Unable to ssh into a linux machine

Question

I have a Linux machine running Centos 7. I am trying to ssh into the machine using putty or WinScp and unable to do that.

I have opened the ssh port using: sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

I have paled the public key inside the authorized_key file. I have edited the /etc/ssh/sshd_config file and enabled the PublickeyAuthentication.

After that i have restarted the sshd service: systemctl restart sshd.service.

Still, i am not able to reach the servers. I thought it might be an internet connection issues but I am able to go online from the machine. The error i receive from putty upon connection is : Network error: Connection timed out.

What am i missing? how do i fix that issue? Any help would be appreciated. Thank you.

Answer 1

You need to properly troubleshoot the issue - step by step.

1. is port opened (netstat)
2. are packets reaching host (tcpdump)
3. how machine replies; where the reply packets go (tcpdump)
4. check all firewall rules; it is possible that other rule matches (iptables)
5. check routing; you may even have multiple routing tables (ip route)

It would be easier to troubleshoot firewall if you could switch it off temporarily but that is not always an option.

Answer 2

I believe --dport is part of the tcp module and requires the -m tcp option to use it. Full command to accept incoming tcp port 22 would then be:

sudo iptables -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT

Answer 3

iptables -A INPUT -i venet0 -m state --state ESTABLISHED,RELATED -j ACCEPT -m comment --comment "ALLOW ESTABLISHED connections"
iptables -A INPUT -i venet0 -p tcp --dport 22 -m state --state NEW -j ACCEPT -m comment --comment "ALLOW new SSH connections"

replace venet0 with your networkinterface, like eth0

if it doesn't work, what is the output of netstat -tulpen | grep ':22' or lsof -4 and iptables -L -v -n

-m tcp is not requierd for --dport