Netstat

Netstat is a command line utility that can be used to list out all the network (socket) connections on a system. It lists out all the tcp, udp socket connections and the unix socket connections.

Apart from connected sockets it can also list listening sockets that are waiting for incoming connections. So by verifying an open port 80 you can confirm if a web server is running on the system or not.

This makes it a very useful tool for network and system administrators.

In this tutorial we shall be checking out few examples of how to use netstat to find information about network connections and open ports on a system.

Here is a quick intro to netstat from the man pages

netstat - Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships

1. List all connections

The first and most simple command is to list out all the current connections. Simply run the netstat command with the a option.

$ netstat -a Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 enlightened:domain *:* LISTEN tcp 0 0 localhost:ipp *:* LISTEN tcp 0 0 enlightened.local:54750 li240-5.members.li:http ESTABLISHED tcp 0 0 enlightened.local:49980 del01s07-in-f14.1:https ESTABLISHED tcp6 0 0 ip6-localhost:ipp [::]:* LISTEN udp 0 0 enlightened:domain *:* udp 0 0 *:bootpc *:* udp 0 0 enlightened.local:ntp *:* udp 0 0 localhost:ntp *:* udp 0 0 *:ntp *:* udp 0 0 *:58570 *:* udp 0 0 *:mdns *:* udp 0 0 *:49459 *:* udp6 0 0 fe80::216:36ff:fef8:ntp [::]:* udp6 0 0 ip6-localhost:ntp [::]:* udp6 0 0 [::]:ntp [::]:* udp6 0 0 [::]:mdns [::]:* udp6 0 0 [::]:63811 [::]:* udp6 0 0 [::]:54952 [::]:* Active UNIX domain sockets (servers and established) Proto RefCnt Flags Type State I-Node Path unix 2 [ ACC ] STREAM LISTENING 12403 @/tmp/dbus-IDgfj3UGXX unix 2 [ ACC ] STREAM LISTENING 40202 @/dbus-vfs-daemon/socket-6nUC6CCx

The above command shows all connections from different protocols like tcp, udp and unix sockets. However this is not quite useful. Administrators often want to pick out specific connections based on protocols or port numbers for example.

2. List only TCP or UDP connections

To list out only tcp connections use the t options.

$ netstat -at Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 enlightened:domain *:* LISTEN tcp 0 0 localhost:ipp *:* LISTEN tcp 0 0 enlightened.local:36310 del01s07-in-f24.1:https ESTABLISHED tcp 0 0 enlightened.local:45038 a96-17-181-10.depl:http ESTABLISHED tcp 0 0 enlightened.local:37892 ABTS-North-Static-:http ESTABLISHED .....

Similarly to list out only udp connections use the u option.

$ netstat -au Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State udp 0 0 *:34660 *:* udp 0 0 enlightened:domain *:* udp 0 0 *:bootpc *:* udp 0 0 enlightened.local:ntp *:* udp 0 0 localhost:ntp *:* udp 0 0 *:ntp *:* udp6 0 0 fe80::216:36ff:fef8:ntp [::]:* udp6 0 0 ip6-localhost:ntp [::]:* udp6 0 0 [::]:ntp [::]:*

The above output shows both ipv4 and ipv6 connections.

3. Disable reverse dns lookup for faster output

By default, the netstat command tries to find out the hostname of each ip address in the connection by doing a reverse dns lookup. This slows down the output. If you do not need to know the host name and just the ip address is sufficient then suppress the hostname lookup with the n option.

$ netstat -ant Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 127.0.1.1:53 0.0.0.0:* LISTEN tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN tcp 0 0 192.168.1.2:49058 173.255.230.5:80 ESTABLISHED tcp 0 0 192.168.1.2:33324 173.194.36.117:443 ESTABLISHED tcp6 0 0 ::1:631 :::* LISTEN

The above command shows ALL TCP connections with NO dns resolution. Got it ? Good.

4. Check listening connections - Open Ports

Any network daemon/service keeps an open port to listen for incoming connections. These too are like socket connections and are listed out by netstat. To view only listening ports use the "-l" option.

$ netstat -tnl Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 127.0.1.1:53 0.0.0.0:* LISTEN tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN tcp6 0 0 ::1:631 :::* LISTEN

Now we can see only listening tcp ports/connections. If you want to see all listening ports, remove the t option. If you want to see only listening udp ports use the "-u" option instead of "-t".

Make sure to remove the "-a" option, otherwise all connections would get listed and not just the listening connections.

If you are running a service like http or smtp on your linux server, then you can use the above command to check whether the service is listening for incoming connections or not.

5. Get process name/pid and user id

When viewing the open/listening ports and connections, its often useful to know the process name/pid which has opened that port or connection.

For example the Apache httpd server opens port 80. So if you want to check whether any http server is running or not, or which http server is running, apache or nginx, then track down the process name.

The process details are made available by the 'p' option.

~$ sudo netstat -nlpt Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 127.0.1.1:53 0.0.0.0:* LISTEN 1144/dnsmasq tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 661/cupsd tcp6 0 0 ::1:631 :::* LISTEN 661/cupsd

When using the p option, netstat must be run with root privileges, otherwise it cannot detect the pids of processes running with root privileges and most services like http and ftp often run with root privileges.

Along with process name/pid its even more useful to get the username/uid owning that particular process. Use the e option along with the p option to get the username too.

$ sudo netstat -ltpe Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name tcp 0 0 enlightened:domain *:* LISTEN root 11090 1144/dnsmasq tcp 0 0 localhost:ipp *:* LISTEN root 9755 661/cupsd tcp6 0 0 ip6-localhost:ipp [::]:* LISTEN root 9754 661/cupsd

The above example lists out Listening connections of Tcp type with Process information and Extended information.

The extended information contains the username and inode of the process. This is a useful command for network administrators.

Note - If you use the n option with the e option, the uid would be listed and not the username.

6. Print statistics

The netstat command can also print out network statistics like total number of packets received and transmitted by protocol type and so on.

To list out statistics of all packet types

$ netstat -s Ip: 32797 total packets received 0 forwarded 0 incoming packets discarded 32795 incoming packets delivered 29115 requests sent out 60 outgoing packets dropped Icmp: 125 ICMP messages received 0 input ICMP message failed. ICMP input histogram: destination unreachable: 125 125 ICMP messages sent 0 ICMP messages failed ICMP output histogram: destination unreachable: 125 ... OUTPUT TRUNCATED ...

To print out statistics of only select protocols like TCP or UDP use the corresponding options like t and u along with the s option. Simple!

7. Display kernel routing information

The kernel routing information can be printed with the r option. It is the same output as given by the route command. We also use the n option to disable the hostname lookup.

$ netstat -rn Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0

8. Print network interfaces

The netstat command can also print out the information about the network interfaces. The "-i" option does the task.

$ netstat -i Kernel Interface table Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg eth0 1500 0 31611 0 0 0 27503 0 0 0 BMRU lo 65536 0 2913 0 0 0 2913 0 0 0 LRU

The above output contains information in a very raw format. To get a more human friendly version of the output use the e option along with i.

$ netstat -ie Kernel Interface table eth0 Link encap:Ethernet HWaddr 00:16:36:f8:b2:64 inet addr:192.168.1.2 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::216:36ff:fef8:b264/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:31682 errors:0 dropped:0 overruns:0 frame:0 TX packets:27573 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:29637117 (29.6 MB) TX bytes:4590583 (4.5 MB) Interrupt:18 Memory:da000000-da020000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:65536 Metric:1 RX packets:2921 errors:0 dropped:0 overruns:0 frame:0 TX packets:2921 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:305297 (305.2 KB) TX bytes:305297 (305.2 KB)

The above output is similar to the output shown by the ifconfig command.

9. Get netstat output continuously

Netstat can output connection information continuously with the c option.

$ netstat -ct

The above command will output tcp connections continuously.

10. Display multicast group information

The g option will display the multicast group information for IPv4 and IPv6 protocols.

$ netstat -g IPv6/IPv4 Group Memberships Interface RefCnt Group --------------- ------ --------------------- lo 1 all-systems.mcast.net eth0 1 224.0.0.251 eth0 1 all-systems.mcast.net lo 1 ip6-allnodes lo 1 ff01::1 eth0 1 ff02::fb eth0 1 ff02::1:fff8:b264 eth0 1 ip6-allnodes eth0 1 ff01::1 wlan0 1 ip6-allnodes wlan0 1 ff01::1

More examples of netstat command

Okay, we covered the basic examples of netstat command above. Now its time to do some geek stuff with style.

Print active connections

Active socket connections are in "ESTABLISHED" state. So to get all current active connections use netstat with grep as follows

$ netstat -atnp | grep ESTA (Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.) tcp 0 0 192.168.1.2:49156 173.255.230.5:80 ESTABLISHED 1691/chrome tcp 0 0 192.168.1.2:33324 173.194.36.117:443 ESTABLISHED 1691/chrome

To watch a continous list of active connections, use the watch command along with netstat and grep

$ watch -d -n0 "netstat -atnp | grep ESTA"

Check if a service is running

If you want to check if a server like http,smtp or ntp is running or not, use grep again.

$ sudo netstat -aple | grep ntp udp 0 0 enlightened.local:ntp *:* root 17430 1789/ntpd udp 0 0 localhost:ntp *:* root 17429 1789/ntpd udp 0 0 *:ntp *:* root 17422 1789/ntpd udp6 0 0 fe80::216:36ff:fef8:ntp [::]:* root 17432 1789/ntpd udp6 0 0 ip6-localhost:ntp [::]:* root 17431 1789/ntpd udp6 0 0 [::]:ntp [::]:* root 17423 1789/ntpd unix 2 [ ] DGRAM 17418 1789/ntpd

So we found that ntp server is running. Grep for http or smtp or whatever you are looking for.

Conclusion

Well, that was most of what netstat is used for. If you are looking for more advanced information or want to dig deeper, read up the netstat manual (man netstat).

And do leave your feedback and suggestions in the comments box below.

Resources

Check out the linux manual page for netstat to learn more.

https://linux.die.net/man/8/netstat

https://man7.org/linux/man-pages/man8/netstat.8.html