I would like to login to a remote server by executing sth. like ssh remoteServer. The remote server can only be reached via a login server (which there are two of).
There are two routes I would like to be able to take (in case one login server cannot be used):
- A → B → D
- A → C → D
Some info:
- A: my local computer
- B: login server; only allows usernameB/password login (no key file); I cannot make changes (save key file etc.)
- C: login server; allows key file login; I can make some changes (not a superuser though)
- D: target server; can only be reached from B or C; allows key file login
I'm using a password protected private key file (on A and C; the same one) which I can use to log in to C and D. Here's what I want:
- ssh B_D: Prompt for password of usernameB@B and then for the password of the private key file for usernameD@D. Since I cannot save the private key on B it has to be forwarded somehow. In case I have loaded it with ssh-agent it shouldn't ask for the password again.
- ssh C_D: Prompt for password of the private key once and then use it to login to usernameC@C and then usernameD@D. Again, if the key was loaded with ssh-agent beforehand, it shouldn't prompt for a password.
Partly I'm able to use the following command to achieve the above (but only if the key was loaded with ssh-agent):
ssh -A -t usernameB@B ssh usernameD@D
ssh -A -t usernameC@C ssh usernameD@D
Is there perhaps a modification that loads the key file first but only for this session? If I log out the key file should then be unloaded.
The above command has two problems:
- I don't want to manually load the key first with ssh-agent. This defeats the purpose of having to enter only one command.
- I want to place this or an equivalent command into the ssh config file. If there is a different way to achieve what I want, that would be fine as well (as long as I don't need to modify anything on B or need superuser rights on C).
I was able to achieve sth. similar using the ssh config file:
Host C
HostName addressOfC
User usernameC
IdentityFile ~/.ssh/id_rsa
ForwardAgent yes
Host C_D
HostName addressD
User usernameD
IdentityFile ~/.ssh/id_rsa
Port 22
ProxyCommand ssh -A -W %h:%p C
Again, two problems:
- I need to load the key with ssh-agent first. Otherwise it prompts twice for the password.
- It takes ages (24 seconds) to connect. I should mention here, that I'm on Windows using mobaXterm, in case this is responsible for the 24 seconds.
There is one final catch: Forwarding the Agent does not seem to be safe according to https://heipei.github.io/2015/02/26/SSH-Agent-Forwarding-considered-harmful/. So a solution which avoids forwarding the agent would be nice.
Edit:
After the remarks from @jakuje I've modified the config file to:
Host C
HostName addressOfC
User usernameC
IdentityFile ~/.ssh/id_rsa
Host C_D
HostName addressD
User usernameD
IdentityFile ~/.ssh/id_rsa
Port 22
ProxyCommand ssh -W %h:%p C
Running ssh -vvv C_D i get the following log (MobaXterm 7.4):
OpenSSH_6.2p2, OpenSSL 1.0.1g 7 Apr 2014
debug1: Reading configuration data /home/mobaxterm/.ssh/config
debug1: /home/mobaxterm/.ssh/config line 24: Applying options for C_D
debug1: Reading configuration data /etc/ssh_config
debug2: ssh_connect: needpriv 0
debug1: Executing proxy command: exec ssh -W addressOfD:22 C
debug1: permanently_drop_suid: 1000
debug3: Incorrect RSA1 identifier
debug3: Could not load "/home/mobaxterm/.ssh/id_rsa" as a RSA1 public key
debug1: identity file /home/mobaxterm/.ssh/id_rsa type 1
debug1: identity file /home/mobaxterm/.ssh/id_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: match: OpenSSH_5.3 pat OpenSSH_5*
debug2: fd 5 setting O_NONBLOCK
debug2: fd 4 setting O_NONBLOCK
debug3: load_hostkeys: loading entries for host "addressOfD" from file "/home/mobaxterm/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/mobaxterm/.ssh/known_hosts:1
debug3: load_hostkeys: loaded 1 keys
debug3: order_hostkeyalgs: prefer hostkeyalgs: [email protected],[email protected],ssh-rsa
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: [email protected],[email protected],ssh-rsa,[email protected],[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-5[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [email protected],zlib,none
debug2: kex_parse_kexinit: [email protected],zlib,none
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[email protected]
debug2: kex_parse_kexinit: none,[email protected]
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_setup: found hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 [email protected]
debug2: mac_setup: found hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 [email protected]
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: dh_gen_key: priv key bits set: 135/256
debug2: bits set: 533/1024
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA [...]
debug3: load_hostkeys: loading entries for host "addressOfD" from file "/home/mobaxterm/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/mobaxterm/.ssh/known_hosts:1
debug3: load_hostkeys: loaded 1 keys
debug1: Host 'addressOfD' is known and matches the RSA host key.
debug1: Found key in /home/mobaxterm/.ssh/known_hosts:1
debug2: bits set: 515/1024
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/mobaxterm/.ssh/id_rsa (0x20072e98), explicit
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug3: start over, passed a different list publickey,gssapi-keyex,gssapi-with-mic,password
debug3: preferred hostbased,publickey,password,keyboard-interactive
debug3: authmethod_lookup publickey
debug3: remaining preferred: password,keyboard-interactive
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/mobaxterm/.ssh/id_rsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug2: input_userauth_pk_ok: fp [...]
debug3: sign_and_send_pubkey: RSA [...]
debug1: Enabling compression at level 6.
Here it pauses for about 22 seconds.
debug1: Authentication succeeded (publickey).
Authenticated to addressOfD (via proxy).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Requesting [email protected]
debug1: Entering interactive session.
debug2: callback start
debug2: x11_get_proto: /bin/xauth list :0.0 2>/dev/null
debug1: Requesting X11 forwarding with authentication spoofing.
debug2: channel 0: request x11-req confirm 1
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: X11 forwarding request accepted on channel 0
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
Edit 2:
I tried the ssh config setup under Ubuntu 16.04 and did not get the 24 second delay (the connection was established in under 2 seconds). I downloaded the latest version of MobaXterm (v9.1) and only the first connection establishment was slow. I also tested MobaXterm on Windows 10 (before: Windows 7), same result. So I guess this is a problem with Windows/MobaXterm or whatever it is using under the hood.
Conclusion:
It seems the best I can do is to move to Linux (bit overkill) or forward the ssh agent (if I'm feeling lucky) or sit back and wait a bit. In both cases I have to load the ssh key in advance to avoid having to enter its password twice.
