What is a network firewall?
Why would I want a firewall?
What can a firewall protect against?
What can't a firewall protect against?
What about viruses?
What are good sources of print information on firewalls?
What are some of the basic design decisions in a firewall?
What are some of the basic types of firewall?
What are proxy servers and how do they work?
What are some cheap packet screening tools?
What are some reasonable filtering rules for a Cisco?
How do I make Web/http work with a firewall?
How do I make DNS work with a firewall?
How do I make FTP work through my firewall?
How do I make Telnet work through my firewall?
How do I make Finger and whois work through my firewall?
How do I make gopher, archie, and other services work through my firewall?
What are the issues about X-Window through a firewall?
What is source routed traffic and why is it a threat?
What are ICMP redirects and redirect bombs?
What about denial of service?
Glossary of firewall related terms
A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. If you don't have a good idea what kind of access you want to permit or deny, or you simply permit someone or some product to configure a firewall based on what they or it think it should do, then they are making policy for your organization as a whole.
The Internet, like any other society, is plagued with the kind of jerks who enjoy the electronic equivalent of writing on other people's walls with spraypaint, tearing their mailboxes off, or just sitting in the street blowing their car horns. Some people try to get real work done over the Internet, and others have sensitive or proprietary data they must protect. Usually, a firewall's purpose is to keep the jerks out of your network while still letting you get your job done.
Many traditional-style corporations and data centers have computing security policies and practices that must be adhered to. In a case where a company's policies dictate how data must be protected, a firewall is very important, since it is the embodiment of the corporate policy. Frequently, the hardest part of hooking to the Internet, if you're a large company, is not justifying the expense or effort, but convincing management that it's safe to do so. A firewall provides not only real security - it often plays an important role as a security blanket for management.
Lastly, a firewall can act as your corporate "ambassador" to the Internet. Many corporations use their firewall systems as a place to store public information about corporate products and services, files to download, bug-fixes, and so forth. Several of these systems have become important parts of the Internet service structure (e.g.: UUnet.uu.net, whitehouse.gov, gatekeeper.dec.com) and have reflected well on their organizational sponsors.
Some firewalls permit only Email traffic through them, thereby protecting the network against any attacks other than attacks against the Email service. Other firewalls provide less strict protections, and block services that are known to be problems.
Generally, firewalls are configured to protect against unauthenticated interactive logins from the "outside" world. This, more than anything, helps prevent vandals from logging into machines on your network. More elaborate firewalls block traffic from the outside to the inside, but permit users on the inside to communicate freely with the outside. The firewall can protect you against any type of network-borne attack if you unplug it.
Firewalls are also important since they can provide a single "choke point" where security and audit can be imposed. Unlike in a situation where a computer system is being attacked by someone dialing in with a modem, the firewall can act as an effective "phone tap" and tracing tool. Firewalls provide an important logging and auditing function; often they provide summaries to the administrator about what kinds and amount of traffic passed through it, how many attempts there were to break into it, etc.
Firewalls can't protect against attacks that don't go through the firewall. Many corporations that connect to the Internet are very concerned about proprietary data leaking out of the company through that route. Unfortunately for those concerned, a magnetic tape can just as effectively be used to export data. Many organizations that are terrified (at a management level) of Internet connections have no coherent policy about how dial-in access via modems should be protected. It's silly to build a 6-foot thick steel door when you live in a wooden house, but there are a lot of organizations out there buying expensive firewalls and neglecting the numerous other back-doors into their network. For a firewall to work, it must be a part of a consistent overall organizational security architecture. Firewall policies must be realistic, and reflect the level of security in the entire network. For example, a site with top secret or classified data doesn't need a firewall at all: they shouldn't be hooking up to the internet in the first place, or the systems with the really secret data should be isolated from the rest of the corporate network.
Another thing a firewall can't really protect you against is traitors or idiots inside your network. While an industrial spy might export information through your firewall, he's just as likely to export it through a telephone, FAX machine, or floppy disk. Floppy disks are a far more likely means for information to leak from your organization than a firewall! Firewalls also cannot protect you against stupidity. Users who reveal sensitive information over the telephone are good targets for social engineering; an attacker may be able to break into your network by completely bypassing your firewall, if he can find a "helpful" employee inside who can be fooled into giving access to a modem pool.
Firewalls can't protect very well against things like viruses. There are too many ways of encoding binary files for transfer over networks, and too many different architectures and viruses to try to search for them all. In other words, a firewall cannot replace security- consciousness on the part of your users. In general, a firewall cannot protect against a data-driven attack -- attacks in which something is mailed or copied to an internal host where it is then executed. This form of attack has occurred in the past against various versions of Sendmail and GhostScript, a freely-available PostScript viewer.
Organizations that are deeply concerned about viruses should implement organization-wide virus control measures. Rather than trying to screen viruses out at the firewall, make sure that every vulnerable desktop has virus scanning software that is run when the machine is rebooted. Blanketting your network with virus scanning software will protect against viruses that come in via floppy disks, modems, and Internet. Trying to block viruses at the firewall will only protect against viruses from the Internet - and the vast majority of viruses are caught via floppy disks.
There are several books that touch on firewalls. The best known are:
There are a number of basic design issues that should be addressed by the lucky person who has been tasked with the responsibility of designing, specifying, and implementing or overseeing the installation of a firewall.
The first and most important is reflects the policy of how your company or organization wants to operate the system: is the firewall in place to explicitly deny all services except those critical to the mission of connecting to the net, or is the firewall in place to provide a metered and audited method of "queuing" access in a non-threatening manner. There are degrees of paranoia between these positions; the final stance of your firewall may be more the result of a political than an engineering decision.
The second is: what level of monitoring, redundancy, and control do you want? Having established the acceptable risk level (e.g.: how paranoid you are) by resolving the first issue, you can form a checklist of what should be monitored, permitted, and denied. In other words, you start by figuring out your overall objectives, and then combine a needs analysis with a risk assessment, and sort the almost always conflicting requirements out into a laundry list that specifies what you plan to implement.
The third issue is financial. We can't address this one here in anything but vague terms, but it's important to try to quantify any proposed solutions in terms of how much it will cost either to buy or to implement. For example, a complete firewall product may cost between $100,000 at the high end, and free at the low end. The free option, of doing some fancy configuring on a Cisco or similar router will cost nothing but staff time and cups of coffee. Implementing a high end firewall from scratch might cost several man- months, which may equate to $30,000 worth of staff salary and benefits. The systems management overhead is also a consideration. Building a home-brew is fine, but it's important to build it so that it doesn't require constant and expensive fiddling-with. It's important, in other words, to evaluate firewalls not only in terms of what they cost now, but continuing costs such as support.
On the technical side, there are a couple of decisions to make, based on the fact that for all practical purposes what we are talking about is a static traffic routing service placed between the network service provider's router and your internal network. The traffic routing service may be implemented at an IP level via something like screening rules in a router, or at an application level via proxy gateways and services.
The decision to make is whether to place an exposed stripped-down machine on the outside network to run proxy services for telnet, ftp, news, etc., or whether to set up a screening router as a filter, permitting communication with one or more internal machines. There are plusses and minuses to both approaches, with the proxy machine providing a greater level of audit and potentially security in return for increased cost in configuration and a decrease in the level of service that may be provided (since a proxy needs to be developed for each desired service). The old trade-off between ease-of-use and security comes back to haunt us with a vengeance.
Conceptually, there are two types of firewalls:
They are not as different as you might think, and latest technologies are blurring the distinction to the point where it's no longer clear if either one is "better" or "worse." As always, you need to be careful to pick the type that meets your needs.
Network level firewalls generally make their decisions based on the source, destination addresses and ports in individual IP packets. A simple router is the "traditional" network level firewall, since it is not able to make particularly sophisticated decisions about what a packet is actually talking to or where it actually came from. Modern network level firewalls have become increasingly sophisticated, and now maintain internal information about the state of connections passing through them, the contents of some of the data streams, and so on. One thing that's an important distinction about many network level firewalls is that they route traffic directly though them, so to use one you usually need to have a validly assigned IP address block. Network level firewalls tend to be very fast and tend to be very transparent to users.
Example Network level firewall: In this example, a network level firewall called a "screened host firewall" is represented. In a screened host firewall, access to and from a single host is controlled by means of a router operating at a network level. The single host is a bastion host; a highly-defended and secured strong-point that (hopefully) can resist attack.
Example Network level firewall: In this example, a network level firewall called a "screened subnet firewall" is represented. In a screened subnet firewall, access to and from a whole network is controlled by means of a router operating at a network level. It is similar to a screened host, except that it is, effectively, a network of screened hosts.
Application level firewalls generally are hosts running proxy servers, which permit no traffic directly between networks, and which perform elaborate logging and auditing of traffic passing through them. Since the proxy applications are sopftware components running on the firewall, it is a good place to do lots of logging and access control. Application level firewalls can be used as network address translators, since traffic goes in one "side" and out the other, after having passed through an application that effectively masks the origin of the initiating connection. Having an application in the way in some cases may impact performance and may make the firewall less transparent. Early application level firewalls such as those built using the TIS firewall toolkit, are not particularly transparent to end users and may require some training. Modern application level firewalls are often fully transparent. Application level firewalls tend to provide more detailed audit reports and tend to enforce more conservative security models than network level firewalls.
Example Application level firewall: In this example, an application level firewall called a "dual homed gateway" is represented. A dual homed gateway is a highly secured host that runs proxy software. It has two network interfaces, one on each network, and blocks all traffic passing through it.
The Future of firewalls lies someplace between network level firewalls and application level firewalls. It is likely that network level firewalls will become increasingly "aware" of the information going through them, and application level firewalls will become increasingly "low level" and transparent. The end result will be a fast packet-screening system that logs and audits data as it passes through. Increasingly, firewalls (network and application layer) incorporate encryption so that they may protect traffic passing between them over the Internet. Firewalls with end-to-end encryption can be used by organizations with multiple points of Internet connectivity to use the Internet as a "private backbone" without worrying about their data or passwords being sniffed.
A proxy server (sometimes referred to as an application gateway or forwarder) is an application that mediates traffic between a protected network and the Internet. Proxies are often used instead of router-based traffic controls, to prevent traffic from passing directly between networks. Many proxies contain extra logging or support for user authentication. Since proxies must "understand" the application protocol being used, they can also implement protocol specific security (e.g., an FTP proxy might be configurable to permit incoming FTP and block outgoing FTP).
Proxy servers are application specific. In order to support a new protocol via a proxy, a proxy must be developed for it. One popular set of proxy servers is the TIS Internet Firewall Toolkit ("FWTK") which includes proxies for Telnet, rlogin, FTP, X-Window, http/Web, and NNTP/Usenet news. SOCKS is a generic proxy system that can be compiled into a client-side application to make it work through a firewall. Its advantage is that it's easy to use, but it doesn't support the addition of authentication hooks or protocol specific logging. For more information on SOCKS, see ftp.nec.com: /pub/security/socks.cstc Users are encouraged to check the file "FILES" for a description of the directory's contents.
The Texas AMU security tools include software for implementing screening routers (FTP net.tamu.edu, pub/security/TAMU). Karlbridge is a PC-based screening router kit ftp://ftp.net.ohio-state.edu/pub/kbridge. A version of the Digital Equipment Corporation "screend" kernel screening software is available for BSD/386, NetBSD, and BSDI. There is a kernel-level packet screen called ipfilter available for free, for BSD-based systems. Many commercial routers support screening of various forms.
The following example shows one possible configuration for using the Cisco as filtering router. It is a sample that shows the implementation of as specific policy. Your policy will undoubtedly vary.
In this example, a company has Class C network address 18.104.22.168. Company network is connected to Internet via IP Service Provider. Company policy is to allow everybody access to Internet services, so all outgoing connections are accepted. All incoming connections go through "mailhost". Mail and DNS are only incoming services.
Only incoming packets from Internet are checked in this configuration. Rules are tested in order and stop when the first match is found. There is an implicit deny rule at the end of an access list that denies everything. This IP access lists assumes that you are running Cisco IOS v. 10.3 or later.
Use at least Cisco version 9.21 so you can filter incoming packets and check for address spoofing. It's still better to use 10.3, where you get some extra features (like filtering on source port) and some improvements on filter syntax.
You have still a few ways to make your setup stronger. Block all incoming TCP-connections and tell users to use passive-FTP clients. You can also block outgoing icmp echo-reply and destination-unreachable messages to hide your network and to prevent use of network scanners. Cisco.com has an archive of examples for building firewalls using Cisco routers (ftp://ftp.cisco.com/pub/acl-examples.tar.Z) Those examples are a bit out-of-date, but there are some perl scripts which are pretty useful, once adjusted for your network.
There are 3 ways to do it - Pick one:
Some organizations want to hide DNS names from the outside. Many experts don't think hiding DNS names is worthwhile, but if site/corporate policy mandates hiding domain names, this is one approach that is known to work. Another reason you may have to hide domain names is if you have a non-standard addressing scheme on your internal network. In that case, you have no choice but to hide those addresses. Don't fool yourself into thinking that if your DNS names are hidden that it will slow an attacker down much if they break into your firewall. Information about what is on your network is too easily gleaned from the networking layer itself. If you want an interesting demonstration of this, ping the subnet broadcast address on your LAN and then do an "arp -a." Note also that hiding names in the DNS doesn't address the problem of host names "leaking" out in mail headers, news articles, etc.
This approach is one of many, and is useful for organizations that wish to hide their host names from the Internet. The success of this approach lies on the fact that DNS clients on a machine don't have to talk to a DNS server on that same machine. In other words, just because there's a DNS server on a machine, there's nothing wrong with (and there are often advantages to) redirecting that machine's DNS client activity to a DNS server on another machine.
First, you set up a DNS server on the bastion host that the outside world can talk to. You set this server up so that it claims to be authoritative for your domains. In fact, all this server knows is what you want the outside world to know; the names and addresses of your gateways, your wildcard MX records, and so forth. This is the "public" server.
Then, you set up a DNS server on an internal machine. This server also claims to be authoritiative for your domains; unlike the public server, this one is telling the truth. This is your "normal" nameserver, into which you put all your "normal" DNS stuff. You also set this server up to forward queries that it can't resolve to the public server (using a "forwarders" line in /etc/named.boot on a UNIX machine, for example).
Finally, you set up all your DNS clients (the /etc/resolv.conf file on a UNIX box, for instance), including the ones on the machine with the public server, to use the internal server. This is the key.
An internal client asking about an internal host asks the internal server, and gets an answer; an internal client asking about an external host asks the internal server, which asks the public server, which asks the Internet, and the answer is relayed back. A client on the public server works just the same way. An external client, however, asking about an internal host gets back the "restricted" answer from the public server.
This approach assumes that there's a packet filtering firewall between these two servers that will allow them to talk DNS to each other, but otherwise restricts DNS between other hosts.
Another trick that's useful in this scheme is to employ wildcard PTR records in your IN-ADDR.ARPA domains. These cause an an address-to-name lookup for any of your non- public hosts to return something like "unknown.YOUR.DOMAIN" rather than an error. This satisfies anonymous FTP sites like ftp.uu.net that insist on having a name for the machines they talk to. This may fail when talking to sites that do a DNS cross-check in which the host name is matched against its address and vice versa.
Generally, making FTP work through the firewall is done either using a proxy server such as the firewall toolkit's ftp-gw or by permitting incoming connections to the network at a restricted port range, and otherwise restricting incoming connections using something like "established" screening rules. The FTP client is then modified to bind the data port to a port within that range. This entails being able to modify the FTP client application on internal hosts.
In some cases, if FTP downloads are all you wish to support, you might want to consider declaring FTP a "dead protocol" and letting you users download files via the Web instead. The user interface certainly is nicer, and it gets around the ugly callback port problem. If you choose the FTP-via-Web approach, your users will be unable to FTP files out, which, depending on what you are trying to accomplish, may be a problem.
A different approach is to use the FTP "PASV" option to indicate that the remote FTP server should permit the client to initiate connections. The PASV approach assumes that the FTP server on the remote system supports that operation. (See RFC1579 for more information)
Other sites prefer to build client versions of the FTP program that are linked against a SOCKS library.
Telnet is generally supported either by using an application proxy such as the firewall toolkit's tn-gw, or by simply configuring a router to permit outgoing connections using something like the "established" screening rules. Application proxies could be in the form of a standalone proxy running on the bastion host, or in the form of a SOCKS server and a modified client.
Many firewall admings permit connections to the finger port from only trusted machines, which can issue finger requests in the form of: finger firstname.lastname@example.org@firewall. This approach only works with the standard UNIX version of finger. Controlling access to services and restricting them to specific machines is managed using either tcp_wrappers or netacl from the firewall toolkit. This approach will not work on all systems, since some finger servers do not permit user@host@host fingering.
Many sites block inbound finger requests for a variety of reasons, foremost being past security bugs in the finger server (the Morris internet worm made these bugs famous) and the risk of proprietary or sensitive information being revealed in user's finger information. In general, however, if your users are accostomed to putting proprietary or sensitive information in their.plan files, you have a more serious security problem than just a firewall can solve.
The majority of firewall administrators choose to support gopher and archie through Web proxies, instead of directly. Proxies such as the firewall toolkit's http-gw convert gopher/gopher+ queries into HTML and vice versa. For supporting archie and other queries, many sites rely on Internet-based Web-to-archie servers, such as ArchiePlex. The Web's tendency to make everything on the Internet look like a Web service is both a blessing and a curse.
There are many new services constantly cropping up. Often they are misdesigned or are not designed with security in mind, and their designers will cheerfully tell you if you want to use them you need to let port xxx through your router. Unfortunately, not everyone can do that, and so a number of interesting new toys are difficult to use for people behind firewalls. Things like RealAudio, which require direct UDP access, are particularly egregious examples. The thing to bear in mind if you find yourself faced with one of these problems is to find out as much as you can about the security risks that the service may present, before you just allow it through. It's quite possible the service has no security implications. It's equally possible that it has undiscovered holes you could drive a truck through.
X Windows is a very useful system, but unfortunately has some major security flaws. Remote systems that can gain or spoof access to a workstation's X display can monitor keystrokes that a user enters, download copies of the contents of their windows, etc.
While attempts have been made to overcome them (E.g., MIT "Magic Cookie") it is still entirely too easy for an attacker to interfere with a user's X display. Most firewalls block all X traffic. Some permit X traffic through application proxies such as the DEC CRL X proxy (FTP crl.dec.com). The firewall toolkit includes a proxy for X, called x-gw, which a user can invoke via the Telnet proxy, to create a virtual X server on the firewall. When requests are made for an X connection on the virtual X server, the user is presented with a pop-up asking them if it is OK to allow the connection. While this is a little unaesthetic, it's entirely in keeping with the rest of X.
Normally, the route a packet takes from its source to its destination is determined by the routers between the source and destination. The packet itself only says where it wants to go (the destination address), and nothing about how it expects to get there.
There is an optional way for the sender of a packet (the source) to include information in the packet that tells the route the packet should get to its destination; thus the name "source routing". For a firewall, source routing is noteworthy, since an attacker can generate traffic claiming to be from a system "inside" the firewall. In general, such traffic wouldn't route to the firewall properly, but with the source routing option, all the routers between the attacker's machine and the target will return traffic along the reverse path of the source route. Implementing such an attack is quite easy; so firewall builders should not discount it as unlikely to happen.
In practice, source routing is very little used. In fact, generally the main legitimate use is in debugging network problems or routing traffic over specific links for congestion control for specialized situations. When building a firewall, source routing should be blocked at some point. Most commercial routers incorporate the ability to block source routing specifically, and many versions of UNIX that might be used to build firewall bastion hosts have the ability to disable or ignore source routed traffic.
An ICMP Redirect tells the recipient system to over-ride something in its routing table. It is legitimately used by routers to tell hosts that the host is using a non-optimal or defunct route to a particular destination, i.e. the host is sending it to the wrong router. The wrong router sends the host back an ICMP Redirect packet that tells the host what the correct route should be. If you can forge ICMP Redirect packets, and if your target host pays attention to them, you can alter the routing tables on the host and possibly subvert the security of the host by causing traffic to flow via a path the network manager didn't intend. ICMP Redirects also may be employed for denial of service attacks, where a host is sent a route that loses it connectivity, or is sent an ICMP Network Unreachable packet telling it that it can no longer access a particular network.
Many firewall builders screen ICMP traffic from their network, since it limits the ability of outsiders to ping hosts, or modify their routing tables.
Denial of service is when someone decides to make your network or firewall useless by disrupting it, crashing it, jamming it, or flooding it. The problem with denial of service on the Internet is that it is impossible to prevent. The reason has to do with the distributed nature of the network: every network node is connected via other networks which in turn connect to other networks, etc. A firewall administrator or ISP only has control of a few of the local elements within reach. An attacker can always disrupt a connection "upstream" from where the victim controls it. In other words, if someone wanted to take a network off the air, they could do it either by taking the network off the air, or by taking the networks it connects to off the air, ad infinitum. There are many, many, ways someone can deny service, ranging from the complex to the brute-force. If you are considering using Internet for a service which is absolutely time or mission critical, you should consider your fall-back position in the event that the network is down or damaged.