ssh takes a long time to connect on some hosts

Question

I have linux machine version red-hat 5.5.0

I have problem

if I perform from my machine ssh to other linux machine - node1 then I login quickly to node1

but if I perform ssh to other linux machine node2 then ssh take a long time

please advice why ?

what need to do in order to perform ssh immediately on node2 ?

remark - in ssh debug I get the - GSS failure ?

Answer 1

Since you are getting GSS failure, you can try adding:

GSSAPIAuthentication no

to /etc/ssh/sshd_config. Then restart the service

/etc/init.d/sshd restart

Answer 2

Try adding the following line to /etc/ssh/sshd_config on node2:

UseDNS no

Then restart sshd:

/etc/init.d/ssh restart

Or if the above doesn't exist:

/etc/init.d/sshd restart

Answer 3

Edit /etc/ssh/sshd_config on the server and add (if it's not there) at the bottom UseDNS no then restart the SSH daemon.

Will stop your machines from resolving DNS and will speed up the process.

Answer 4

2. Take a look here: OpenSSH FAQ especially chapter 3.3. It also points to some other possible delay causes.

3. or Most appropriate method to know the problem is to connect using ssh in debug mode:
   

   # ssh -v <Server name>
   
   OpenSSH_5.8p1 Debian-1ubuntu3, OpenSSL 0.9.8o 01 Jun 2010
   debug1: Reading configuration data /etc/ssh/ssh_config
   debug1: Applying options for *
   debug1: Connecting to mysql [192.168.0.29] port 22.
   debug1: Connection established.
   debug1: permanently_set_uid: 0/0
   debug1: identity file /root/.ssh/id_rsa type -1
   debug1: identity file /root/.ssh/id_rsa-cert type -1
   debug1: identity file /root/.ssh/id_dsa type -1
   debug1: identity file /root/.ssh/id_dsa-cert type -1
   debug1: identity file /root/.ssh/id_ecdsa type -1
   debug1: identity file /root/.ssh/id_ecdsa-cert type -1
   debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
   debug1: match: OpenSSH_5.3 pat OpenSSH*
   debug1: Enabling compatibility mode for protocol 2.0
   debug1: Local version string SSH-2.0-OpenSSH_5.8p1 Debian-1ubuntu3
   debug1: SSH2_MSG_KEXINIT sent
   debug1: SSH2_MSG_KEXINIT received
   debug1: kex: server->client aes128-ctr hmac-md5 none
   debug1: kex: client->server aes128-ctr hmac-md5 none
   debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
   debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
   debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
   debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
   debug1: Server host key: RSA 1a:2c:c4:62:cc:27:1b:76:6b:f7:b2:38:00:7b:3f:63
   debug1: Host 'mysql' is known and matches the RSA host key.
   debug1: Found key in /root/.ssh/known_hosts:5
   debug1: ssh_rsa_verify: signature correct
   debug1: SSH2_MSG_NEWKEYS sent
   debug1: expecting SSH2_MSG_NEWKEYS
   debug1: SSH2_MSG_NEWKEYS received
   debug1: Roaming not allowed by server
   debug1: SSH2_MSG_SERVICE_REQUEST sent
   debug1: SSH2_MSG_SERVICE_ACCEPT received
   debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
   ->> debug1: Next authentication method: gssapi-keyex
   debug1: No valid Key exchange context
   debug1: Next authentication method: gssapi-with-mic
   debug1: Unspecified GSS failure. Minor code may provide more information Credentials cache file '/tmp/krb5cc_0' not found<br/>
   

   Line marked with arrow was causing the delay in my case. I commented out following line on the destination server and it resolved the issue in my case
   

   #GSSAPI options
   #GSSAPIAuthentication no
   #GSSAPIAuthentication yes
   #GSSAPICleanupCredentials yes
   #GSSAPICleanupCredentials yes
   #GSSAPIStrictAcceptorCheck yes
   #GSSAPIKeyExchange no
   

   restart the SSH daemon on the remote server and try to reconnect.. it s fine!
   

4. Some versions of glibc (notably glibc 2.1 shipped with Red Hat 6.1) can take a long time to resolve “IPv6 or IPv4″ addresses from domain names. This can be worked around with by specifying AddressFamily inet option in ssh_config.
   
5. 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.

Answer 5

I found this answer too:

1. Specify the option to disable GSSAPI authentication when using SSH or SCP command, e.g.: ssh -o GSSAPIAuthentication=no [email protected]

-OR-

2. Explicitly disable GSSAPI authentication in SSH client program configuration file, i.e. edit the /etc/ssh/ssh_config and add in this configuration (if it’s not already in the config file): GSSAPIAuthentication no

-OR-

3. Like 2 but in your private ssh config
   Edit /home/YOURUSERNAME/.ssh/config and add GSSAPIAuthentication no

=== BUG ===

When I try to connect to an ssh server (with ssh -v) I always have (my system is an Ubuntu 8.04):

debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure. Minor code may provide more information
No credentials cache found
debug1: Unspecified GSS failure. Minor code may provide more information
No credentials cache found
debug1: Unspecified GSS failure. Minor code may provide more information
debug1: Next authentication method: publickey

The fact is that on many servers the establishment of ssh connection is very slow because of this issue.

https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/416264

Answer 6

If you changed your hostname recently, update your /etc/hosts.

I have changed my hostname using hostnamectl set-hostname and updated IPv4 address in /etc/hosts, but forgot to update IPv6 address:

127.0.0.1              localhost
45.12.34.56            srv1.foobar.com
2a00:abc:123:6432::1   srv1.foobar.com

Answer 7

sshd created systemd sessions. In my case, with systemd with high load (near 100%), the ssh session only started after the systemd session request expired..

in my case, stopping other tasks, trying to restart systemd with systemctl daemon-reexec solved