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?
traceroute -T -p telnet 1.2.3.4, i.e. using the same TCP port as telnet does. (You can also try the mtr version of traceroute, it may return results faster).tracerouteoutput + recording: asciinema.org/a/e16ndw5jm81yyzfswrieofu83route -nas requested, to confirm that your computers are using the default route to reach 1.2.3.4. If so, hop 1 should represent an IP address which is handled directly on the router. Are the machines you tested on all connected to the same router? What model is the router?