Linux: Check Network Connection Command

1. 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.
2. netstat command: It can display network connections, routing
   tables, interfaces and much more.
3. 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
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Linux: Check Network Connection Command                            http://www.cyberciti.biz/faq/check-network-connec...
        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
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            *:*
# 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.