Why I can't I telnet to a specific port?

Question

the port 7778 is in listen,

netstat -tulpn
tcp        0      0 127.0.0.1:7778   0.0.0.0:*        LISTEN      22776/java

but I can't telnet that port from remote machine while I can telnet other port by using this command

telnet 192.168.1.100 port_number

What do you mean "remote machine"? A machine outside your LAN, or a machine in your LAN? — zhongfu, Apr 14 '15 at 13:26
I am pretty sure its because you are only listening on ip address 127.0.0.1 so only on loopback. You cant connect to that from anywhere. — kubacapek, Apr 14 '15 at 13:31
You said you couldn't "telnet that port" (7778) from the other machine, but what follows that suggests that you could do that. Could you explain what you're doing that doesn't work? — zhongfu, Apr 14 '15 at 13:32
i edit my question, i meant that i can telnet other ports from remote machine — Nadya Nux, Apr 14 '15 at 13:37
this looks like a XY problem, was java runtime meant to be use from external hosts ? is yes, you should configure it. — Archemar, Apr 14 '15 at 13:41
yes it will be used from external hosts, this was my question in comment bellow, how to configure java to listen on IP address instead of localhost to be able to use it from remote machines. — Nadya Nux, Apr 14 '15 at 13:45
@heemayl : first time heard about XY question, because i still new here — Nadya Nux, Apr 14 '15 at 14:54

score 2 · Answer 1 · answered Apr 14 '15 at 13:32

2

The address 127.0.0.1 is the loopback address.

As you have the 127.0.0.1:7788 in the "Local Address" of netstat output, this means that the connection is only listening for connections originating from this computer only on the loopback interface. No other computers on the network can reach your loopback address directly hence the telnet is failing from other computers.

answered Apr 14 '15 at 13:32

heemayl

94,145

yes I can only telnet that port from localhost but i can't telnet it from remote host, so howto change it? i mean to use the address it instead of localhost and make it accessible by other machines? – Nadya Nux Apr 14 '15 at 13:35

Archemar · Answer 2 · 2015-04-14T13:53:47.783

2

java is listening in 127.0.0.1, that is localhost.

You can't connect from outside, unless you do some kind of forwarding, using ssh for instance.

Edit:

from external hosts,

if unix/linux

ssh -L 1234:127.0.0.1:7778 runtime

then from that external host, telnet 127.0.0.1 1234 (ssh will forward you)

if windows, use putty or bitwise to forward local port 1234 to 7778 on host holding runtime.

edited Apr 14 '15 at 13:53

answered Apr 14 '15 at 13:35

Archemar

682

there is no way to force java to listen in IP address? – Nadya Nux Apr 14 '15 at 13:41
You can alway rewrite the part of code that listen(), then recompile. Or ask your Oracle support. – Archemar Apr 14 '15 at 13:53

Why I can't I telnet to a specific port?

2 Answers2