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Securing Debian Manual
Appendix F - Security update protected by a firewall

After a standard installation, a system may still have some security vulnerabilities. Unless you can download updates for the vulnerable packages on another system (or you have mirrored security.debian.org for local use), the system will have to be connected to the Internet for the downloads.

However, as soon as you connect to the Internet you are exposing this system. If one of your local services is vulnerable, you might be compromised even before the update is finished! This may seem paranoid but, in fact, analysis from the Honeynet Project has shown that systems can be compromised in less than three days, even if the system is not publicly known (i.e., not published in DNS records).

When doing an update on a system not protected by an external system like a firewall, it is possible to properly configure your local firewall to restrict connections involving only the security update itself. The example below shows how to set up such local firewall capabilities, which allow connections from security.debian.org only, logging all others.

The following example can be use to setup a restricted firewall ruleset. Run this commands from a local console (not a remote one) to reduce the chances of locking yourself out of the system.

       # iptables -F
       # iptables -L
       Chain INPUT (policy ACCEPT)
       target     prot opt source               destination
     
       Chain FORWARD (policy ACCEPT)
       target     prot opt source               destination
     
       Chain OUTPUT (policy ACCEPT)
       target     prot opt source               destination
       # iptables -A OUTPUT -d security.debian.org --dport 80 -j ACCEPT
       # iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
       # iptables -A INPUT -p icmp -j ACCEPT
       # iptables -A INPUT -j LOG
       # iptables -A OUTPUT -j LOG
       # iptables -P INPUT DROP
       # iptables -P FORWARD DROP
       # iptables -P OUTPUT DROP
       # iptables -L
       Chain INPUT (policy DROP)
       target     prot opt source               destination
       ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0          state RELATED,ESTABLISHED
       ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0
       LOG        all  --  anywhere             anywhere           LOG level warning
     
       Chain FORWARD (policy DROP)
       target     prot opt source               destination
     
       Chain OUTPUT (policy DROP)
       target     prot opt source               destination
       ACCEPT     80   --  anywhere             security.debian.org
       LOG        all  --  anywhere             anywhere           LOG level warning

Note: Using a DROP policy in the INPUT chain is the most correct thing to do, but be very careful when doing this after flushing the chain from a remote connection. When testing firewall rulesets from a remote location it is best if you run a script with the firewall ruleset (instead of introducing the ruleset line by line through the command line) and, as a precaution, keep a backdoor[85] configured so that you can re-enable access to the system if you make a mistake. That way there would be no need to go to a remote location to fix a firewall ruleset that blocks you.

FIXME: This needs DNS to be working properly since it is required for security.debian.org to work. You can add security.debian.org to /etc/hosts but now it is a CNAME to several hosts (there is more than one security mirror)

FIXME: this will only work with HTTP URLs since ftp might need the ip_conntrack_ftp module, or use passive mode.

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Securing Debian Manual