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How to understand Linux netstat command, many novices are not very clear about this, in order to help you solve this problem, the following small series will explain in detail for everyone, there are people who need this can learn, I hope you can gain something.

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The Netstat command is used to display various network-related information, such as network connections, routing tables, Interface statistics, masquerade connections, multicast Memberships, and so on.

Output Message Meaning

When netstat is executed, the output is

Active Internet connections (w/o servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 2 210.34.6.89:telnet 210.34.6.96:2873 ESTABLISHED tcp 296 0 210.34.6.89:1165 210.34.6.84:netbios-ssn ESTABLISHED tcp 0 0 localhost.localdom:9001 localhost.localdom:1162 ESTABLISHED tcp 0 0 localhost.localdom:1162 localhost.localdom:9001 ESTABLISHED tcp 0 80 210.34.6.89:1161 210.34.6.10:netbios-ssn CLOSE Active UNIX domain sockets (w/o servers) Proto RefCnt Flags Type State I-Node Path unix 1 [ ] STREAM CONNECTED 16178 @000000dd unix 1 [ ] STREAM CONNECTED 16176 @000000dc unix 9 [ ] DGRAM 5292 /dev/log unix 1 [ ] STREAM CONNECTED 16182 @000000df

Overall, netstat's output can be divided into two parts:

One is Active Internet connections, called Active TCP connections, where "Recv-Q" and "Send-Q" mean %0A's are Receive Queue and Send Queue. These numbers should usually be 0. If not, the package is piling up in the queue. This can only be seen in very rare cases.

The other is Active UNIX domain sockets, called Active Unix Domain Sockets (the same as network sockets, but only for native communication, which doubles performance). Proto shows the protocol used for the connection,RefCnt indicates the number of the process connected to the socket,Types shows the type of socket,State shows the current state of the socket, and Path indicates the path name used by other processes connected to the socket.

common parameters

-a (all) shows all options, default does not show LISTEN related-t (tcp) shows tcp related options only-u (udp) shows udp related options only-n rejects alias, can display all numbers converted to numbers. - l List only the service status in Listen-p Display the program name that established the relevant link-r Display routing information, routing table-e Display extended information, such as uid, etc. -s Perform statistics according to each protocol-c Every fixed time, execute the netstat command.

Tip: The status of LISTEN and LISTENING can only be seen with-a or-l.

Practical command examples

1. List all ports (listening and unlistening)

List all ports netstat -a

# netstat -a | more Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:30037 *:* LISTEN udp 0 0 *:bootpc *:* Active UNIX domain sockets (servers and established) Proto RefCnt Flags Type State I-Node Path unix 2 [ ACC ] STREAM LISTENING 6135 /tmp/.X11-unix/X0 unix 2 [ ACC ] STREAM LISTENING 5140 /var/run/acpid.socket

List all tcp ports netstat -at

# netstat -at Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:30037 *:* LISTEN tcp 0 0 localhost:ipp *:* LISTEN tcp 0 0 *:smtp *:* LISTEN tcp6 0 0 localhost:ipp [::]:* LISTEN

List all udp ports netstat -au

# netstat -au Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State udp 0 0 *:bootpc *:* udp 0 0 *:49119 *:* udp 0 0 *:mdns *:*

2. List all Sockets that are listening

Show only listening ports netstat -l

# netstat -l Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:ipp *:* LISTEN tcp6 0 0 localhost:ipp [::]:* LISTEN udp 0 0 *:49119 *:*

List only all listening tcp ports netstat -lt

# netstat -lt Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:30037 *:* LISTEN tcp 0 0 *:smtp *:* LISTEN tcp6 0 0 localhost:ipp [::]:* LISTEN

List only all listening udp ports netstat -lu

# netstat -lu Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State udp 0 0 *:49119 *:* udp 0 0 *:mdns *:*

List only all listening UNIX ports netstat -lx

# netstat -lx Active UNIX domain sockets (only servers) Proto RefCnt Flags Type State I-Node Path unix 2 [ ACC ] STREAM LISTENING 6294 private/maildrop unix 2 [ ACC ] STREAM LISTENING 6203 public/cleanup unix 2 [ ACC ] STREAM LISTENING 6302 private/ifmail unix 2 [ ACC ] STREAM LISTENING 6306 private/bsmtp Did reading the above help you? If you still want to have further understanding of related knowledge or read more related articles, please pay attention to the industry information channel, thank you for your support.

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