Limit access to openssh features with the Match option
Published: 24-11-2012 | Author: Remy van Elst | Text only version of this article
Table of Contents
OpenSSH has a lot of nice features which let you control how it is used. Forexample, you can disallow the root account to login, set the port number,protocol version and a lot of other features. This tutorial will show you how toenable certain features for certain hosts, users, groups and addresses with theMatch keyword in sshd_config. And as a bonus it also covers the iptablesfirewall.
First a few general tips about securing ssh.
- Disable login with username/pasword and use key based authentication.
- Do not allow root to login, limit permissions with sudo or use su.
- Only allow protocol 2, not any of the earlier ones.
- Not a real security measure but makes your logs less dirty: set sshd to a higher port.
You of course have disabled password authentication and you use public/privatekey based authentication, right? Buy, you might have that one host which is forwhatever reason is not able to use ssh keys. The below config line disablespassword authentication for everyone, and then it enables it for the IP address184.108.40.206:
PasswordAuthentication no### this should be on the bottom of the config file### Enable password authentication for IP 220.127.116.11Match Address 18.104.22.168 PasswordAuthentication yes
Or you might have a user which needs to use a graphical application on a server.But all the other users do not have to use that. For example, you might haveMatlab for one user. You can install a desktop for them, but you can also letthem use X forwarding. The below config allows X forwarding for the user John,but disallows it for everyone else.
X11Forwarding no### add this to the bottom of the sshd_configMatch User John X11Forwarding yes
But lets say John can only use matlab (X-forwarding) from the internal networkand you want to X forwarding for every other user, only allowing it from thelocal 172.16.1.* network, you might want to use this config lines:
X11Forwarding no### add this to the bottom of the sshd_configMatch User John Address 172.16.1.* X11Forwarding yes
And what if you want to allow a few IP addresses and one hostname to login witha password, as root? Note that this is a bad thing to do, you should not allowroot to login but use su or sudo, and preferably all users should login with sshkeys.
PaswordAuthentication noPermitRootLogin no### Add this to the end of the config fileMatch Address 10.20.30.40,22.214.171.124 Host dispatch.raymii.org PasswordAuthentication Yes PermitRootLogin yes
Restrict access via the firewall
Using iptables is also a good way to restrict access to a few IP addresses. Thebelow example allows the IP addresses 126.96.36.199, 188.8.131.52 and 10.2.3.40 to talk toport 22, and discards all the other traffic.
iptables -I INPUT -s 184.108.40.206 -p tcp -m tcp --dport 22 -j ACCEPTiptables -I INPUT -s 220.127.116.11 -p tcp -m tcp --dport 22 -j ACCEPTiptables -I INPUT -s 10.2.3.40 -p tcp -m tcp --dport 22 -j ACCEPTiptables -I INPUT -p tcp -m tcp --dport 22 -j REJECT
All the options
The list below are all the options supported in an SSH Match pattern: