Linux Systems Security. Firewalls and Filters NETS1028 Fall 2016

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1 Linux Systems Security Firewalls and Filters NETS1028 Fall 2016

2 Firewall A physical barrier designed to slow or prevent the spread of fire In computer networks, a mechanism to slow or prevent the passage of network traffic Several firewall software packages have come and gone over the past 20 years, iptables is ubiquitous for Linux now

3 netfilter.org Netfilter is the home of several packet filtering projects including iptables, which is used in more or less all modern Linux kernels GPLv2 licensed, open source, in active development since approximately 1999 Corporate sponsors include Watchguard, LinuxCare Inc., Connectiva, Sophos, and many others

4 Stateful vs. Stateless 1st generation packet filters were stateless network layer filters - each packet was examined on an individual basis and decisions about it were based solely on the contents of that packet 2nd generation packet filters incorporated connection information and could make stateful decisions as well - SPI 3rd generation adds application awareness and can make decisions based on unexpected traffic patterns - deep packet inspection

5 NAT NAT was developed to deal with limited address space in IPV4 It was quickly recognized that it also provided the function of hiding internal addresses making reconnaissance more difficult for attackers Many firewalls provide NAT as an added tool for slowing attackers

6 Proxies A proxy is a software device which provides a middleman for connections and can perform additional filtering of traffic Useful for implementing more complex applicationspecific rules such as url-based filtering MTAs can perform a proxy function for Firewalling external connections from non-proxy hosts can add a layer of protection against internal hosts which have been compromised or have misuse attempted on them

7 iptables Tables iptables uses 3 built-in tables as the basis for managing traffic The filter table is the default table used to filter traffic The NAT table is used to perform address modifications in order to provide NAT The mangle table is used to modify packets in other ways Tables contain chains of rules

8 Packet flow "Netfilter-packet-flow" by Jengelh - Own work, Origin SVG PNG. Licensed under CC BY-SA 3.0 via Commons -

9 iptables Chains A chain is a sequence of rules INPUT, OUTPUT, and FORWARD are the built-in chains INPUT is applied to packets destined for this host from network interfaces OUTPUT is applied to packets generated by this host FORWARD is applied to packets not generated by, or destined for, this host A chain also has a policy, which is what happens to packets not specified in the rules Create your own chains with iptables -N, delete them with iptables -X

10 iptables Chain Policy Each rule in a chain can specify parameters to identify packets that the rule applies to and an action to take if the packet matches the parameters If a packet is compared to all the rules and does not match any of them, the policy for the chain is applied to the packet The default policy after installation is ACCEPT Other policies available include DROP and REJECT

11 iptables Rules Each rule in a chain can have a number of parameters including a target Typical parameters might include chain name interface name protocol (name or number from etc/protocols) source address name/number/cidr range and/or port name or number from /etc/services destination address name/number/cidr range and/or port name or number from /etc/services jump target Builtin targets include ACCEPT, DROP, REJECT, LOG Additional targets can be other chains which allows you to clarify your chains Extensions can also be targets - see iptables-extensions(8)

12 iptables Command iptables -V to get version info iptables -L [-v] to get config summary iptables -S to show rules in iptables command line format iptables -A to append rules to a chain iptables -I to insert rules into a chain other than at the end iptables -F to flush rules from a chain ip6tables command builds rules for IPV6

13 iptables Examples iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport ssh -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT iptables -P INPUT DROP iptables -P OUTPUT DROP iptables -A INPUT -p tcp -j LOG --log-prefix "INPUTLOG " What common network traffic might break because of this? How would you discover what was broken? Logging only input traffic only tells you who is trying to break in, not who is trying to get out.

14 Exercises Create a set of firewall rules to allow traffic on loopback, allow only ssh on eth0, and set the INPUT and OUTPUT policy to DROP Verify you cannot connect to your vm using a protocol you are serving but not permitting through the firewall (install something like a telnet service for testing purposes), and that ssh still works Add a rule to log all non-ssh tcp packets, retry the telnet and check your /var/log/kern.log to see what got logged Reboot your vm, and check your iptables rules using the -L option

15 iptables Persistence iptables is a memory-based utility To have the rules take effect at boot, we need to use software that not only installs the rules, but saves those rules ofr reinstallation at next boot Most higher level packages that try to automate firewall management save the rules you create you can install the iptables-persistent package and save your rules to /etc/iptables/rules.v[46] using ip[6]tables-save You can use one or more of several packages intended to manage an iptables configuration

16 Exercises With your own iptables rules installed, install the iptables-persistent package, having it save your IPV4 rules Examine the contents of /etc/iptables/rules.v4 and compare it to the output of iptables-save Reboot and verify your rules are automatically reinstalled Remove the iptables-persistent package

17 iptables Extensions Extensions exist for iptables and add packet matching capabilities using modules as well as new targets to give more options about what to do with matched packets -m option can be used to enable modules to extend the capabilities of iptables Some modules permit options Interesting modules: limit, connlimit, conntrack, iprange, multiport, comment Interesting targets: LOG, REDIRECT, TEE

18 Common Attack Handling Drop or limit pings from all non-local hosts, limiting icmp rates across the board can help against smurfs Drop packets sourced from private netblocks which you aren t using yourself Drop malformed packets using --tcp-flags, port scans often use these Configure appropriate kernel tuning parameters to increase resilience to attacks Modern Linux kernel is quite robust in major distros, most attacks are on services so block or limit them and use whatever config options are available to you in those services

19 Exercises Run sysctl -a to get an idea of the kernel parameters currently set up on your system What do you suppose are the implications of being able to retrieve this type of information as an ordinary user? has excellent sysctl documentation for kernel version 2.6, find the swappiness parameter in that document to see what it can do for you, check out the wikipedia article for more info Performance tuning also affects resiliency, example references on tuning for performance include:

20 iptstate top-style tool for observing connection states Requires at least one rule that uses conntrack or state extension in order to provide state capture help screen available with h key, shows current sort and display settings buggy on Ubuntu currently

21 Exercises Install iptstate package Add a rule to your INPUT table for protocol tcp, destination port ssh, module conntrack, option ctstate INVALID Run iptstate and observe the various connections being tracked by iptables Use iptables -L -v to see the packet and byte counts being seen by the various rules you have in place

22 UFW Uncomplicated Firewall A command line utility to simplify firewall management Uses pre-configured rulesets for common configurations, with catch-all rules in /etc/ufw It is a front end to the iptables command, but conflicts are probable if you use both to set up your firewall - instead use the pre and post rules files in ufw to set up custom rulesets Provides enable/disable and configuration save gufw is a graphical frontend to ufw

23 Exercises Install the ufw package Use ufw to allow ssh traffic Check your status with ufw, enable it, recheck your status Run iptables -L -v with the ufw firewall tool in enabled state Disable the ufw firewall tool and see what is left behind in your live iptables Reboot to clear out your tables for the next exercise

24 ipkungfu Another frontend to iptables (there are many, e.g. Uses a relatively friendly configuration file and supports automatic config at boot Groups many rule ideas into simpler concepts and makes them options in config files

25 Exercises Install the ipkungfu package Review the configuration files in /etc/ipkungfu Modify ipkungfu.conf to set GATEWAY=0, DISALLOW_PRIVATE=0 Modify services.conf to ACCEPT ftp and ssh traffic Run ipkungfu show-vars to see your current configuration with ipkungfu s guesses Run ipkungfu -t to test and install your new configuration Use iptables -L to see the new iptables configuration Check /etc/default/ipkungfu to see if it is enabled on system startup (IPKFSTART setting)

26 fail2ban fail2ban is a package that can scan log files looking for repeated login failures and then block the source hosts using iptables It does not require chain DROP policy, so if you don t have a deny policy, it will still work fail2ban knows many common log file formats such as ssh, web servers, servers, ftp, and many applications that sit on top of those services see /etc/fail2ban/filter.d for the logs it knows, /var/log/fail2ban.log to see what it has been doing when running copy /etc/fail2ban/jail.conf to /etc/fail2ban/jail.local and modify to enable or configure jails fail2ban.org, 2014 PyCon video: v=xcxheawy7cu#t=190

27 Exercises Install vsftpd and fail2ban, you may need a reboot to have a clean set of iptables to work with depending on the state you left things in from previous exercises Configure the vsftpd jail to be enabled in jail.local and restart fail2ban service - use iptables -L -v to see what it installed Use a second terminal window to perform several login failures using ftp Watch the fail2ban.log using tail -f to see what it does While you have a vsftpd ban in place, try: fail2ban-client status vsftpd fail2ban-client get vsftpd bantime fail2ban-client -help

28 Additional Filtering Reduce DNS spoofing by setting nospoof on in /etc/ host.conf - see manpages/precise/man5/host.conf.5.html for additional host name lookup filtering Proxy servers ( , web, etc.) can be set up, use iptables to prevent connections for proxied services that try to bypass the proxies, proxies can do application-level filtering

iptables and ip6tables An introduction to LINUX firewall

7 19-22 November, 2017 Dhaka, Bangladesh iptables and ip6tables An introduction to LINUX firewall Imtiaz Rahman SBAC Bank Ltd AGENDA iptables and ip6tables Structure Policy (DROP/ACCEPT) Syntax Hands on