TCP (Transmission Control Protocol) is a standard that defines how
network conversation between two systems is established and maintained
to facilitate an exchange of data between applications. Internet
Protocol (IP) defines how systems send packets of data to each other.
The TCP States in Linux
Below is a list of TCP connection states that can be viewed using netstat or ss command on Linux.
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.
For the difference in usage between ss and netstat command, check netstat vs ss usage guide on Linux.
Use the command below to check all applications TCP states on your
Linux server, it will give you the number of processes in each state.
# netstat -nat | awk '{print $6}' | sort | uniq -c | sort -r
8959 CLOSE_WAIT
887 FIN_WAIT2
6 SYN_RECV
5597 TIME_WAIT
472 ESTABLISHED
24 LISTEN
1 SYN_SENT
1 Foreign
1 FIN_WAIT1
1 established)
183 LAST_ACKTo understand the options used in the command, read netstat vs ss usage guide on Linux.
You can also get the list of processes in a particular state by piping
the output to grep. For example to get processes in CLOSEWAIT state, use # netstat -apn | grep CLOSE_WAIT You can further filter this output to get process ID of the processes in CLOSEWAIT state.
netstat -apn | grep CLOSE_WAIT | awk '{ print $7 }' | sort | uniq -c | sort -nr
If you want to limit the output to top 10 processes with CLOSE_WAIT TCP connection state, use head
# netstat -apn | grep CLOSE_WAIT | awk '{ print $7 }' | sort | uniq -c | sort -nr | head -n 10
3856 8166/jsvc.exec
1783 15643/jsvc.exec
1313 26749/jsvc.exec
1203 11450/jsvc.exec
563 22495/jsvc.exec
270 6698/jsvc.exec
229 22625/jsvc.exec
9 9729/jsvc.exec
2 32038/httpd
2 29352/httpdThis shows that the Process with ID 8166 has 3856 CLOSE_WAIT connection states.
If you’re running short of TCP connections or doing troubleshooting,
you may need to identify this process with a large number of CLOSE_WAIT
connection states. It could mean that the application doesn’t close
connections as expected.
# ps 8166
PID TTY STAT TIME COMMAND
8166 ? Sl 242:29 jsvc.exec -debug -pidfile /var/run/myapp.pid myapp.jarI made a simple bash script which uses the commandnetstat to identify count for TCP connection states and the processes with many states in CLOSE_WAIT.
#!/bin/bash
# Script to print Linux TCP connections using netstat
# Github: https://github.com/jmutai
#
# vvvv vvvv-- the code from above
RED='\033[0;31m'
NC='\033[0m' # No Color
echo ""
echo -en "${RED} ALL TCP Connections Count: ${NC}\n"
netstat -nat | awk '{print $6}' | sort | uniq -c | sort -r
echo ""
echo -en "${RED} Top CLOSE_WAIT state TCP Connections: ${NC}\n"
netstat -apn | grep CLOSE_WAIT | awk '{ print $7 }' | sort | uniq -c | sort -nr | head -n 10Sample output:
Thanks for your time.
