I have two Ubuntu 18.04 and 20.x installs at home and trying to ssh between them, I can connect by Internal IP but not by hostname
Setup:

Machine A: 192.x.x.115

Machine B: 192.x.x.125

both installs have ssh running and I can connect fine using internal IP.

From Machine B I would like to connect to Machine A using hostname or hostname.ddns.net

Here are things I have setup:

Machine A:

machineA> /etc/hosts

127.0.0.1    machineA.ddns.net machineA



192.x.x.115  machineA

machineA> /etc/hostname

machineA.ddns.net

machineA> /etc/resolv.conf

nameserver 127.0.0.53

options edns0 trust-ad

search hsd1.xx.isp.net

Machine B:

machineB> ssh [email protected] # i.e machineA is successful

machineB> ssh -vvv [email protected]
gives

Connecting to machineA.ddns.net [ISP assigned address] port 22.



ssh: connect to host machineA.ddns.net port 22: Connection timed out

machineB> ssh -vvv user@machineA

ssh: Could not resolve hostname machineA: Name or service not known

SSH_from_external_to_machineA is successful

ssh [email protected] # is successful

commands tried on machineA:

machineA> sudo service ssh status # gives active(running)

machineA> sudo netstat -alnp | grep :22 # shows :22 with LISTEN

machineA> sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT

1. I feel ISP is not blocking ssh from ssh since I can connect from external using machineA.ddns.net

   
   



2. Router is allowing port 22 since I can connect using IP.

   
   

I feel hostname is not resolved correctly, please review and let me know if I am missing any additional steps.

Thanks,

Additional edits:

machineB> ping machineA_Hostname is successful

machineB> nslookup machineA.ddns.net is showing external IP as expected

machineB> ssh machineA # Could not resolve hostname: Name or service not known

machineB> ssh machineA.ddns.net
attempts to connect to exact external IP that was reported by ping and 'Connection timed out'