How do I check network connections under Linux using command line options?
You need to use any one of the following tool or command under Linux to check network connections including their state, source/destination, and addresses and bandwidth usage etc:
- ss command: It dump socket (network connection) statistics such as all TCP / UDP connections, established connection per protocol (e.g., display all established ssh connections), display all the tcp sockets in various state such as ESTABLISHED or FIN-WAIT-1 and so on.
- netstat command: It can display network connections, routing tables, interfaces and much more.
- tcptrack and iftop commands: Displays information about TCP connections it sees on a network interface and display bandwidth usage on an interface by host respectively.
Display Currently Established, Closed, Orphaned and Waiting TCP sockets, enter:
# ss -s
Sample outputs:
Total: 529 (kernel 726) TCP: 1403 (estab 286, closed 1099, orphaned 1, synrecv 0, timewait 1098/0), ports 774 Transport Total IP IPv6 * 726 - - RAW 0 0 0 UDP 27 13 14 TCP 304 298 6 INET 331 311 20 FRAG 0 0 0
Or you can use the netstat command as follows:
# netstat -s
Sample outputs:
Ip:
102402748 total packets received
3 with invalid addresses
0 forwarded
0 incoming packets discarded
102192035 incoming packets delivered
95627316 requests sent out
Icmp:
6726 ICMP messages received
167 input ICMP message failed.
ICMP input histogram:
destination unreachable: 2353
timeout in transit: 4
echo requests: 4329
10323 ICMP messages sent
0 ICMP messages failed
ICMP output histogram:
destination unreachable: 5994
echo replies: 4329
IcmpMsg:
InType3: 2353
InType8: 4329
InType11: 4
OutType0: 4329
OutType3: 5994
Tcp:
839222 active connections openings
2148984 passive connection openings
1480 failed connection attempts
1501 connection resets received
281 connections established
101263451 segments received
94668430 segments send out
9820 segments retransmited
0 bad segments received.
1982 resets sent
Udp:
1024635 packets received
18 packets to unknown port received.
0 packet receive errors
1024731 packets sent
TcpExt:
592 invalid SYN cookies received
396 resets received for embryonic SYN_RECV sockets
2 packets pruned from receive queue because of socket buffer overrun
3 ICMP packets dropped because they were out-of-window
2166428 TCP sockets finished time wait in fast timer
2773 time wait sockets recycled by time stamp
11 packets rejects in established connections because of timestamp
1223930 delayed acks sent
298 delayed acks further delayed because of locked socket
Quick ack mode was activated 263 times
16413851 packets directly queued to recvmsg prequeue.
627914668 packets directly received from backlog
1146535624 packets directly received from prequeue
43628505 packets header predicted
10588802 packets header predicted and directly queued to user
8646100 acknowledgments not containing data received
44710342 predicted acknowledgments
5 times recovered from packet loss due to fast retransmit
596 times recovered from packet loss due to SACK data
2 bad SACKs received
Detected reordering 1 times using FACK
Detected reordering 4 times using time stamp
3 congestion windows fully recovered
7 congestion windows partially recovered using Hoe heuristic
TCPDSACKUndo: 44
208 congestion windows recovered after partial ack
2743 TCP data loss events
190 timeouts after SACK recovery
90 timeouts in loss state
2268 fast retransmits
220 forward retransmits
3704 retransmits in slow start
1318 other TCP timeouts
TCPRenoRecoveryFail: 2
98 sack retransmits failed
226 packets collapsed in receive queue due to low socket buffer
267 DSACKs sent for old packets
1 DSACKs sent for out of order packets
272 DSACKs received
34 connections reset due to unexpected data
184 connections reset due to early user close
147 connections aborted due to timeout
IpExt:
InBcastPkts: 10
Display All Open Network Ports
Use the ss command as follows:
# ss -l
Sample outputs:
Recv-Q Send-Q Local Address:Port Peer Address:Port 0 50 127.0.0.1:mysql *:* 0 128 127.0.0.1:11211 *:* 0 128 *:sunrpc *:* 0 128 :::www :::* 0 128 *:55153 *:* 0 3 10.1.11.27:domain *:* 0 3 192.168.1.101:domain *:* 0 3 127.0.0.1:domain *:* 0 3 :::domain :::* 0 128 :::ssh :::* 0 128 *:ssh *:* 0 128 127.0.0.1:ipp *:* 0 128 ::1:ipp :::* 0 128 :::afs3-callback :::* 0 128 *:afs3-callback *:* 0 100 *:smtp *:* 0 128 ::1:953 :::* 0 128 127.0.0.1:953 *:* 0 5 *:8000 *:*
OR Use the netstat command as follows:
# netstat -tulpn
Sample outputs:
Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN 1380/mysqld tcp 0 0 127.0.0.1:11211 0.0.0.0:* LISTEN 1550/memcached tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN 936/portmap tcp 0 0 0.0.0.0:55153 0.0.0.0:* LISTEN 1025/rpc.statd tcp 0 0 10.1.11.27:53 0.0.0.0:* LISTEN 1343/named tcp 0 0 192.168.1.101:53 0.0.0.0:* LISTEN 1343/named tcp 0 0 127.0.0.1:53 0.0.0.0:* LISTEN 1343/named tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 979/sshd tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 1828/cupsd tcp 0 0 0.0.0.0:7001 0.0.0.0:* LISTEN 10129/transmission tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 1694/master tcp 0 0 127.0.0.1:953 0.0.0.0:* LISTEN 1343/named tcp 0 0 0.0.0.0:8000 0.0.0.0:* LISTEN 1539/icecast2 tcp6 0 0 :::80 :::* LISTEN 1899/apache2 tcp6 0 0 :::53 :::* LISTEN 1343/named tcp6 0 0 :::22 :::* LISTEN 979/sshd tcp6 0 0 ::1:631 :::* LISTEN 1828/cupsd tcp6 0 0 :::7001 :::* LISTEN 10129/transmission tcp6 0 0 ::1:953 :::* LISTEN 1343/named udp 0 0 239.255.255.250:1900 0.0.0.0:* 11937/opera udp 0 0 239.255.255.250:1900 0.0.0.0:* 11937/opera udp 0 0 0.0.0.0:111 0.0.0.0:* 936/portmap udp 0 0 0.0.0.0:777 0.0.0.0:* 1025/rpc.statd udp 0 0 0.0.0.0:38297 0.0.0.0:* 1025/rpc.statd udp 0 0 192.168.1.101:33843 0.0.0.0:* 11937/opera udp 0 0 10.1.11.27:53 0.0.0.0:* 1343/named udp 0 0 192.168.1.101:53 0.0.0.0:* 1343/named udp 0 0 127.0.0.1:53 0.0.0.0:* 1343/named udp 0 0 0.0.0.0:68 0.0.0.0:* 5840/dhclient udp 0 0 127.0.0.1:11211 0.0.0.0:* 1550/memcached udp 0 0 0.0.0.0:7001 0.0.0.0:* 10129/transmission udp 0 0 10.1.11.27:33372 0.0.0.0:* 11937/opera udp6 0 0 :::53 :::* 1343/named
Display All TCP Sockets
Type the ss command as follows:
# ss -t -a
Or use the netstat command as follows:
# netstat -nat
Display All UDP Sockets
Type the ss command as follows:
# ss -u -a
Or use the netstat command as follows:
# netstat -nau
lsof Command
You can use the lsof command follows to list more information about open ports:
# lsof -i :portNumber
# lsof -i tcp:portNumber
# lsof -i udp:portNumber
# lsof -i :80 | grep LISTEN
View Established Connections Only
Use the netstat command as follows:
# netstat -natu | grep 'ESTABLISHED'
Say Hello To tcptrack
The tcptrack command displays the status of TCP connections that it sees on a given network interface. tcptrack monitors their state and displays information such as state, source/destination addresses and bandwidth usage in a sorted, updated list very much like the top command.
# tcptrack -i eth0
Sample outputs:
Fig.01: tcptrack in Action
See how to install and use the tcptrack tool under Linux for more information.
iftop command
The iftop command listens to network traffic on a given network interface such as eth0, and displays a table of current bandwidth usage by pairs of hosts:
# iftop -i eth1
It can display or analyses packet flowing in and out of the 192.168.1.0/24 network:
# iftop -F 192.168.1.0/24
Sample outputs:
