[ILUG] Slow SSH between machines on Debian cluster

[Cian Davis]

Thanks to Steph and Brian who pointed out the potential problem with it
resolving IPv6 addresses as well.

Adding AddressFamily inet to /etc/ssh/ssh_config sorted it.

Thanks,
Cian

Cian Davis wrote:

>Hi All,
>I have a cluster of 9 Debian machines all on a GigE switch. They were
>RH8 a while ago but I wiped them all and put Debian on them (viva apt!).
>One of the machines acts as the master node to the others, providing
>connection to the outside UL network, firewall (with NAT), DNS and proxy
>(primarily for apt) for the other 8 machines.
>
>Problem I'm having is that ssh'ing between the machines is very slow -
>there's between and 5 and 15 second delay (at the same place each time
>when doing a ssh -v)
>
>I have heard that reverse DNS lookups can slow down the process but the
>master node is the DNS server for the cluster and responds very fast.
>The problem does not manifest itself if you SSH by IP, which would
>suggest a DNS problem. I tried adding CheckHostIP no to
>/etc/ssh/ssh_config on both machines. However, even when you are SSH'ing
>by IP and get no delay, it correctly reverse resolves the IP.
>
>master:~# ssh root at 10.10.10.11
>Last login: Thu Aug 18 15:01:01 2005 from master.cfd.ul.ie
>
>I know that if reverse DNS isn't setup, it will show the IP address
>instead of the DNS name because at first I forgot to setup reverse DNS
>for the cfd.ul.ie domain and it showed the IP address. Adding a reverse
>lookup zone solved the problem.
>
>The only other option in ssh_config is ForwardX11 yes on the master
>node. sshd_config attached.
>
>Also, adding
>10.10.10.11     node01.cfd.ul.ie        node01
>to /etc/hosts solves the problem but I would really like not to have to
>add the machine names to /etc/hosts as it almost defeats the purpose of
>having a cfd.ul.ie DNS zone. The problem isn't apparent when you SSH
>from outside (i.e. from the UL network using the DNS name).
>
>Anyone suggest a way to ssh'ing by name but get rid of the speed
>problems without an /etc/hosts solution?
>
>Regards,
>Cian
>
>  
>