Q: Telnet connect to non-existing adress I'm having unexpected behavior using the telnet command on various linux (Linux Mint, Ubuntu server).
When trying to connect to a non-existent device, it succeed. I tested with 1.2.3.4 which is not a placeholder.
Telnet
$ telnet 1.2.3.4 9100
Trying 1.2.3.4...
Connected to 1.2.3.4.
Escape character is '^]'.
^]
telnet> Connection closed.

Ping
fails as expected, this is not the problem!
$ ping 1.2.3.4 -c 5
PING 1.2.3.4 (1.2.3.4) 56(84) bytes of data.

--- 1.2.3.4 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4076ms

Traceroute (update)
$ sudo traceroute -T -p telnet 1.2.3.4
traceroute to 1.2.3.4 (1.2.3.4), 30 hops max, 60 byte packets
 1  1.2.3.4 (1.2.3.4)  2.693 ms  3.166 ms  3.178 ms

Route
$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.2.240   0.0.0.0         UG    600    0        0 wlp4s0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 br-427309471a28
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 docker0
172.18.0.0      0.0.0.0         255.255.0.0     U     0      0        0 br-427309471a28
192.168.2.0     0.0.0.0         255.255.255.0   U     600    0        0 wlp4s0

Even when stopping docker I still can reproduce:
$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.2.240   0.0.0.0         UG    600    0        0 wlp4s0
192.168.2.0     0.0.0.0         255.255.255.0   U     600    0        0 wlp4s0

Question
Here is a recording.
Why is telnet connecting on a device that doesn't exist?
