Linux Commands Examples

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netstat

Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships


see also : route - ifconfig - iptables

Synopsis

netstat [address_family_options] [--tcp|-t] [--udp|-u] [--raw|-w] [--listening|-l] [--all|-a] [--numeric|-n] [--numeric-hosts] [--numeric-ports] [--numeric-users] [--symbolic|-N] [--extend|-e[--extend|-e]] [--timers|-o] [--program|-p] [--verbose|-v] [--continuous|-c]

netstat {--route|-r} [address_family_options] [--extend|-e[--extend|-e]] [--verbose|-v] [--numeric|-n] [--numeric-hosts] [--numeric-ports] [--numeric-users] [--continuous|-c]

netstat {--interfaces|-i} [--all|-a] [--extend|-e[--extend|-e]] [--verbose|-v] [--program|-p] [--numeric|-n] [--numeric-hosts] [--numeric-ports] [--numeric-users] [--continuous|-c]

netstat {--groups|-g} [--numeric|-n] [--numeric-hosts] [--numeric-ports] [--numeric-users] [--continuous|-c]

netstat {--masquerade|-M} [--extend|-e] [--numeric|-n] [--numeric-hosts] [--numeric-ports] [--numeric-users] [--continuous|-c]

netstat {--statistics|-s} [--tcp|-t] [--udp|-u] [--raw|-w]

netstat {--version|-V}

netstat {--help|-h}

address_family_options:

[-4] [-6] [--protocol={inet,unix,ipx,ax25,netrom,ddp}[,...]] [--unix|-x] [--inet|--ip] [--ax25] [--ipx] [--netrom] [--ddp]


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examples

0
I use

netstat -tapn

to have instant current network connections 
example added by LeBerger
0
I use

netstat -tapn

to have instant current network connections
example added by LeBerger
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source
            
netstat -tulpn
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netstat -nlp
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netstat -tlnp
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echo "netstat -rn"
netstat -rn
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netstat -autn
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Where does netstat get the process name?

There is nothing wrong with reading things from /proc. In fact, that is where those tools get that information from. There are actually three files with the command name.

/proc/PID/cmdline
/proc/PID/stat
/proc/PID/status

I believe it is usually obtained from stat.

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How can I determine the process for what is listening on the network in linux?

When you display this list, for lines that have no process name, can you check the state of the tcp socket?

If it is a closing socket, the process may have disconnected and the TCP Stack might be just cleaning up the connection.

Secondly, are you running the netstat command with root rights?
If you do not have rights to the process, its name will not be listed.
Actually, if that happens, most netstat versions will show a warning about this before listing the output.

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Where are the open ports ? Am I getting an incomplete list?

What you are looking for is netstat with the -l switch, as in "listen".

What we usually call an "open port" is actually a listening socket on which a program is bound. It can be on any interface (0.0.0.0:port) or interface-specific (127.0.0.1:port for 'loopback'). When a program is bound to this socket, the kernel allows packets destinating there and forwards them. If no program is bound on the said socket, the packet is dropped. hence the "open/closed" terminology.

You may anlso want to use the -n switch to prevent netstat from transforming port numbers and host addresses in to their respective names.

To do the test remotely, please have a look at the nmapcommand.

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Count exchanged bytes per TCP connection

If you want something fancy and graphical you might take a look at ntop - it's in most distro's apt / yum / whatever.

It monitors the network traffic creating nice graphs and charts on a per-connection, protocol, host, etc basis and displays it all through your web browser (has a built in web server).

It's nice, but fairly heavyweight.

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How to interpret the output of netstat -o / netstat --timers

The first number is clearly the countdown timer. Depending on status type, once the timer times out, it will retry and send out another FIN or whatever needed packet to try again. So the second number keeps track of the number of retries. Notice that the timer increases because TCP has backoff timers in case there is a traffic condition. The backoff timer avoids excessively frequent retries. Each failure results in a larger and larger backoff. The backoff could be exponential growth or linear, depending on TCP stack. I have not figured out what the 3 figure is for. Always zero in my displays.

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What does netstat mean by "acknowledgments not containing data received"?

When a TCP connection is established, there is a so-called "three-way handshake" at the start, this consists of packets containing no data, just the TCP headers with the "Ack" bit set and various sequence numbers to satisfy both ends that everything is OK.

After that, it is usual for the receiver to periodically acknowledge data being received. Often this is done by setting the Ack bit on a convenient outgoing chunk of data. If the communication is mostly one way I expect the receiver will have to occasionally send an empty packet to acknowledge receipt of data received so far. A large block of data is usually split into many small packets to traverse networks with low maximum transmission units (MTUs). I don't have the details to hand but I suspect the transmitting end may become anxious if it sends many many packets and doesn't ever receive an ack, even if the receiver has nothing else to say.

So acknowledgemet packets not containing data are not necessarily a cause for concern.

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checking ssh tunnel

netstat -anp | grep 60000.*ssh

should do the job.

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TCP number of connections differ from client to server

If one side closes the connection without signaling that it will be closing you will be left with a Half Open connection.

Intentionally closing only one side of the connection is the way old SYN Flood Denial of Service attacks where performed.

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how to use netstat on a specific port in Linux

use netstat -anp | grep portNumber

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Interpreting statistics from netstat -s

Looking at the relevant source code of netstat or using strace -e open netstat -s, one can see that the above information is fetched from /proc/net/snmp and /proc/net/netstat. This proc-fs information is filled up by the Linux networking stack and initialised in net/ipv4/proc.c. This part of the kernel, if enabled, starts its functionality when the kernel boots up.

Since you have not specified the exactitude of the answer you're looking for, and to make a long story short, I suppose you might be happy enough with this:

uptime

You can also get it in seconds resolution using something along the lines of:

set -- $(grep btime /proc/stat) && btime = $2
bc -l < <(printf "%s - %s\n" "$(date +%s)" "$btime")

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What does *:* in lsof output stand for?

The digit after : denotes the portnumber of the service.

CLOSED denotes that the particular port is a closed port.

In security parlance, the term open port is used to mean a TCP or UDP port number that is configured to accept packets. In contrast, a port which rejects connections or ignores all packets directed at it is called a "closed port"

Malicious hackers (or crackers) commonly use port scanning software to find which ports are "open" (unfiltered) in a given computer, and whether or not an actual service is listening on that port. They can then attempt to exploit potential vulnerabilities in any services they find.

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What is ::: in the Local Address of netstat output?

:::* Would be your localhost/loop back in IPv6 :)

Basically, you have services listening and connecting to services locally.

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Filtering UDS from output of netstat without grep

Use

netstat -46

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What is the difference between :::: and 0.0.0.0 from the netstat -an output?

As you have correctly identified, an IPv4 address of 0.0.0.0 is a "catch-all" listen address.

:: is the short-form of 0:0:0:0:0:0:0:0, which is the equivalent address in IPv6. The third colon separates this address from the port number.

This question has been asked on the web before1.


1. discovered via a Google search for netstat colons

description

Netstat prints information about the Linux networking subsystem. The type of information printed is controlled by the first argument, as follows:

(none)
By default, netstat displays a list of open sockets. If you don’t specify any address families, then the active sockets of all configured address families will be printed.

--route , -r
Display the kernel routing tables. See the description in route(8) for details. netstat -r and route -e produce the same output.

--groups , -g
Display multicast group membership information for IPv4 and IPv6.

--interfaces, -i
Display a table of all network interfaces.

--masquerade , -M
Display a list of masqueraded connections.

--statistics , -s
Display summary statistics for each protocol.

options

--verbose , -v
Tell the user what is going on by being verbose. Especially print some useful information about unconfigured address families.

--wide , -W
Do not truncate IP addresses by using output as wide as needed. This is optional for now to not break existing scripts.

--numeric , -n
Show numerical addresses instead of trying to determine symbolic host, port or user names.

--numeric-hosts
shows numerical host addresses but does not affect the resolution of port or user names.

--numeric-ports
shows numerical port numbers but does not affect the resolution of host or user names.

--numeric-users
shows numerical user IDs but does not affect the resolution of host or port names.

--protocol=family , -A
Specifies the address families (perhaps better described as low level protocols) for which connections are to be shown. family is a comma (’,’) separated list of address family keywords like inet, unix, ipx, ax25, netrom, and ddp. This has the same effect as using the --inet, --unix (-x), --ipx, --ax25, --netrom, and --ddp options.

The address family inet includes raw, udp and tcp protocol sockets.

-c, --continuous
This will cause netstat to print the selected information every second continuously.

-e, --extend
Display additional information. Use this option twice for maximum detail.

-o, --timers
Include information related to networking timers.

-p, --program
Show the PID and name of the program to which each socket belongs.

-l, --listening
Show only listening sockets. (These are omitted by default.)

-a, --all
Show both listening and non-listening sockets. With the --interfaces option, show interfaces that are not up

-F
Print routing information from the FIB. (This is the default.)

-C
Print routing information from the route cache.

files

/etc/services -- The services translation file

/proc -- Mount point for the proc filesystem, which gives access to kernel status information via the following files.

/proc/net/dev -- device information

/proc/net/raw -- raw socket information

/proc/net/tcp -- TCP socket information

/proc/net/udp -- UDP socket information

/proc/net/igmp -- IGMP multicast information

/proc/net/unix -- Unix domain socket information

/proc/net/ipx -- IPX socket information

/proc/net/ax25 -- AX25 socket information

/proc/net/appletalk -- DDP (appletalk) socket information

/proc/net/nr -- NET/ROM socket information

/proc/net/route -- IP routing information

/proc/net/ax25_route -- AX25 routing information

/proc/net/ipx_route -- IPX routing information

/proc/net/nr_nodes -- NET/ROM nodelist

/proc/net/nr_neigh -- NET/ROM neighbours

/proc/net/ip_masquerade -- masqueraded connections

/proc/net/snmp -- statistics

notes

Starting with Linux release 2.2 netstat -i does not show interface statistics for alias interfaces. To get per alias interface counters you need to setup explicit rules using the ipchains(8) command.

output

Active Internet connections (TCP, UDP, raw)
Proto

The protocol (tcp, udp, raw) used by the socket.

Recv-Q
The count of bytes not copied by the user program connected to this socket.

Send-Q
The count of bytes not acknowledged by the remote host.

Local Address
Address and port number of the local end of the socket. Unless the --numeric (-n) option is specified, the socket address is resolved to its canonical host name (FQDN), and the port number is translated into the corresponding service name.

Foreign Address
Address and port number of the remote end of the socket. Analogous to "Local Address."

State
The state of the socket. Since there are no states in raw mode and usually no states used in UDP, this column may be left blank. Normally this can be one of several values:
ESTABLISHED

The socket has an established connection.

SYN_SENT

The socket is actively attempting to establish a connection.

SYN_RECV

A connection request has been received from the network.

FIN_WAIT1

The socket is closed, and the connection is shutting down.

FIN_WAIT2

Connection is closed, and the socket is waiting for a shutdown from the remote end.

TIME_WAIT

The socket is waiting after close to handle packets still in the network.

CLOSE

The socket is not being used.

CLOSE_WAIT

The remote end has shut down, waiting for the socket to close.

LAST_ACK

The remote end has shut down, and the socket is closed. Waiting for acknowledgement.

LISTEN

The socket is listening for incoming connections. Such sockets are not included in the output unless you specify the --listening (-l) or --all (-a) option.

CLOSING

Both sockets are shut down but we still don’t have all our data sent.

UNKNOWN

The state of the socket is unknown.

User
The username or the user id (UID) of the owner of the socket.

PID/Program name
Slash-separated pair of the process id (PID) and process name of the process that owns the socket. --program causes this column to be included. You will also need superuser privileges to see this information on sockets you don’t own. This identification information is not yet available for IPX sockets.

Timer
(this needs to be written)

Active UNIX domain Sockets
Proto

The protocol (usually unix) used by the socket.

RefCnt
The reference count (i.e. attached processes via this socket).

Flags
The flags displayed is SO_ACCEPTON (displayed as ACC), SO_WAITDATA (W) or SO_NOSPACE (N). SO_ACCECPTON is used on unconnected sockets if their corresponding processes are waiting for a connect request. The other flags are not of normal interest.

Type
There are several types of socket access:
SOCK_DGRAM

The socket is used in Datagram (connectionless) mode.

SOCK_STREAM

This is a stream (connection) socket.

SOCK_RAW

The socket is used as a raw socket.

SOCK_RDM

This one serves reliably-delivered messages.

SOCK_SEQPACKET

This is a sequential packet socket.

SOCK_PACKET

Raw interface access socket.

UNKNOWN

Who ever knows what the future will bring us - just fill in here :-)

State
This field will contain one of the following Keywords:

FREE

The socket is not allocated

LISTENING

The socket is listening for a connection request. Such sockets are only included in the output if you specify the --listening (-l) or --all (-a) option.

CONNECTING

The socket is about to establish a connection.

CONNECTED

The socket is connected.

DISCONNECTING

The socket is disconnecting.

(empty)

The socket is not connected to another one.

UNKNOWN

This state should never happen.

PID/Program name
Process ID (PID) and process name of the process that has the socket open. More info available in Active Internet connections section written above.

Path
This is the path name as which the corresponding processes attached to the socket.

Active IPX sockets
(this needs to be done by somebody who knows it)

Active NET/ROM sockets
(this needs to be done by somebody who knows it)

Active AX.25 sockets
(this needs to be done by somebody who knows it)


bugs

Occasionally strange information may appear if a socket changes as it is viewed. This is unlikely to occur.


see also

route , ifconfig , ipchains, iptables , proc


authors

The netstat user interface was written by Fred Baumgarten <dc6iq[:at:]insu1.etec[:dot:]uni-karlsruhe.de>, the man page basically by Matt Welsh <mdw[:at:]tc.cornell[:dot:]edu>. It was updated by Alan Cox <Alan.Cox[:at:]linux[:dot:]org> but could do with a bit more work. It was updated again by Tuan Hoang <tqhoang[:at:]bigfoot[:dot:]com>.
The man page and the command included in the net-tools package is totally rewritten by Bernd Eckenfels <ecki[:at:]linux[:dot:]de>.

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