=^.^=

Bad Snort Rules

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Protecting a busy web platform or ISP with snort is made challenging due to the large number of false positives it generates in these environments. This post will serve as a compilation of snort rules I think should be disabled or tuned out of the box, it will be updated as I find new ones. The only way to tell if snort is interfering with legitimate traffic is to monitor its logs over a long period and look up the rules which appear most often. Make sure these rules don't do anything that could interfere with your regular operations - for example a rule to block the slammer worm that hits a dozen times a day probably has nothing to do with your e-mail troubles but a rule for an outdated MTA might explain why Blackberry's servers are being blocked every time a client tries to check their mail.

On ClearOS systems snort rules are located in /var/lib/suva/services/intrusion-protection/rules/ (formerly /var/lib/suva/services/snort/rules/).  To find a rule change to that directory then

# grep sid:number *

and the file name and line number will be printed next to the rule's line. Disabling a rule is as simple as prefixing it with a hash mark (#) and restarting snort:

# /etc/init.d/snort restart

I've started a thread in the Clear forums so everyone can share and discuss their false positives: http://www.clearfoundation.com/component/option,com_kunena/Itemid,232/catid,7/func,view/id,12205/

SID 1091:
False positive on any request containing ?????????? in the URL

SID 1233:
"I noticed, what I believe to be a false positive in rule 1233 (Outlook EML access).: a user was accessing hotmail and the GET was for /i.p.emlips.gif. Since the rule is looking for uricontent:".eml", an alert was triggered. "
http://www.networksecurityarchive.org/html/Snort-Signatures/2005-06/msg00008.html

SID 1390:
False positive on payloads containing CCCCCCCCCCCCCCCCCCCCCCCC, brought to my attenti0n on the ClearOS forums (thanks Kevin and Tim)
http://www.clearfoundation.com/component/option,com_kunena/Itemid,232/catid,8/func,view/id,9967

SID 1394:
False positive on payloads containing 'aaaaaaaaaaaaaaaaaaaaa'

SID 1807:
False positive on any request where the string "chunked" preceeds the Transfer-Encoding header. A fix is available at:
http://osdir.com/ml/security.ids.snort.sigs/2004-07/msg00018.html

SID  2109:
False positive on any attempt to retrieve e-mail from BlackBerry servers. An interesting note about this, I had dozens of users with blackberries on the network running just fine for years then all at once a few months ago - without even touching the software - snort started blocking the BlackBerry pop3 IP pool and the complaints came flooding in. When I researched it at the time it seemed as though this false positive was reported on at least one mailing list between 2004-6, perhaps RIM switched "back" to the would-be offending platform.

SID 2229:
This rule is false positive for ANY request to viewtopic.php, taking out any visitors to a phpBB installation. There is a fix for this rule outlined in this e-mail from 2004:
http://www.webservertalk.com/archive253-2004-11-452217.html

SID 2000540:
False positive on SSL connections with Google Gmail web servers
It appears to be generating false positives for fragmented ACK packets intended to match a form of Nmap scan
http://www.clearfoundation.com/component/option,com_kunena/Itemid,232/catid,8/func,view/id,19544/#19551

SIDs 1000000211, 1000000212, 1000000213, 1000000214:
"These are known to cause issues with Gallery2 (part of the web PHP rulesets)"
Tim Burgess supplied these on Clear's forum, I think the 1000000000-range rules may be unique to ClearOS.
http://www.clearfoundation.com/component/option,com_kunena/Itemid,232/catid,7/func,view/id,12205/#12227

SID 966
This rule has caused me a lot of problems when uploading large binary files, though I'm not sure why since only the URI should be scanned and not entire POST variables. May be a bad one for me only.

IP Subnets and Netmasks Demystified

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I had to explain the purpose of netmasks to my field tech yesterday and recalled the great difficulty I had with the concept when I started taking an interest in networking. In this article I will attempt to put the matter in terms I wish I had heard first.

The Internet is a network composed of networks which may be composed of smaller networks ad infinitum. These networks are called sub-networks or "subnets." Hosts on the same subnet can generally communicate directly with one another. Hosts on different subnets communicate with one another by sending the traffic via one or more gateways or "routers" which have a presence on each subnet. Some say proper protocol is to only call a router a gateway when it connects two or more different layer 2 technologies, however in OS network configuration common parlance is usually just gateway.

Hosts on a network need to know what traffic they can send directly over the layer 2 (or otherwise transparently local) network to other hosts and what packets need to be passed to the gateway. The value of the configured netmask is laid over the host's own IP address in such a way that is difficult to visualize in dotted-decimal notation, so let's take the IP address 192.168.0.1 and netmask 255.255.255.0 and look at them in binary:

11000000101010000000000000000001
11111111111111111111111100000000

A netmask of 255.255.255.0 when applied to 192.168.0.1 means "anything that doesn't match the first 24 digits of your IP is NOT local traffic and must be sent to the gateway." The bits in the local address which are in the same position as the 1 bits of the netmask determine if an address is local or not, so it could be said that the "1 bits" are masked out. The "0 bits" are called the rest field and by ignoring them the host can quickly tell 192.168.0.22 is on the local subnet but 192.168.1.22 is not, and neither is 222.222.222.22:

11000000101010000000000000000001 192.168.0.1 local host
11111111111111111111111100000000 255.255.255.0 netmask
11000000101010000000000000010110 192.168.0.22
11000000101010000000000100010110 192.168.1.22
11011110110111101101111000010110 222.222.222.22

There are 8 rest bits in this example so we can tell that the address space of this subnet is 256 addresses deep and they range from 192.168.0.0 to 192.168.0.255 (00000000 - 11111111).

Before Classless Inter-Domain Routing or CIDR, the subnets of the Internet were divided into fixed-width blocks called classes. Class names were given to subnets of various sizes and specializations, starting with large Class A subnets at the beginning of Internet address space and advancing to smaller and even experimental subnets as it descends. CIDR did away with the concept of classes and opened the netmask to single-bit-level control; subnets could now be created at sizes of any power of two, in any position of the global address space. CIDR also brought with it a new notation for routing prefixes, a slash after an IP followed by the number of "1 bits" in the netmask. In this way the IP 192.168.0.1 netmask 255.255.255.0 can be written 192.168.0.1/24. Whle CIDR notation can be used to determine the netmask of a subnet and vice versa it is typically used only to describe whole subnets or convey that a given IP is part of a certain sized subnet.

Netmask Netmask (binary) CIDR Contains
255.255.255.255 11111111.11111111.11111111.11111111 /32 Host (single address)
255.255.255.254 11111111.11111111.11111111.11111110 /31 Unusable
255.255.255.252 11111111.11111111.11111111.11111100 /30 2 usable
255.255.255.248 11111111.11111111.11111111.11111000 /29 6 usable
255.255.255.240 11111111.11111111.11111111.11110000 /28 14 usable
255.255.255.224 11111111.11111111.11111111.11100000 /27 30 usable
255.255.255.192 11111111.11111111.11111111.11000000 /26 62 usable
255.255.255.128 11111111.11111111.11111111.10000000 /25 126 usable
255.255.255.0 11111111.11111111.11111111.00000000 /24 "Class C" 254 usable
-
255.255.254.0 11111111.11111111.11111110.00000000 /23 2 Class C's
255.255.252.0 11111111.11111111.11111100.00000000 /22 4 Class C's
255.255.248.0 11111111.11111111.11111000.00000000 /21 8 Class C's
255.255.240.0 11111111.11111111.11110000.00000000 /20 16 Class C's
255.255.224.0 11111111.11111111.11100000.00000000 /19 32 Class C's
255.255.192.0 11111111.11111111.11000000.00000000 /18 64 Class C's
255.255.128.0 11111111.11111111.10000000.00000000 /17 128 Class C's
255.255.0.0 11111111.11111111.00000000.00000000 /16 "Class B"
-
255.254.0.0 11111111.11111110.00000000.00000000 /15 2 Class B's
255.252.0.0 11111111.11111100.00000000.00000000 /14 4 Class B's
255.248.0.0 11111111.11111000.00000000.00000000 /13 8 Class B's
255.240.0.0 11111111.11110000.00000000.00000000 /12 16 Class B's
255.224.0.0 11111111.11100000.00000000.00000000 /11 32 Class B's
255.192.0.0 11111111.11000000.00000000.00000000 /10 64 Class B's
255.128.0.0 11111111.10000000.00000000.00000000 /9 128 Class B's
255.0.0.0 11111111.00000000.00000000.00000000 /8 "Class A"
-
254.0.0.0 11111110.00000000.00000000.00000000 /7
252.0.0.0 11111100.00000000.00000000.00000000 /6
248.0.0.0 11111000.00000000.00000000.00000000 /5
240.0.0.0 11110000.00000000.00000000.00000000 /4
224.0.0.0 11100000.00000000.00000000.00000000 /3
192.0.0.0 11000000.00000000.00000000.00000000 /2
128.0.0.0 10000000.00000000.00000000.00000000 /1
0.0.0.0 00000000.00000000.00000000.00000000 /0 This Network

It should be noted here that in any size subnet you create you will be shy two usable IP addresses from the full number of slots. This is because subnets have a broadcast address that reaches every host at the last IP and their routes are represented by their lowest, therefore 192.168.0.0 represents the subnet, 192.168.0.255 is the broadcast address and 192.168.0.1 - 192.168.0.254 can be used for real hosts, a total of 254 usable addresses.

ClearOS Installation Checklist

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I'm writing this checklist as I setup a new router for the home office to remind me of the modifications I need to make to get a fresh deployment "just right" the first time.  ClearOS is a CentOS-based router distribution that lets one rapidly and easily deploy and manage routers and miscellaneous network services. CentOS itself is a de-branded flavour of Red Hat Enterprise Linux. Back when ClearOS flew under the ClarkConnect label if one wanted certain parts of the product one had to either pay for an Office or Enterprise license or use well crafted google queries to find the ftp credentials for Enterprise repositories, grab the RPMs, rpm2tgz/rpm2cpio them and overlay them on the filesystem (and don't forget to fix the permissions!) to avoid unresolvable dependencies.

Fortunately with the morph to ClearOS the Clear Foundation folks stopped charging for commodity software (i.e. web configuration modules for DMZ and Multi-WAN (load balancing/failover)) and started focusing on services you can live without. I warn against paying just for the tech support, you're much better off in the community forum - sparse though the posts are. From my experience (last dating in 2008 mind you) their Level 1 cuts off at putting the cd in the drive and may flat out refuse to support their product if you reveal it's being used in any setting more complex than a small office. If you're an intermediate *nix user and you can't figure out the problem on your own or with google chances are tech support can't (or won't) help you anyway; drop by the users forum.

Most of the routers I make these days are virtual machines and that goes beyond the scope of this checklist, however I plan to cover my process in a future article. The machine this checklist will be based on is an AthlonXP 2500+ 1.83GHz with 384MB DDR266 and an 8GB CompactFlash card plugged into a CF-IDE adapter, which you can get on eBay for about $1CAD. The machine has an onboard NIC and two PCI NICs as well as a wifi card so it can be turned into an access point. I like to use CF cards instead of hard drives on my physical routers because - although the cheap ones can be quite slow - you don't have to worry about them up and dying on you for several years. The cards are worn out with repeated write cycles (though often in the high thousands) so if you choose to use them you should try to minimize the amount of data written to disk during day-to-day operation. A remote syslogd might be of great help.

We're going to assume you've already downloaded and burned the installation ISO to disc. At the time of writing the current version is 5.1 SP1, the instructions below may not apply to future versions. Once you've booted you'll eventually be asked if you would like to let the installer automatically partition your hard drive or if you'd like to manually configure the partitioning. It's usually fine to let the installer do its thing but if you're working in confined spaces like our 8GB flash card or have plenty of ram the oft-defaulted swap partition size of 1GB is a tad generous. Also choose to manually configure the partitioning if you would like to use software RAID. If you choose to RAID your drives use anything but level 0; there is no need which I could conceive for high performance storage on a router (almost everything needed is loaded into ram on startup) and reliability is priority one on mission-critical systems like these.

Chances are if you want to do anything with the storage on your router you want to outsource that operation to another machine. Your router is the gatekeeper for the network and if it becomes compromised the consequences could be worse than with any single workstation. The simplest way to reduce the risk of a service being exploited is to not run it, so your router shouldn't run anything it doesn't need for management or to route and protect the network (firewall, IPS, IDS etc). In following with that notion keep the number of users on the system to an absolute minimum. If you're the only person who should have access to the box you should be the only person with a user account. ClearOS allows root logins via SSH by default so you should create at least one user account for yourself in order to separate privileges.

Once you've completed the installation and rebooted you can connect to the management interface at https://lan-ip:81 and log in as root. You'll be asked to fill in a number of details to complete the installation process. Once that's completed register your router with ClearSDN. If you have not made an account with them yet you should do so at the ClearSDN portal first. I generally disable "send diagnostic reports" when I register the routers but you may be less paranoid and more helpful. Once your router has been registered go to Software Updates. You can enable or disable automatic updates. They are enabled by default and I don't like that one bit: what if one of the repositories gets hacked? What if a new RPM breaks something critical and I'm not around to fix it?

Don't waste your time with all the checkboxes, shell into your router and run the following:

  • yum update
    • Updates all packages currently installed
  • yum install screen
    • Screen is a handy tool for multitasking shells
  • yum install lynx
    • There's already a version of lynx that comes as part of the ClearOS text console and you can use it by symlinking it to /usr/bin and its config file to /etc but that's messy.
  • yum install links
    • You don't need this if you have lynx, I just like to install them both so I can type either. Depends on my mood.
  • yum install nmap
    • nmap is an invaluable network diagnostic and analysis tool.
  • yum groupinstall "Development Tools"
    • Install this on systems where you expect to be compiling third-party software. Wherever possible use RHEL/CentOS RPMs for the corresponding version of ClearOS. You probably don't want to install this on space-restricted systems. If you have space to kill it never hurts to be prepared.
  • yum install ncurses-devel
  • yum install kernel-devel
    • Install these on systems where you expect to be modifying the kernel, you will need the Development Tools group to compile the kernel or modules.
  • yum install net-snmp
    • Install this so you can monitor system statistics remotely. (i.e. with Cacti)
  • yum install wpa-supplicant
    • You need this if you want to run an access point, ClearOS is only configured for WEP by default and can't be set up through the web config.

The ClearOS web config has an embedded MRTG package that graphs system vitals, but if you plan on remotely monitoring your router's statistics (load average, network traffic, etc.) you will probably want to install net-snmp. Depending on your configuration you may need open port 161UDP. Here's a very short configuration sample that you can drop into /etc/snmp/snmpd.conf:

rouser  public
rocommunity  public localhost
syslocation  "Server Room"
syscontact  your@email
com2sec local     127.0.0.1/32    public
com2sec local     192.168.0.0/24    public
group MyROGroup v1         local
group MyROGroup v2c        local
group MyROGroup usm        local
view all    included  .1  80
access MyROGroup ""      any       noauth    exact  all    none   none

Replace 192.168.0.0/24 with the subnet or IP that should have access to SNMP data.

ClearOS is one of the few distributions that enable syncookies by default. You probably don't need to add these lines since syncookies override tcp_max_syn_backlog. I like to do it anyway just in case something fails on bootup. Per my previous article Defending Against the SYN Flood add these lines to /etc/rc.d/rc.firewall.local:

echo 3096 > /proc/sys/net/ipv4/tcp_max_syn_backlog
echo 2 > /proc/sys/net/ipv4/tcp_syn_retries
echo 1 > /proc/sys/net/ipv4/tcp_synack_retries
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

Or not. It won't kill you either way.

ClearOS comes with a special rule that snort will use to detect local SSH brute force attempts but as we covered in my previous article Stifling Brute Force Attacks with fail2ban fail2ban is highly extensible and can perform any operation that can be executed from the command line in response to any pattern match found in a given log file.  Fail2ban is not available in the default ClearOS repositories but we can use the RHEL 5 rpm available at http://dag.wieers.com/rpm/packages/fail2ban/. After installing the packages listed above the RPM should have only one dependency: gamin-python. Install fail2ban thus:

# yum install gamin-python
# wget http://rpmforge.sw.be/redhat/el5/en/i386/rpmforge/RPMS/fail2ban-0.8.1-1.el5.rf.noarch.rpm
# rpm -iv fail2ban-0.8.1-1.el5.rf.noarch.rpm

Return to the webconfig and make sure you have installed all the components and third party applications listed that you need, like the Advanced Firewall Module which is not installed by default. Configure your firewall, DHCP and VPN(s). Back at the command line let's clean out all the packages we just downloaded:

# yum clean all

At the command line run chkconfig --list:

acpid           0:off   1:off   2:on    3:on    4:on    5:on    6:off
avahi-daemon    0:off   1:off   2:off   3:on    4:on    5:on    6:off
avahi-dnsconfd  0:off   1:off   2:off   3:off   4:off   5:off   6:off
clamd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
cpuspeed        0:off   1:on    2:on    3:on    4:on    5:on    6:off
crond           0:off   1:off   2:on    3:on    4:on    5:on    6:off
cups            0:off   1:off   2:off   3:off   4:off   5:off   6:off
dansguardian-av 0:off   1:off   2:off   3:off   4:off   5:off   6:off
dnsmasq         0:off   1:off   2:on    3:on    4:on    5:on    6:off
fail2ban        0:off   1:off   2:off   3:on    4:on    5:on    6:off
firewall        0:off   1:off   2:on    3:on    4:on    5:on    6:off
freshclam       0:off   1:off   2:off   3:off   4:off   5:off   6:off
haldaemon       0:off   1:off   2:off   3:on    4:on    5:on    6:off
httpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
ipsec           0:off   1:off   2:off   3:off   4:off   5:off   6:off
iscsi           0:off   1:off   2:off   3:off   4:off   5:off   6:off
iscsid          0:off   1:off   2:on    3:on    4:on    5:on    6:off
kudzu           0:off   1:off   2:off   3:on    4:on    5:on    6:off
l7-filter       0:off   1:off   2:off   3:off   4:off   5:off   6:off
ldap            0:off   1:off   2:off   3:on    4:on    5:on    6:off
ldapsync        0:off   1:off   2:off   3:on    4:on    5:on    6:off
lm_sensors      0:off   1:off   2:on    3:on    4:on    5:on    6:off
lvm2-monitor    0:off   1:on    2:on    3:on    4:on    5:on    6:off
mcstrans        0:off   1:off   2:off   3:off   4:off   5:off   6:off
mdmonitor       0:off   1:off   2:off   3:off   4:off   5:off   6:off
mdmpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
messagebus      0:off   1:off   2:off   3:on    4:on    5:on    6:off
multipathd      0:off   1:off   2:off   3:off   4:off   5:off   6:off
mysqld          0:off   1:off   2:off   3:off   4:off   5:off   6:off
netconsole      0:off   1:off   2:off   3:off   4:off   5:off   6:off
netfs           0:off   1:off   2:off   3:on    4:on    5:on    6:off
netplugd        0:off   1:off   2:off   3:off   4:off   5:off   6:off
network         0:off   1:off   2:on    3:on    4:on    5:on    6:off
nmb             0:off   1:off   2:off   3:off   4:off   5:off   6:off
nscd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
ntpd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
openvpn         0:off   1:off   2:off   3:off   4:off   5:off   6:off
pptpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
rdisc           0:off   1:off   2:off   3:off   4:off   5:off   6:off
restorecond     0:off   1:off   2:on    3:on    4:on    5:on    6:off
saslauthd       0:off   1:off   2:off   3:on    4:on    5:on    6:off
smb             0:off   1:off   2:off   3:off   4:off   5:off   6:off
snmpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
snmptrapd       0:off   1:off   2:off   3:off   4:off   5:off   6:off
snort           0:off   1:off   2:off   3:off   4:off   5:off   6:off
snortsam        0:off   1:off   2:off   3:off   4:off   5:off   6:off
squid           0:off   1:off   2:off   3:off   4:off   5:off   6:off
sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
suvad           0:off   1:off   2:on    3:on    4:on    5:on    6:off
syslog          0:off   1:off   2:on    3:on    4:on    5:on    6:off
system-mysqld   0:off   1:off   2:on    3:on    4:on    5:on    6:off
syswatch        0:off   1:off   2:on    3:on    4:on    5:on    6:off
vpnwatchd       0:off   1:off   2:off   3:off   4:off   5:off   6:off
webconfig       0:off   1:off   2:on    3:on    4:on    5:on    6:off
winbind         0:off   1:off   2:off   3:off   4:off   5:off   6:off
wpa_supplicant  0:off   1:off   2:off   3:off   4:off   5:off   6:off

We want to turn off anything that starts in runlevels 2.3.4 or 5 that we don't need. This will make the router boot faster and use less ram, which is particularly important if you're building a virtual machine. A fast power-cycle has obvious advantages for any connectivity device and one can comfortably fit a low-traffic ClearOS router into 96MB of RAM with room to breathe by disabling the right services. Based on the defaults shown here, these are some services you probably want to turn off - your mileage of course may vary:

  • avahi-daemon
  • avahi-dnsconfd
    • Zeroconf stuff. You only want it if you know what that means, and probably not even then.
  • haldaemon
    • Practically unused by anything but X
  • iscsi
  • iscsid
    • Obviously you want these if you really are using iSCSI.
  • kudzu
    • Checks for new hardware and can interrupt boot process, can be run from the command line anyway
  • lvm2-monitor
    • You only want this if you're using LVM
  • messagebus
    • Same with HALD
  • netfs
    • Leave this on if you're doing anything with NFS

Services you may want to disable include:

  • suvad
    • Talks to ClearSDN, disabling interferes with updates and ClearSDN services but it can be started on demand
  • lm_sensors
    • There's no hardware to monitor on a xen virtual machine
  • cpuspeed
    • Ditto
  • acpid
    • Ditto

Use the following syntax to remove init scripts from these runlevels:

# chkconfig --level 2345 iscsid off

And enable anything that should be turned on:

# chkconfig --level 2345 snmpd on

Be sure not to touch any of the numerous LDAP services, ClearOS uses that internally to manage the user accounts. If you don't know what a service does be sure to look it up before you disable it.

If your router includes a wireless card that requires firmware do not forget to download it to /lib/firmware.

Updatedb indexes the files on your mounted partitions for fast searching with the locate or slocate tool. You should run it once now that you have most of the files installed. By default, cron runs updatedb every night. This causes high I/O load and can be disabled by disabling its cron script's execute bit:

# chmod -x /etc/cron.daily/mlocate.cron

While the web config makes ClearOS what it is, I don't like the console configuration (slow, featureless and requires two logins...) - and I really don't like the graphical one (wtf?). These are a severe obstacle on Xen installations where it's difficult to navigate serial-based (no ptys to alt+Fx to) xenconsole out of the bottomless ncurses pit. Rather than loop-mount the image and configure the networking and reboot the VMs and shell in I prefer to disable that rubbish altogether. Edit /etc/inittab to reflect:

# Run gettys in standard runlevels
#1:2345:respawn:/sbin/mingetty --autologin=clearconsole tty1
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2

If this router is going to be in a high traffic environment you may find that snortreport.sh progressively uses more and more resources to compile the webconfig-accessible IPS/IDS reports until it starts overlapping with itself, causing extreme load. You can delete it, move it, or remove its execute bit:

# chmod -x /usr/sbin/snortreport.sh

ClarkConnect used to come with some fairly dangerous default snort rules, particularly if your router is intended to be the firewall for a public network. Things look a lot better now, the two rules I could remember always having to comment out now come commented by default; as things run I'll keep a list of false-positive generating rules here.

NOTE: That list can be found here: Bad Snort Rules

Snort rules take the form of lines in files located in /var/lib/suva/services/intrusion-protection/rules/ (formerly /var/lib/suva/services/snort/rules/).  Disabling a rule is as simple as prefixing it with a hash mark (#) and restarting snort:

# /etc/init.d/snort restart

Check your Intrusion Prevention reports in the web config regularly when you first deploy your new firewall. Investigate any rule that appears multiple times to determine if your particular environment is triggering false positives. This is critical if you are protecting a public network, say a farm of web servers. One rule (that is now commented by default) would block an IP that sent or received a string of ascii 'a's, like: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaa' because a long string of 'a's  is one signature of a certain buffer overflow attack. One day one of my users said "Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaah!" in a web-based chat room when I had a freshly deployed router out and he and everyone who saw it were swiftly blocked from the network. One can either live without IPS (I wouldn't recommend it) or one can mitigate the downtime through careful monitoring.

You may wish to lock down SSH by following my article on key-exchange. While by default (in gateway mode) ssh is not accessible on the external addresses one should never discount the possibility of attack from within. Any machine behind your firewall that can be compromised will be in a unique position to compromise other machines if attention is not paid to internal network security. There is no such thing as a trusted network, only more trustworthy.

Passwordless or Single Password SSH with Key Exchange

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IMPORTANT UPDATE In the ten years since this article was published a lot has changed. Please see my updated article Generate and Automatically Load SSH Keys for Convenient Passwordless Authentication for a more secure and convenient implementation.

In the last two articles we have covered in detail the main flaw of any username-password authentication scheme and how to defend against attacks by increasing their time/resource cost. Unfortunately this does nothing to eliminate the problem but key-exchange authentication - while not unbreakable - changes the shape of the playing field and it's becoming an increasingly favoured authentication scheme for a myriad of applications including SSH and VPN protocols. This article will show you how to quickly generate and exchange keys with remote hosts and disable traditional password authentication.

First you will need to generate a key pair:

ssh-keygen -t rsa

You are going to have to decide here whether you want to encrypt your private key with a passphrase and enter one password every time you use key exchange or make login instant at the expense of a more vulnerable key. You need to consider the possible damage that could be done if a given machine with unrestricted shell access to other hosts is compromised.

The least level of protection you can apply is exchanging only lesser-privileged accounts as a first step toward higher levels, i.e. by using su. Key exchange suffers from this weakness only when the private key is stored locally and unencrypted. One could keep the private key (~/.ssh/id_rsa) on a USB stick, however your key is vulnerable when the device is mounted. Even when using an encrypted key if the file is intercepted it can eventually be cracked. Smart cards (themselves) do not share these weaknesses and will be the topic of an upcoming article.

Never allow root to directly shell into a machine regardless of the authentication scheme you choose to use, make sure your target's sshd_conf includes:

PermitRootLogin no
AllowUsers user1 user2 user3

Where user1,2,3 are the names of specific users permitted to login. This may not be practical for larger or public installations.

If the target account on the remote host has not yet used ssh you may have to create ~/.ssh. Add the new private key to the remote host's authorized keys list:

cat ~/.ssh/id_rsa.pub | ssh xxx.xxx.xxx.xxx "xargs --null echo >> ~/.ssh/authorized_keys"

If you have not already shelled into the remote host from this account you will be prompted to accept its public key. You may then be prompted for your password. If the copy was successful you will be returned to the command line without a message. Try logging into the remote host, if you are not asked for a password or you are asked your private key's passphrase you have successfully performed the key exchange.

Once you've finished exchanging keys with all of the hosts which should have access to the target you might proceed to disable password based authentication. The previous command will no longer work to import new keys, you will have to transfer them via other means (i.e. a host that has already exchanged keys with the target). Edit /etc/ssh/sshd_config to reflect:
PasswordAuthentication no ChallengeResponseAuthentication no
and restart sshd. Try logging in from a machine that has not exchanged keys with it. You should see something like:
Permission denied (publickey).
In an ideal world you wouldn't run management services (SSH, webmin, snmp etc) on public address space. One can keep SSH from being exposed in the first place by making it listen on a private subnet and connecting to it via VPN. The only time I can see someone wanting to expose SSH in particular is to provide sftp and chances are you'll be dealing with a number of users where key exchange isn't practical. You can address this by keeping password authentication and enabling a chroot jail, which I'll cover in a future article. If an attacker does manage to break into an account despite your fail2ban setup they will at least be confined to their own little slice of the filesystem.

Brute Force and Flood Protection for Web Forms

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In the last article I told you any username-and-password authentication system that is exposed to the Internet is inherently vulnerable to dictionary and brute force attack. If you must use such an authentication scheme you can defend it by implementing rate control. If you block an attacker from trying to log in for one hour after three failed attempts it would take them a year to try just under 3,000 combinations. In cryptanalytic terms that is abysmal and the odds are on your side that the attacker will have moved on by then.

While porting your ban system to fail2ban might be a great idea it's probably overkill for situations where you have hundreds of legitimate users who might often forget their credentials; IP-bans are not generally considered good customer service. Many sites, including Google, will present the user with a CAPTCHA after three failed attempts and that's great but those are getting easier to crack every day.

For the sake of the pseudocode in this article we're going to assume you want to block the  potential attacker and politely tell them they have either a) failed to log in too many times, please come back in an hour or b) posted too recently, please try again. Since we want to be able to rate control two (and perhaps more in the future) different things and we don't want to make a mess of our database let's make one table called 'greylist' and use the type column to differentiate:

CREATE TABLE `demo_cat`.`greylist` ( `type` VARCHAR( 30 ) NOT NULL, `date` INT NOT NULL, `ip` VARCHAR( 15 ) NOT NULL, PRIMARY KEY ( `ip` ), INDEX ( `date` ) );

Now in your login script for argument's sake we'll say $outcome is a boolean representation of if the authentication was successful or not and $delay is the period of time we want to measure for in seconds. We'll start off by clearing everything that's out of date, a relatively inexpensive query to run every time there's a failure. After the table has been updated we'll add an entry for the current failure and take a tally of all the entries for the user's IP. If the tally exceeds the retry $threshold we'll tell them to buzz off for an hour, change their password, show a captcha or whatever suits your site best.

<?php if(!$outcome) { $ip = mysql_real_escape_string($_SERVER['REMOTE_ADDR']); mysql_query("delete from `greylist` where `type` = 'login' and `date` < '".time()-$offset."'"); mysql_query("insert into `greylist` (`type`, `date`, `ip`) values ('login', '".time()."', '$ip'')"); $result = mysql_query("select `ip` from `greylist` where `type` = 'login' and `ip` = '$ip'"); if(mysql_num_rows($result > $threshold)) { // Too many tries, what now? } else { // Please try again } } ?>

It is as simple as that. Now let's use this to flood-protect our comments box:
<?php if($_POST) { $ip = mysql_real_escape_string($_SERVER['REMOTE_ADDR']); mysql_query("delete from `greylist` where `type` = 'comment' and `date` < '".time()-$offset."'"); mysql_query("insert into `greylist` (`type`, `date`, `ip`) values ('comment', '".time()."', '$ip'')"); $result = mysql_query("select `ip` from `greylist` where `type` = 'comment' and `ip` = '$ip'"); if(mysql_num_rows($result > $threshold)) { // You posted too recently, please wait x seconds before trying again. } else { // Continue... } } ?>

A more sophisticated implementation of this concept is in use at Ychan, where users' posting patterns are analyzed to determine if they are computers, legitimate humans or computers trying to look like humans.