Why does ssh's “password” prompt take so long to appear?

Question

When I try to ssh, the password prompt takes too long (almost two minutes) to appear.

Why does this happen?

Answer 1

There are several things that can go wrong. Add -vvv to make ssh print a detailed trace of what it's doing, and see where it's pausing.

The problem could be on the client or on the server.

A common problem on the server is if you're connecting from a client for which reverse DNS lookups time out. (A “reverse DNS lookup” means getting back from the client machine's IP address to a host name. It isn't really useful for security, only slightly helpful to diagnose breakin attempts from log entries, but the default configuration does it anyway.) To turn off reverse DNS lookups, add UseDNS no to /etc/ssh/sshd_config (you need to be root on the server; remember to restart the SSH service afterwards).

Another thing that can go wrong is GSSAPI authentication timing out. If you don't know what that is, you're probably not relying on it; you can turn it off by adding the line GSSAPIAuthentication no to /etc/ssh/ssh_config or ~/.ssh/config (that's on the client side).

Answer 2

Let time the login process and see how long it will take:

[root@gislab00207 ~]# time ssh root@ISSLABNTL01
root@isslabntl01's password:
Last login: Fri Oct  4 07:55:03 2013 from 3.60.40.232

[root@ISSLABNTL01 ~]# exit
logout
Connection to ISSLABNTL01 closed.

real    0m45.192s
user    0m0.003s
sys     0m0.005s

You have new mail in /var/spool/mail/root
[root@gislab00207 ~]#

See above it took about 45 seconds to login -------- VERY SLOW

Once You login as root edit sshd_config file and change the UseDNS entry as below. Here I am using sed instead of editing the file.

[root@ISSLABNTL01 ~]# grep -i dns /etc/ssh/sshd_config
#UseDNS yes

[root@ISSLABNTL01 ~]# sed -i 's/#UseDNS yes/UseDNS no/g' /etc/ssh/sshd_config
[root@ISSLABNTL01 ~]# grep -i dns /etc/ssh/sshd_config
UseDNS no

[root@ISSLABNTL01 ~]# service sshd restart
Stopping sshd:                                             [  OK  ]
Starting sshd:                                             [  OK  ]
[root@ISSLABNTL01 ~]# exit

Let us time the login process and see now how long it will takes.

[root@gislab00207 ~]# time ssh root@ISSLABNTL01
root@isslabntl01's password:
Last login: Fri Oct  4 07:55:03 2013 from 3.60.40.232

[root@ISSLABNTL01 ~]# exit
logout

Connection to ISSLABNTL01 closed.

real    0m6.192s
user    0m0.003s
sys     0m0.005s

You have new mail in /var/spool/mail/root
[root@gislab00207 ~]#

See it took now 6 seconds, the time for me to type the password.

Answer 3

It is something that comes wrong from Ubuntu's installation.

To fix it you have to change this line in /etc/nsswitch.conf:

hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4

And change it for this one:

hosts:          files dns

Answer 4

In my case, the issue can be solved by restarting systemd-logind:

systemctl restart systemd-logind

This is mentioned on Serverfault.

I have to do this regularly though, and I don’t know what the root cause of the problem is.

Answer 5

The debug output for ssh in my case just stopped for 30 seconds while it was 'connecting'. The solution turned out to be related to the DNS settings on my local system. A previous network configuration had left behind a bogus DNS server in the /etc/resolv.conf file. Replacing it with a current DNS server fixed the problem.

Answer 6

For me was changing system dns to 127.0.0.1, prior to this is was a non-existing host.

nano /etc/resolv.conf

And write the following

domain localdomain
search localdomain
(Removed this line) ===> nameserver 10.0.0.1
(Added this line) ===> nameserver 127.0.0.1

Answer 7

I don't know as much about the poster's environment as I'd like to, but for others with a similar problem, it may be an issue with sssd, which we use to tie-in ldap mojo.

This happens to me:

$ time ssh server.example.com

real  2m0.018s
user  0m0.006s
sys   0m0.004s

I have to gain access to the server (in my case via the console) then do a:

service restart sssd

After that, things just work. I haven't had time to debug the root cause, but this bandaid works for me.

Answer 8

I could solve the slow password prompt via ssh - issue by checking Enable DNS Relay in DHCP settings on my dlink router. Afterwards connections with SSH worked within a second.

Network Settings -> Router Settings -> Enable DNS Relay [x]

The default configuration forwards every DNS request to the provider. It was slow although I was connecting with ssh pi@10.0.0.103. A hint to the solution was an entry in /etc/resolv.conf "search upc.at" which is provided via dhcp.

The dlink manual states:

When DNS Relay is enabled, DHCP clients of the router will be assigned 
the router's LAN IP address as their DNS server. All DNS requests that 
the router receives will be forwarded to your ISPs DNS servers. 
When DNS relay is disabled, all DHCP clients of the router will 
be assigned the ISP's DNS server.

After a dhcp release on client and server, connecting via SSH was fast again. HTH.