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I have exactly same setup in two offices. From one office, if I try to do ftp (or ssh) to anywhere in the world, it takes a long time before the login prompt comes (roughly 10-15 seconds minimum). Ftp normally takes far more time though.
However, from another office, I get it almost instantaneously!
I was told by several people that it could be "Reverse DNS Mapping" which is causing this trouble. If that is really the case, then I should get the trouble in both offices since they have exactly same kind of DNS setup (of course, with the difference of their own external/internal IPs).
I also have similar type of "/etc/hosts", "/etc/resolv.conf" and "/etc/nsswitch.conf" files at both ends. I have also checked "/etc/hosts.allow" and "/etc/hosts.deny" files at both ends. They are essentially empty.
The machines where I try this, are the main masquerading machines connected directly to the internet via modem/router and firewall. There is of course, internal network at both offices which is connected via VPN (ipsec).
Could it be the case that some DNS entries at ISP of problematic office is causing some trouble here?
Distribution: Ubuntu currently, also Fedora, RHEL, CentOS
really sounds like dns. You can change your /etc/ssh/sshd_config to not do reverse dns lookup, that could save you time. Also, most ftp daemons can be changed. Sounds like dns is hosed at one end, or your NAT has no reverse record.
I think the following entry would do it (in /etc/ssh/sshd_config):
Note that you would need to make this change on the servers you are trying to reach from the affected office, not the servers located at this office.
The problem is that when you contact a host from this office, the host tries to do an inverse lookup on your public IP address. If there is no reply, pretty much everything that uses IP will be delayed. The real solution, therefore, is to contact your ISP and get a reverse DNS entry set up for that address.