Skillfully using Netstat to troubleshoot network problems

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Ping and traceroute are two common commands when encountering a network failure on a Linux server, but many times you need to know more about the details of the network to help solve the problem. To do this, you can use the Netstat command, which provides detailed information about the network sockets and other useful information. As with the ping and traceroute commands, you can simply use netstat at the command line and get the results immediately.

First, what is netstat

The netstat command is a very useful tool for dealing with network problems. Netstat is the abbreviation for network Statistics, which can display incoming and outgoing network connections, and can also be used to obtain network statistics, protocol statistics, routing table information, and so on.

We can use Netstat to find network problems and measure network traffic, so it can be used to collect network outages, spin down, or network bottlenecks.

Second, the basic netstat

To get a list of all current connections, just use the-a option.

# netstat -aActive Internet connections (servers and established)Proto Recv-Q Send-Q Local Address           Foreign Address         State      tcp        0      0 *:1922                  *:*                     LISTEN     tcp        0    216 chdc154:1922            223.99.111.233:11303    ESTABLISHEDtcp6       0      0 [::]:9000               [::]:*                  LISTEN     tcp6       0      0 [::]:8009               [::]:*                  LISTEN     tcp6       0      0 [::]:mysql              [::]:*                  LISTEN     tcp6       0      0 [::]:1922               [::]:*                  LISTEN     tcp6       0      0 [::]:9090               [::]:*                  LISTEN     tcp6       0      0 localhost:8005          [::]:*                  LISTEN     ......

It provides basic information about the connections of different types of protocols, such as TCP and UDP, and the active UNIX domain socket information. However, Natstat also allows users to obtain more specific information to help with debugging.

Third, filter by connection type

Filtering results based on connection types can help you find the information you need. For example, if you want to view a TCP connection, you can immediately follow a T option on the-a option above, as follows:

# netstat -aActive Internet connections (servers and established)Proto Recv-Q Send-Q Local Address           Foreign Address         State      tcp        0      0 *:1922                  *:*                     LISTEN     tcp        0    216 chdc154:1922            223.99.111.233:11303    ESTABLISHEDtcp6       0      0 [::]:9000               [::]:*                  LISTEN     tcp6       0      0 [::]:8009               [::]:*                  LISTEN   ......

Similarly, if the-a option is followed by the U option, the value lists the UDP connection.

Iv. filtering according to the listening connection

If you want to see the connection you are listening on, you can use the-l option (remove-a option), such as:

# netstat -lActive Internet connections (only servers)......Active UNIX domain sockets (only servers)Proto RefCnt Flags       Type       State         I-Node   Pathunix  2      [ ACC ]     STREAM     LISTENING     47834116 /run/systemd/privateunix  2      [ ACC ]     STREAM     LISTENING     1661287  /run/user/0/systemd/privateunix  2      [ ACC ]     SEQPACKET  LISTENING     15450    /run/udev/controlunix  2      [ ACC ]     STREAM     LISTENING     96528873 /run/snapd-snap.socketunix  2      [ ACC ]     STREAM     LISTENING     10581    /var/lib/lxd/unix.socketunix  2      [ ACC ]     STREAM     LISTENING     10578    /run/uuidd/requestunix  2      [ ACC ]     STREAM     LISTENING     10582    /run/acpid.socket......

Similar to the-a option, the-l option immediately follows the T option, the-LT option, to view the TCP connection being listened to, and-lu to view the TCP connection being listened to. This way, you can easily see if the specified port is open and listening, and determine whether the site app or app is running as expected.

V. Viewing network statistics

# netstat -sIp:    1473970908 total packets received    17795365 with invalid addresses    0 forwarded    0 incoming packets discarded    1453512118 incoming packets delivered    2392531460 requests sent out    40 outgoing packets dropped    3 fragments dropped after timeout    48 reassemblies required    15 packets reassembled ok    3 packet reassembles failedIcmp:    3589646 ICMP messages received    37 input ICMP message failed.    ICMP input histogram:        destination unreachable: 178        timeout in transit: 18        echo requests: 3589445        echo replies: 5......

As you can see, the-S option provides some statistical information that may be useful when debugging, such as total, incoming and outgoing packets, and ICMP messages that are received, sent, and failed.

Skillfully using Netstat to troubleshoot network problems