see also :
route - ifconfig - iptables
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]
Step 2
I use netstat -tapn to have instant current network connectionsexample added by LeBerger
I use netstat -tapn to have instant current network connectionsexample added by LeBerger
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.
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.
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.
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.
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.
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.
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")
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.
:::* Would be your localhost/loop back in IPv6 :)
Basically, you have services listening and connecting to services locally.
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
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.
--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.
/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
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.
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)
Occasionally strange information may appear if a socket changes as it is viewed. This is unlikely to occur.
route , ifconfig , ipchains, iptables , proc
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>.