What might increase delay of ssh connection?
SSH does a reverse lookup to resolve the connecting host, the delay you encounter is either due to slow response or more likely a time out.
If you cannot perform reverse resolution from that host, you can alternatively disable the reverse lookup of sshd in the configuration file sshd_config.
Look for and change the following entry:
UseDNS no
Yes. It is a misconfigured DNS installation. You need to make sure that the DNS resolution works.
SSH tries to get the rDNS entry from the host that connects to the server. If it can't be resolved you run into a timeout.
As others have said, this is likely a DNS resolution problem.
For future reference, note that the OpenSSH FAQ describes this problem under '3.3 - ssh(1) takes a long time to connect or log in'. There are a few other possible causes, but DNS resolution is the most common issue:
There may be a DNS lookup problem, either at the client or server. You can use the nslookup command to check this on both client and server by looking up the other end's name and IP address. In addition, on the server look up the name returned by the client's IP-name lookup. You can disable most of the server-side lookups by setting UseDNS no in sshd_config.
Some admins set UseDNS no as a protection against DNS Spoofing. For example, see the book SSH, the Secure Shell, 2nd Edition, section '10.3.3.4 TCP/IP settings' by O'Reilly, which says:
We also disable reverse DNS lookups on incoming connections:
# OpenSSH UseDNS noYou might think security is increased by reverse DNS lookups, but in fact, DNS isn’t secure enough to guarantee accurate lookups. Also, due to other issues in your Unix and network environment, reverse DNS mappings might not even work properly [5.3.3.8]. Finally, SSH connections can be tremendously slowed down or fail altogether if the client's DNS is hosed (e.g., lots of nameservers, all unresponsive, so sshd times out). The IP addresses of connecting hosts end up in your logs anyway, so you can look them up later.