ssh public key authentication

teacup · 07-14-2004, 07:27 PM

This is continued from my unresolved problem here.

Let me explain my problem in context. My sister, a technophobe, uses a pentium 60 MHz computer. She is running debian with X and windomaker (hostname lisacomp). She only uses her computer to check email with sylpheed, ocassionally play xgalaga, and talk to friends with gaim. She occasionally needs/wants to browse the web. I used to have dillo installed on her computer, but it didn't (and still doesn't) render many pages correctly. It also didn't support flash which one of her textbook's website required. I tried installing mozilla firebird on her computer. This didn't work very well -- it took way too long to start and worked slowly. I set up ssh with pubkey authentication so that she could log into my computer (hostname teacup) without a password. I then placed an icon on her desktop that ran

Code:

ssh -X teacup firewhatever

This worked magnificently for a long period of time. Then, my hard drive kicked the bucket, I lost all of my data, and I reinstalled slackware. Since I reinstalled slackware, I cannot ssh from lisacomp to teacup without a password. I tried sshing with my account and my sister's account, but I had no success. Public key authentication from teacup to lisacomp works fine. I correctly regenerated all of the necessary keys. I searched all over this forum. I deleted all the keys and regenerated them as rsa instead of dsa. I tried giving authorized_keys, id_rsa.pub, id_rsa the most liberal permissions possible. I scoured the manpages for ssh, sshd, ssh_config, and sshd_config. I diffed her ssh_config with my ssh_config, and I diffed her sshd_config with my sshd_config. I found no significant differences. Remember, ssh with a password works correctly, my problem is getting it to work without one. Here is some sample output:

Code:

pcurry@lisacomp:~$ ssh -vv teacup
OpenSSH_3.8.1p1 Debian 1:3.8.1p1-4, OpenSSL 0.9.7d 17 Mar 2004
debug1: Reading configuration data /etc/ssh/ssh_config
debug2: ssh_connect: needpriv 0
debug1: Connecting to teacup [192.168.0.2] port 22.
debug1: Connection established.
debug1: identity file /home/pcurry/.ssh/identity type -1
debug2: key_type_from_name: unknown key type '-----BEGIN'
debug2: key_type_from_name: unknown key type '-----END'
debug1: identity file /home/pcurry/.ssh/id_rsa type 1
debug1: identity file /home/pcurry/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_3.8p1
debug1: match: OpenSSH_3.8p1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_3.8.1p1 Debian 1:3.8.1p1-4
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@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@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@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@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@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@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: server->client aes128-cbc hmac-md5 none
debug2: mac_init: found hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
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: 125/256
debug2: bits set: 501/1024
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'teacup' is known and matches the RSA host key.
debug1: Found key in /home/pcurry/.ssh/known_hosts:1
debug2: bits set: 508/1024
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: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/pcurry/.ssh/identity ((nil))
debug2: key: /home/pcurry/.ssh/id_rsa (0x808b1b8)
debug2: key: /home/pcurry/.ssh/id_dsa ((nil))
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Trying private key: /home/pcurry/.ssh/identity
debug1: Offering public key: /home/pcurry/.ssh/id_rsa
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Trying private key: /home/pcurry/.ssh/id_dsa
debug2: we did not send a packet, disable method
debug1: Next authentication method: keyboard-interactive
debug2: userauth_kbdint
debug2: we sent a keyboard-interactive packet, wait for reply
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug2: we did not send a packet, disable method
debug1: Next authentication method: password
pcurry@teacup's password:

Here is the sshd_config file on teacup:

Code:

#	$OpenBSD: sshd_config,v 1.65 2003/08/28 12:54:34 markus Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options change a
# default value.

Port 22
Protocol 2
#ListenAddress 0.0.0.0
#ListenAddress ::

# HostKey for protocol version 1
#HostKey /etc/ssh/ssh_host_key
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 768

# Logging
#obsoletes QuietMode and FascistLogging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

LoginGraceTime 120
PermitRootLogin no
StrictModes yes

RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
PasswordAuthentication yes
PermitEmptyPasswords no

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCreds yes

# Set this to 'yes' to enable PAM authentication (via challenge-response)
# and session processing. Depending on your PAM configuration, this may
# bypass the setting of 'PasswordAuthentication'
#UsePAM no

#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes 
X11DisplayOffset 10
X11UseLocalhost yes
PrintMotd yes
PrintLastLog yes
#KeepAlive yes
UseLogin no
UsePrivilegeSeparation yes
#PermitUserEnvironment no
#Compression yes
#ClientAliveInterval 0
#ClientAliveCountMax 3
UseDNS yes
#PidFile /var/run/sshd.pid
#MaxStartups 10

# no default banner path
#Banner /some/path

# override default of no subsystems
Subsystem	sftp	/usr/libexec/sftp-server

Here is the ssh_config file on lisacomp:

Code:

 #       $OpenBSD: ssh_config,v 1.19 2003/08/13 08:46:31 markus Exp $

# This is the ssh client system-wide configuration file.  See
# ssh_config(5) for more information.  This file provides defaults for
# users, and the values can be changed in per-user configuration files
# or on the command line.

# Configuration data is parsed as follows:
#  1. command line options
#  2. user-specific file
#  3. system-wide file
# Any configuration value is only changed the first time it is set.
# Thus, host-specific definitions should be at the beginning of the
# configuration file, and defaults at the end.

PubkeyAuthentication yes
# Site-wide defaults for various options

# Host *
#   ForwardAgent no
#   ForwardX11 no
#   ForwardX11Trusted yes
#   RhostsRSAAuthentication no
#   RSAAuthentication yes
#   PasswordAuthentication yes
#   HostbasedAuthentication no
#   BatchMode no
#   CheckHostIP yes
#   AddressFamily any
#   ConnectTimeout 0
#   StrictHostKeyChecking ask
#   IdentityFile ~/.ssh/identity
#   IdentityFile ~/.ssh/id_rsa
#   IdentityFile ~/.ssh/id_dsa
#   Port 22
#   Protocol 2,1
#   Cipher 3des
#   Ciphers aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc
#   EscapeChar ~

Here is the .ssh directory on lisacomp:

Code:

pcurry@lisacomp:~/.ssh$ ls -la ~ | grep ssh; ls -l
drwx------    2 pcurry   pcurry       4096 Jul 14 09:14 .ssh
total 16
-rw-------    1 pcurry   pcurry        223 Jul 14 09:12 authorized_keys
-rw-------    1 pcurry   pcurry        883 Jul 14 09:14 id_rsa
-rw-------    1 pcurry   pcurry        225 Jul 14 09:14 id_rsa.pub
-rw-------    1 pcurry   pcurry        228 Jul 14 09:14 known_hosts

Here is the .ssh directory on teacup:

Code:

drwx------   2 pcurry users    168 Jul 14 09:15 .ssh/
total 16
-rw-------  1 pcurry users 225 Jul 14 09:15 authorized_keys
-rw-------  1 pcurry users 883 Jul 14 09:12 id_rsa
-rw-------  1 pcurry users 223 Jul 14 09:12 id_rsa.pub
-rw-------  1 pcurry users 230 Jul 14 09:11 known_hosts

I've spent many hours trying to get this to work and would appreciate assistance. Thank you.

mlp68 · 07-15-2004, 12:12 AM

Do you have root access on teacup?

My suspicion is that "the most liberal permissions" get you. sshd will not honor a key file if it thinks that the permissions are too open and the file can be tampered with.

My generic way to shake down ssh problems is to also look at the sshd response in addition to the client response. Maybe the answer is already in teacup's syslog, but you can do, as root on teacup

sshd -d -p 2000

that starts a one-shot sshd in debug mode (all verbose output goes to the terminal). Pick a port (2000 in this example) that's > 1024 and not blocked by firewalls. Then do, on the other system,

ssh -vv -p 2000 teacup

and you will get some hints why it doesn't work. When you connect and get out, that special sshd terminates, and you have to restart it for the next try. The really interesting answers come from sshd, not from the client.

Hope it helps,
mlp

teacup · 07-16-2004, 09:30 PM

Quote:

My suspicion is that "the most liberal permissions" get you. sshd will not honor a key file if it thinks that the permissions are too open and the file can be tampered with.

I should have just said:
authorized_keys 666
id_rsa.pub 666
id_rsa 600
The man page says that authorized_keys and id_rsa.pub can be accessible to others.

Quote:

$HOME/.ssh/authorized_keys
Lists the public keys (RSA/DSA) that can be used for logging in
as this user. The format of this file is described in the
sshd(8) manual page. In the simplest form the format is the same
as the .pub identity files. This file is not highly sensitive,
but the recommended permissions are read/write for the user, and
not accessible by others.

Quote:

$HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
Contains the public key for authentication (public part of the
identity file in human-readable form). The contents of the
$HOME/.ssh/identity.pub file should be added to the file
$HOME/.ssh/authorized_keys on all machines where the user wishes
to log in using protocol version 1 RSA authentication. The con-
tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
should be added to $HOME/.ssh/authorized_keys on all machines
where the user wishes to log in using protocol version 2 DSA/RSA
authentication. These files are not sensitive and can (but need
not) be readable by anyone. These files are never used automati-
cally and are not necessary; they are only provided for the con-
venience of the user.

but id_rsa cannot

Quote:

$HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
Contains the authentication identity of the user. They are for
protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
These files contain sensitive data and should be readable by the
user but not accessible by others (read/write/execute). Note
that ssh ignores a private key file if it is accessible by oth-
ers.

The debugging message is here:

Code:

root@teacup:/home/pcurry# sshd -d
debug1: sshd version OpenSSH_3.8p1
debug1: read PEM private key done: type RSA
debug1: private host key: #0 type 1 RSA
debug1: read PEM private key done: type DSA
debug1: private host key: #1 type 2 DSA
socket: Address family not supported by protocol
debug1: Bind to port 22 on 0.0.0.0.
Server listening on 0.0.0.0 port 22.
debug1: Server will not fork when running in debugging mode.
Connection from 192.168.0.3 port 1210
debug1: Client protocol version 2.0; client software version OpenSSH_3.8.1p1 Debian 1:3.8.1p1-4
debug1: match: OpenSSH_3.8.1p1 Debian 1:3.8.1p1-4 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_3.8p1
debug1: permanently_set_uid: 33/33
debug1: list_hostkey_types: ssh-rsa,ssh-dss
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: client->server aes128-cbc hmac-md5 none
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
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: KEX done
debug1: userauth-request for user pcurry service ssh-connection method none
debug1: attempt 0 failures 0
Failed none for pcurry from 192.168.0.3 port 1210 ssh2
Failed none for pcurry from 192.168.0.3 port 1210 ssh2
debug1: userauth-request for user pcurry service ssh-connection method publickey
debug1: attempt 1 failures 1
debug1: test whether pkalg/pkblob are acceptable
debug1: temporarily_use_uid: 1000/100 (e=0/0)
debug1: trying public key file /home/pcurry/.ssh/authorized_keys
Authentication refused: bad ownership or modes for directory /home/pcurry
debug1: restore_uid: 0/0
debug1: temporarily_use_uid: 1000/100 (e=0/0)
debug1: trying public key file /home/pcurry/.ssh/authorized_keys
Authentication refused: bad ownership or modes for directory /home/pcurry
debug1: restore_uid: 0/0
Failed publickey for pcurry from 192.168.0.3 port 1210 ssh2
debug1: userauth-request for user pcurry service ssh-connection method keyboard-interactive
debug1: attempt 2 failures 2
debug1: keyboard-interactive devs
debug1: auth2_challenge: user=pcurry devs=
debug1: kbdint_alloc: devices ''
Failed keyboard-interactive for pcurry from 192.168.0.3 port 1210 ssh2

The headsmacker is right here:

Quote:

Authentication refused: bad ownership or modes for directory /home/pcurry

Apparantly, ssh doesn't like it if your home directory has group write access. My home dir on teacup was 770. I changed it to 750 and ssh works without a password now. Thanks. Does anyone know why ssh requires this? I don't remember reading anything about this in the documentation.

balpo · 03-23-2010, 01:00 AM

Quote:

Originally Posted by teacup

I should have just said:
authorized_keys 666
id_rsa.pub 666
id_rsa 600
The man page says that authorized_keys and id_rsa.pub can be accessible to others.

Another thing you can try is setting StrictMode No in your sshd_config file, this disables Strict modes on directories.

akshay_n · 11-27-2011, 11:27 PM

You mentioned :
Apparantly, ssh doesn't like it if your home directory has group write access.

I need to keep the group write permissions on my machine for /home/user.

Isn't there any other way in which this could be done.
Because i don't want to revoke the group write permissions.

There has to be another way to bypass this SecurityCheck.