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显示网络状态：netstat

netstat 显示的是网络连接、路由表 (-r)、接口 (-i)、伪装连接 (-M)、多点广播成员 (-g) 和统计信息 (-s)。

tux@mercury:~> netstat -r
Kernel IP routing table
Destination     Gateway         Genmask        Flags  MSS Window  irtt Iface
192.168.2.0     *               255.255.254.0  U        0 0          0 eth0
link-local      *               255.255.0.0    U        0 0          0 eth0
loopback        *               255.0.0.0      U        0 0          0 lo
default         192.168.2.254   0.0.0.0        UG       0 0          0 eth0

tux@mercury:~> netstat -i
Kernel Interface table
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR  TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0   1500   0 1624507 129056      0      0   7055      0      0      0 BMNRU
lo    16436   0   23728      0      0      0  23728      0      0      0 LRU

显示网络连接或统计数据时，可指定要显示的套接字类型：TCP (-t)、UDP (-u) 或 raw (-r)。-p 选项显示套接字所属程序的 PID 和名称。

下例列出了所有 TCP 连接和使用这些连接的程序。

mercury:~ # netstat -t -p
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address  Foreign Address       State       PID/Pro

tcp      0      0 mercury:33513    www.novell.com:www-http ESTABLISHED 6862/fi
tcp      0    352 mercury:ssh      mercury2.:trc-netpoll     ESTABLISHED 19422/s
tcp      0      0 localhost:ssh  localhost:17828         ESTABLISHED -

下面，显示 TCP 协议的统计信息：

tux@mercury:~> netstat -s -t
Tcp:
    2427 active connections openings
    2374 passive connection openings
    0 failed connection attempts
    0 connection resets received
    1 connections established
    27476 segments received
    26786 segments send out
    54 segments retransmited
    0 bad segments received.
    6 resets sent
[...]
    TCPAbortOnLinger: 0
    TCPAbortFailed: 0
    TCPMemoryPressures: 0