Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!newsfeed.utk.edu!news-hog.berkeley.edu!ucberkeley!news.cis.ohio-state.edu!not-for-mail From: C Matthew Curtin Newsgroups: comp.security.firewalls,comp.security.unix,comp.security.misc,comp.answers,news.answers Subject: Firewalls FAQ Followup-To: poster Date: 2 Jul 2001 05:39:01 GMT Organization: The Ohio State University Dept. of Computer and Info. Science Lines: 2108 Approved: news-answers-request@MIT.EDU Message-ID: <9hp1dl$3r2$1@news.cis.ohio-state.edu> NNTP-Posting-Host: epsilon.cis.ohio-state.edu Summary: Answers to Frequently Asked Questions about Internet Firewalls X-PostedBy: pfaq X-Face: L"IcL.b%SDN]0Kql2b`e.}+i05V9fi\yX#H1+Xl)3!+n/3?5`%-SA-HDgPk9uTk<3dv^J5DCgal)-E{`zN#*o6F|y>r)\<1024) * Try to protect services that live on high port numbers 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 list assumes that you are running Cisco IOS v. 10.3 or later. no ip source-route ! interface ethernet 0 ip address 195.55.55.1 no ip directed-broadcast ! interface serial 0 no ip directed-broadcast ip access-group 101 in ! access-list 101 deny ip 127.0.0.0 0.255.255.255 any access-list 101 deny ip 10.0.0.0 0.255.255.255 any access-list 101 deny ip 172.16.0.0 0.15.255.255 any access-list 101 deny ip 192.168.0.0 0.0.255.255 any access-list 101 deny ip any 0.0.0.255 255.255.255.0 access-list 101 deny ip any 0.0.0.0 255.255.255.0 ! access-list 101 deny ip 195.55.55.0 0.0.0.255 access-list 101 permit tcp any any established ! access-list 101 permit tcp any host 195.55.55.10 eq smtp access-list 101 permit tcp any host 195.55.55.10 eq dns access-list 101 permit udp any host 192.55.55.10 eq dns ! access-list 101 deny tcp any any range 6000 6003 access-list 101 deny tcp any any range 2000 2003 access-list 101 deny tcp any any eq 2049 access-list 101 deny udp any any eq 2049 ! access-list 101 permit tcp any 20 any gt 1024 ! access-list 101 permit icmp any any ! snmp-server community FOOBAR RO 2 line vty 0 4 access-class 2 in access-list 2 permit 195.55.55.0 0.0.0.255 3.6.2 Explanations * Drop all source-routed packets. Source routing can be used for address spoofing. * Drop directed broadcasts, which are used in smurf attacks. * If an incoming packet claims to be from a local net, loopback network, or private network, drop it. * All packets which are part of already established TCP-connections can pass through without further checking. * All connections to low port numbers are blocked except SMTP and DNS. * Block all services that listen for TCP connections on high port numbers. X-windows (port 6000+), OpenWindows (port 2000+) are a few candidates. NFS (port 2049) runs usually over UDP, but it can be run over TCP, so you should block it. * Incoming connections from port 20 into high port numbers are supposed to be FTP data connections. * Access-list 2 limits access to router itself (telnet & SNMP) * All UDP traffic is blocked to protect RPC services 3.6.3 Shortcomings * You cannot enforce strong access policies with router access lists. Users can easily install backdoors to their systems to get over ``no incoming telnet'' or ``no X'' rules. Also crackers install telnet backdoors on systems where they break in. * You can never be sure what services you have listening for connections on high port numbers. * Checking the source port on incoming FTP data connections is a weak security method. It also breaks access to some FTP sites. It makes use of the service more difficult for users without preventing bad guys from scanning your systems. 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 use to have an archive of examples for building firewalls using Cisco routers, but it doesn't seem to be online anymore. There are some notes on Cisco access control lists, at least, at ftp://ftp.cisco.com/pub/mibs/app_notes/access-lists. 3.7 What are the critical resources in a firewall? It's important to understand the critical resources of your firewall architecture, so when you do capacity planning, performance optimizations, etc., you know exactly what you need to do, and how much you need to do it in order to get the desired result. What exactly the firewall's critical resources are tends to vary from site to site, depending on the sort of traffic that loads the system. Some people think they'll automatically be able to increase the data throughput of their firewall by putting in a box with a faster CPU, or another CPU, when this isn't necessarily the case. Potentially, this could be a large waste of money that doesn't do anything to solve the problem at hand or provide the expected scalability. On busy systems, memory is extremely important. You have to have enough RAM to support every instance of every program necessary to service the load placed on that machine. Otherwise, the swapping will start and the productivity will stop. Light swapping isn't usually much of a problem, but if a system's swap space begins to get busy, then it's usually time for more RAM. A system that's heavily swapping is often relatively easy to push over the edge in a denial-of-service attack, or simply fall behind in processing the load placed on it. This is where long email delays start. Beyond the system's requirement for memory, it's useful to understand that different services use different system resources. So the configuration that you have for your system should be indicative of the kind of load you plan to service. A 700 MHz processor isn't going to do you much good if all you're doing is netnews and mail, and are trying to do it on an IDE disk with an ISA controller. Table 1: Critical Resources for Firewall Services Service Critical Resource Email Disk I/O Netnews Disk I/O Web Host OS Socket Performance IP Routing Host OS Socket Performance Web Cache Host OS Socket Performance, Disk I/O 3.8 What is a DMZ, and why do I want one? ``DMZ'' is an abbreviation for ``demilitarized zone''. In the context of firewalls, this refers to a part of the network that is neither part of the internal network nor directly part of the Internet. Typically, this is the area between your Internet access router and your bastion host, though it can be between any two policy-enforcing components of your architecture. A DMZ can be created by putting access control lists on your access router. This minimizes the exposure of hosts on your external LAN by allowing only recognized and managed services on those hosts to be accessible by hosts on the Internet. Many commercial firewalls simply make a third interface off of the bastion host and label it the DMZ. The point is that the network is neither ``inside'' nor ``outside''. For example, a web server running on NT might be vulnerable to a number of denial-of-service attacks against such services as RPC, NetBIOS and SMB. These services are not required for the operation of a web server, so blocking TCP connections to ports 135, 137, 138, and 139 on that host will reduce the exposure to a denial-of-service attack. In fact, if you block everything but HTTP traffic to that host, an attacker will only have one service to attack. This illustrates an important principle: never offer attackers more to work with than is absolutely necessary to support the services you want to offer the public. 3.9 How might I increase the security and scalability of my DMZ? A common approach for an attacker is to break into a host that's vulnerable to attack, and exploit trust relationships between the vulnerable host and more interesting targets. If you are running a number of services that have different levels of security, you might want to consider breaking your DMZ into several ``security zones''. This can be done by having a number of different networks within the DMZ. For example, the access router could feed two ethernets, both protected by ACLs, and therefore in the DMZ. On one of the ethernets, you might have hosts whose purpose is to service your organization's need for Internet connectivity. These will likely relay mail, news, and host DNS. On the other ethernet could be your web server(s) and other hosts that provide services for the benefit of Internet users. In many organizations, services for Internet users tend to be less carefully guarded and are more likely to be doing insecure things. (For example, in the case of a web server, unauthenticated and untrusted users might be running CGI or other executable programs. This might be reasonable for your web server, but brings with it a certain set of risks that need to be managed. It is likely these services are too risky for an organization to run them on a bastion host, where a slip-up can result in the complete failure of the security mechanisms.) By putting hosts with similar levels of risk on networks together in the DMZ, you can help minimize the effect of a breakin at your site. If someone breaks into your web server by exploiting some bug in your web server, they'll not be able to use it as a launching point to break into your private network if the web servers are on a separate LAN from the bastion hosts, and you don't have any trust relationships between the web server and bastion host. Now, keep in mind that we're running ethernet here. If someone breaks into your web server, and your bastion host is on the same ethernet, an attacker can install a sniffer on your web server, and watch the traffic to and from your bastion host. This might reveal things that can be used to break into the bastion host and gain access to the internal network. Splitting services up not only by host, but by network, and limiting the level of trust between hosts on those networks, you can greatly reduce the likelihood of a breakin on one host being used to break into the other. Succinctly stated: breaking into the web server in this case won't make it any easier to break into the bastion host. You can also increase the scalability of your architecture by placing hosts on different networks. The fewer machines that there are to share the available bandwidth, the more bandwidth that each will get. 3.10 What is a `single point of failure', and how do I avoid having one? An architecture whose security hinges upon one mechanism has a single point of failure. Software that runs bastion hosts has bugs. Applications have bugs. Software that controls routers has bugs. It makes sense to use all of these components to build a securely designed network, and to use them in redundant ways. If your firewall architecture is a screened subnet, you have two packet filtering routers and a bastion host. (See question 3.2 from this section.) Your Internet access router will not permit traffic from the Internet to get all the way into your private network. However, if you don't enforce that rule with any other mechanisms on the bastion host and/or choke router, only one component of your architecture needs to fail or be compromised in order to get inside. On the other hand, if you have a redundant rule on the bastion host, and again on the choke router, an attacker will need to defeat three mechanisms. Further, if the bastion host or the choke router needs to invoke its rule to block outside access to the internal network, you might want to have it trigger an alarm of some sort, since you know that someone has gotten through your access router. 3.11 How can I block all of the bad stuff? For firewalls where the emphasis is on security instead of connectivity, you should consider blocking everything by default, and only specifically allowing what services you need on a case-by-case basis. If you block everything, except a specific set of services, then you've already made your job much easier. Instead of having to worry about every security problem with everything product and service around, you only need to worry about every security problem with a specific set of services and products. :-) Before turning on a service, you should consider a couple of questions: * Is the protocol for this product a well-known, published protocol? * Is the application to service this protocol available for public inspection of its implementation? * How well known is the service and product? * How does allowing this service change the firewall architecture? Will an attacker see things differently? Could it be exploited to get at my internal network, or to change things on hosts in my DMZ? When considering the above questions, keep the following in mind: * ``Security through obscurity'' is no security at all. Unpublished protocols have been examined by bad guys and defeated. * Despite what the marketing representatives say, not every protocol or service is designed with security in mind. In fact, the number that are is very few. * Even in cases where security is a consideration, not all organizations have competent security staff. Among those who don't, not all are willing to bring a competent consultant into the project. The end result is that otherwise-competent, well-intended developers can design insecure systems. * The less that a vendor is willing to tell you about how their system really works, the more likely it is that security (or other) problems exist. Only vendors with something to hide have a reason to hide their designs and implementations. 3.12 How can I restrict web access so users can't view sites unrelated to work? A few years ago, someone got the idea that it's a good idea to block ``bad'' web sites, i.e., those that contain material that The Company views ``inappropriate''. The idea has been increasing in popularity, but there are several things to consider when thinking about implementing such controls in your firewall. * It is not possible to practically block everything that an employer deems ``inappropriate''. The Internet is full of every sort of material. Blocking one source will only redirect traffic to another source of such material, or cause someone to figure a way around the block. * Most organizations do not have a standard for judging the appropriateness of material that their employees bring to work, i.e., books, magazines, etc. Do you inspect everyone's briefcase for ``inappropriate material'' every day? If you do not, then why would you inspect every packet for ``inappropriate material''? Any decisions along those lines in such an organization will be arbitrary. Attempting to take disciplinary action against an employee where the only standard is arbitrary typically isn't wise, for reasons well beyond the scope of this document. * Products that perform site-blocking, commercial and otherwise, are typically easy to circumvent. Hostnames can be rewritten as IP addresses. IP addresses can be written as a 32-bit integer value, or as four 8-bit integers (the most common form). Other possibilities exist, as well. Connections can be proxied. Web pages can be fetched via email. You can't block them all. The effort that you'll spend trying to implement and manage such controls will almost certainly far exceed any level of damage control that you're hoping to have. The rule-of-thumb to remember here is that you cannot solve social problems with technical solutions. If there is a problem with someone going to an ``inappropriate'' web site, that is because someone else saw it and was offended by what he saw, or because that person's productivity is below expectations. In either case, those are matters for the personnel department, not the firewall administrator. 4 Various Attacks 4.1 What is source routed traffic and why is it a threat? 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 take to 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. 4.2 What are ICMP redirects and redirect bombs? 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. Before you decide to completely block ICMP, you should be aware of how the TCP protocol does ``Path MTU Discovery'', to make certain that you don't break connectivity to other sites. If you can't safely block it everywhere, you can consider allowing selected types of ICMP to selected routing devices. If you don't block it, you should at least ensure that your routers and hosts don't respond to broadcast ping packets. 4.3 What about denial of service? 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. TCP/IP's UDP echo service is trivially abused to get two servers to flood a network segment with echo packets. You should consider commenting out unused entries in /etc/inetd.conf of Unix hosts, adding no ip small-servers to Cisco routers, or the equivalent for your components. 4.4 What are some common attacks, and how can I protect my system against them? Each site is a little different from every other in terms of what attacks are likely to be used against it. Some recurring themes do arise, though. 4.4.1 SMTP Server Hijacking (Unauthorized Relaying) This is where a spammer will take many thousands of copies of a message and send it to a huge list of email addresses. Because these lists are often so bad, and in order to increase the speed of operation for the spammer, many have resorted to simply sending all of their mail to an SMTP server that will take care of actually delivering the mail. Of course, all of the bounces, spam complaints, hate mail, and bad PR come for the site that was used as a relay. There is a very real cost associated with this, mostly in paying people to clean up the mess afterward. The Mail Abuse Prevention System Transport Security Initiative maintains a complete description of the problem, and how to configure about every mailer on the planet to protect against this attack. 4.4.2 Exploiting Bugs in Applications Various versions of web servers, mail servers, and other Internet service software contain bugs that allow remote (Internet) users to do things ranging from gain control of the machine to making that application crash and just about everything in between. The exposure to this risk can be reduced by running only necessary services, keeping up to date on patches, and using products that have been around a while. 4.4.3 Bugs in Operating Systems Again, these are typically initiated by users remotely. Operating systems that are relatively new to IP networking tend to be more problematic, as more mature operating systems have had time to find and eliminate their bugs. An attacker can often make the target equipment continuously reboot, crash, lose the ability to talk to the network, or replace files on the machine. Here, running as few operating system services as possible can help. Also, having a packet filter in front of the operating system can reduce the exposure to a large number of these types of attacks. And, of course, chosing a stable operating system will help here as well. When selecting an OS, don't be fooled into believing that ``the pricier, the better''. Free operating systems are often much more robust than their commercial counterparts 5 How Do I... 5.1 Do I really want to allow everything that my users ask for? It's entirely possible that the answer is ``no''. Each site has its own policies about what is and isn't needed, but it's important to remember that a large part of the job of being an organization's gatekeeper is education. Users want streaming video, real-time chat, and to be able to offer services to external customers that require interaction with live databases on the internal network. That doesn't mean that any of these things can be done without presenting more risk to the organization than the supposed ``value'' of heading down that road is worth. Most users don't want to put their organization at risk. They just read the trade rags, see advertisements, and they want to do those things, too. It's important to look into what it is that they really want to do, and to help them understand how they might be able to accomplish their real objective in a more secure manner. You won't always be popular, and you might even find yourself being given direction to do something incredibly stupid, like ``just open up ports foo through bar''. If that happens, don't worry about it. It would be wise to keep all of your exchanges on such an event so that when a 12-year-old script kiddie breaks in, you'll at least be able to separate yourself from the whole mess. 5.2 How do I make Web/HTTP work through my firewall? There are three ways to do it. 1. Allow ``established'' connections out via a router, if you are using screening routers. 2. Use a web client that supports SOCKS, and run SOCKS on your bastion host. 3. Run some kind of proxy-capable web server on the bastion host. Some options include Squid , Apache , Netscape Proxy , and http-gw from the TIS firewall toolkit. Most of these can also proxy other protocols (such as gopher and ftp), and can cache objects fetched, which will also typically result in a performance boost for the users, and more efficient use of your connection to the Internet. Essentially all web clients (Mozilla, Internet Explorer, Lynx, etc.) have proxy server support built directly into them. 5.3 How do I make SSL work through the firewall? SSL is a protocol that allows secure connections across the Internet. Typically, SSL is used to protect HTTP traffic. However, other protocols (such as telnet) can run atop SSL. Enabling SSL through your firewall can be done the same way that you would allow HTTP traffic, if it's HTTP that you're using SSL to secure, which is usually true. The only difference is that instead of using something that will simply relay HTTP, you'll need something that can tunnel SSL. This is a feature present on most web object caches. You can find out more about SSL from Netscape . 5.4 How do I make DNS work with a firewall? 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 authoritative 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. 5.5 How do I make FTP work through my firewall? 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 ``Firewall-Friendly FTP'' [1].) Other sites prefer to build client versions of the FTP program that are linked against a SOCKS library. 5.6 How do I make Telnet work through my firewall? 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. 5.7 How do I make Finger and whois work through my firewall? Many firewall admins permit connections to the finger port from only trusted machines, which can issue finger requests in the form of: finger user@host.domain@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 accustomed to putting proprietary or sensitive information in their .plan files, you have a more serious security problem than just a firewall can solve. 5.8 How do I make gopher, archie, and other services work through my firewall? 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. 5.9 What are the issues about X11 through a firewall? The X Windows System 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. 5.10 How do I make RealAudio work through my firewall? RealNetworks maintains some information about how to get RealAudio working through your firewall . It would be unwise to make any changes to your firewall without understanding what the changes will do, exactly, and knowing what risks the new changes will bring with them. 5.11 How do I make my web server act as a front-end for a database that lives on my private network? The best way to do this is to allow very limited connectivity between your web server and your database server via a specific protocol that only supports the level of functionality you're going to use. Allowing raw SQL, or anything else where custom extractions could be performed by an attacker isn't generally a good idea. Assume that an attacker is going to be able to break into your web server, and make queries in the same way that the web server can. Is there a mechanism for extracting sensitive information that the web server doesn't need, like credit card information? Can an attacker issue an SQL select and extract your entire proprietary database? ``E-commerce'' applications, like everything else, are best designed with security in mind from the ground up, instead of having security ``added'' as an afterthought. Review your architecture critically, from the perspective of an attacker. Assume that the attacker knows everything about your architecture. Now ask yourself what needs to be done to steal your data, to make unauthorized changes, or to do anything else that you don't want done. You might find that you can significantly increase security without decreasing functionality by making a few design and implementation decisions. Some ideas for how to handle this: * Extract the data you need from the database on a regular basis so you're not making queries against the full database, complete with information that attackers will find interesting. * Greatly restrict and audit what you do allow between the web server and database. 5.12 But my database has an integrated web server, and I want to use that. Can't I just poke a hole in the firewall and tunnel that port? If your site firewall policy is sufficiently lax that you're willing to manage the risk that someone will exploit a vulnerability in your web server that will result in partial or complete exposure of your database, then there isn't much preventing you from doing this. However, in many organizations, the people who are responsible for tying the web front end to the database back end simply do not have the authority to take that responsibility. Further, if the information in the database is about people, you might find yourself guilty of breaking a number of laws if you haven't taken reasonable precautions to prevent the system from being abused. In general, this isn't a good idea. See question 5.11 for some ideas on other ways to accomplish this objective. 5.13 How Do I Make IP Multicast Work With My Firewall? IP multicast is a means of getting IP traffic from one host to a set of hosts without using broadcasting; that is, instead of every host getting the traffic, only those that want it will get it, without each having to maintain a separate connection to the server. IP unicast is where one host talks to another, multicast is where one host talks to a set of hosts, and broadcast is where one host talks to all hosts. The public Internet has a multicast backbone (``MBone'') where users can engage in multicast traffic exchange. Common uses for the MBone are streams of IETF meetings and similar such interaction. Getting one's own network connected to the MBone will require that the upstream provider route multicast traffic to and from your network. Additionally, your internal network will have to support multicast routing. The role of the firewall in multicast routing, conceptually, is no different from its role in other traffic routing. That is, a policy that identifies which multicast groups are and aren't allowed must be defined and then a system of allowing that traffic according to policy must be devised. Great detail on how exactly to do this is beyond the scope of this document. Fortunately, RFC 2588 [2] discusses the subject in more detail. Unless your firewall product supports some means of selective multicast forwarding or you have the ability to put it in yourself, you might find forwarding multicast traffic in a way consistent with your security policy to be a bigger headache than it's worth. A Some Commercial Products and Vendors We feel this topic is too sensitive to address in a FAQ, however, an independently maintained list (no warranty or recommendations are implied) can be found online. B Glossary of Firewall-Related Terms Abuse of Privilege When a user performs an action that they should not have, according to organizational policy or law. Access Control Lists Rules for packet filters (typically routers) that define which packets to pass and which to block. Access Router A router that connects your network to the external Internet. Typically, this is your first line of defense against attackers from the outside Internet. By enabling access control lists on this router, you'll be able to provide a level of protection for all of the hosts ``behind'' that router, effectively making that network a DMZ instead of an unprotected external LAN. Application-Layer Firewall A firewall system in which service is provided by processes that maintain complete TCP connection state and sequencing. Application layer firewalls often re-address traffic so that outgoing traffic appears to have originated from the firewall, rather than the internal host. Authentication The process of determining the identity of a user that is attempting to access a system. Authentication Token A portable device used for authenticating a user. Authentication tokens operate by challenge/response, time-based code sequences, or other techniques. This may include paper-based lists of one-time passwords. Authorization The process of determining what types of activities are permitted. Usually, authorization is in the context of authentication: once you have authenticated a user, they may be authorized different types of access or activity. Bastion Host A system that has been hardened to resist attack, and which is installed on a network in such a way that it is expected to potentially come under attack. Bastion hosts are often components of firewalls, or may be ``outside'' web servers or public access systems. Generally, a bastion host is running some form of general purpose operating system (e.g., Unix, VMS, NT, etc.) rather than a ROM-based or firmware operating system. Challenge/Response An authentication technique whereby a server sends an unpredictable challenge to the user, who computes a response using some form of authentication token. Chroot A technique under Unix whereby a process is permanently restricted to an isolated subset of the filesystem. Cryptographic Checksum A one-way function applied to a file to produce a unique ``fingerprint'' of the file for later reference. Checksum systems are a primary means of detecting filesystem tampering on Unix. Data Driven Attack A form of attack in which the attack is encoded in innocuous-seeming data which is executed by a user or other software to implement an attack. In the case of firewalls, a data driven attack is a concern since it may get through the firewall in data form and launch an attack against a system behind the firewall. Defense in Depth The security approach whereby each system on the network is secured to the greatest possible degree. May be used in conjunction with firewalls. DNS spoofing Assuming the DNS name of another system by either corrupting the name service cache of a victim system, or by compromising a domain name server for a valid domain. Dual Homed Gateway A dual homed gateway is a system that has two or more network interfaces, each of which is connected to a different network. In firewall configurations, a dual homed gateway usually acts to block or filter some or all of the traffic trying to pass between the networks. Encrypting Router see Tunneling Router and Virtual Network Perimeter. Firewall A system or combination of systems that enforces a boundary between two or more networks. Host-based Security The technique of securing an individual system from attack. Host based security is operating system and version dependent. Insider Attack An attack originating from inside a protected network. Intrusion Detection Detection of break-ins or break-in attempts either manually or via software expert systems that operate on logs or other information available on the network. IP Spoofing An attack whereby a system attempts to illicitly impersonate another system by using its IP network address. IP Splicing / Hijacking An attack whereby an active, established, session is intercepted and co-opted by the attacker. IP Splicing attacks may occur after an authentication has been made, permitting the attacker to assume the role of an already authorized user. Primary protections against IP Splicing rely on encryption at the session or network layer. Least Privilege Designing operational aspects of a system to operate with a minimum amount of system privilege. This reduces the authorization level at which various actions are performed and decreases the chance that a process or user with high privileges may be caused to perform unauthorized activity resulting in a security breach. Logging The process of storing information about events that occurred on the firewall or network. Log Retention How long audit logs are retained and maintained. Log Processing How audit logs are processed, searched for key events, or summarized. Network-Layer Firewall A firewall in which traffic is examined at the network protocol packet layer. Perimeter-based Security The technique of securing a network by controlling access to all entry and exit points of the network. Policy Organization-level rules governing acceptable use of computing resources, security practices, and operational procedures. Proxy A software agent that acts on behalf of a user. Typical proxies accept a connection from a user, make a decision as to whether or not the user or client IP address is permitted to use the proxy, perhaps does additional authentication, and then completes a connection on behalf of the user to a remote destination. Screened Host A host on a network behind a screening router. The degree to which a screened host may be accessed depends on the screening rules in the router. Screened Subnet A subnet behind a screening router. The degree to which the subnet may be accessed depends on the screening rules in the router. Screening Router A router configured to permit or deny traffic based on a set of permission rules installed by the administrator. Session Stealing See IP Splicing. Trojan Horse A software entity that appears to do something normal but which, in fact, contains a trapdoor or attack program. Tunneling Router A router or system capable of routing traffic by encrypting it and encapsulating it for transmission across an untrusted network, for eventual de-encapsulation and decryption. Social Engineering An attack based on deceiving users or administrators at the target site. Social engineering attacks are typically carried out by telephoning users or operators and pretending to be an authorized user, to attempt to gain illicit access to systems. Virtual Network Perimeter A network that appears to be a single protected network behind firewalls, which actually encompasses encrypted virtual links over untrusted networks. Virus A replicating code segment that attaches itself to a program or data file. Viruses might or might not not contain attack programs or trapdoors. Unfortunately, many have taken to calling any malicious code a ``virus''. If you mean ``trojan horse'' or ``worm'', say ``trojan horse'' or ``worm''. Worm A standalone program that, when run, copies itself from one host to another, and then runs itself on each newly infected host. The widely reported ``Internet Virus'' of 1988 was not a virus at all, but actually a worm. C TCP and UDP Ports by Mikael Olsson This appendix will begin at a fairly ``basic'' level, so even if the first points seem childishly self-evident to you, you might still learn something from skipping ahead to something later in the text. C.1 What is a port? A ``port'' is ``virtual slot'' in your TCP and UDP stack that is used to map a connection between two hosts, and also between the TCP/UDP layer and the actual applications running on the hosts. They are numbered 0-65535, with the range 0-1023 being marked as ``reserved'' or ``privlileged'', and the rest (1024-65535) as ``dynamic'' or ``unprivileged''. There are basically two uses for ports: * ``Listening'' on a port. This is used by server applications waiting for users to connect, to get to some ``well known service'', for instance HTTP (TCP port 80), Telnet (TCP port 21), DNS (UDP and sometimes TCP port 53). * Opening a ``dynamic'' port. Both sides of a TCP connection need to be identified by IP addresses and port numbers. Hence, when you want to ``connect'' to a server process, your end of the communications channel also needs a ``port''. This is done by choosing a port above 1024 on your machine that is not currently in use by another communications channel, and using it as the ``sender'' in the new connection. Dynamic ports may also be used as ``listening'' ports in some applications, most notably FTP. Ports in the range 0-1023 are almost always server ports. Ports in the range 1024-65535 are usually dynamic ports (i.e., opened dynamically when you connect to a server port). However, any port may be used as a server port, and any port may be used as an ``outgoing'' port. So, to sum it up, here's what happens in a basic connection: * At some point in time, a server application on host 1.2.3.4 decides to ``listen'' at port 80 (HTTP) for new connections. * You (5.6.7.8) want to surf to 1.2.3.4, port 80, and your browser issues a connect call to it. * The connect call, realising that it doesn't yet have local port number, goes hunting for one. The local port number is necessary since when the replies come back some time in the future, your TCP/IP stack will have to know to what application to pass the reply. It does this by remembering what application uses which local port number. (This is grossly simplified, no flames from programmers, please.) * Your TCP stack finds an unused dynamic port, usually somewhere above 1024. Let's assume that it finds 1029. * Your first packet is then sent, from your local IP, 5.6.7.8, port 1029, to 1.2.3.4, port 80. * The server responds with a packet from 1.2.3.4, port 80, to you, 5.6.7.8, port 1029. * This procedure is actually longer than this, read on for a more in-depth explanation of TCP connect sequences. C.2 How do I know which application uses what port? There are several lists outlining the ``reserved'' and ``well known'' ports, as well as ``commonly used'' ports, and the best one is: ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers. For those of you still reading RFC 1700 to find out what port number does what, STOP DOING IT. It is horribly out of date, and it won't be less so tomorrow. Now, as for trusting this information: These lists do not, in any way, constitute any kind of holy bible on which ports do what. Wait, let me rephrase that: THERE IS NO WAY OF RELIABLY DETERMINING WHAT PORT DOES WHAT SIMPLY BY LOOKING IN A LIST. C.3 What are LISTENING ports? Suppose you did ``netstat -a'' on your machine and ports 1025 and 1030 showed up as LISTENing. What do they do? Right, let's take a look in the assigned port numbers list. blackjack 1025/tcp network blackjack iad1 1030/tcp BBN IAD Wait, what's happening? Has my workstation stolen my VISA number and decided to go play blackjack with some rogue server on the internet? And what's that software that BBN has installed? This is NOT where you start panicking and send mail to the firewalls list. In fact, this question has been asked maybe a dozen times during the past six months, and every time it's been answered. Not that THAT keeps people from asking the same question again. If you are asking this question, you are most likely using a windows box. The ports you are seeing are (most likely) two listening ports that the RPC subsystem opens when it starts up. This is an example of where dynamicly assigned ports may be used by server processes. Applications using RPC will later on connect to port 135 (the netbios ``portmapper'') to query where to find some RPC service, and get an answer back saying that that particular service may be contacted on port 1025. Now, how do we know this, since there's no ``list'' describing these ports? Simple: There's no substitute for experience. And using the mailing list search engines also helps a hell of a lot. C.4 How do I determine what service the port is for? Since it is impossible to learn what port does what by looking in a list, how do i do it? The old hands-on way of doing it is by shutting down nearly every service/daemon running on your machine, doing netstat -a and taking note of what ports are open. There shouldn't be very many listening ones. Then you start turning all the services on, one by one, and take note of what new ports show up in your netstat output. Another way, that needs more guess work, is simply telnetting to the ports and see what comes out. If nothing comes out, try typing some gibberish and slamming Enter a few times, and see if something turns up. If you get binary garble, or nothing at all, this obviously won't help you. :-) However, this will only tell you what listening ports are used. It won't tell you about dynamically opened ports that may be opened later on by these applications. There are a few applications that might help you track down the ports used. On Unix systems, there's a nice utility called lsof that comes preinstalled on many systems. It will show you all open port numbers and the names of the applications that are using them. This means that it might show you a lot of locally opened files aswell as TCP/IP sockets. Read the help text. :-) On windows systems, nothing comes preinstalled to assist you in this task. (What's new?) There's a utility called ``Inzider'' which installs itself inside the windows sockets layer and dynamically remembers which process opens which port. The drawback of this approach is that it can't tell you what ports were opened before inzider started, but it's the best that you'll get on windows (to my knowledge). http://ntsecurity.nu/toolbox/inzider/. C.5 What ports are safe to pass through a firewall? ALL. No, wait, NONE. No, wait, uuhhh... I've heard that all ports above 1024 are safe since they're only dynamic?? No. Really. You CANNOT tell what ports are safe simply by looking at its number, simply because that is really all it is. A number. You can't mount an attack through a 16-bit number. The security of a ``port'' depends on what application you'll reach through that port. A common misconception is that ports 25 (SMTP) and 80 (HTTP) are safe to pass through a firewall. *meep* WRONG. Just because everyone is doing it doesn't mean that it is safe. Again, the security of a port depends on what application you'll reach through that port. If you're running a well-written web server, that is designed from the ground up to be secure, you can probably feel reasonably assured that it's safe to let outside people access it through port 80. Otherwise, you CAN'T. The problem here is not in the network layer. It's in how the application processes the data that it receives. This data may be received through port 80, port 666, a serial line, floppy or through singing telegram. If the application is not safe, it does not matter how the data gets to it. The application data is where the real danger lies. If you are interested in the security of your application, go subscribe to bugtraq or try searching their archives. This is more of an application security issue rather than a firewall security issue. One could argue that a firewall should stop all possible attacks, but with the number of new network protocols, NOT designed with security in mind, and networked applications, neither designed with security in mind, it becomes impossible for a firewall to protect against all data-driven attacks. C.6 The behavior of FTP Or, ``Why do I have to open all ports above 1024 to my FTP server?'' FTP doesn't really look a whole lot like other applications from a networking perspective. It keeps one listening port, port 21, which users connect to. All it does is let people log on, and establish ANOTHER connection to do actual data transfers. This second connection is usually on some port above 1024. There are two modes, ``active'' (normal) and ``passive'' mode. This word describes the server's behaviour. In active mode, the client (5.6.7.8) connects to port 21 on the server (1.2.3.4) and logs on. When file transfers are due, the client allocates a dynamic port above 1024, informs the server about which port it opened, and then the server opens a new connection to that port. This is the ``active'' role of the server: it actively establishes new connections to the client. In passive mode, the connection to port 21 is the same. When file transfers are due, the SERVER allocates a dynamic port above 1024, informs the client about which port it opened, and then the CLIENT opens a new connection to that port. This is the ``passive'' role of the server: it waits for the client to establish the second (data) connection. If your firewall doesn't inspect the application data of the FTP command connection, it won't know that it needs to dynamically open new ports above 1024. On a side note: The traditional behaviour of FTP servers in active mode is to establish the data session FROM port 20, and to the dynamic port on the client. FTP servers are steering away from this behaviour somewhat due to the need to run as ``root'' on unix systems in order to be able to allocate ports below 1024. Running as ``root'' is not good for security, since if there's a bug in the software, the attacker would be able to compromise the entire machine. The same goes for running as ``Administrator'' or ``SYSTEM'' (``LocalSystem'') on NT machines, although the low port problem does not apply on NT. To sum it up, if your firewall understands FTP, it'll be able to handle the data connections by itself, and you won't have to worry about ports above 1024. If it does NOT, there are four issues that you need to address: * Firewalling an FTP server in active mode You need to let your server open new connections to the outside world on ports 1024 and above * Firewalling an FTP server in passive mode You need to let the outside world connect to ports 1024 and above on your server. CAUTION!!!! There may be applications running on some of these ports that you do NOT want outside people using. Disallow access to these ports before allowing access to the 1024-65535 port range. * Firewalling FTP clients in active mode You need to let the outside world connect to ports 1024 and above on your clients. CAUTION!!!! There may be applications running on some of these ports that you do NOT want outside people using. Disallow access to these ports before allowing access to the 1024-65535 port range. * Firewalling FTP clients in passive mode You need to let your clients open new connections to the outside world on ports 1024 and above. Again, if your firewall understands FTP, none of the four points above apply to you. Let the firewall do the job for you. C.7 What software uses what FTP mode? It is up to the client to decide what mode to use; the default mode when a new connection is opened is ``active mode''. Most FTP clients come preconfigured to use active mode, but provide an option to use ``passive'' (``PASV'') mode. An exception is the windows command line FTP client which only operates in active mode. Web Browsers generally use passive mode when connecting via FTP, with a weird exception: MSIE 5 will use active FTP when FTP:ing in ``File Explorer'' mode and passive FTP when FTP:ing in ``Web Page'' mode. There is no reason whatsoever for this behaviour; my guess is that someone in Redmond with no knowledge of FTP decided that ``Of course we'll use active mode when we're in file explorer mode, since that looks more active than a web page''. Go figure. C.8 Is my firewall trying to connect outside? My firewall logs are telling me that my web server is trying to connect from port 80 to ports above 1024 on the outside. What is this?! If you are seeing dropped packets from port 80 on your web server (or from port 25 on your mail server) to high ports on the outside, they usually DO NOT mean that your web server is trying to connect somewhere. They are the result of the firewall timing out a connection, and seeing the server retransmitting old responses (or trying to close the connection) to the client. TCP connections always involve packets traveling in BOTH directions in the connection. If you are able to see the TCP flags in the dropped packets, you'll see that the ACK flag is set but not the SYN flag, meaning that this is actually not a new connection forming, but rather a response of a previously formed connection. Read point 8 below for an in-depth explanation of what happens when TCP connections are formed (and closed) C.9 The anatomy of a TCP connection TCP is equipped with 6 ``flags'', which may be ON or OFF. These flags are: FIN ``Controlled'' connection close SYN Open new connection RST ``Immediate'' connection close PSH Instruct receiver host to push the data up to the application rather than just queue it ACK ``Acknowledge'' a previous packet URG ``Urgent'' data which needs to be processed immediately In this example, your client is 5.6.7.8, and the port assigned to you dynamically is 1049. The server is 1.2.3.4, port 80. You begin the connection attempt: 5.6.7.8:1049 -> 1.2.3.4:80 SYN=ON The server receives this packet and understands that someone wants to form a new connection. A response is sent: 1.2.3.4:80 -> 5.6.7.8:1049 SYN=ON ACK=ON The client receives the response, and informs that the response is received 5.6.7.8:1049 -> 1.2.3.4:80 ACK=ON Here, the connection is opened. This is called a three-way handshake. Its purpose is to verify to BOTH hosts that they have a working connection between them. The internet being what it is, unreliable and flooded, there are provisions to compensate for packet loss. If the client sends out the initial SYN without receiving a SYN+ACK within a few seconds, it'll resend the SYN. If the server sends out the SYN+ACK without receiving an ACK in a few seconds, it'll resend the SYN+ACK packet. The latter is actually the reason that SYN flooding works so well. If you send out SYN packets from lots of different ports, this will tie up a lot of resources on the server. If you also refuse to respond to the returned SYN+ACK packets, the server will KEEP these connections for a long time, resending the SYN+ACK packets. Some servers will not accept new connections while there are enough connections currently forming; this is why SYN flooding works. All packets transmitted in either direction after the three-way handshake will have the ACK bit set. Stateless packet filters make use of this in the so called ``established'' filters: They will only let packets through that have the ACK bit set. This way, no packet may pass through in a certain direction that could form a new connection. Typically, you don't allow outside hosts to open new connections to inside hosts by requiring the ACK bit set on these packets. When the time has come to close the connection, there are two ways of doing it: Using the FIN flag, or using the RST flag. Using FIN flags, both implementations are required to send out FIN flags to indicate that they want to close the connection, and then send out acknowledgements to these FINs, indicating that they understood that the other end wants to close the connection. When sending out RST's, the connection is closed forcefully, and you don't really get an indication of whether the other end understood your reset order, or that it has in fact received all data that you sent to it. The FIN way of closing the connection also exposes you to a denial-of-service situation, since the TCP stack needs to remember the closed connection for a fairly long time, in case the other end hasn't received one of the FIN packets. If sufficiently many connections are opened and closed, you may end up having ``closed'' connections in all your connection slots. This way, you wouldn't be able to dynamically allocate more connections, seeing that they're all used. Different OSes handle this situation differently. References 1 Steven M. Bellovin. Firewall-friendly FTP. RFC 1579. 2 R. Finlayson. Ip multicast and firewalls. RFC 2588, May 1999. 3 Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear. Address allocation for private internets. RFC 1918, February 1996. 4 R. Thayer, N. Doraswamy, and R. Glenn. IP Security Document Roadmap. RFC 2411, November 1998.