When I’m trying to ssh to a remote server, after I enter the username, it takes a lot of time before it displays the password prompt. Basically, my SSH ( openSSH ) is slow during authentication process. How do I solve this problem?’

$ ssh -v root@remote-host
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to remote-host [1.1.1.1] port 22.
debug1: Connection established.
debug1: identity file /home/allen/.ssh/identity type -1
debug1: identity file /home/allen/.ssh/id_rsa type -1
debug1: identity file /home/allen/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.9p1 Debian-5ubuntu1.1
debug1: match: OpenSSH_5.9p1 Debian-5ubuntu1.1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.3
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: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent

debug1: SSH2_MSG_SERVICE_ACCEPT received <-- OpenSSH hanging here for 1 min

debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Trying private key: /home/root/.ssh/identity
debug1: Trying private key: /home/root/.ssh/id_rsa
debug1: Trying private key: /home/root/.ssh/id_dsa
debug1: Next authentication method: password

Solution: set UseDNS to no in sshd_config file

To fix this performance issue while connecting to a remote server using ssh, set the UseDNS to no as shown below in your /etc/ssh/sshd_config file.

$ vi /etc/ssh/sshd_config
UseDNS no

Restart the openssh and connect to the remote server again, which should be quick this time and will not hang at SSH2_MSG_SERVICE_ACCEPTED.

# service sshd restart

$ ssh -v root@remote-host