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I am trying to ssh remote server from my local server. But whenever i run ssh command:

ssh root@x.x.x.x

I get error:

Connection closed by x.x.x.x

Output of ssh -v -v -v -v root@x.x.x.x is:

OpenSSH_5.9p1 Debian-5ubuntu1.1, OpenSSL 1.0.1 14 Mar 2012
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug2: ssh_connect: needpriv 0
debug1: Connecting to x.x.x.x [x.x.x.x] port 22.
debug1: Connection established.
debug3: Incorrect RSA1 identifier
debug3: Could not load "/home/mona/.ssh/id_rsa" as a RSA1 public key
debug1: identity file /home/mona/.ssh/id_rsa type 1
debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-2048
debug1: Checking blacklist file /etc/ssh/blacklist.RSA-2048

debug1: identity file /home/mona/.ssh/id_rsa-cert type -1
debug1: identity file /home/mona/.ssh/id_dsa type -1
debug1: identity file /home/mona/.ssh/id_dsa-cert type -1
debug1: identity file /home/mona/.ssh/id_ecdsa type -1
debug1: identity file /home/mona/.ssh/id_ecdsa-cert 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_5.9p1 Debian-5ubuntu1.1
debug2: fd 3 setting O_NONBLOCK
debug3: load_hostkeys: loading entries for host "151.236.220.15" from file "/home/mona/.ssh/known_hosts"
debug3: load_hostkeys: loaded 0 keys
debug1: SSH2_MSG_KEXINIT sent
Connection closed by x.x.x.x

I have loaded content of my id_rsa.pub in known_hosts keys.

I am not able to ssh login.

Can anybody help me in this? Will really appreciate it.

Thank you.

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That looks like either tcpwrappers or a firewall closing the connection. –  Drav Sloan Aug 10 '13 at 11:32
    
@DravSloan Can u tell how to know which one is closing and how to stop it? Thanks. –  user1957141 Aug 10 '13 at 12:22
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3 Answers 3

Following Fred's point in the comments (and actually reading the error message), I was incorrect and ssh was connecting. I will leave my original response at the bottom and additionally answer the question of not being able to connect to a running ssh.

Another good way to diagnose ssh issues when the sshd server refuses connections, and if the OP is correct nothing is getting logged in auth.log or syslog, is to run it on a separate port with debugging enabled (I've picked the arbitrary port of 44).

/full/path/to/sshd -p 44 -d 

You can then connect with your ssh client and get further debugging of the issue:

ssh -p 44 root@x.x.x.x

Root (as Fred pointed out in his answer) is a user that can potentially be restricted via the ssh option PermitRootLogin option in your sshd_config. Also the types of authentication methods used by your sshd_config can further restrict how you can access your host:

RSAAuthentication
PubkeyAuthentication
RhostsRSAAuthentication
HostbasedAuthentication
ChallengeResponseAuthentication
PasswordAuthentication
KerberosAuthentication
GSSAPIAuthentication

Look at the man page for sshd_config (man 5 sshd_config) for more information on those options. Usually most sshds have RSAAuthentication, PubkeyAuthentication and sometimes PasswordAuthentication. RSAAuthentication is specific to Protocol 1 and most hosts use Protocol 2 which uses PubkeyAuthentication. Both rely on root having a key file (usually found in /root/.ssh/authorized_keys), but this location can be overridden by the AuthorizedKeysFile option. It looks like PasswordAuthentication is not enabled on your sshd.

For RSA and Pubkey authentication you need a keypair. Which you have generated and they live on your client machine in /home/mona/.ssh/id_rsa and /home/mona/.ssh/id_rsa.pub. The public half of these two files (the key contained in /home/mona/.ssh/id_rsa.pub) you would need to put in root's authorized_key file mentioned above.

Original Answer, referring to a failure to connect remotely to the sshd process

That looks like either TCPWrappers or a firewall closing the initial connection.

Check your auth.log or syslog files in /var/log as these may provide some clues as to which is blocking the connection.

TCPwrappers is usually implemented via a /etc/hosts.allow file and on some unixes an additional or just the /etc/hosts.deny file (i.e without a hosts.allow file).

Entries are usually of the form:

<service> : <access list> : <allow|deny>

OR

<service> : <access list>

depending on the type of tcp wrapper being used. The format of these files can usually be found with the hosts_access man page man 5 hosts_access. You may have to add an entry to allow your remote IP access.

sshd : my.ip.address.here : allow

Most distributions with a Linux kernel tend to use iptables as the main firewall, though some use ipchains. (I know FreeBSD uses ipfw which is ported from NetBSD). You service provider may also have a firewall, or router with a firewall in front of your service which is blocking these requests. As to which firewall your hosts uses will need some investigation.

iptables firewall rules can be listed via the iptables -nvL command (which must be ran as root, or via sudo). the INPUT chain is the ruleset used to allow/disallow incoming connections your host. You may have to add a rule to allow SSH connections inbound:

iptables -I INPUT -p tcp --dport 22 -j ACCEPT -m comment --comment "Allow SSH connections from all hosts"

You may want to make it only allow connections from a specific IP:

iptables -I INPUT -s 10.10.10.10 -p tcp --dport 22 -j ACCEPT -m comment --comment "Allow SSH connections from the 10.10.10.10 host"

If your service provider blocks port 22, then you will probably need to put the service on a different port (port 2222 is quite popular) via the Port option in your sshd_config file (which usually lives in /etc/ssh).

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Hey, I checked in logs, can't see error related to ssh. My hosts.allow and hosts.deny files are commented. What should i do then? –  user1957141 Aug 10 '13 at 15:16
    
Check that you are running an sshd? Does pgrep sshd return anything? If it returns nothing then try: /etc/init.d/ssh start. If pgrep does return a PID, then is there a firewall blocking the connections to sshd? –  Drav Sloan Aug 10 '13 at 15:27
    
pgrep sshd returned number '895'. Is this pid? –  user1957141 Aug 10 '13 at 15:59
    
Yep that's it's process id or pid. So sshd is running. So it's either not listening on port 22, or a firewall or similar is blocking connections. –  Drav Sloan Aug 10 '13 at 16:02
    
What should i do to make this work then? –  user1957141 Aug 10 '13 at 16:09
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Just to be sure, you have #PermitRootLogin yes either with or without the # in your sshd_config file? You'd need that to ssh in as root. And really, I'd suggest not permitting root to ssh into your server (change the line to PermitRootLogin no if it isn't set to that already). Force everyone to login as a normal account, then su root if they need privileges. That way, you can see who logged in and became root and you prevent everyone without a login from trying to guess your root password.

The server machine's public key should be in known_hosts on the client machine to authenticate the server so you know you aren't connecting to some rogue server impersonating the server you want. The first time you ssh to a server, you'll be asked to approve the key that goes in known_hosts. Afterwards, the server authentication occurs automatically.

You put your account's public key (from the .pub file) in authorized_keys on the server. Then when you connect to the server, your client encodes a message with the private key and sends it to the server, which uses the corresponding public key from the authorized_key file to decrypt the message. If the server can do so, that proves the client has the private key, and therefore is authorized to log in.

My reading of the debug data says that the server can't find the public key of your account. I'd user ssh-copy-id to put my public key on the server.

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This could be a problem with the SSH host key on the remote server. See this question and this apple support thread (don't worry--it's not an apple-specific issue).

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