Using a firewall is as much about making intelligent policy decisions as it is about learning the syntax. Firewalls like
iptables are designed to enforce policies by interpreting rules set by the administrator. However, as an administrator, you need to know what types of rules make sense for your infrastructure.
While other guides focus on the commands needed to get up and running, in this guide, we will discuss some of the decisions you will have to make when implementing a firewall. These choices will affect how your firewall behaves, how locked down your server is, and how it will respond to various conditions that occur. We will be using
iptables as a specific example, but most of the concepts will be broadly applicable.
When constructing a firewall, one of the most important decisions to make is the default policy. This determines what happens when traffic is not matched by any other rules. By default, a firewall can either
ACCEPT any traffic unmatched by previous rules, or
DROP that traffic.
A default policy of
ACCEPT means that any unmatched traffic is allowed to enter the server. This is generally not recommended, because it means that you would need to work backwards from there, blocking all unwanted traffic. Blocklist-type approaches are difficult to manage, because you’d need to anticipate and block every type of unwanted traffic. This can lead to maintenance headaches and is generally prone to mistakes, misconfigurations, and unanticipated holes in the established policy.
The alternative is a default policy of
DROP. This means that any traffic not matched by an explicit rule will not be allowed. Each and every service must be explicitly allowed, which might seem like a significant amount of up-front configuration. However, this means that your policy tends towards security and that you know exactly what is permitted to receive traffic on your server. Also, nearly all preconfigured policies will follow this approach, meaning that you can build on existing defaults.
The choice of a default drop policy leads to another subtle decision. With
iptables and other similar firewalls, the default policy can be set using the built-in policy functionality of the firewall, or implemented by adding a catch-all drop rule at the end of the list of rules.
The distinction between these two methods comes down to what happens if the firewall rules are flushed.
If your firewall’s built-in policy function is set to
DROP and your firewall rules are ever flushed (reset), or if certain matching rules are removed, your services will instantly become inaccessible remotely. This is often a good idea when setting policy for non-critical services so that your server is not exposed to malicious traffic if the rules are removed.
The downside to this approach is that your services will be completely unavailable to your clients until you re-establish permissive rules. You could even potentially lock yourself out of the server if you do not have local or web-based remote access as an alternative.
The alternative to setting a drop policy using the built-in policy functionality is to set your firewall’s default policy to
ACCEPT and then implement a
DROP policy with regular rules. You can add a normal firewall rule at the end of your chain that matches and denies all remaining unmatched traffic.
In this case, if your firewall rules are flushed, your services will be accessible but unprotected. Depending on your options for local or alternative access, this might be a necessary evil to ensure that you can re-enter your server if the rules are flushed. If you decide to use this option, ensure that the catch-all rule always remains the last rule in your rule set.
There are a few different ways of preventing a packet from reaching its intended destination. The choice between these has an impact on how the client perceives its connection attempt and how quickly they are able to determine that their request will not be served.
The first way that packets can be denied is with
DROP. Drop can be used as a default policy or as a target for match rules. When a packet is dropped,
iptables just throws it away. It sends no response back to the client trying to connect and does not give any indication that it has ever even received the packets in question. This means that clients (legitimate or not) will not receive any confirmation of the receipt of their packets.
For TCP connection attempts (such as connections made by a web browser), the connection will stall until the timeout limit has been reached. The lack of response for UDP clients is even more ambiguous. In fact, not receiving a UDP packet back is often an indication that the packet was accepted. If the UDP client cares about receipt of its packets, it will have to resend them to try to determine whether they were accepted, lost in transit, or dropped. This can increase the amount of time that a malicious actor will have to spend to get information about the state of your server ports, but it could also cause problems with legitimate traffic.
An alternative to dropping traffic is to explicitly reject packets that you do not allow. ICMP, or Internet Control Message Protocol, is a meta-protocol used throughout the internet to send status, diagnostic, and error messages between hosts as an out-of-band channel that does not rely on conventional communication protocols like TCP or UDP. When you use the
REJECT target instead of the
DROP target, the traffic is denied and an ICMP packet is returned to the sender to inform them that their traffic was received but will not be accepted. A status message can also be included to provide a reason.
This has a number of consequences. Assuming that ICMP traffic is allowed to reach the client, they will immediately be informed that their traffic is blocked. For legitimate clients, this means that they can contact the administrator or check their connection options to ensure that they are reaching out to the correct port. For malicious users, this means that they can complete their scans and map out the open, closed, and filtered ports in a shorter period of time.
There is a lot to consider when deciding whether to drop or reject traffic. One important consideration is that most malicious traffic will actually be perpetrated by automated scripts. Since these scripts are typically not supervised, dropping illegitimate traffic will not meaningfully discourage them, and will have negative effects for legitimate users. More on this subject can be found on Peter Benie’s website.
The table below shows how a server protected by a firewall will react to different requests depending on the policy being applied to the destination port.
|Client Packet Type
|Inferred Port State
|nmap [-sT | -sS] -Pn <server>
|nmap [-sT | -sS] -Pn <server>
|nmap [-sT | -sS] -Pn <server>
|nmap -sU -Pn <server>
|Open or Filtered
|nmap -sU -Pn <server>
|Open or Filtered
|nmap -sU -Pn <server>
|ICMP Port Unreachable
The first column indicates the packet type sent by the client. The second column contains the
nmap commands that can be used to test each scenario. The third column indicates the port policy being applied to the port. The fourth column is the response the server will send back and the fifth column is what the client can infer about the port based on the response it has received.
As with deciding whether to drop or reject denied traffic, you have the option to accept or reject ICMP packets destined for your server.
ICMP is a protocol used for many things. As mentioned, it is often sent back to give status information about requests using other protocols. One of its most popular functions is to send and respond to network pings to verify connectability to remote hosts. There are many other uses for ICMP that are not as widely known, but still useful.
ICMP packets are organized by “type” and then further by “code”. A type specifies the general meaning of the message. For instance, Type 3 means that the destination was unreachable. A code is often used to give further information about a type. For example, ICMP Type 3 Code 3 means that the destination port was unavailable, while ICMP Type 3 Code 0 means that the destination network could not be reached.
Some ICMP types are deprecated, so they can be blocked unconditionally. Among these are ICMP source quench (type 4 code 0) and alternate host (type 6). Types 1, 2, 7 and type 15 and above are all deprecated, reserved for future use, or experimental. Many upstream firewall templates will handle this by default.
Some ICMP types are useful in certain network configurations, but should be blocked in others.
For instance, ICMP redirect messages (type 5) can be useful to illuminate bad network design. An ICMP redirect is sent when a better route is directly available to the client. So if a router receives a packet that will have to be routed to another host on the same network, it sends an ICMP redirect message to tell the client to send the packets through the other host in the future.
This is useful if you trust your local network and want to spot inefficiencies in your routing tables during initial configuration. On an untrusted network, a malicious user could potentially send ICMP redirects to manipulate the routing tables on hosts.
Other ICMP types that are useful in some networks and potentially harmful in others are ICMP router advertisement (type 9) and router solicitation (type 10) packets. Router advertisement and solicitation packets are used as part of IRDP (ICMP Internet Router Discovery Protocol), a system that allows hosts, upon booting up or joining a network, to dynamically discover available routers.
In most cases, it is better for a host to have static routes configured for the gateways it will use. These packets should be accepted in the same situations as the ICMP redirect packets. In fact, since the host will not know the preferred route for traffic of any discovered routes, redirect messages are often needed directly after discovery. If you are not running a service that sends router solicitation packets or modifies your routes based on advertisement packets (like
rdisc), you can safely block these packets.
ICMP types that are usually safe to allow are below, but you may want to disable them if you want to be extra careful.
The types below can usually be allowed without explicit rules by configuring your firewall to allow responses to requests it has made (by using the
conntrack module to allow
Blocking all incoming ICMP traffic is still recommended by some security experts, however many people now encourage intelligent ICMP acceptance policies. These two Stackexchange threads have more information.
For some services and traffic patterns, you may want to allow access only as long as the client is not abusing that access. Two ways of constraining resource usage are connection limiting and rate limiting.
Connection limiting can be implemented using extensions like
connlimit to check how many active connections a client has open. This can be used to restrict the number of connections allowed at one time. If you decide to impose connection limits, you will have some decisions to make:
Connections can be limited on a host-by-host basis, or a limit can be set for a network segment by supplying a network prefix (such as an IP address range for an entire organization). You can also set a global maximum number of connections for a service or the entire machine. Keep in mind that it is possible to mix and match these to create more complex policies to control your connection numbers.
Rate limiting allows you to construct rules that govern the rate or frequency at which traffic will be accepted by your server. There are a number of different firewall extensions that can be used for rate limiting including
recent. The choice of the extension you use will depend largely on the way that you want to limit traffic.
limit extension will cause the rule in question to be matched until the limit is hit, after which further packets are dropped. A limit like
5/sec will allow 5 packets to match per second, after which the rule no longer matches. This is good for setting a global rate-limit for a service. You can also deploy an additional service like Fail2ban to block repeated connection attempts.
hashlimit extension is more flexible, allowing you to specify some of the values that
iptables will hash to evaluate a match. For instance, it can look at the source address, source port, destination address, destination port, or a combination of those four values to evaluate each entry. It can limit by packets or by bytes received. This provides flexible per-client or per-service rate limiting.
recent extension dynamically adds client IP addresses to a list or checks against an existing list when the rule matches. This allows you to spread your limiting logic across a number of different rules for complex patterns. It has the ability to specify a hit count and a time range like the other limiters, but can also reset the time range if additional traffic is seen, forcing a client to stop all traffic if they are being limited.
nftables firewall policy is essentially rooted in extending the built-in chains. For a start, this usually means changing the default policy for the existing chains and adding rules. For more complex firewalls, it is often a good idea to extend the management framework by creating additional chains.
User-created chains are called secondarily, and inherently tied to their “calling chain,” that they originate from. User-created chains have no default policy, so if a packet falls through a user-created chain, it will return to the calling chain and continue evaluation. With that in mind, user-created chains are mainly useful for organizational purposes, to make rule matching conditions more maintainable, and to improve readability by splitting match conditions.
If you find yourself repeating certain match criteria for a significant number of rules, it might be worthwhile to create a jump rule with the shared match criteria to a new chain. Inside the new chain, you can add that set of rules with the redundant matching criteria omitted.
The decision as to whether to lump all of your rules into one of the built-in chains or whether to create and utilize additional chains will depend on how complex your rule set is.
You should now have a better understanding of the decisions you’ll have to make when designing firewall policies for your servers. Usually the time investment involved with firewalls skews heavily towards the initial setup. While it may take some time and experimentation to come up with a policy that best serves your needs, doing so will give you more control over the security of your server.
If you would like to know more about firewalls and
iptables specifically, check out the following articles:
The following guides can help you implement your policies. Choose the guide that matches your firewall to get started:
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