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Protecting sshd (and others) with BlockHosts

What is BlockHosts?

Blockhosts is a python script which records how many times a system service has been probed, using configurable pattern matching to recognize failed accesses (such as for "sshd" or "proftpd" or any service), and when a particular IP address exceeds a certain number of failed attempts that IP address is blocked by using one of the following techniques, e.g.:

using TCP_WRAPPERS (writes to /etc/hosts.allow) using "ip route" commands to setup null-routing for attackers using IPtables to setup packet filtering for attackers


BlockHosts can prevent attacks from SSHd and many other services (such as proftpd, vsftpd) innately. It provides multiple ways to block the attacker It's very easy to set up


Ebuild Currently there's no BlockHosts package in the Portage tree. I've written an ebuild (thanks again to #gentoo-sunrise for reviewing it) which should work (for all archs?). If you do not know how to cope with 3rd party ebuilds, refer to the handbook. download (save as blockhosts-2.6.0)

Emerge Warning: I assume that you've set your PORTDIR_OVERLAY to /usr/local/portage.

First, copy the ebuild to /usr/local/portage/app-admin/blockhosts/. Then do this:

  1. Create blockhosts digest
    • ebuild /usr/local/portage/app-admin/blockhosts/blockhosts-2.6.0.ebuild digest
  2. Unmask blockhosts
    • echo "app-admin/blockhosts ~x86" >> /etc/portage/package.keywords
  3. Emerge it
    • emerge -va app-admin/blockhosts

That's everything.


First, you need to create and edit some files

  1. Create /etc/hosts.allow if it does not exists (required by BlockHosts)
    • touch /etc/hosts.allow
  2. Append the following lines to /etc/hosts.allow (BlockHosts will write its own stuff between them)
echo "#---- BlockHosts Additions" >> /etc/hosts.allow
echo "#---- BlockHosts Additions" >> /etc/hosts.allow

Setting up BlockHosts protecting sshd

Setting up openssh:

Check if openssh was merged with the tcpd useflag enabled:

equery uses openssh

If not, add this useflag (it's necessary to work with TCP_WRAPPERS) to the openssh package

echo "net-misc/openssh tcpd" >> /etc/portage/package.use

Re-emerge to apply use flags

emerge -va net-misc/openssh

Add this to your /etc/ssh/sshd_config (disables DNS lookups which may confuse blockhosts when scanning the logs)

UseDNS no

Check if your SSHd logs to /var/log/messages

cat /var/log/messages | grep sshd

If there's some recent output, everything's ok. Proceed.

Setting up BlockHosts:

Edit /etc/blockhosts.cfg

nano /etc/blockhosts.cfg

All occurences of LOGFILES are commented, uncomment the first occurence, and change "secure" to "messages".

File: /etc/blockhosts.cfg

LOGFILES = [ "/var/log/messages", ]
#LOGFILES = [ "/var/log/auth.log", ]
#LOGFILES = [ "/var/log/secure", "/var/log/vsftpd.log", ]

Save, close nano again, this should do it.


You can tune some settings in the config file if you like to (THRESHOLD and stuff), but the defaults are ok.


Run in --dry-run mode (simulation):

  1. This will check your logs (configured in /etc/blockhosts.cfg) for potential attacks
/usr/bin/ --dry-run --verbose

The script should output something like this (assumed there were some failed login attempts already):

#---- BlockHosts Additions
ALL: : deny 
#bh: ip: :   8 : 2007-04-10 00:52:23 CEST

#bh: logfile: /var/log/sshd
#bh: offset: 13083
#bh: first line:Apr  9 23:49:37 hostname sshd(pam_unix)[29697]: authentication$
#---- BlockHosts Additions

The lines starting with "#bh: ip:" count how many times a host has failed to login to any of your services. The lines starting with "ALL:" are the blocked hosts. Now, if everything seems to be ok, drop the --dry-run parameter. BlockHosts will now write to the /etc/hosts.allow file and every service that uses TCP_WRAPPERS (mod_wrap for proftpd) refuses connections from banned ips.

/usr/bin/ --verbose


Now we want to have a cronjob or something which will run again and again, to check the logs frequently. Alternatively, we can have TCP_WRAPPERS run the script every time someone connects to our watched service by modifying /etc/hosts.allow.


Add a cronjob which runs every five minutes

nano /etc/crontab

Add cron

*/5 * * * * /usr/bin/ --verbose >> /var/log/blockhosts.log 2>&1

Save, close. BlockHosts should now update hosts.allow every five minutes.


Add a couple of lines to the end of /etc/hosts.allow to have the script called at every connection attempt.

File: /etc/hosts.allow

sshd : ALL : spawn (/usr/bin/ )
sshd : ALL : allow

This will cause /usr/bin/ to be called every time someone connects - much more likely to stop an attack before it fills your logs. However, if you log in incorrectly too many times and get blocked, you will be blocked until someone from a different ip logs in after the timeout has expired (set in /etc/blockhosts.cfg). To remove your dependency on another user attempting to log in, have a cron job also running the script, although possibly less frequently than every five minutes.

See also BlockHosts Homepage

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