Adrian's KB

Linux Networking

3 min read

1. Open SSH Tunnels in Linux shell

 
ssh -L 9876:192.168.100.110:443 root@10.60.9.46

Command composition

SyntaxDescription
sshProgram to start Secure Shell
-L
:192.168.100.110ip address of the destination
:443Port in the destination
rootusername in the tunnel/jump serve
@10.60.9.46tunnel server

2. Enable SSH Server on Linux

Check if openssh-server is installed

 
kali@kali:~$ dpkg -l | grep openssh-server
ii  openssh-server                         1:9.9p1-3                                amd64        secure shell (SSH) server, for secure access from remote machines

Check if telnet is allowed

 
kali@kali:~$ telnet 127.0.0.1 22          
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused

Check if the service is running

kali@kali:~$ service ssh status 
○ ssh.service - OpenBSD Secure Shell server
     Loaded: loaded (/usr/lib/systemd/system/ssh.service; disabled; preset: >
     Active: inactive (dead)
       Docs: man:sshd(8)
             man:sshd_config(5)

Start SSH service if it’s not running

 
kali@kali:~$ service ssh start

Verify the service was successfully started

 
kali@kali:~$ service ssh status
● ssh.service - OpenBSD Secure Shell server
     Loaded: loaded (/usr/lib/systemd/system/ssh.service; disabled; preset: >
     Active: active (running) since Wed 2025-06-18 18:06:51 EDT; 1s ago
 Invocation: f9b83328674d42bfb87bbc57ac93df1c
       Docs: man:sshd(8)
             man:sshd_config(5)
    Process: 242265 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SU>
   Main PID: 242268 (sshd)
      Tasks: 1 (limit: 2199)
     Memory: 2.1M (peak: 2.6M)
        CPU: 47ms
     CGroup: /system.slice/ssh.service
             └─242268 "sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startu>
 
Jun 18 18:06:51 kali systemd[1]: Starting ssh.service - OpenBSD Secure Shell>
Jun 18 18:06:51 kali sshd[242268]: Server listening on 0.0.0.0 port 22.
Jun 18 18:06:51 kali sshd[242268]: Server listening on :: port 22.
Jun 18 18:06:51 kali systemd[1]: Started ssh.service - OpenBSD Secure Shell >

3. DIG Diagnostic Tools

DNS Diagnostics (Dig)

Monday, June 26, 2023

9:50 AM

Attempt to use Dig instead of NSLookup

dig +noall +noedns +answer -x 127.0.0.1

dig +noall +noedns +answer localhost

Check a range (10.2.4.99-112)

for ((i=99; i⇐112; i++)); do dig +noall +noedns +answer -x “10.2.4.$i”; done

Dig specify DNS server

dig @x.x.x.x +noall +noedns +answer localhost

Where x.x.x.x is the DNS IP address

4. Tcpdump

TCP DUMP

Tuesday, September 26, 2023

10:36 AM

tcpdumps -i la interface

Sample

root@avamar#: tcpdump -i any -n host datadomainipaddress and port (161 or 162 or 163) -vv

systadmin@datadomain#: se tcpdump -n host avamaripaddress and port (161 or 162 or 163) -vv

-n host | specifies the host ip/dns address -n port | specifies the port address grep | can be used to filter output grep -v | can be used to exclude output -w /path/to/file.pcap | can be used to specify output.pcap

5. Iperf

iperf server

iperf -s

IPerf client

iperf -c SourceIP -i 30 -t 300

Where -c | is xxxxxx -i | is xxxxxx -t | is xxxxxx

6. ifconfig / route / ip

How to add ip address to eth

ifconfig add -i eth........ netmask ..

How to add routes and default gateway

route -add .......

Default

route -add default gw ......

ip (modern replacement for ifconfig and route)

Show interfaces
ip a
Show routes
ip r
Add IP
ip addr add 192.168.1.100/24 dev eth0
Add route
ip route add default via 192.168.1.1

7. Telnet / curl portest

Test port with telnet

telnet 127.0.0.1 22

where 127.0.0.1 | this is the ip address 22 | this is the port

Curl

curl -v ..... 127.0.0.1:22 ....

where curl | .. -v | … 127.0.0.1 | ip address 22 | port

8. Netcat (nc)

Test TCP/UDP connections, listen on ports:

nc -zv 127.0.0.1 22

9. Netstat and SS

netstat (deprecated, but still used)

View open ports, connections, and routing tables:

netstat -tuln

ss (modern replacement for netstat)

ss -tuln

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