statistics display networking statistics (like SNMP)
-M, --masquerade display masqueraded connections
-v, --verbose be verbose
-W, --wide don't truncate IP addresses
-n, --numeric don't resolve names
--numeric-hosts don't resolve host names
--numeric-ports don't resolve port names
--numeric-users don't resolve user names
-N, --symbolic resolve hardware names
-e, --extend display other/more information
-p, --programs display PID/Program name for sockets
-o, --timers display timers
-c, --continuous continuous listing
-l, --listening display listening server sockets
-a, --all display all sockets (default: connected)
-F, --fib display Forwarding Information Base (default)
-C, --cache display routing cache instead of FIB
-Z, --context display SELinux security context for sockets
<Socket>={-t|--tcp} {-u|--udp} {-U|--udplite} {-S|--sctp} {-w|--raw}
{-x|--unix} --ax25 --ipx --netrom
<AF>=Use '-6|-4' or '-A <af>' or '--<af>'; default: inet
List of possible address families (which support routing):
inet (DARPA Internet) inet6 (IPv6) ax25 (AMPR AX.25)
netrom (AMPR NET/ROM) ipx (Novell IPX) ddp (Appletalk DDP)
x25 (CCITT X.25)