[SOLVED] Telnet / Postfix connection refused question for CENTOS 6.4

person_of_interest · 02-04-2015, 03:03 AM

Good Morning! I am supporting the coffee and Doritos industry with my frustration with this problem. I have been battling it for two months. I have several Red Hat books, the official Red Hat study guide, and have searched the forums for weeks with no avail. I was wondering if you have ever had this?

I am trying to "telnet server.example.com 25" or telnet in general. Both servers are running on a KVM on the 192.168.122.0/24 network.

[root@server ~]# telnet server.example.com
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused
Trying 192.168.122.191...
telnet: connect to address 192.168.122.191: Connection refused

I can ssh, run NFS, and also run internal postfix e-mails.

IPTABLES IS TURNED OFF ON BOTH CLIENTS

Postfix and Telnet is configured on both clients, with both listening:

[root@server ~]# netstat -tulpn |grep :25
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 1802/

When I Telnet from client named 'Server' to client named 'ISCSI', tpcdump on client 'ISCSI' gives the following:

19:01:23.579330 STP 802.1d, Config, Flags [none], bridge-id 8000.52:54:00:2e:85:0e.8003, length 43
19:01:23.632750 IP server.57477 > iscsi.example.com.telnet: Flags [S], seq 2403459934, win 14600, options [mss 1460,sackOK,TS val 2679191 ecr 0,nop,wscale 6], length 0
19:01:23.632882 IP iscsi.example.com.telnet > server.57477: Flags [R.], seq 0, ack 2403459935, win 0, length 0
19:01:24.428091 IP server.57478 > iscsi.example.com.telnet: Flags [S], seq 942066757, win 14600, options [mss 1460,sackOK,TS val 2679990 ecr 0,nop,wscale 6], length 0
19:01:24.428163 IP iscsi.example.com.telnet > server.57478: Flags [R.], seq 0, ack 942066758, win 0, length 0
19:01:25.006222 IP server.57479 > iscsi.example.com.telnet: Flags [S], seq 2689610466, win 14600, options [mss 1460,sackOK,TS val 2680564 ecr 0,nop,wscale 6], length 0

person_of_interest · 02-04-2015, 05:55 AM

I noticed that my paste showed a bad connection on the telnet loop.

Here is the correction. I think I found the problem

From client ISCSI:

[root@iscsi ~]# telnet iscsi.example.com 25
Trying 192.168.122.136...
telnet: connect to address 192.168.122.136: Connection refused
[root@iscsi ~]# vim /etc/sysconfig/network
[root@iscsi ~]# vim /etc/resolv.conf
[root@iscsi ~]# telnet iscsi.example.com
Trying 192.168.122.136...
telnet: connect to address 192.168.122.136: Connection refused

[root@iscsi ~]# telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 iscsi.example.com ESMTP Sendmail 8.14.4/8.14.4; Tue, 3 Feb 2015 21:55:21 -0500

Loop from client "Server"

[root@server ~]# telnet server.example.com 25
Trying 127.0.0.1...
Connected to server.example.com.
Escape character is '^]'.
220 server.example.com ESMTP Sendmail 8.14.4/8.14.4; Tue, 3 Feb 2015 21:53:25 -0500

person_of_interest · 02-04-2015, 05:59 AM

The loop now works for both clients. The /etc/host looks like this:

client "ISCSI"

[root@iscsi ~]# cat /etc/hosts
127.0.0.1 localhost.localdomain localhost
127.0.0.1 iscsi iscsi.example.com
::1 localhost6.localdomain6 localhost6
192.168.122.136 iscsi iscsi.example.com
192.168.122.191 server server.example.com

client "Server"
[root@server ~]# cat /etc/hosts
127.0.0.1 localhost.localdomain localhost
127.0.0.1 server server.example.com
192.168.122.191 server server.example.com
::1 localhost6.localdomain6 localhost6
192.168.122.136 iscsi iscsi.example.com

person_of_interest · 02-04-2015, 06:03 AM

Now when I telnet, the TCPDUMP output is:

22:05:27.111413 IP iscsi.42519 > server.telnet: Flags [S], seq 1215495062, win 14600, options [mss 1460,sackOK,TS val 623072 ecr 0,nop,wscale 6], length 0
22:05:27.114956 IP server.telnet > iscsi.42519: Flags [R.], seq 0, ack 1215495063, win 0, length 0
22:05:28.669995 IP iscsi.42520 > server.telnet: Flags [S], seq 3574758482, win 14600, options [mss 1460,sackOK,TS val 624630 ecr 0,nop,wscale 6], length 0
22:05:28.672073 IP server.telnet > iscsi.42520: Flags [R.], seq 0, ack 3574758483, win 0, length 0

person_of_interest · 02-04-2015, 12:19 PM

So I built two more VMs. "Server2" and "Client2". I can telnet on port 22, but no other port:

[root@server2 ~]# telnet client2 22
Trying 192.168.122.5...
Connected to client2.
Escape character is '^]'.
SSH-2.0-OpenSSH_5.3
^\
Connection closed by foreign host.
[root@server2 ~]# telnet client2 23
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 24
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 25
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 26
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 27
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 28
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 29
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused
[root@server2 ~]# telnet client2 69
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused

descendant_command · 02-04-2015, 12:32 PM

So what are you trying to achieve?

You can't connect to a port that doesn't have a listening process attached.

person_of_interest · 02-04-2015, 12:33 PM

Update,
So for no reason whatsoever, I put in the FQDN and it worked from client2 --> server2. However, still can't go from server2 --> client2

[root@client2 ~]# telnet server2.example.com 25
Trying 192.168.122.33...
Connected to server2.example.com.
Escape character is '^]'.
220 server2.example.com ESMTP Postfix
^]

[root@server2 ~]# telnet client2 25
Trying 192.168.122.5...
telnet: connect to address 192.168.122.5: Connection refused

server2 netstat
[root@server2 ~]# netstat -ln |grep :25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 :::25 :::* LISTEN

client2 netstat
[root@client2 ~]# netstat -ln |grep :25
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN

person_of_interest · 02-04-2015, 12:37 PM

Quote:

Originally Posted by descendant_command

So what are you trying to achieve?

You can't connect to a port that doesn't have a listening process attached.

Thank you for your reply!

I am trying to troubleshoot postfix within an internal network. I am using telnet to troubleshoot. I need to open a listener, like in httpd. I will research that. Thank you!

person_of_interest · 02-04-2015, 12:47 PM

Update,
I did a reboot on both, and now I have two sets of listening ports....I wonder why it only worked after a reboot.

[student@client2 ~]$ netstat -ln |grep :25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 :::25 :::* LISTEN

[student@server2 ~]$ netstat -ln |grep :25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 :::25 :::* LISTEN

person_of_interest · 02-04-2015, 12:56 PM

SOLVED....I guess?

So I did a reboot, and now it works. I have rebooted literally hundreds of time over the past eight weeks, so I'm not sure why today is the special day, but I can telnet and use postfix on port 25 without a firewall on. Now I will turn on IPTABLES and make sure that works. The only thing that I can think is that my BIND/NAMED hasn't been configured, and I am setting up my DNS wrong. Thanks again for your time!

[root@client2 ~]# netstat -ln |grep :25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 :::25 :::* LISTEN

[root@client2 ~]# mail
Heirloom Mail version 12.4 7/29/08. Type ? for help.
"/var/spool/mail/root": 2 messages 2 new
>N 1 root Wed Feb 4 12:04 18/570 "loop"
N 2 root Wed Feb 4 13:48 21/803 "3client2"

[root@server2 ~]# netstat -ln |grep :25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 :::25 :::* LISTEN

[root@server2 ~]# mail
Heirloom Mail version 12.4 7/29/08. Type ? for help.
"/var/spool/mail/root": 5 messages 5 new
>N 1 root Wed Feb 4 12:06 18/594 "loop"
N 2 Mail Delivery System Wed Feb 4 12:06 75/2551 "Undelivered Mail Returned to Sender"
N 3 root Wed Feb 4 13:24 21/775 "2srv2"
N 4 root Wed Feb 4 13:31 21/775 "2srv2"
N 5 root Wed Feb 4 13:48 21/772 "3srv2"