Learning to Use the ss Tool to its Full Potential

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The study of socket connections is important for every UNIX and network administrator because it allows you to better understand your Linux system’s status. Written by Alexey Kuznetosv to replace the famous netstat utility , the more capable ss (socket statistics) utility allows you to monitor TCP, UDP, and UNIX sockets. The purpose of this guide is to help you learn the ss utility and to use it as productively as possible.

Running ss without using the sudo utility will result in different output. Practically, this means that running ss without root privileges will show the results available to the current user only. If you are not familiar with the sudo command, see the Users and Groups guide.

Command Line Options

The ss(8) binary supports many command line options, including the following:

Option Definition
-h The -h option shows a summary of all options.
-V The -V option displays the version of ss
-H The -H option tells ss to suppress the header line – this is useful when you want to process the generated output using a scripting language.
-t The -t parameter tells ss to show TCP connections only.
-u The –u parameter tells ss to show UDP connections only.
-d The –d parameter tells ss to show DCCP sockets only.
-S The –S parameter tells ss to show SCTP sockets only.
-a The -a option tells ss to display both listening and non-listening sockets of every kind.
-l The -l parameter tells ss to display listening sockets, which are omitted by default.
-e The -e option tells ss to display detailed socket information.
-x The -x parameter tells ss to display UNIX domain sockets only.
-A The -A option allows you to select the socket types that you want to see. The -A option accepts the following set of identifiers that can be combined and separated by commas: all, inet, tcp, udp, raw, unix, packet, netlink, unix_dgram, unix_stream, unix_seqpacket, packet_raw and packet_dgram.
-4 The -4 command line option tells ss to display IPv4 connections only.
-6 The -6 command line option tells ss to display IPv6 connections only.
-f FAMILY The -f tells ss to display sockets of type FAMILY. The supported values are unix, inet, inet6 and netlink.
-s The -s option displays useful statistics about the current connections.
-o The -o option displays timer information. There are five types of timers: on, which is either a TCP retrans timer, a TCP early retrans timer, or a tail loss probe timer; keepalive, which is the TCP keep alive timer; timewait, which is the timewait stage timer; persist, which is the zero window probe timer; and unknown, which is a timer that is none of the other timers.
-n The -n option tells ss to disable the resolving of service names.
-r The -r option tells ss to enable DNS resolving in the output, which is turned off by default.
-m The -m parameter tells ss to display socket memory usage information.
-p The -p parameter tells ss to display the process that is using a socket.
-D FILE The -D parameter tells ss to save the output in the FILE file.
The -A tcp option is equivalent to -t, the -A udp option is equivalent to -u and the –A unix option is equivalent to -x.

Installing ss

The ss tool is part of the IPROUTE2 Utility Suite. Since the ss command line tool is usually installed by default, you will not need to install it yourself. On a Debian Linux system, you can find the ss executable inside /bin.

If for some reason ss is not installed on your Linux system, you should install the iproute2 package using your favorite package manager.


Basic Usage

The simplest way to use ss is without any command line parameters. When ss is used without any command line arguments, it prints all TCP, UDP and socket connections. The list might get big on busy machines, which means that it can become more difficult to parse – the output of wc(1), (a word count utility), shows that the list is long yet manageable:

ss | wc
     94     750    7926

If you also use the -a parameter to show all listening and non-listening sockets, the output will be much higher:

ss -a | wc
    224    1682   19562

Listing Sockets


The following command displays all listening and non-listening (-a) TCP (-t) sockets:

ss -t -a
State    Recv-Q  Send-Q  Local Address:Port   Peer Address:Port
LISTEN   0       80      *:*
LISTEN   0       128     *:ssh                *:*
LISTEN   0       100     *:smtp               *:*
ESTAB    0       204
LISTEN   0       128     :::http              :::*
LISTEN   0       128     :::ssh               :::*
LISTEN   0       128     :::https             :::*

The output is separated into columns. The first column, state, shows the state of the TCP connection. As the example is using the -a option, both listening and non-listening states are included in the output. The second and the third columns, Recv-Q and Send-Q, show the amount of data queued for receive and transmit operations. The Local Address:Port column shows the IP address the process listens to as well as the port number that is used - you can connect the name of the service with a numeric value by looking at the /etc/services file. The last column, Peer Address:Port, is useful when there is an active connection because it shows the address and port number of the client machine, though here it is without any real values for TCP connections that are in the LISTEN state. As the -r option is not used, you only see IP addresses in the output.

Running ss -t without –a will display established TCP connections only:

ss -t
State  Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
ESTAB  0       204


The following command displays all UDP (-u) sockets:

ss -u -a
State    Recv-Q  Send-Q  Local Address:Port                  Peer Address:Port
UNCONN   0       0       *:mdns                              *:*
UNCONN   1536    0               *:*
UNCONN   0       0       *:54087                             *:*
UNCONN   0       0       *:bootpc                            *:*
UNCONN   0       0                  *:*
UNCONN   0       0                       *:*
UNCONN   0       0       *:ntp                               *:*
UNCONN   0       0       :::mdns                             :::*
UNCONN   0       0       :::48582                            :::*
UNCONN   0       0       fe80::f03c:91ff:fe69:1381%eth0:ntp  :::*
UNCONN   0       0       2a01:7e00::f03c:91ff:fe69:1381:ntp  :::*
UNCONN   0       0       ::1:ntp                             :::*
UNCONN   0       0       :::ntp                              :::*

Running ss -u without –a will display established UDP connections only. In this case there are no established UDP connections:

ss -u
Recv-Q Send-Q  Local Address:Port  Peer Address:Port

Display Statistics

You can display statistics about the current connections using the -s option:

ss -s
Total: 199 (kernel 228)
TCP:   9 (estab 1, closed 2, orphaned 0, synrecv 0, timewait 0/0), ports 0

Transport  Total  IP  IPv6
*          228    -   -
RAW        0      0   0
UDP        13     7   6
TCP        7      4   3
INET       20     11  9
FRAG       0      0   0

Filter by TCP State

ss allows you to filter its output by state using the state and exclude keywords followed by a state identifier. The state keyword displays output that matches the provided identifier, whereas the exclude keyword displays everything except the output that matches the identifier.

The use of state is illustrated in the next example:

ss -t4 state established
Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
0       0

The use of exclude is illustrated in the next example:

ss -t4 exclude established
State      Recv-Q  Send-Q  Local Address:Port   Peer Address:Port
LISTEN     0       80      *:*
LISTEN     0       128     *:ssh                *:*
LISTEN     0       100     *:smtp               *:*
TIME-WAIT  0       0

The -t4 command option returns IPv4 TCP connections.

Filter Output by IP Address and Port Number

The more you filter the output of ss, the more accurate and relevant information you will receive. There exist two ss options that allow you to include connections from certain IP addresses and port numbers.

The following command shows traffic from a given IP address only, using the dst keyword:

ss -nt dst
State        Recv-Q  Send-Q  Local Address:Port         Peer Address:Port
ESTAB        0      0
FIN-WAIT-1   0       32      ::ffff:  ::ffff:
ESTAB        0       0       ::ffff:  ::ffff:
ESTAB        0       0       ::ffff:  ::ffff:
ESTAB        0       0       ::ffff:  ::ffff:

If you want to display traffic from an entire network, you can replace the IP address with a network address such as 2.86.7/24.

The following command displays information about the HTTP and the HTTPS protocols, which are associated with port numbers 80 and 443 as defined in /etc/services:

ss -at '( dport = :http or dport = :https or sport = :http or sport = :https )'
State      Recv-Q  Send-Q  Local Address:Port           Peer Address:Port
LISTEN     0       128     :::http                      :::*
LISTEN     0       128     :::https                     :::*
ESTAB      0       0       ::ffff:  ::ffff:
ESTAB      0       0       ::ffff:  ::ffff:
ESTAB      0       0       ::ffff:  ::ffff:
ESTAB      0       0       ::ffff:  ::ffff:
ESTAB      0       0       ::ffff:  ::ffff:
ESTAB      0       0       ::ffff:  ::ffff:
TIME-WAIT  0       0       ::ffff:   ::ffff:

dport means destination port and sport means source port.

The following command is equivalent to the previous command:

ss -at '( dport = :80 or dport = :443 or sport = :80 or sport = :443 )'

Display Timer Information

The -o option displays timer information:

ss -nt dst -o
State  Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
ESTAB  0       0     timer:(keepalive,72min,0)

Enable IP Address Resolving

The -r parameter enables IP address resolving, which returns the domain names of the IP addresses:

ss -r -t
State  Recv-Q  Send-Q    Local Address:Port                  Peer Address:Port
ESTAB  0       168       li140-253.members.linode.com:ssh    ppp-2-86-7-61.home.otenet.gr:50939
ESTAB  0       0         li140-253.members.linode.com:https  ::ffff:

A side effect of the -r command line option is that it slows the execution of the ss command due to the DNS lookups that need to be performed.

Display Detailed Socket Information

The -e option tells ss to display detailed socket information. The -e option is illustrated in the following example:

ss -t -e
State  Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
ESTAB  0       0    timer:(keepalive,54min,0) ino:10195329 sk:11e <->

Show a Connection’s UNIX Process

The -p option displays the process ID(s) and the process name of a connection:

ss -t -p
State  Recv-Q  Send-Q  Local Address:Port           Peer Address:Port
ESTAB  0       204             users:(("sshd",pid=3964,fd=3),("sshd",pid=3951,fd=3))
ESTAB  0       51      ::ffff:  ::ffff:  users:(("apache2",pid=30871,fd=29))

The following command shows SSH-related processes on the current machine:

ss -t -p -a | grep ssh
LISTEN  0  128  *:ssh               *:*                    users:(("sshd",pid=812,fd=3))
ESTAB   0  36        users:(("sshd",pid=3964,fd=3),("sshd",pid=3951,fd=3))
ESTAB   0  0  users:(("sshd",pid=8538,fd=3),("sshd",pid=8537,fd=3))
LISTEN  0  128  :::ssh              :::*                   users:(("sshd",pid=812,fd=4))

Find Which Process is Using a Given Port Number

With the help of ss and grep(1), you can discover which process is using a given port number:

ss -tunap | grep :80
tcp  LISTEN  0  128  :::80 :::*  users:(("apache2",pid=8772,fd=4),("apache2",pid=8717,fd=4),("apache2",pid=8715,fd=4),("apache2",pid=8714,fd=4),("apache2",pid=8713,fd=4),("apache2",pid=8712,fd=4),("apache2",pid=8711,fd=4),("apache2",pid=8709,fd=4))

As Apache uses multiple child processes, you receive a list of processes for port number 80.

The next command will do exactly the same thing without using grep(1):

ss -tup -a sport = :80
Netid  State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
tcp    LISTEN  0       128     :::http             :::*               users:(("apache2",pid=8715,fd=4),("apache2",pid=8714,fd=4),("apache2",pid=8713,fd=4),("apache2",pid=8712,fd=4),("apache2",pid=8711,fd=4),("apache2",pid=8709,fd=4))

Find Open Ports Above Port Number 1024

ss supports ranges when working with port numbers. This feature is illustrated in the following example that finds open port above port number 1024:

ss -t -a sport \> :1024
State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
LISTEN  0       80     *:*

The ss -t -a sport \> :1024 command can be also written as ss -t -a sport '> :1024'.

Search for Specific TCP Characteristics

The following command shows all TCP connections that use IPv4 that are in listening state, as well as the name of the process using the socket without resolving the IP addresses and the port number:

ss -t -4nlp
State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
LISTEN  0       80      *:*                users:(("mysqld",pid=1003,fd=17))
LISTEN  0       128     *:22                *:*                users:(("sshd",pid=812,fd=3))
LISTEN  0       100     *:25                *:*                users:(("smtpd",pid=9011,fd=6),("master",pid=1245,fd=13))

The following command shows all SSH related connections and sockets:

ss -at '( dport = :ssh or sport = :ssh )'
State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
LISTEN  0       128     *:ssh               *:*
ESTAB   0       0
LISTEN  0       128     :::ssh              :::*

Show Sockets in a Listening State

The following command shows TCP sockets in listening (-l) state:

ss -l -t
State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
LISTEN  0       80     *:*
LISTEN  0       128     \*:ssh              *:*
LISTEN  0       100     *:smtp              *:*
LISTEN  0       128     :::http             :::*
LISTEN  0       128     :::ssh              :::*
LISTEN  0       128     :::https            :::*

The following command shows IPv4 UDP sockets in listening state:

ss -l -u -4
State   Recv-Q  Send-Q  Local Address:Port      Peer Address:Port
UNCONN  0       0       *:mdns                  *:*
UNCONN  1536    0  *:*
UNCONN  0       0        *:54087                *:*
UNCONN  0       0        *:bootpc               *:*
UNCONN  0       0     *:*
UNCONN  0       0          *:*
UNCONN  0       0        *:ntp                  *:*

Advanced Filtering with ss

The following ss command will list all TCP sockets that are in the ESTABLISHED state, use HTTP or HTTPS on the local machine and belong to the 2.86.7/24 network and display their timers:

ss -o state established '( sport = :http or sport = :https )' dst 2.86.7/24
Netid  Recv-Q  Send-Q  Local Address:Port           Peer Address:Port
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)
tcp    0       0       ::ffff:  ::ffff:  timer:(keepalive,119min,0)

Apart from the standard TCP state names (established, syn-sent, syn-recv, fin-wait-1, fin-wait-2, time-wait, closed, close-wait, last-ack, listen and closing), you can also use the following states:

  • all: For all the states.
  • bucket: For TCP minisockets (TIME-WAIT|SYN-RECV) states.
  • big: For all states except for minisockets - this is the opposite of bucket.
  • connected: For the not closed and not listening states.
  • synchronized: For connected and not SYN-SENT states.

Using AWK to Process ss Output

The following command displays a summary of all sockets based on their state:

ss -t -u -a | awk '{print $1}' | grep -v State | sort | uniq -c | sort -nr
     13 udp
      7 tcp
      1 Netid

The following command displays a summary of all sockets based on their protocol:

ss -a | awk '{print $1}' | grep -v State | sort | uniq -c | sort -nr
    133 u_str
     37 u_dgr
     34 nl
     13 udp
      8 tcp
      1 u_seq
      1 p_raw
      1 Netid

The last command will create a summary of all IPv6 TCP connections that are in the CONNECTED state:

ss -t6 state connected | awk '{print $1}' | grep -v State | sort | uniq -c | sort -nr

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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