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how to check ssh connect failed reason

neo created at7 months ago view count: 34

ssh command add v flag, you will look ssh full verbose info.

ssh [email protected] -lroot -v

Verbose:

OpenSSH_8.1p1, LibreSSL 2.7.3
debug1: Reading configuration data /Users/neo/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 47: Applying options for *
debug1: Connecting to github.com port 22.
debug1: Connection established.
debug1: identity file /Users/neo/.ssh/id_rsa type 0
debug1: identity file /Users/neo/.ssh/id_rsa-cert type -1
debug1: identity file /Users/neo/.ssh/id_dsa type -1
debug1: identity file /Users/neo/.ssh/id_dsa-cert type -1
debug1: identity file /Users/neo/.ssh/id_ecdsa type -1
debug1: identity file /Users/neo/.ssh/id_ecdsa-cert type -1
debug1: identity file /Users/neo/.ssh/id_ed25519 type -1
debug1: identity file /Users/neo/.ssh/id_ed25519-cert type -1
debug1: identity file /Users/neo/.ssh/id_xmss type -1
debug1: identity file /Users/neo/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.1
debug1: Remote protocol version 2.0, remote software version babeld-4f04c79d
debug1: no match: babeld-4f04c79d
debug1: Authenticating to github.com:22 as 'git'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: rsa-sha2-512
debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ssh-rsa SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8
debug1: Host 'github.com' is known and matches the RSA host key.
debug1: Found key in /Users/neo/.ssh/known_hosts:12
debug1: rekey out after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 134217728 blocks
debug1: Will attempt key: /Users/neo/.ssh/id_rsa RSA SHA256:2jOGgfNJAljQt1+eyplRcemv5jBoFx+3j8upzO/9VEw
debug1: Will attempt key: /Users/neo/.ssh/id_dsa
debug1: Will attempt key: /Users/neo/.ssh/id_ecdsa
debug1: Will attempt key: /Users/neo/.ssh/id_ed25519
debug1: Will attempt key: /Users/neo/.ssh/id_xmss
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /Users/neo/.ssh/id_rsa RSA SHA256:2jOGgfNJAljQt1+eyplRcemv5jBoFx+3j8upzO/9VEw
debug1: Server accepts key: /Users/neo/.ssh/id_rsa RSA SHA256:2jOGgfNJAljQt1+eyplRcemv5jBoFx+3j8upzO/9VEw
debug1: Authentication succeeded (publickey).
Authenticated to github.com ([198.18.1.134]:22).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype [email protected] want_reply 0
debug1: Sending environment.
debug1: Sending env LC_TERMINAL_VERSION = 3.4.10
debug1: Sending env LANG = zh_CN.UTF-8
debug1: Sending env LC_TERMINAL = iTerm2
debug1: Sending env LC_CTYPE = zh_CN.UTF-8
debug1: Sending env LC_NUMERIC = en_US.UTF-8
debug1: Sending env LC_TIME = en_US.UTF-8
debug1: Sending env LC_COLLATE = en_US.UTF-8
debug1: Sending env LC_MONETARY = en_US.UTF-8
debug1: Sending env LC_MESSAGES = en_US.utf-8
debug1: Sending env LC_PAPER = en_US.UTF-8
debug1: Sending env LC_NAME = en_US.UTF-8
debug1: Sending env LC_ADDRESS = en_US.UTF-8
debug1: Sending env LC_TELEPHONE = en_US.UTF-8
debug1: Sending env LC_MEASUREMENT = en_US.UTF-8
debug1: Sending env LC_IDENTIFICATION = en_US.UTF-8
PTY allocation request failed on channel 0
Hi neo! You've successfully authenticated, but GitHub does not provide shell access.
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: channel 0: free: client-session, nchannels 1
Connection to github.com closed.
Transferred: sent 3768, received 3012 bytes, in 0.8 seconds
Bytes per second: sent 4595.2, received 3673.2
debug1: Exit status 1

ssh

ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections, arbitrary TCP ports and UNIX sockets can also be forwarded over the secure channel.

ssh connects and logs into the specified hostname (with optional user name). The user must prove his/her identity to the remote machine using one of several methods (see below).

If command is specified, it is executed on the remote host instead of a login shell.

The options are as follows:

-4
Forces ssh to use IPv4 addresses only.

-6
Forces ssh to use IPv6 addresses only.

-A
Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.

Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's UNIX socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.

-a
Disables forwarding of the authentication agent connection.

-b bind_address
Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address.

-C
Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and UNIX connections). The compression algorithm is the same used by gzip(1). Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option.

-c cipher_spec
Selects the cipher specification for encrypting the session. cipher_spec is a comma-separated list of ciphers listed in order of preference. See the Ciphers keyword in ssh_config5 for more information.

-D [bind_address : ] port
Specifies a local ``dynamic'' application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.

IPv6 addresses can be specified by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of ``localhost'' indicates that the listening port be bound for local use only, while an empty address or `*' indicates that the port should be available from all interfaces.

-E log_file
Append debug logs to log_file instead of standard error.

-e escape_char
Sets the escape character for sessions with a pty (default: `~' ) . The escape character is only recognized at the beginning of a line. The escape character followed by a dot (`.' ) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to ``none'' disables any escapes and makes the session fully transparent.

-F configfile
Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config ) will be ignored. The default for the per-user configuration file is ~/.ssh/config

-f
Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm

If the ExitOnForwardFailure configuration option is set to ``yes'' then a client started with -f will wait for all remote port forwards to be successfully established before placing itself in the background.

-G
Causes ssh to print its configuration after evaluating Host and Match blocks and exit.

-g
Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.

-I pkcs11
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user's private RSA key.

-i identity_file
Selects a file from which the identity (private key) for public key authentication is read. The default is ~/.ssh/id_dsa ~/.ssh/id_ecdsa ~/.ssh/id_ed25519 and ~/.ssh/id_rsa Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files). If no certificates have been explicitly specified by the CertificateFile directive, ssh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.

-J [user @ ] host [: port ]
Connect to the target host by first making a ssh connection to the jump host and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump hops may be specified separated by comma characters. This is a shortcut to specify a ProxyJump configuration directive.

-K
Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.

-k
Disables forwarding (delegation) of GSSAPI credentials to the server.

-L [bind_address : ] port : host : hostport
-L [bind_address : ] port : remote_socket
-L local_socket : host : hostport
-L local_socket : remote_socket
Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote side. This works by allocating a socket to listen to either a TCP port on the local side, optionally bound to the specified bind_address or to a Unix socket. Whenever a connection is made to the local port or socket, the connection is forwarded over the secure channel, and a connection is made to either host port hostport or the Unix socket remote_socket from the remote machine.

Port forwardings can also be specified in the configuration file. Only the superuser can forward privileged ports. IPv6 addresses can be specified by enclosing the address in square brackets.

By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of ``localhost'' indicates that the listening port be bound for local use only, while an empty address or `*' indicates that the port should be available from all interfaces.

-l login_name
Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file.

-M
Places the ssh client into ``master'' mode for connection sharing. Multiple -M options places ssh into ``master'' mode with confirmation required before slave connections are accepted. Refer to the description of ControlMaster in ssh_config5 for details.

-m mac_spec
A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the MACs keyword for more information.

-N
Do not execute a remote command. This is useful for just forwarding ports.

-n
Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.)

-O ctl_cmd
Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: ``check'' (check that the master process is running), ``forward'' (request forwardings without command execution), ``cancel'' (cancel forwardings), ``exit'' (request the master to exit), and ``stop'' (request the master to stop accepting further multiplexing requests).

-o option
Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag. For full details of the options listed below, and their possible values, see ssh_config5.

AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
Ciphers
ClearAllForwardings
Compression
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIDelegateCredentials
HashKnownHosts
Host
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
HostName
IdentitiesOnly
IdentityAgent
IdentityFile
Include
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
Match
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteCommand
RemoteForward
RequestTTY
SendEnv
ServerAliveInterval
ServerAliveCountMax
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
UsePrivilegedPort
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation

-p port
Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.

-Q query_option
Queries ssh for the algorithms supported for the specified version 2. The available features are: cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenticated encryption), mac (supported message integrity codes), kex (key exchange algorithms), key (key types), key-cert (certificate key types), key-plain (non-certificate key types), and protocol-version (supported SSH protocol versions).

-q
Quiet mode. Causes most warning and diagnostic messages to be suppressed.

-R [bind_address : ] port : host : hostport
-R [bind_address : ] port : local_socket
-R remote_socket : host : hostport
-R remote_socket : local_socket
-R [bind_address : ] port
Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side.

This works by allocating a socket to listen to either a TCP port or to a Unix socket on the remote side. Whenever a connection is made to this port or Unix socket, the connection is forwarded over the secure channel, and a connection is made from the local machine to either an explicit destination specified by host port hostport or local_socket or, if no explicit destination was specified, ssh will act as a SOCKS 4/5 proxy and forward connections to the destinations requested by the remote SOCKS client.

Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square brackets.

By default, TCP listening sockets on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address An empty bind_address or the address `*' , indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config5).

If the port argument is `0' , the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with -O forward the allocated port will be printed to the standard output.

-S ctl_path
Specifies the location of a control socket for connection sharing, or the string ``none'' to disable connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config5 for details.

-s
May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)). The subsystem is specified as the remote command.

-T
Disable pseudo-terminal allocation.

-t
Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.

-V
Display the version number and exit.

-v
Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

-W host : port
Requests that standard input and output on the client be forwarded to host on port over the secure channel. Implies -N -T ExitOnForwardFailure and ClearAllForwardings though these can be overridden in the configuration file or using -o command line options.

-w local_tun [: remote_tun ]
Requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun ) and the server (remote_tun )

The devices may be specified by numerical ID or the keyword ``any'' which uses the next available tunnel device. If remote_tun is not specified, it defaults to ``any'' See also the Tunnel and TunnelDevice directives in ssh_config5. If the Tunnel directive is unset, it is set to the default tunnel mode, which is ``point-to-point''

-X
Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.

X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.

For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Please refer to the ssh -Y option and the ForwardX11Trusted directive in ssh_config5 for more information.

(Debian-specific: X11 forwarding is not subjected to X11 SECURITY extension restrictions by default, because too many programs currently crash in this mode. Set the ForwardX11Trusted option to ``no'' to restore the upstream behaviour. This may change in future depending on client-side improvements.)

-x
Disables X11 forwarding.

-Y
Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.

(Debian-specific: This option does nothing in the default configuration: it is equivalent to ``ForwardX11Trusted yes '' which is the default as described above. Set the ForwardX11Trusted option to ``no'' to restore the upstream behaviour. This may change in future depending on client-side improvements.)

-y
Send log information using the syslog(3) system module. By default this information is sent to stderr.

ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described in ssh_config5.  

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