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      Inspecting

      Inspecting Network Information with netstat


      Updated by Linode

      Contributed by

      Mihalis Tsoukalos

      The netstat command line utility shows information about the network status of a workstation or server. netstat is available on Unix-like and Windows operating systems, with some differences in its usage between these systems.

      netstat is an older utility, and some components of its functionality have been superseded by newer tools, like the ss command. A primary benefit of using netstat is that it is frequently pre-installed on Linux systems, while other tools might not be. As well, many (but not all) of the command line options for netstat can be run without root privileges, so it can still be useful on a system where you do not have root or sudo privileges.


      Assumptions

      This guide assumes some basic knowledge of networking in Linux, including network interfaces, routing tables, and network connections and sockets.

      In This Guide

      This guide will explore the options available when running netstat on Linux. netstat can be used to inspect:

      A list of the command line options can be found below, and some advanced examples of using netstat with the AWK command will be introduced at the end of the guide.

      Note

      This guide is written for a non-root user. Depending on your configuration, some commands might require the help of sudo in order to properly execute. If you are not familiar with the sudo command, see the Users and Groups guide.

      Basic Usage

      Installing netstat

      If netstat is not present on your Linux server or workstation, it can be added by installing the net-tools package:

      sudo apt install net-tools # Debian-based systems
      sudo yum install net-tools # CentOS and RHEL systems
      

      Running netstat without Any Options

      If you execute netstat without any command line arguments and options, the utility will display all open sockets and network connections, formatted in two tables. This will most likely be a relatively long list:

      netstat
      
        
      Active Internet connections (w/o servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        0      0 li140-253.members.l:ssh 185.232.67.121:43556    TIME_WAIT
      tcp        0      0 li140-253.members.:smtp 37.252.14.141:64553     SYN_RECV
      tcp        0      0 li140-253.members.l:ssh 37.252.14.141:43860     SYN_RECV
      tcp        0      0 li140-253.members.:smtp 37.252.14.141:44909     SYN_RECV
      tcp        0      0 li140-253.members.l:ssh ppp-2-86-7-61.hom:54757 ESTABLISHED
      tcp        0      0 li140-253.members.l:ssh 37.252.14.141:62736     SYN_RECV
      tcp6       0      0 li140-253.members.:http 37.252.14.141:63805     SYN_RECV
      
      Active UNIX domain sockets (w/o servers)
      Proto RefCnt Flags       Type       State         I-Node   Path
      unix  2      [ ]         DGRAM                    20972    /var/spool/postfix/dev/log
      unix  3      [ ]         DGRAM                    18134    /run/systemd/notify
      unix  3      [ ]         STREAM     CONNECTED     24059
      unix  2      [ ]         DGRAM                    22790
      unix  2      [ ACC ]     STREAM     LISTENING     24523    public/showq
      unix  2      [ ACC ]     STREAM     LISTENING     24526    private/error
      
      

      The first table displays network connections, and the columns of this table are interpreted as follows:

      Column Description
      Proto The protocol of the connection: TCP, UDP, or raw.
      Recv-Q When in reference to a TCP connection, this column shows the number of bytes received by the local network interface but not read by the connected process.
      Send-Q When in reference to a TCP connection, this column shows the number of bytes sent to the other side of the connection but not acknowledged by the remote host.
      Local Address The local address and port for the connection. By default, this will display the host name for the address, if it can be resolved. The service name for the port (e.g. SSH for port 22) will also be displayed by default.
      Foreign Address The address and port number for the connected host. The host name and service name will be displayed by default, similar to the behavior for the Local Address column.
      State The state of the connection. Because raw and UDP connections will generally not have state information, this column will usually be blank for those connection types. For TCP connections, the State column will have a value that matches one of the states specified by TCP: SYN_RECV, SYN_SENT, ESTABLISHED, etc. By default, connections in the LISTEN state will not be displayed.

      The second table displays unix sockets, and the columns of this table are interpreted as follows:

      Column Description
      Proto The protocol of the socket (unix).
      RefCnt The reference count, which is the number of attached processes connected via this socket.
      Flags Any flags associated with the socket. This will most often display ACC, short for SO_ACCEPTON, which is shown for unconnected sockets whose processes are waiting for connection requests.
      Type The type of the socket: datagram/connectionless (SOCK_DGRAM), stream/connection (SOCK_STREAM), raw (SOCK_RAW), reliably-delivered messages (SOCK_RDM), sequential packet (SOCK_SEQPACKET), or the obsolete SOCK_PACKET.
      State The state of the socket: FREE for unallocated sockets, LISTENING for sockets listening for connections, CONNECTING for sockets that are about to be connected, CONNECTED for connected sockets, and DISCONNECTING for disconnecting sockets. If the state is empty, the socket is not connected. Sockets in the LISTENING state will not be displayed by default.
      I-Node The filesystem inode of the socket.
      Path The filesystem path of the socket.

      Command Line Options

      Some important and frequently-used command line options of netstat are as follows:

      Option Definition
      -v Shows verbose output.
      -r Displays the kernel routing tables.
      -e Displays extended information for network connections.
      -i Displays a table of all network interfaces. When used with -a, the output also includes interfaces that are not up.
      -s Displays summary statistics for each protocol.
      -W Avoids truncating IP addresses and provides as much screen space as needed to display them.
      -n Displays numerical (IP) addresses, instead of resolving them to hostnames.
      -A Allows you to specify the protocol family. Valid values are inet, inet6, unix, ipx, ax25, netrom, econet, ddp and bluetooth.
      -t Displays TCP data only.
      -u Displays UDP data only.
      -4 Displays IPv4 connections only.
      -6 Displays IPv6 connections only.
      -c Displays information continuously (every second).
      -p Displays the process ID and the name of the program that owns the socket. It requires root privileges for this.
      -o Displays timer information.
      -a Shows both listening and non-listening network connections and unix sockets.
      -l Displays listening network connections and unix sockets, which are not shown by default.
      -C Displays routing information from the route cache.
      -g Displays multicast group membership information for IPv4 and IPv6.

      The rest of this guide will put the most important of these command line options to work in order to help you learn their usage. However, nothing can replace experimenting with netstat on your own.

      Sockets/Network Connections

      Include the LISTENING State

      Run netstat with the -a option to show both listening and non-listening network connections and sockets:

      netstat -a
      
        
      Active Internet connections (servers and established)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN
      tcp        0    316 li1076-154.members.:ssh 192.0.2.4:51109       ESTABLISHED
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN
      Active UNIX domain sockets (servers and established)
      Proto RefCnt Flags       Type       State         I-Node   Path
      unix  2      [ ACC ]     STREAM     LISTENING     15668    /run/systemd/private
      unix  6      [ ]         DGRAM                    9340     /run/systemd/journal/dev-log
      unix  3      [ ]         DGRAM                    9096     /run/systemd/notify
      unix  2      [ ]         DGRAM                    9098     /run/systemd/cgroups-agent
      unix  2      [ ACC ]     STREAM     LISTENING     9107     /run/systemd/fsck.progress
      unix  2      [ ACC ]     SEQPACKET  LISTENING     9117     /run/udev/control
      unix  2      [ ]         DGRAM                    9119     /run/systemd/journal/syslog
      unix  2      [ ]         DGRAM                    50340    /run/user/1000/systemd/notify
      unix  2      [ ACC ]     STREAM     LISTENING     50344    /run/user/1000/systemd/private
      ...
      
      

      Only Show the LISTENING State

      Run netstat with the -l option to only show listening network connections and sockets:

        
      Active Internet connections (only servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN
      Active UNIX domain sockets (only servers)
      Proto RefCnt Flags       Type       State         I-Node   Path
      unix  2      [ ACC ]     STREAM     LISTENING     15668    /run/systemd/private
      unix  2      [ ACC ]     STREAM     LISTENING     9107     /run/systemd/fsck.progress
      unix  2      [ ACC ]     SEQPACKET  LISTENING     9117     /run/udev/control
      unix  2      [ ACC ]     STREAM     LISTENING     50344    /run/user/1000/systemd/private
      unix  2      [ ACC ]     STREAM     LISTENING     50349    /run/user/1000/gnupg/S.gpg-agent.ssh
      unix  2      [ ACC ]     STREAM     LISTENING     50352    /run/user/1000/gnupg/S.gpg-agent.extra
      unix  2      [ ACC ]     STREAM     LISTENING     50354    /run/user/1000/gnupg/S.gpg-agent.browser
      unix  2      [ ACC ]     STREAM     LISTENING     50356    /run/user/1000/gnupg/S.gpg-agent
      unix  2      [ ACC ]     STREAM     LISTENING     9210     /run/systemd/journal/stdout
      unix  2      [ ACC ]     STREAM     LISTENING     11261    /var/run/dbus/system_bus_socket
      
      

      Show IPv4 Connections Only

      The -A inet, --inet and -4 command line options will all tell netstat to show IPv4 connections (both TCP and UDP) only. Because listening connections are not shown by default, this command displays connections that are in a non-listening state:

      netstat -4
      
        
      Active Internet connections (w/o servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        1      0 li140-253.members.:smtp 193.32.160.143:41356    CLOSE_WAIT
      tcp        0    300 li140-253.members.l:ssh athedsl-405473.ho:64917 ESTABLISHED
      tcp        1      0 li140-253.members.:smtp 193.32.160.136:37752    CLOSE_WAIT
      tcp        1      0 li140-253.members.:smtp 193.32.160.136:49900    CLOSE_WAIT
      tcp        1      0 li140-253.members.:smtp 193.32.160.136:49900    CLOSE_WAIT
      
      

      Note

      If you want to display IPv4 connections that are in both listening and non-listening state, add the -a command line option:

      netstat -4a
      

      Show IPv6 Connections Only

      The -A inet6, --inet6 and -6 command line options will all tell netstat to show IPv6 connections (both TCP and UDP) only. Because listening connections are not shown by default, this command displays connections that are in a non-listening state:

      netstat -6
      
        
      Active Internet connections (servers and established)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      udp6       0      0 [::]:mdns               [::]:*
      udp6       0      0 [::]:58949              [::]:*
      udp6       0      0 fe80::f03c:91ff:fe6:ntp [::]:*
      udp6       0      0 2a01:7e00::f03c:91f:ntp [::]:*
      udp6       0      0 localhost:ntp           [::]:*
      udp6       0      0 [::]:ntp                [::]:*
      
      

      Note

      If you want to display IPv4 connections that are in both listening and non-listening state, add the -a command line option:

      netstat -6a
      

      Show Listening UNIX Sockets

      The -x option limits netstat to showing Unix sockets. If you want to only display listening UNIX sockets, use the following command:

      netstat -lx
      
        
      Active UNIX domain sockets (only servers)
      Proto RefCnt Flags       Type       State         I-Node   Path
      unix  2      [ ACC ]     STREAM     LISTENING     21569793 /run/user/1000/gnupg/S.gpg-agent.extra
      unix  2      [ ACC ]     STREAM     LISTENING     21569796 /run/user/1000/gnupg/S.gpg-agent.ssh
      unix  2      [ ACC ]     STREAM     LISTENING     21569798 /run/user/1000/gnupg/S.gpg-agent.browser
      unix  2      [ ACC ]     STREAM     LISTENING     21569800 /run/user/1000/gnupg/S.dirmngr
      unix  2      [ ACC ]     STREAM     LISTENING     21569802 /run/user/1000/gnupg/S.gpg-agent
      unix  2      [ ACC ]     STREAM     LISTENING     24485    public/cleanup
      unix  2      [ ACC ]     STREAM     LISTENING     20306    /var/run/dbus/system_bus_socket
      unix  2      [ ACC ]     STREAM     LISTENING     24490    private/tlsmgr
      unix  2      [ ACC ]     STREAM     LISTENING     24493    private/rewrite
      unix  2      [ ACC ]     STREAM     LISTENING     24496    private/bounce
      unix  2      [ ACC ]     STREAM     LISTENING     24499    private/defer
      unix  2      [ ACC ]     STREAM     LISTENING     24502    private/trace
      unix  2      [ ACC ]     STREAM     LISTENING     24505    private/verify
      unix  2      [ ACC ]     STREAM     LISTENING     20319    /var/run/avahi-daemon/socket
      ...
      
      

      Show TCP Connections Only

      The -t option limits netstat to showing TCP network connections. Because listening connections are not shown by default, the following command displays connections that are in a non-listening state:

      netstat -nt
      
        
      Active Internet connections (w/o servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        1      0 109.74.193.253:25       193.32.160.143:41356    CLOSE_WAIT
      tcp        0      0 109.74.193.253:22       79.131.135.223:64917    ESTABLISHED
      tcp        1      0 109.74.193.253:25       193.32.160.136:37752    CLOSE_WAIT
      tcp        1      0 109.74.193.253:25       193.32.160.136:49900    CLOSE_WAIT
      tcp6       0      0 109.74.193.253:80       104.18.40.175:26111     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.40.175:47427     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.41.175:24763     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.41.175:32295     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.41.175:53268     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.40.175:4436      SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.40.175:17099     SYN_RECV
      tcp6       0      0 109.74.193.253:80       104.18.41.175:12892     SYN_RECV
      
      

      Note

      The -n option in the previous command tells netstat to not resolve IP addresses to hostnames.

      Note

      If you want to display both listening and non-listening TCP connections, add the -a command line option:

      netstat -ta
      

      Show IPv4 TCP Connections Only

      If you are only interested in IPv4 TCP connections, use the -t and -4 options together. Because listening connections are not shown by default, the following command displays connections that are in a non-listening state:

      netstat -nt4
      
        
      Active Internet connections (w/o servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        1      0 109.74.193.253:25       193.32.160.143:41356    CLOSE_WAIT
      tcp        0      0 109.74.193.253:22       79.131.135.223:64917    ESTABLISHED
      tcp        1      0 109.74.193.253:25       193.32.160.136:37752    CLOSE_WAIT
      tcp        1      0 109.74.193.253:25       193.32.160.136:49900    CLOSE_WAIT
      
      

      Note

      If you want to display both listening and non-listening IPv4 TCP connections, add the -a command line option:

      netstat -t4a
      

      Show Listening TCP Connections Only

      If you want to display listening TCP connections only, combine -l and -t:

      netstat -lt
      
        
      Active Internet connections (only servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      tcp        0      0 localhost:mysql         0.0.0.0:*               LISTEN
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN
      tcp      101      0 0.0.0.0:smtp            0.0.0.0:*               LISTEN
      tcp6       0      0 [::]:http               [::]:*                  LISTEN
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN
      tcp6       0      0 [::]:https              [::]:*                  LISTEN
      
      

      Show UDP Connections Only

      If you are only interested in seeing UDP connections, use the -u option:

      netstat  -u
      
        
      Active Internet connections (servers and established)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State
      udp        0      0 0.0.0.0:mdns            0.0.0.0:*
      udp        0      0 li140-253.member:syslog 0.0.0.0:*
      udp        0      0 0.0.0.0:60397           0.0.0.0:*
      udp        0      0 0.0.0.0:bootpc          0.0.0.0:*
      udp        0      0 li140-253.members.l:ntp 0.0.0.0:*
      udp        0      0 localhost:ntp           0.0.0.0:*
      udp        0      0 0.0.0.0:ntp             0.0.0.0:*
      udp6       0      0 [::]:mdns               [::]:*
      udp6       0      0 [::]:58949              [::]:*
      udp6       0      0 fe80::f03c:91ff:fe6:ntp [::]:*
      udp6       0      0 2a01:7e00::f03c:91f:ntp [::]:*
      udp6       0      0 localhost:ntp           [::]:*
      udp6       0      0 [::]:ntp                [::]:*
      
      

      Note

      To show only IPv4 or IPv6 UDP connections, combine -u with -4 or -6:

      netstat -u4
      netstat -u6
      

      Show Extended Output

      The -e command line parameter tells netstat to show extended output, which will add the User and Inode columns to the displayed table (but only for network connections, not unix sockets). For example, this command will show extended output for a system’s listening TCP connections:

      netstat -lte
      
        
      Active Internet connections (only servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode
      tcp        0      0 localhost:mysql         0.0.0.0:*               LISTEN      mysql      35862475
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN      root       35572959
      tcp      101      0 0.0.0.0:smtp            0.0.0.0:*               LISTEN      root       35544149
      tcp6       0      0 [::]:http               [::]:*                  LISTEN      root       35577141
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN      root       35572961
      tcp6       0      0 [::]:https              [::]:*                  LISTEN      root       35577145
      
      

      Show the PID and Program Name

      The -p option displays the process ID and program name that corresponds to a network connection or unix socket.

      Note

      netstat requires root privileges to show the PID and program name of processes that are not owned by your user.

      This command will display the PID and program name for a system’s listening TCP connections:

      sudo netstat -ltp
      
        
      Active Internet connections (only servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
      tcp        0      0 localhost:mysql         0.0.0.0:*               LISTEN      24555/mysqld
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN      1008/sshd
      tcp      101      0 0.0.0.0:smtp            0.0.0.0:*               LISTEN      8576/master
      tcp6       0      0 [::]:http               [::]:*                  LISTEN      1808/apache2
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN      1008/sshd
      tcp6       0      0 [::]:https              [::]:*                  LISTEN      1808/apache2
      
      

      Note

      In particular, the previous example’s command is a fast way to learn about which networked services are running on your system.

      Combining -p and -e

      Combining -p with -e while having root privileges will simultaneously reveal the user, inode, and PID/program name of your network connections. The following example command will show all of this information for a system’s listening TCP connections:

      sudo netstat -ltpe
      
        
      Active Internet connections (only servers)
      Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode      PID/Program name
      tcp        0      0 localhost:mysql         0.0.0.0:*               LISTEN      mysql      35862475   24555/mysqld
      tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN      root       35572959   1008/sshd
      tcp      101      0 0.0.0.0:smtp            0.0.0.0:*               LISTEN      root       35544149   8576/master
      tcp6       0      0 [::]:http               [::]:*                  LISTEN      root       35577141   1808/apache2
      tcp6       0      0 [::]:ssh                [::]:*                  LISTEN      root       35572961   1008/sshd
      tcp6       0      0 [::]:https              [::]:*                  LISTEN      root       35577145   1808/apache2
      
      

      Routing Tables

      One of the most frequent uses of netstat is for showing the routing table of a machine:

      netstat -nr
      
        
      Kernel IP routing table
      Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
      0.0.0.0         109.74.193.1    0.0.0.0         UG        0 0          0 eth0
      109.74.193.0    0.0.0.0         255.255.255.0   U         0 0          0 eth0
      
      

      In this output, the U flag means that the route is in use and the G flag denotes the default gateway. The H flag, which is not displayed here, would mean that the route is to a host and not to a network.

      Network Interfaces

      The -i option shows network statistics on a per-interface basis:

      netstat -i
      
        
      Kernel Interface table
      Iface      MTU    RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
      eth0      1500  7075525      0      0 0       6830902      0      0      0 BMRU
      lo       65536   573817      0      0 0        573817      0      0      0 LRU
      
      
      Column Description
      Iface The name of the interface.
      MTU The value of the Maximum Transmission Unit.
      RX-OK The number of error free packets received.
      RX-ERR The number of packets received with errors.
      RX-DRP The number of dropped packets when receiving.
      RX-OVR The number of packets lost due to the overflow when receiving.
      TX-OK The number of error-free packets transmitted.
      RX-ERR The number of transmitted packets with errors.
      TX-DRP The number of dropped packets when transmitting.
      TX-OVR The number of packets lost due to the overflow when transmitting.
      Flag Flag values for the interface.

      If you combine -a with -i, netstat will also display interfaces that are not up:

      netstat -ia
      
        
      Kernel Interface table
      Iface      MTU    RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
      dummy0    1500        0      0      0 0             0      0      0      0 BO
      erspan0   1450        0      0      0 0             0      0      0      0 BM
      eth0      1500 13128358      0      0 0      15677694      0      0      0 BMRU
      gre0      1476        0      0      0 0             0      0      0      0 O
      gretap0   1462        0      0      0 0             0      0      0      0 BM
      ip6_vti0  1364        0      0      0 0             0      0      0      0 O
      ip6gre0   1448        0      0      0 0             0      0      0      0 O
      ip6tnl0   1452        0      0      0 0             0      0      0      0 O
      ip_vti0   1480        0      0      0 0             0      0      0      0 O
      lo       65536   846097      0      0 0        846097      0      0      0 LRU
      sit0      1480        0      0      0 0             0      0      0      0 O
      teql0     1500        0      0      0 0             0      0      0      0 O
      tunl0     1480        0      0      0 0             0      0      0      0 O
      
      

      Network Protocols

      Use the -s option to see network statistics on a per-protocol basis:

      netstat -s
      
        
      Ip:
          Forwarding: 2
          6775334 total packets received
          11 with invalid addresses
          0 forwarded
          0 incoming packets discarded
          6775323 incoming packets delivered
          7339283 requests sent out
      Icmp:
          10531 ICMP messages received
          4415 input ICMP message failed
          InCsumErrors: 3
          ICMP input histogram:
              destination unreachable: 6035
              timeout in transit: 93
              redirects: 1
              echo requests: 4379
              timestamp request: 20
          16939 ICMP messages sent
          0 ICMP messages failed
          ICMP output histogram:
              destination unreachable: 12540
              echo replies: 4379
              timestamp replies: 20
      IcmpMsg:
              InType3: 6035
              InType5: 1
              InType8: 4379
              InType11: 93
              InType13: 20
              OutType0: 4379
              OutType3: 12540
              OutType14: 20
      Tcp:
          38781 active connection openings
          330301 passive connection openings
          10683 failed connection attempts
          26722 connection resets received
          1 connections established
          6580191 segments received
          10797407 segments sent out
          654603 segments retransmitted
          748 bad segments received
          408640 resets sent
          InCsumErrors: 747
      Udp:
          212303 packets received
          13230 packets to unknown port received
          126 packet receive errors
          213173 packets sent
          0 receive buffer errors
          0 send buffer errors
          InCsumErrors: 126
      UdpLite:
      TcpExt:
          10451 resets received for embryonic SYN_RECV sockets
          9 ICMP packets dropped because they were out-of-window
          41710 TCP sockets finished time wait in fast timer
          294 packetes rejected in established connections because of timestamp
          161285 delayed acks sent
          22 delayed acks further delayed because of locked socket
          Quick ack mode was activated 20984 times
          43 SYNs to LISTEN sockets dropped
          1199311 packet headers predicted
          1851531 acknowledgments not containing data payload received
          919487 predicted acknowledgments
          114 times recovered from packet loss due to fast retransmit
          TCPSackRecovery: 5474
          TCPSACKReneging: 2
          Detected reordering 8235 times using SACK
          Detected reordering 21 times using reno fast retransmit
          Detected reordering 219 times using time stamp
          154 congestion windows fully recovered without slow start
          80 congestion windows partially recovered using Hoe heuristic
          TCPDSACKUndo: 142
          1009 congestion windows recovered without slow start after partial ack
          TCPLostRetransmit: 33008
          68 timeouts after reno fast retransmit
          TCPSackFailures: 302
          599 timeouts in loss state
          57605 fast retransmits
          2647 retransmits in slow start
          TCPTimeouts: 618841
          TCPLossProbes: 35168
          TCPLossProbeRecovery: 12069
          TCPRenoRecoveryFail: 60
          TCPSackRecoveryFail: 668
          TCPBacklogCoalesce: 54624
          TCPDSACKOldSent: 20866
          TCPDSACKOfoSent: 56
          TCPDSACKRecv: 5136
          TCPDSACKOfoRecv: 76
          20881 connections reset due to unexpected data
          1466 connections reset due to early user close
          3960 connections aborted due to timeout
          TCPSACKDiscard: 54
          TCPDSACKIgnoredOld: 28
          TCPDSACKIgnoredNoUndo: 2114
          TCPSpuriousRTOs: 23
          TCPSackShifted: 33515
          TCPSackMerged: 56742
          TCPSackShiftFallback: 24412
          TCPDeferAcceptDrop: 127813
          TCPRcvCoalesce: 258864
          TCPOFOQueue: 24749
          TCPOFOMerge: 56
          TCPChallengeACK: 238
          TCPSYNChallenge: 3
          TCPFastOpenCookieReqd: 6
          TCPFromZeroWindowAdv: 32
          TCPToZeroWindowAdv: 32
          TCPWantZeroWindowAdv: 187
          TCPSynRetrans: 517978
          TCPOrigDataSent: 7250401
          TCPHystartTrainDetect: 102
          TCPHystartTrainCwnd: 15000
          TCPHystartDelayDetect: 1533
          TCPHystartDelayCwnd: 101578
          TCPACKSkippedSynRecv: 16
          TCPACKSkippedPAWS: 160
          TCPACKSkippedSeq: 54
          TCPACKSkippedTimeWait: 140
          TCPACKSkippedChallenge: 91
          TCPWinProbe: 1552
          TCPDelivered: 7093769
          TCPAckCompressed: 12241
          TCPWqueueTooBig: 222
      IpExt:
          InMcastPkts: 104
          OutMcastPkts: 106
          InOctets: 3072954621
          OutOctets: 10300134722
          InMcastOctets: 8757
          OutMcastOctets: 8837
          InNoECTPkts: 6759736
          InECT1Pkts: 312
          InECT0Pkts: 54355
          InCEPkts: 8644
      Sctp:
          0 Current Associations
          0 Active Associations
          0 Passive Associations
          0 Number of Aborteds
          0 Number of Graceful Terminations
          14 Number of Out of Blue packets
          0 Number of Packets with invalid Checksum
          14 Number of control chunks sent
          0 Number of ordered chunks sent
          0 Number of Unordered chunks sent
          14 Number of control chunks received
          0 Number of ordered chunks received
          0 Number of Unordered chunks received
          0 Number of messages fragmented
          0 Number of messages reassembled
          14 Number of SCTP packets sent
          14 Number of SCTP packets received
          SctpInPktSoftirq: 14
          SctpInPktDiscards: 14
      
      

      Including the -w option will tell netstat to display raw network statistics:

      sudo netstat -sw
      
        
      Ip:
          Forwarding: 2
          6775954 total packets received
          11 with invalid addresses
          0 forwarded
          0 incoming packets discarded
          6775943 incoming packets delivered
          7339740 requests sent out
      Icmp:
          10531 ICMP messages received
          4415 input ICMP message failed
          InCsumErrors: 3
          ICMP input histogram:
              destination unreachable: 6035
              timeout in transit: 93
              redirects: 1
              echo requests: 4379
              timestamp request: 20
          16942 ICMP messages sent
          0 ICMP messages failed
          ICMP output histogram:
              destination unreachable: 12543
              echo replies: 4379
              timestamp replies: 20
      IcmpMsg:
              InType3: 6035
              InType5: 1
              InType8: 4379
              InType11: 93
              InType13: 20
              OutType0: 4379
              OutType3: 12543
              OutType14: 20
      UdpLite:
      IpExt:
          InMcastPkts: 104
          OutMcastPkts: 106
          InOctets: 3072998471
          OutOctets: 10300305693
          InMcastOctets: 8757
          OutMcastOctets: 8837
          InNoECTPkts: 6760354
          InECT1Pkts: 312
          InECT0Pkts: 54357
          InCEPkts: 8644
      
      

      Multicast Group Membership

      The netstat -g command displays multicast group membership information:

      netstat -g
      
        
      IPv6/IPv4 Group Memberships
      Interface       RefCnt Group
      --------------- ------ ---------------------
      lo              1      all-systems.mcast.net
      eth0            1      224.0.0.251
      eth0            1      all-systems.mcast.net
      lo              1      ip6-allnodes
      lo              1      ff01::1
      dummy0          1      ip6-allnodes
      dummy0          1      ff01::1
      eth0            1      ff02::202
      eth0            1      ff02::fb
      ...
      
      

      Note

      The default output of netstat -g displays both IPv4 and IPv6 data.

      Using AWK to process netstat output

      The AWK programming language can help you process netstat output and generate handy reports.

      Showing the Number of Listening Processes Per Username

      The following command calculates the total number of listening processes per username:

      sudo netstat -lte | awk '{print $7}' | grep -v Address | grep -v "^$" | sort | uniq -c | awk '{print $2 ": " $1}'
      
        
      mysql: 1
      root: 5
      
      
      • The netstat command collects the listening TCP connections and includes the users for the connections’ processes in the output.
      • The first awk command limits the output to the column that displays the user.
      • The first grep command deletes the line with the header information generated by netstat.
      • The second grep command deletes empty lines from the output.
      • The sort command sorts the users alphabetically.
      • After that, the uniq command counts line occurrences while omitting repeated output.
      • Lastly, the second awk command reverses the two columns of the uniq command’s output and prints the data on screen.

      HTTP Connections

      The following command, which needs root privileges to run, extracts the IP address from all established Apache connections and calculates the number of connections per IP address:

      sudo netstat -anpt | grep apache2 | grep ESTABLISHED | awk -F "[ :]*" '{print $4}' | uniq -c
      
        
            4 109.74.193.253
      
      

      TCP Connections

      The following command calculates the number of TCP connections per IP address, sorted by the number of connections:

      netstat -nt | awk '/^tcp/ {print $5}' | awk -F: '{print $1}' | sort | uniq -c | sort -nr
      
        
            2 193.32.160.136
            1 79.131.135.223
            1 193.32.160.143
            1 106.13.205.251
      
      

      Counting TCP States

      The next command counts the various types of TCP states:

      netstat -ant | awk '{print $6}' | grep -v established) | grep -v Foreign | sort | uniq -c | sort -n
      
        
            2 ESTABLISHED
            3 CLOSE_WAIT
            6 LISTEN
      
      

      Summary

      Even if there exists other more modern tools that can replace netstat, netstat remains a handy tool that will definitely help you if you ever have networking problems on your Linux machine. However, never forget to check your log files for errors or warnings related to your network problem before troubleshooting.

      More Information

      You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

      Find answers, ask questions, and help others.

      This guide is published under a CC BY-ND 4.0 license.



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