List all IP addresses connected to Server

Below is an example of Unix command to list all the IP addresses connected to your server on port 80. You can customize for another port.

netstat -tn 2>/dev/null | grep :80 | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -nr | head

Output – Total connections by IP, from highest to lowest.

97 114.198.236.100
56 67.166.157.194
44 170.248.43.76
38 141.0.9.20
37 49.248.0.2
37 153.100.131.12
31 223.62.169.73
30 65.248.100.253
29 203.112.82.128
29 182.19.66.187

Note
This command is useful to detect if your server is under attack, and null route those IPs. Read this null route attacker IP story.

Let break above lengthy command into pieces :

1. netstat -tn 2>/dev/null

Uses netstat to list all network connections, ins and outs.

-n – Display numeric only, don’t resolve into name.
-t – Display only TCP connections.

Output

#Examples - 7 connections tcp        0      0 64.91.*.*:80            114.198.236.100:12763       TIME_WAIT    tcp        0      0 64.91.*.*:80            175.136.226.244:51950       TIME_WAIT    tcp        0      0 64.91.*.*:80            175.136.226.244:51951       TIME_WAIT    tcp        0      0 64.91.*.*:23            202.127.210.2:14517         TIME_WAIT    tcp        0      0 64.91.*.*:80            149.238.193.121:65268       TIME_WAIT    tcp        0      0 64.91.*.*:80            114.198.236.100:44088       ESTABLISHED tcp        0      0 64.91.*.*:80            175.136.226.244:51952       TIME_WAIT

2>/dev/null
Redirect all unwanted output to /dev/null, a special place to absorb all output and clear it.

2. grep :80

Only display the IP address that connected to server on port 80.

tcp        0      0 64.91.*.*:80            114.198.236.100:12763       TIME_WAIT    tcp        0      0 64.91.*.*:80            175.136.226.244:51950       TIME_WAIT    tcp        0      0 64.91.*.*:80            175.136.226.244:51951       TIME_WAIT    tcp        0      0 64.91.*.*:80            149.238.193.121:65268       TIME_WAIT    tcp        0      0 64.91.*.*:80            114.198.236.100:44088       ESTABLISHED tcp        0      0 64.91.*.*:80            175.136.226.244:51952       TIME_WAIT

3. awk ‘{print $5}’

Uses awk to display the 5th field only.

114.198.236.100:12763  
175.136.226.244:51950
175.136.226.244:51951
149.238.193.121:65268
114.198.236.100:44088
175.136.226.244:51952
Copy

4. cut -d: -f1

Uses cut to extract the content.

-d – Character immediately following the -d option is use as delimiter, default is tab.
-f – Specifies a field list, separated by a delimiter.

114.198.236.100 175.136.226.244 175.136.226.244 149.238.193.121 114.198.236.100 175.136.226.244

5. sort | uniq -c | sort -nr

Sort the list, group it and sort it again in reverse order.

sort

114.198.236.100 114.198.236.100 149.238.193.121 175.136.226.244 175.136.226.244 175.136.226.244

uniq -c – Group it.

2 114.198.236.100 1 149.238.193.121 3 175.136.226.244

sort -nr – sort by numeric, and reverse order (highest display first)

3 175.136.226.244 2 114.198.236.100 1 149.238.193.121

Done.

6. head

This is optional, to display the first 10 result.