Block repeated illegal or failed SSH logins

Introduction

We're starting to see a rash of password guessing attacks via SSH on exposed BSD servers which are running the SSH daemon. These login attempts are coming from multiple addresses, which makes some people suspect that they're being carried out by a network of "bots" rather than a single attacker.

Limiting SSH login sessions

In your sshd_config file the following settings can also help slow down such attacks.

• LoginGraceTime

The server disconnects after this time if the user has not successfully logged in. If the value is 0, there is no time limit. The default is 120 seconds.

• MaxStartups

Specifies the maximum number of concurrent unauthenticated connections to the sshd daemon. Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10. Alternatively, random early drop can be enabled by specifying the three colon separated values "start:rate:full" (e.g.,"10:30:60"). sshd will refuse connection attempts with a probability of "rate/100" (30%) if there are currently "start" (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches "full" (60).

Firewall repeated illegal or failed SSH logins attempts

To firewall failed login attemps, a simple script that will scan the log file for illegal or failed attempts and firewall repeated IP's will do the trick. It will slow down and stop a brute force dictionary login attack.

Using the examples below you can create a file called sshd-fwscan.sh, then use cron to run the file every x minutes and it will automatically firewall the IP once it detects 5 or more failed login attempts.

/etc/syslog.conf

You need an auth.* line in your syslog.conf file in order to log all authentications.

auth.*                                          /var/log/auth.log

Using IPFW

sshd-fwscan.sh

#!/bin/sh
if ipfw show | awk '{print $1}' |