[Bug 883201] Re: Several second hang on ssh login

[Joel]

I've tracked the problem down to something DNS related. But I'm still
not sure exactly what the issue is.

Other people seem to be having the problem as well, for a couple of
Ubuntu releases at least. The instructions for fixing the problem on the
internet say to set "UseDNS = no" in client sshd config (this is silly
and does nothing of course), and then to add the server hostname in
/etc/hosts (which is actually what fixes the problem).

Troubleshooting reveals the following interesting facts:

1) The problem does not occur with IP addresses or FQDNs. So 'ssh
servername.domain.com' or 'ssh 192.168.0.2' is fast but 'ssh servername'
is slow.

2) 'host servername' is consistently fast, and the only search line in
/etc/resolv.conf is 'search domain.com'

3) The root user does not experience this issue.

4) Normal users with all userland config wiped (.bashrc and
.bash_profile) still experience the issue.

5) The problem is not reproducible with any other network tool other
than ssh. Ping, telnet, wget, etc., do not experience any DNS-lookup
hang.

So what is ssh doing special with regards to DNS that other network
tools don't do?

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https://bugs.launchpad.net/bugs/883201

Title:
  Several second hang on ssh login

Status in “openssh” package in Ubuntu:
  New

Bug description:
  This problem is new since 11.04. It only occurs when connecting to a
  server that I have not recently connected to. The problem still
  remains after commenting out all lines in /etc/ssh_config.

  To reproduce:

  Check that DNS is operating correctly:

  joel at joel-vm:~$ host testp1-db  # this is instant
  testp1-db.mydomain.com has address 192.168.2.50
  joel at joel-vm:~$ host 192.168.2.50 # this is instant
  50.2.168.192.in-addr.arpa domain name pointer testp1-db.mydomain.com.

  Connect to the server:

  joel at joel-vm:~$ ssh -vvv root at testp1-db
  OpenSSH_5.8p1 Debian-7ubuntu1, OpenSSL 1.0.0e 6 Sep 2011
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: Applying options for *
  debug2: ssh_connect: needpriv 0

  <At this point, there is a hang of several seconds and then everything
  follows normally>

  debug1: Connecting to testp1-db [192.168.2.50] port 22.
  debug1: Connection established.

  
  Connecting to the server subsequently works at normal speed.

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