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I did setup openwrt on a tp-link router. There is a simple configuration now, WAN link I simulate connecting ethernet RJ45 port with my laptop's RJ45 network adapter port (IP on WAN interface is set DHCP from range 192.168.137.0/24, where my laptop's inteface has 192.168.137.1)

In openwrt firewall settings I configured access to drobbear/ssh server by forwarded port 22001:

config redirect
        option src              wan
        option src_dport        22001
        option dest             lan
        option dest_port        22
        option proto            tcp

I can login from my laptop to openwrt via "network cable" (WAN interface on router), however one thing is strange - I can use any IP from range 192.168.137.2-254 to access my router's 22001 port. Why?

cygwin$ ./Fping.exe -n 1 -w 2 -A -g 192.168.137.1/192.168.137.254|grep -E '^Reply'
Reply[1] from 192.168.137.1: bytes=32 time=0.2 ms TTL=128
Reply[42] from 192.168.137.42: bytes=32 time=0.5 ms TTL=64

Nmap finds only one IP with 22001 port opened (openwrt's):

cygwin$ ./drives/c/Program\ Files\ \(x86\)/Nmap/nmap.exe -Pn -p22001 192.168.137.0/24
Starting Nmap 7.80 ( https://nmap.org ) at 2019-12-23 21:22 CET.
Nmap scan report for 192.168.137.42
Host is up (0.00s latency).

PORT      STATE SERVICE
22001/tcp open  optocontrol
MAC Address: 68:FF:7C:B3:C1:14 (Tp-link Technologies)

Nmap scan report for 192.168.137.1
Host is up (0.00s latency).

PORT      STATE  SERVICE
22001/tcp closed optocontrol

Nmap done: 256 IP addresses (2 hosts up) scanned in 6.26 seconds

But telnet on port 22001 (and ssh) works on any IP from range 192.168.137.2-254, example:

cygwin$ telnet 192.168.137.123 22001
Trying 192.168.137.123...
Connected to 192.168.137.123.
Escape character is '^]'.
SSH-2.0-dropbear
[...]
telnet> q
Connection closed.
cygwin$ telnet 192.168.137.222 22001
Trying 192.168.137.222...
Connected to 192.168.137.222.
Escape character is '^]'.
SSH-2.0-dropbear
[...]
telnet> q
Connection closed.

And indeed, I can 'ssh -p 22001' any such IP to login my router... Could somebody explain it, please?

  • Can you add to question the IP address of the machines that you are telneting /sshing from. I think the command ipconfig (for your OS) may provide this. – ctrl-alt-delor Dec 23 '19 at 22:28

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