Netstat

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Screenshot of netstat

netstat (network statistics) is a command-line tool that displays network connections (both incoming and outgoing), routing tables, and a number of network interface statistics. It is available on Unix, Unix-like, and Windows NT-based operating systems.

It is used for finding problems in the network and to determine the amount of traffic on the network as a performance measurement. [1]

Parameters

Parameters used with this command must be prefixed with a hyphen (-) rather than a slash (/).

-a : Displays all active TCP connections and the TCP and UDP ports on which the computer is listening.

-b : Displays the binary (executable) program's name involved in creating each connection or listening port. (Windows only)

-e : Displays ethernet statistics, such as the number of bytes and packets sent and received. This parameter can be combined with -s.

-f : Displays fully qualified domain names <FQDN> for foreign addresses.(not available under Windows)

-i : Displays network interfaces and their statistics (not available under Windows)

-n : Displays active TCP connections, however, addresses and port numbers are expressed numerically and no attempt is made to determine names.

-o : Displays active TCP connections and includes the process ID (PID) for each connection. You can find the application based on the PID on the Processes tab in Windows Task Manager. This parameter can be combined with -a, -n, and -p. This parameter is available on Microsoft Windows XP, 2003 Server (not Microsoft Windows 2000)).

-p Windows: Protocol : Shows connections for the protocol specified by Protocol. In this case, the Protocol can be tcp, udp, tcpv6, or udpv6. If this parameter is used with -s to display statistics by protocol, Protocol can be tcp, udp, icmp, ip, tcpv6, udpv6, icmpv6, or ipv6.

-p Linux: Process : Show which processes are using which sockets (similar to -b under Windows) (you must be root to do this)

-r : Displays the contents of the IP routing table. (This is equivalent to the route print command under Windows.)

-s : Displays statistics by protocol. By default, statistics are shown for the TCP, UDP, ICMP, and IP protocols. If the IPv6 protocol for Windows XP is installed, statistics are shown for the TCP over IPv6, UDP over IPv6, ICMPv6, and IPv6 protocols. The -p parameter can be used to specify a set of protocols.

-v : When used in conjunction with -b it will display the sequence of components involved in creating the connection or listening port for all executables.

Interval : Redisplays the selected information every Interval seconds. Press CTRL+C to stop the redisplay. If this parameter is omitted, netstat prints the selected information only once.

/? : Displays help at the command prompt. (only on Windows)

Statistics provided

Netstat provides statistics for the following:

• Proto - The name of the protocol (TCP or UDP).

• Local Address - The IP address of the local computer and the port number being used. The name of the local computer that corresponds to the IP address and the name of the port is shown unless the -n parameter is specified. If the port is not yet established, the port number is shown as an asterisk (*).

• Foreign Address - The IP address and port number of the remote computer to which the socket is connected. The names that corresponds to the IP address and the port are shown unless the -n parameter is specified. If the port is not yet established, the port number is shown as an asterisk (*).

• State - Indicates the state of a TCP connection. The possible states are as follows: CLOSE_WAIT, CLOSED, ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, LAST_ACK, LISTEN, SYN_RECEIVED, SYN_SEND, and TIME_WAIT. For more information about the states of a TCP connection, see RFC 793.

Examples

To display the statistics for only the TCP or UDP protocols, type one of the following commands:

netstat -s -p tcp

netstat -s -p udp

To display active TCP connections and the process IDs every 5 seconds, type the following command (works on Microsoft XP and 2003 only):

netstat -o 5

Mac OS X version

netstat -w 5

To display active TCP connections and the process IDs using numerical form, type the following command (works on Microsoft XP and 2003 only):

netstat -n -o

Caveats

Some versions of netstat lack explicit field delimiters in their printf-generated output, leading to numeric fields running together and thus corrupting the output data.

Platform Specific Remarks

Under Linux, raw data can often be obtained from the /proc/net/dev to work around the printf output corruption arising in netstat's network interface statistics summary, netstat -i, until such time as the problem is corrected.

On the Windows platform, netstat information can be retrieved by calling the GetTcpTable and GetUdpTable functions in the IP Helper API, or IPHLPAPI.DLL. Information returned includes local and remote IP addresses, local and remote ports, and (for GetTcpTable) TCP status codes. In addition to the command-line netstat.exe tool that ships with Windows, there are GUI-based netstat programs available.

On the Windows platform, this command is available only if the Internet Protocol (TCP/IP) protocol is installed as a component in the properties of a network adapter in Network Connections.

With Mac OS X 10.5, the /Applications/Utilities folder contains a network utility called: Network Utility, see tab Netstat for these stats presented in a gui application, along with Ping, Lookup, Traceroute, Whois, Finger and Port Scan.

See also

• lsof -i

External links

• Ports & Services Database
• Microsoft TechNet Netstat article – documentation for the netstat.exe command-line program
• The netstat Command (Linux) – a guide to using the netstat command in Linux.
• Security Now #49 - The NETSTAT Command – podcast guide to netstat from Security Now!.