Suggestion: SSHD pseudo/fake mode. Source available.

[Daniel Kastenholz]

Hope this helps.



[PROBLEM]
---
[Description:]
- Difference in sshd behaviour with option "DenyUsers=*" (Daemon closes connection after first wrong set of credentials)
---
[Applies to version:]
OpenSSH_3.7.1p2 (Suse 9.0)
---
[Steps to reproduce:]
- Launch sshd from command line
- Launch ssh from command line
- Enter wrong password
---



[TEST CASE 1]
---
[Server command line:]
sshd -o Port=2222 -d -d -d > out.txt 2>&1
---
[Client command line:]
ssh -p 2222 root at localhost
---
[Expected behaviour:]
- Daemon should deny login and ask 2 more times
---
[Actual behaviour:]
- Daemon behaves like expected
---
[out.txt:]
debug2: read_server_config: filename /etc/ssh/sshd_config
debug1: sshd version OpenSSH_3.7.1p2
debug1: private host key: #0 type 0 RSA1
debug3: Not a RSA1 key file /etc/ssh/ssh_host_rsa_key.
debug1: read PEM private key done: type RSA
debug1: private host key: #1 type 1 RSA
debug3: Not a RSA1 key file /etc/ssh/ssh_host_dsa_key.
debug1: read PEM private key done: type DSA
debug1: private host key: #2 type 2 DSA
debug1: Bind to port 2222 on ::.
Server listening on :: port 2222.
Generating 768 bit RSA key.
RSA key generation complete.
debug1: Server will not fork when running in debugging mode.
Connection from ::ffff:127.0.0.1 port 32775
debug1: Client protocol version 2.0; client software version OpenSSH_3.7.1p2
debug1: match: OpenSSH_3.7.1p2 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-1.99-OpenSSH_3.7.1p2
debug1: list_hostkey_types: ssh-rsa,ssh-dss
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
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-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: first_kex_follows 0 
debug2: kex_parse_kexinit: reserved 0 
debug2: mac_init: found hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
debug2: mac_init: found hmac-md5
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug2: dh_gen_key: priv key bits set: 134/256
debug2: bits set: 1624/3191
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug2: bits set: 1605/3191
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
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: KEX done
debug1: userauth-request for user root service ssh-connection method none
debug1: attempt 0 failures 0
debug2: input_userauth_request: setting up authctxt for root
debug1: PAM: initializing for "root"
debug3: Trying to reverse map address 127.0.0.1.
debug1: PAM: setting PAM_RHOST to "localhost"
debug1: PAM: setting PAM_TTY to "ssh"
debug2: input_userauth_request: try method none
Failed none for root from ::ffff:127.0.0.1 port 32775 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 1 failures 1
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_recv entering
debug3: ssh_msg_send: type 1
Postponed keyboard-interactive for root from ::ffff:127.0.0.1 port 32775 ssh2
debug3: ssh_msg_recv entering
debug2: PAM: sshpam_respond
debug3: ssh_msg_send: type 6
debug3: ssh_msg_send: type 7
debug3: ssh_msg_recv entering
PAM: Authentication failure
debug2: auth2_challenge_start: devices <empty>
Failed keyboard-interactive/pam for root from ::ffff:127.0.0.1 port 32775 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 2 failures 2
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_send: type 1
debug3: ssh_msg_recv entering
debug3: ssh_msg_recv entering
Postponed keyboard-interactive for root from ::ffff:127.0.0.1 port 32775 ssh2
debug2: PAM: sshpam_respond
debug3: ssh_msg_send: type 6
debug3: ssh_msg_recv entering
debug3: ssh_msg_send: type 7
PAM: Authentication failure
debug2: auth2_challenge_start: devices <empty>
Failed keyboard-interactive/pam for root from ::ffff:127.0.0.1 port 32775 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 3 failures 3
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_recv entering
debug3: ssh_msg_send: type 1
Postponed keyboard-interactive for root from ::ffff:127.0.0.1 port 32775 ssh2
debug3: ssh_msg_recv entering
debug2: PAM: sshpam_respond
debug3: ssh_msg_send: type 6
debug3: ssh_msg_recv entering
debug3: ssh_msg_send: type 7
PAM: Authentication failure
debug2: auth2_challenge_start: devices <empty>
Failed keyboard-interactive/pam for root from ::ffff:127.0.0.1 port 32775 ssh2
Connection closed by ::ffff:127.0.0.1
debug1: Calling cleanup 0x8066f50(0x0)
debug1: PAM: cleanup
debug1: Calling cleanup 0x80733b0(0x0)



[TEST CASE 2]
---
[Server command line:]
sshd -o Port=2222 -d -d -d -o DenyUsers="*" > out.txt 2>&1
---
[Client command line:]
ssh -p 2222 root at localhost
---
[Expected behaviour:]
- Daemon should deny login and ask 2 more times
---
[Actual behaviour:]
- Daemon denies first login and closes connection
---
[out.txt:]
debug2: read_server_config: filename /etc/ssh/sshd_config
debug1: sshd version OpenSSH_3.7.1p2
debug1: private host key: #0 type 0 RSA1
debug3: Not a RSA1 key file /etc/ssh/ssh_host_rsa_key.
debug1: read PEM private key done: type RSA
debug1: private host key: #1 type 1 RSA
debug3: Not a RSA1 key file /etc/ssh/ssh_host_dsa_key.
debug1: read PEM private key done: type DSA
debug1: private host key: #2 type 2 DSA
debug1: Bind to port 2222 on ::.
Server listening on :: port 2222.
Generating 768 bit RSA key.
RSA key generation complete.
debug1: Server will not fork when running in debugging mode.
Connection from ::ffff:127.0.0.1 port 32772
debug1: Client protocol version 2.0; client software version OpenSSH_3.7.1p2
debug1: match: OpenSSH_3.7.1p2 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-1.99-OpenSSH_3.7.1p2
debug1: list_hostkey_types: ssh-rsa,ssh-dss
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
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-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc at lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160 at openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: first_kex_follows 0 
debug2: kex_parse_kexinit: reserved 0 
debug2: mac_init: found hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
debug2: mac_init: found hmac-md5
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug2: dh_gen_key: priv key bits set: 130/256
debug2: bits set: 1626/3191
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug2: bits set: 1576/3191
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
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: KEX done
debug1: userauth-request for user root service ssh-connection method none
debug1: attempt 0 failures 0
debug3: Trying to reverse map address 127.0.0.1.
User root not allowed because listed in DenyUsers
input_userauth_request: illegal user root
debug1: PAM: initializing for "root"
debug1: PAM: setting PAM_RHOST to "localhost"
debug1: PAM: setting PAM_TTY to "ssh"
debug2: input_userauth_request: try method none
Failed none for illegal user root from ::ffff:127.0.0.1 port 32772 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 1 failures 1
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_send: type 1
debug3: ssh_msg_recv entering
debug3: ssh_msg_recv entering
Postponed keyboard-interactive for illegal user root from ::ffff:127.0.0.1 port 32772 ssh2
debug2: auth2_challenge_start: devices <empty>
Failed keyboard-interactive/pam for illegal user root from ::ffff:127.0.0.1 port 32772 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 2 failures 2
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_send: type 7
debug3: ssh_msg_recv entering
PAM: System error
Failed keyboard-interactive for illegal user root from ::ffff:127.0.0.1 port 32772 ssh2
debug1: userauth-request for user root service ssh-connection method keyboard-interactive
debug1: attempt 3 failures 3
debug2: input_userauth_request: try method keyboard-interactive
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=root devs=
debug1: kbdint_alloc: devices 'pam'
debug2: auth2_challenge_start: devices pam
debug2: kbdint_next_device: devices <empty>
debug1: auth2_challenge_start: trying authentication method 'pam'
debug3: ssh_msg_send: type 7
debug3: ssh_msg_recv entering
PAM: System error
Failed keyboard-interactive for illegal user root from ::ffff:127.0.0.1 port 32772 ssh2
Connection closed by ::ffff:127.0.0.1
debug1: Calling cleanup 0x8066f50(0x0)
debug1: PAM: cleanup
debug1: Calling cleanup 0x80733b0(0x0)