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Monday, March 11, 2013

ASA ssh login problem

Working for ISP is big fun. From all the work you do there is one routine like swapping of network devices (for example Cisco ASA firewall) that you are going to do. Not going into too much details the process is straight forward and requires:
  • copy the config to new device
  • rack the new device
  • make sure that the switches and VLANs are configured properly
  • change routing info if needed 

After putting new ASA FW into rack you can connect using serial line but you can't access it over SSH. You getting this error message.
$ ssh
ssh_exchange_identification: Connection closed by remote host

Troubleshooting and solution

From serial console access enable debugging:
# debug ssh

Connect over ssh. You are going to see this logs on console:
Device ssh opened successfully.
SSH0: SSH client: IP = ''  interface # = 2
SSH: unable to retrieve default host public key.  Please create a defauth RSA key pair before using SSH
SSH0: Session disconnected by SSH server - error 0x00 "Internal error"

Searching for 'unable to retrieve default host public key' finds the links in reference sections.  To fix this we need:
fw-asa(config)# crypto key generate rsa
INFO: The name for the keys will be: 
Keypair generation process begin. Please wait...

Once ASA has its own RSA key to use for SSH handshaking the logs from a sucessful SSH session looks like:
Device ssh opened successfully.
SSH0: SSH client: IP = ''  interface # = 2
SSH: host key initialised
SSH: license supports 3DES: 2
SSH: license supports DES: 2
SSH0: starting SSH control process
SSH0: Exchanging versions - SSH-2.0-Cisco-1.25
SSH0: send SSH message: outdata is NULL
server version string:SSH-2.0-Cisco-1.25SSH0: receive SSH message: 83 (83)
SSH0: client version is - SSH-2.0-OpenSSH_4.3
client version string:SSH-2.0-OpenSSH_4.3SSH0: begin server key generation
SSH0: complete server key generation, elapsed time = 1830 ms
SSH2 0: SSH2_MSG_KEXINIT received
SSH2: kex: client->server aes128-cbc hmac-md5 none
SSH2: kex: server->client aes128-cbc hmac-md5 none
SSH2 0: expecting SSH2_MSG_KEXDH_INIT
SSH2 0: SSH2_MSG_KEXDH_INIT received
SSH2 0: signature length 143
SSH2: kex_derive_keys complete
SSH2 0: newkeys: mode 1
SSH2 0: waiting for SSH2_MSG_NEWKEYSSSH0: TCP read failed, error code = 0x86300003 "TCP connection closed"
SSH0: receive SSH message: [no message ID: variable *data is NULL]

SSH2 0: Unexpected mesg type receivedSSH0: Session disconnected by SSH server - error 0x00 "Internal error"


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