Sam Stelfox

Thoughts from a software engineer, systems hacker and Linux gubernāre.

SSHd

Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. Used primarily on GNU/Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which send information, notably passwords, in plaintext, rendering them susceptible to packet analysis. The encryption used by SSH provides confidentiality and integrity of data over an insecure network, such as the Internet.

Security Notes

SSH is a crux service. An improperly configured SSH could be equivalent to letting an attacker have physical access to the box.

Generally it's recommended to run any instances of SSH exposed to the public internet on an alternate port. Alternate ports won't protect you against vulnerabilities but it will drastrically reduce the number of automated and scripted attacks your server will be subjected to. If nothing else it will allow real attacks to stand out more clearly in your logs.

Firewall Adjustments

My default IPTables firewall already has the following rules in place to allow access to SSH by default while still providing a modicum of protection from attackers.

# Allow SSH, but no more than 5 new connections every minute Note: This has
# extreme repercussions if I was to use sftp as each file transfer initiates a
# new connection. Since it's rare for me to use sftp this isn't really an
# issue, however, when I do want to use it this rule will be the cause of
# failed transfers. Hopefully I will save myself the diagnostic nightmare
# scenario I went through last time
-A INPUT -m tcp -p tcp --dport 22 -m state --state NEW -m recent --name SSH --set
-A INPUT -m tcp -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 5 --rttl --name SSH -j LOG --log-prefix "SSH Brute Force"
-A INPUT -m tcp -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 5 --rttl --name SSH -j DROP
-A INPUT -m tcp -p tcp --dport 22 -m state --state NEW -j ACCEPT

Configuration

/etc/ssh/sshd_config

The following is a minimal SSHd config relying mostly on the defaults. It requires a group named sshers to be created before hand and any user that should have legitimate access via SSH should be added to the group.

Banner /etc/issue.net

ClientAliveInterval 60
TCPKeepAlive no

SyslogFacility AUTHPRIV
UseDNS no

PermitRootLogin no
PasswordAuthentication no
UsePAM yes

AllowTcpForwarding no
AllowGroups sshers

# Accept locale-related environment variables
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE

To create the group and add the user 'test' to it you can run the following commands:

groupadd sshers
usermod -a -G sshers test

/etc/issue.net

The net issue file is what gets displayed before a user logs in. While I'm under no false pretense that having a policy displayed to a potential attacker will prevent them from their malicious activity, it is generally recommended by law enforcement to ensure that notice of unauthorized access was given prior.

At a minimum it will show the server has been professionally configured and that they are not dealing with a service that was left unconfigured.

************************************************************************
This system is privately owned. If you are not authorized to access this
system, exit immediately. Unauthorized access to this system is
forbidden by organization policies, national, and international laws.
Unauthorized users are subject to criminal and civil penalties as well
as organization initiated disciplinary proceedings.

By entry into this system you acknowledge that you are authorized to
access and have been granted the level of privileges you will
subsequently execute on this system. You further acknowledge that by
entry into this system you expect no privacy from monitoring.
************************************************************************

Default SSH Client Configuration

# /etc/ssh/ssh_config
Host *
  Compression yes
  HashKnownHosts yes
  Protocol 2
  SendEnv LC_ALL
  VerifyHostKeyDNS ask
  VisualHostKey yes

SSH Authorized Keys

SSH authorized keys provide a considerably stronger authentication method than a user's password as long as the key is protected by a pass-phrase. If the key is left without a pass-phrase, anyone who manages to get access to the system can immediately login as that user anywhere they have deployed the key. Ensure there is a strong pass-phrase on the key.

The keys are normally created as ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub, however I like to physically separate my keys from my computers so I'll give my keys a descriptive name of [email protected] and put them on a pendrive located at /media/pendrive. Personally I to use keys with 4096 bits. The following command will create the public / private keys:

[[email protected] ~]$ ssh-keygen -t rsa -b 4096 -f [email protected]
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in [email protected]
Your public key has been saved in [email protected]
The key fingerprint is:
xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx [email protected]

Now that we have a strong key we just need to place it on the servers we want to access. Conveniently enough there is a utility that makes this quick and simple, if for some reason you don't trust this utility or want to do it by hand just copy the contents of [email protected] into ~/.ssh/authorized_keys on the remote host and make sure the file's permissions are set to 0400.

You can use the utility by performing the following command, replacing [email protected] with a valid username and hostname for the remote host.

[[email protected] ~]$ ssh-copy-id -i [email protected] [email protected]
[email protected]'s password: 
Now try logging into the machine, with "ssh [email protected]'", and check in:

  .ssh/authorized_keys

to make sure we haven't added extra keys that you weren't expecting.

[[email protected] ~]$ ssh -i [email protected] [email protected]
Enter passphrase for key [email protected]':
[[email protected] ~]$

Once your sure that the key based login is working you can safely disable password based logins through ssh to the system entirely by changing the following line in /etc/ssh/sshd_config:

PasswordAuthentication yes
---- Replace with ----
PasswordAuthentication no

This replaces the 'something you know' of a password based login, with 'something you have' (the key, especially if it's on a pendrive) and 'something you know' (the key's pass-phrase).

User Jails

Jails can isolate users from sensitive parts of the system, creating a 'fake' environment for them to execute their programs. In the event that a user's account becomes compromised, the jail can limit what an attacker has access too though it should be considered a delay rather than a security measure since a determined hacker might be able to break out of an otherwise secure jail.

To start off we're going to need to setup an environment for our jail. The following commands create the base directories and the /dev/null device within the jail.

[[email protected]]# mkdir -p /var/jail/{dev,etc,lib,lib64,usr/bin,bin,home}
[[email protected]]# chown -R root:root /var/jail/
[[email protected]]# mknod -m 666 /var/jail/dev/null c 1 3

There is something serious to note here. The partitioning security guidelines add two flags to the mount options of the /var partition. You will need to remove the 'nosuid', and 'noexec' options from this partition to be able use a chroot jail.

Next we need to copy a few minimum files into the jail's etc directory. Hard links may work better for these as they will get properly updated with the rest of the system, however, I am unsure whether or not that would actually work.

[[email protected]]# cp /etc/ld.so.cache /etc/ld.so.conf /etc/nsswitch.conf /etc/hosts /var/jail/etc/

At this point you need to decide what commands you want you're user to have access too and copy the appropriate binaries into place. You can use the which command to locate a binary and then copy it into the same directory within the jail. The following is an example for moving bash over.

[[email protected]]# which bash
/bin/bash
[[email protected]]# cp /bin/bash /var/jail/bin/

The trickiest part of setting up a jail is the required libraries for the allowed executables. Conveniently, all the Linux distributions that I have tried come with a tool too determine what libraries are required. ldd. Here's a little snippet that will give you a run down of the counts for the number of times each library is used by a system:

find /bin -type f -perm /a+x -exec ldd {} \; \
| grep so \
| sed -e '/^[^\t]/ d' \
| sed -e 's/\t//' \
| sed -e 's/.*=..//' \
| sed -e 's/ (0.*)//' \
| sort \
| uniq -c \
| sort -n

If for some reason ldd isn't reporting all of the libraries, while the app is running you can use this command to track down what it has actually loaded:

lsof -P -T -p Application_PID

A few references:

Multiplex-Sessions & Signed PubKeys

Published SSH Host Keys

LXC Specific

If you want to run sshd in a LXC container you will need to disable pam_loginuid.so in /etc/pam.d/sshd by commenting out the appropriate line.

Misc

I've play around with implementing a gatekeeper style script to provide an additional layer of security. In practice real multi-factor authentication is more reliable and should be used instead.