telnet: Unable to connect to remote host: Network is unreachable

Question

I am not able to connect to gmail with telnet using any of the ports. I am using this command:

telnet smtp.gmail.com 25

and it gives following error:

Trying 74.125.68.108...
Trying 74.125.68.109...
Trying 2404:6800:4003:c02::6d...
telnet: Unable to connect to remote host: Network is unreachable

I used this to configure my firewall using method 2 thinking this problem may be because of it.

output of command sudo ufw status is:

Status: active

To                         Action      From
--                         ------      ----
25,53,80,110,443/tcp       ALLOW OUT   Anywhere
53,67,68/udp               ALLOW OUT   Anywhere
51413/tcp                  ALLOW OUT   Anywhere
51413/udp                  ALLOW OUT   Anywhere
6969/tcp                   ALLOW OUT   Anywhere
587,2525/tcp               ALLOW OUT   Anywhere
587,2525/udp               ALLOW OUT   Anywhere
25,53,80,110,443/tcp (v6)  ALLOW OUT   Anywhere (v6)
53,67,68/udp (v6)          ALLOW OUT   Anywhere (v6)
51413/tcp (v6)             ALLOW OUT   Anywhere (v6)
51413/udp (v6)             ALLOW OUT   Anywhere (v6)
6969/tcp (v6)              ALLOW OUT   Anywhere (v6)
587,2525/tcp (v6)          ALLOW OUT   Anywhere (v6)
587,2525/udp (v6)          ALLOW OUT   Anywhere (v6)

I even tried to use my mobile network instead of lan thinking my ISP may have blocked ports but I got the same error. Can someone help please?

UPDATE: sudo netstat -tunlp gives:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:23              0.0.0.0:*               LISTEN      8390/xinetd     
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      4310/cupsd      
tcp        0      0 127.0.0.1:5432          0.0.0.0:*               LISTEN      1405/postgres   
tcp        0      0 127.0.0.1:5433          0.0.0.0:*               LISTEN      1779/postgres   
tcp        0      0 127.0.0.1:9306          0.0.0.0:*               LISTEN      3275/searchd    
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      1171/mysqld     
tcp        0      0 127.0.1.1:53            0.0.0.0:*               LISTEN      2465/dnsmasq    
udp        0      0 0.0.0.0:5353            0.0.0.0:*                           3414/chrome     
udp        0      0 0.0.0.0:5353            0.0.0.0:*                           749/avahi-daemon: r
udp        0      0 0.0.0.0:48695           0.0.0.0:*                           749/avahi-daemon: r
udp        0      0 0.0.0.0:46846           0.0.0.0:*                           9389/dhclient   
udp        0      0 127.0.1.1:53            0.0.0.0:*                           2465/dnsmasq    
udp        0      0 0.0.0.0:68              0.0.0.0:*                           9389/dhclient   
udp6       0      0 :::47892                :::*                                749/avahi-daemon: r
udp6       0      0 :::5353                 :::*                                749/avahi-daemon: r
udp6       0      0 :::14340                :::*                                9389/dhclient 

Solution

Many ISPs will block connections to port 25 as an anti-spam measure. My ISP (Cox) does this, only allowing connections to their own mail servers on port 25. For other email services, an alternate port has to be used which may necessitate the use of SSL or TLS rather than an open connection like port 25 gives you.

Have a look at your ISP's website specifically regarding port 25.

First try connecting to your ISP's email servers if available using port 25. If that works, it's probably an ISP restriction.

As a test, you can play around with netcat a bit:

sudo nc -l -p 25

Then in a new terminal, try a telnet connection to localhost:25. If the things you type are reflected in netcat, then it's again possible that it's a restriction on your ISP.

To further test and validate this, you can set up a port forward in your router for port 25 to your current PC and then attempt the same netcat trick. Instead, though, telnet to your public IP, port 25. If this fails, then it's almost surely an ISP restriction.

Note: many ISPs use this practice -- including mobile providers such as Verizon. So, just because it didn't work on a tethered connection either doesn't mean it's not an ISP problem.

---

Solution

It's possible to connect to Gmail in plain text (no secure negotiation) on port 587. Therefore, you should see that telnet smtp.gmail.com 587 works just fine!

Comments

[heemayl]

Can you ping smtp.gmail.com?

[sonalkr132]

I used: ping -c 10 mail.google.com. It seemed to work. Output had line: 10 packets transmitted, 9 received, 10% packet loss, time 9990ms

[heemayl]

smtp.gmail.com and mail.google.com resolve to different IP addresses so you should try to ping smtp.gmail.com as you are telnetting this host on port 25.

[heemayl]

Could you try telnet smtp.gmail.com 465?

[sonalkr132]

telnet localhost 25 gave output: Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. and nothing appeared in other terminal window. I just want to establish a connection with gmail, doesn't matter if its port 25 or any other port.

[heemayl]

Could you try telnet smtp.gmail.com 465?

[Chuck R]

Updated answer.

[sonalkr132]

@heemayl telnet smtp.gmail.com 465 doesn't work either

[sonalkr132]

@Githlar I just want to establish a connection with gmail so that I can send automated mails for my website. I was able to send mails earlier using port '587' but its not working now. I am going to try your solution now.

[Chuck R]

Use port 587 rather than 465, 465 is the legacy port. telnet smtp.gmail.com 587 works in plain text, responds to HELO and everything. So, configure your email client with port 587 and it should work as expected.

[Chuck R]

Whoops, used smtp.google.com instead of smtp.gmail.com

[heemayl]

This narrowed down the problem to only server & client i.e. either your firewall is blocking or gmail's server has blocked your ISP's IP.