For production environments, community strings should be chosen with caution and should consist of a series of alphabetical, numerical, and nonalphanumeric symbols.
Refer to Use a Strong Password for more information on the selection of nontrivial passwords. MIBs are either standard or enterprise specific. The firewall can support a variety of MIBs. Cisco ASA version 8. A recommended minimum list of MIBs and traps to monitor that focus on device health, resources, and normal operation follows:. SNMPv3 provides secure access to devices by authenticating and optionally encrypting packets over the network. SNMPv3 consists of three primary configuration options:.
The local-engine and remote-engine IDs are not configurable. There is no support for SNMP views. If needed, SNMP users and groups should also be removed in the correct order. Note that snmp-server user configuration commands are not displayed in the configuration output of the device as required by RFC ; therefore, the user password is not viewable from the configuration.
The show snmp user command in the following example allows administrators to view the configured users:. Event logging provides visibility into the operation of a Cisco ASA device and the network where it is deployed. Cisco ASA Software provides several flexible logging options that can help achieve an organization's network management and visibility goals.
These sections provide some basic logging best practices that can help an administrator use logging successfully while minimizing the impact of logging on a Cisco ASA device. Sending logging information to a remote syslog server allows administrators to correlate and audit network and security events across network devices more effectively.
Note that, by default, syslog messages are transmitted unreliably by UDP and in clear text. For this reason, any protections that a network provides for management traffic for example, encryption or out-of-band access should be applied to syslog traffic as well.
The following configuration example configures a Cisco ASA device to send logging information to a remote syslog server:. It offers proactive diagnostics and real-time alerts on the Cisco ASA and provides higher network availability and increased operational efficiency.
SCH can also collect syslogs to the central portal page hosted on Cisco's servers. Note that SCH does not serve as a syslog collecting service because certain limitations apply. However, it can collect syslogs at higher levels warning or error , and under certain conditions it can proactively open service requests and notify the administrators. Each log message that is generated by a Cisco ASA device is assigned one of eight severity levels that range from level 0, emergency, through level 7, debugging.
Unless specifically required, it is advisable to avoid logging at level 7. This level produces an elevated CPU load on the device that can lead to device and network instability.
The global configuration command logging trap level is used to specify which logging messages are sent to remote syslog servers. The specified level indicates the lowest severity message that is sent. For buffered logging, the logging buffered level command is used. The following configuration example limits log messages that are sent to remote syslog servers and the local log buffer to levels 0 emergency through 6 information :.
Refer to Configuring Logging for more information. Monitor sessions are interactive management sessions in which the EXEC command terminal monitor has been issued.
Instead, administrators are advised to send logging information to the local log buffer, which can be viewed using the show logging command. Use the global configuration commands no logging console and no logging monitor to disable logging to the console sessions and terminal lines. The following configuration example shows the use of these commands:.
Refer to Configuring Logging for more information about global configuration commands. Cisco ASA software supports the use of a local log buffer so that an administrator can view locally generated log messages. The use of buffered logging is highly recommended versus logging to either the console or monitor sessions. There are two configuration options that are relevant when configuring buffered logging: the logging buffer size and the message severities that are stored in the buffer.
The size of the logging buffer is configured with the global configuration command logging buffer-size. The lowest severity included in the buffer is configured using the logging buffered command. An administrator is able to view the contents of the logging buffer through the show logging EXEC command. The following configuration example includes the configuration of a logging buffer of 16, bytes and a severity of 6, information, indicating that messages at levels 0 emergency through 6 information are stored:.
The configuration of logging time stamps helps administrators and engineers correlate events across network devices. It is important to implement a correct and consistent logging time stamp configuration to enable correlation of logging data.
Logging time stamps should be configured to include the date and time with millisecond precision and to include the time zone in use on the device. The following example includes the configuration of logging time stamps with millisecond precision:. Administrators are encouraged to follow standard configuration management and logging procedures that will enable configuration rollback, configuration restoration, or misconfiguration tracking.
AAA accounting can be used to track configuration changes on a firewall. In addition, if the firewall is managed through an external management tool, it should be able to provide configuration management logs. The Cisco Security Manager platform manages firewall devices and can provide change management and configuration change logging functionality. The configuration archive can then be used to replace or roll back the current running configuration.
Note : This link requires login because the Smart Call Home feature is a registered service. Control plane functions consist of the protocols and processes that communicate between network devices to move data from source to destination. It is important that events in the management and data planes do not adversely affect the control plane. If a data plane event such as a DoS attack impacts the control plane, the entire network can become unstable.
The information that follows provides features and configurations that can help ensure the resilience of the control plane. Protection of the control plane of a network device is critical because the control plane ensures that the management and data planes are maintained and operational.
If the control plane becomes unstable during a security incident, it may not be possible for administrators and engineers to recover the stability of the network. Because of the secure nature and operations of Cisco firewall platforms, the platforms do not support ICMP redirects.
Filtering with an interface access list elicits the transmission of ICMP unreachable messages back to the source of the filtered traffic. Generating these messages can increase CPU utilization on the device. Cisco firewalls can be configured to elicit or suppress ICMP unreachable messages.
ICMP unreachables should be filtered to allow only known sources, for example those from management subnets. The following example illustrates filtering ICMP unreachable messages to permit only messages to known sources:. ICMP unreachable rate limiting can be changed from the default using the icmp unreachable rate-limit rate burst-size size global configuration command. ICMP responses are often used for troubleshooting and monitoring services.
Because of the secure nature and operations of Cisco firewall platforms, ICMP responses from the firewall should be limited by filtering traffic to permit only what is necessary or expected.
ICMP responses can also be limited by disabling ICMP responses on interfaces, specifically the outside or "untrusted" interface s at a minimum. The following command syntax limits ICMP responses on interfaces:.
To enhance security, routing updates may be authenticated using a simple password or keys depending on the routing protocol being used.
Use routing protocol authentication to prevent spoofing and routing attacks on firewalls. To enable authentication of EIGRP packets and specify the authentication key leveraging MD5 , use the authentication mode eigrp and authentication key eigrp commands as follows:. To enable authentication of Routing Information Protocol RIP version 2 packets and specify the authentication key, use the rip authentication mode and rip authentication key commands as follows:.
Note: By default "text" authentication is used. We recommend the use of "MD5. To enable authentication of OSPF packets and specify the authentication key, use the ospf authentication and ospf authentication-key commands as follows:.
Note: MD5 is the recommended configuration for ospf authentication,! The firewall data plane handles most of the traff i c that traverses the firewall. Data plane protection can prevent attacks for both the firewall and devices to which the firewall sends traffic. Securing the control plane and management plane is essential, but all control plane and data plane traffic traverses through the data plane. Because the data plane is responsible for processing and forwarding traffic, protecting the firewall data plane plays an important part in firewall hardening and security.
Any activated firewall feature may affect data plane traffic, so it is important to keep the firewall software version updated to the latest stable code that meets business requirements. It is also important to back up all firewall rulebase and configuration files regularly on a separate, accessible location. Backups can be used after a system failure and helps reduce total downtime. The Adaptive Security Algorithm ensures the secure use of applications and services. Some applications require special handling in the Adaptive Security Algorithm firewall application inspection function.
These applications embed IP addressing information in the user data packet or open secondary channels on dynamically assigned ports. A host on one firewall interface can create any type of connection to a host on another interface of the same firewall as long as any required address translation can be made and relevant interface access lists permit it. When address translation methods are required and after they have been configured between pairs of firewall interfaces, the administrator must configure and apply access lists to the interfaces.
The steps required for placing an ACL on the firewall include configuring the ACL and binding it to a firewall interface. Any source and destination address specified in the ACL is relative to any address translation that occurs on the interface where the ACL is applied.
ACEs can classify packets by inspecting Layer 2 through Layer 4 headers for a number of parameters, including the following:.
After an ACL has been properly configured, the administrator can apply it to an interface to filter traffic. The security appliance can filter packets in both the inbound and outbound direction on an interface. An ACL must be applied to each lower-security interface so that specific inbound connections are permitted.
For information about security levels, refer to the Security Levels section of this document. Once the packet is allowed, the flow is created in the Adaptive Security Algorithm connection table, and all further packets in the flow are permitted based on the connection entry, bypassing the ACL check. You can use the show conn command to view the connection table. Note: ACLs are normally evaluated in the order in which they appear in the firewall configuration.
It is important to configure and use an ACL to limit the types of traffic in a specific direction. When traffic is permitted by an ACL, connections are allowed to pass; when traffic is denied, all corresponding packets are dropped at the firewall. In addition, when an xlate entry is created for a new connection and the interface ACLs permit the initial traffic, the return traffic specific to that connection is also permitted because the firewall has built the proper xlate and conn entries for it.
Therefore, ACL changes should be made when traffic through the firewall is low. This section lists some best practices to be followed for ACL configuration on firewalls. However, the list is not exhaustive and should serve as a guideline for firewall hardening. To control access to an interface, use the access-group command in interface configuration mode.
This rule determines whether there any ACLs are defined that are not applied to an interface. The permit ip any any command is not recommended. Allowing access to all destinations provides access to all the hosts inside the perimeter, including the firewall itself, and to all Internet hosts.
Traffic should be carefully filtered to meet the organization's requirements. The permit icmp any any command is also not recommended. It is not secure to permit all ICMP traffic on firewalls, which would allow an attacker to exploit the network using ICMP attacks such as ping sweeps and ping floods. Without stateful inspection, ICMP can be used to attack your network.
The ICMP inspection engine ensures that there is only one response for each request, and that the sequence number is correct. The best practice is to use ACLs to limit as much traffic as possible.
Administrators are advised to create exact matches of host and network addresses rather than using the generic keyword any in access lists. Specifying the exact port numbers is recommended rather than opening all ports by not specifying anything in the ports field. Increased granularity increases security and also makes it easier to troubleshoot any malicious behavior.
It is a best practice to have an explicit deny statement at the end and log all the denied packets. The log keyword at the end of the individual ACL entries shows the ACL number and whether the packet was permitted or denied in addition to port-specific information.
By default, logging message default severity level 4, warning is generated when a deny access list entry is matched with a traffic flow. One can also log the rate at which traffic flows match specific access list entries. This can be useful to gauge the volume of attacks or exploits that are occurring over time. One can also set the logging severity level on a per-ACE basis if needed. Otherwise, severity level 6 is the default.
Note: Although all ACLs contain an implicit deny statement, Cisco recommends use of an explicit deny statement, for example, deny ip any any. On most platforms, such statements maintain a count of the number of denied packets. This count can be displayed using the show access-list command. The ability to configure security levels is a necessary firewall feature.
A security-level value from 0 through defines the trustworthiness of networks reachable through an interface. A value of 0 indicates the least trusted, and a value of indicates the most trusted. Administrators are advised to correctly configure security levels for traffic traversal before ACLs are applied.
The following are the key points:. For more details regarding security levels, see the Security Levels section of the Cisco Series Configuration Guide. Based on an organization's security policy, the security appliance can either pass or drop the packets if they contain content not allowed in the network. Cisco firewalls support two types of application layer filtering: content filtering and URL filtering. Cisco firewalls can differentiate friendly applets from untrusted applets.
If a trusted website sends Java or ActiveX applets, the security appliance can forward them to the host requesting the connection. If the applets are sent from untrusted web servers, the security appliance can modify the content and remove the applets from the packets. This way, end users are not making decisions regarding which applet to accept or refuse.
They can download any applets without taking extra precautions. The security appliance searches for these tags for traffic that originated on a preconfigured port. A local content filtering server can be set up on the security appliance by using the filter command, followed by the name of the type of content to be removed.
The following shows the complete command syntax:. Cisco firewalls can delegate packet-filtering responsibilities to an external server. Administrators can define an external filtering server by using the url-server command. For example, the complete command syntax to specify a Websense server is:.
Note: Users may experience longer access times if the response from the filtering server is slow or delayed. This may happen if the filtering server is located at a remote location and the WAN link is slow.
In addition, slow response times may also result if the URL server cannot keep up with the number of requests being sent to it. The url-server command does not verify whether a Websense or SmartFilter server is reachable from the security appliance.
You can specify up to 16 filtering servers for redundancy. If the security appliance is not able to reach the first server in the list, it tries the second server from the list, and so on. One must be deleted before the other is set up. Firewall software offers an adaptable and scalable modular policy framework.
For traffic flows traversing the firewall, flow-based policies can be established for any administratively defined criteria and then applied to a set of security services, such as firewall policies, inspection engine policies, quality of service QoS policies, and VPN policies, with each specified traffic flow providing more granular and flexible inspection control.
IP spoofing occurs when a potential intruder copies or falsifies a trusted source IP address. This is typically employed as an auxiliary technique for countless types of network-based attacks. Cisco firewalls contain several features to enhance the ability of the network to defend itself. Antispoofing is one such feature, which helps to protect an interface of the ASA by verifying that the source of network traffic is valid. This section discusses some antispoofing features. This security feature works by enabling a router to verify the reachability of the source address in packets being forwarded.
This capability can limit the appearance of spoofed addresses on a network. If the source IP address is not valid, the packet is discarded. Normally, the security appliance examines only the destination address when determining where to forward the packet. For any traffic to be allowed through the security appliance, the security appliance routing table must include a route back to the source address. See RFC for more information. To enable uRPF, enter this command:. When administrators use uRPF in strict mode, the packet must be received on the interface that the security device would use to forward the return packet.
Dropping this legitimate traffic could occur when asymmetric routing paths exist in the network. When administrators use uRPF in loose mode, the source address must appear in the routing table. Administrators can change this behavior using the allow-default option, which allows the use of the default route in the source verification process.
In addition, a packet that contains a source address for which the return route points to the Null 0 interface will be dropped. An access list may also be specified that permits or denies certain source addresses in uRPF loose mode.
Care must be taken to ensure that the appropriate uRPF mode loose or strict is configured during the deployment of this feature because it can drop legitimate traffic. Although asymmetric traffic flows may be a concern when deploying this feature, uRPF loose mode is a scalable option for networks that contain asymmetric routing paths. This RFC is a widespread resource, particularly for the Internet edge, because in such an environment the boundary between private and public addresses in the sense of RFC is clearly demarcated.
It is usually appropriate for an antispoofing access list to filter out all ICMP redirects regardless of source or destination address. These are just basic guidelines and can be further fine tuned with other filtering such as anti-bogon, which filters traffic that claims to be sourced from reserved addresses or from an IPv4 block that has yet to be allocated by the Internet Assigned Numbers Authority IANA.
In general, antispoofing filters are best deployed as input access lists; that is, packets must be filtered at the arriving interfaces, not at the interfaces through which they exit. The input access list also protects the firewall itself from spoofing attacks, whereas an output list protects only devices behind the firewall.
Through the stateful application inspection used by the Adaptive Security Algorithm, the Cisco ASA tracks each connection that traverses the firewall and ensures that it is valid. The firewall, through stateful inspection, also monitors the state of the connection to compile information to place in a state table. With the use of the state table in addition to administrator-defined rules, filtering decisions are based on context that is established by packets previously passed through the firewall.
The implementation of application inspections consists of these actions:. By default, the configuration includes a policy that matches all default application inspection traffic and applies certain inspections to the traffic on all interfaces a global policy. Not all inspections are enabled by default. Only one global policy can be applied. If it is necessary to alter the global policy, one must either edit the default policy or disable it and apply a new one.
An interface policy overrides the global policy. The default policy configuration includes these commands:. To disable global inspection for an application, use the no version of the inspect command. Enhanced HTTP inspection is disabled by default.
To enable HTTP application inspection or change the ports on which the security appliance listens, use the inspect http command in class configuration mode. Class configuration mode is accessible from policy map configuration mode. Hollie's Hub for Good Supporting each other to make an impact. Write for DigitalOcean You get paid, we donate to tech non-profits. As part of your setup and deployment process, it is important to include building in robust and thorough security measures for your systems and applications before they are publicly available.
Implementing the security measures in this tutorial before you deploy your applications will ensure that any software that you run on your infrastructure has a secure base configuration, as opposed to ad-hoc measures that may be implemented post-deploy.
This guide highlights some practical security measures that you can take while you are configuring and setting up your server infrastructure. This list is not an exhaustive list of everything that you can do to secure your servers, but this offers you a starting point that you can build upon. Over time you can develop a more tailored security approach that suits the specific needs of your environments and applications.
SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. A more secure alternative to password-based logins, SSH keys use encryption to provide a secure way of logging into your server and are recommended for all users. With SSH keys, a private and public key pair are created for the purpose of authentication. The private key is kept secret and secure by the user, while the public key can be shared. When your client first connects to the server, the server will ask for proof that you have the associated private key.
It does this by generating a random value and sending it to your SSH client. Your SSH client will then use your private key to encrypt the response and then send the encrypted reply to the server.
If the server can decrypt the random value, then it means that your client possesses the private key andthe server will let you connect without a password.
With SSH, any kind of authentication — including password authentication — is completely encrypted. However, when password-based logins are allowed, malicious users can repeatedly attempt to access a server, especially if it has a public-facing IP address.
With modern computing power, it is possible to gain entry to a server by automating these attempts and trying combination after combination until the right password is found. Setting up SSH key authentication allows you to disable password-based authentication. SSH keys generally have many more bits of data than a password, meaning that there are significantly more possible combinations that an attacker would have to run through.
Many SSH key algorithms are considered uncrackable by modern computing hardware because they would require too much time to run through all of the feasible matches. SSH keys are the recommended way to log into any Linux server environment remotely.
A pair of SSH keys can be generated on your local machine and you can transfer the public key to your servers within a few minutes. If you would still like password authentication, consider implementing a solution like fail2ban on your servers to limit password guesses.
In either case, it is a best practice to not allow the root user to login directly over SSH. Instead, login as an unprivileged user and then escalate privileges as needed using a tool like sudo. This approach to limiting permissions is known as the principle of least privilege. A firewall is a software or hardware device that controls how services are exposed to the network, and what types of traffic are allowed in and out of a given server or servers.
A properly configured firewall will ensure that only services that should be publicly available can be reached from outside your servers or network. On a typical server, a number of services may be running by default. These can be categorized into the following groups:. Firewalls can ensure that access to your software is restricted according to the categories above with varying degrees of granularity. Public services can be left open and available to the internet, and private services can be restricted based on different criteria, such as connection types.
Internal services can be made completely inaccessible to the internet. For ports that are not being used, access is blocked entirely in most configurations. A properly configured firewall will restrict access to everything except the specific services you need to remain open.
Exposing only a few pieces of software reduces the attack surface of your server, limiting the components that are vulnerable to exploitation. There are many firewalls available for Linux systems, some are more complex than others. Here are some options to get up and running:. If you would like to learn how to use Iptables, our Iptables Essentials: Common Firewall Rules and Commands tutorial demonstrates how to use Iptables directly.
With any of the tutorials mentioned here, be sure that your firewall configuration defaults to blocking unknown traffic. That way any new services that you deploy will not be inadvertently exposed to the Internet.
Instead you will have to allow access explicitly, which will force you to evaluate how the service is run, accessed, and who should be able to use it. Using private instead of public networking for internal communication is preferable given the choice between the two, as VPC networks allow you to isolate groups of resources into specific private networks. VPC networks will only connect to each other using their private network interfaces over an internal network, which means that the traffic among your systems will not be routed through the public internet where it could be exposed or intercepted.
VPC networks can also be used to isolate execution environments and tenants. Many cloud infrastructure providers enable you to create and add resources to a VPC network inside their data centers. DigitalOcean places each applicable resource Droplets, load balancers, Kubernetes Clusters, and databases into a VPC upon creation at no additional cost.
Manually configuring your own private network can require advanced server configurations and networking knowledge. A big portion of security involves analyzing our systems, understanding the available attack surfaces, and locking down the components as best as we can. Service auditing is a way of knowing what services are running on a given system, which ports they are using for communication, and what protocols are accepted.
This information can help you configure which services should be publicly accessible, firewall settings, and monitoring and alerting. Servers can run processes for internal purposes and to handle external clients. Each running service, whether it is intended to be internal or public, represents an expanded attack surface for malicious users. The more services that you have running, the greater the chance of a vulnerability affecting your software. Once you have a good idea of what network services are running on your machine, you can begin to analyze these services.
When you perform a service audit, ask yourself the following questions about each running service:. This type of service audit should be standard practice when configuring any new server in your infrastructure. Performing service audits every few months will also help you catch any services with configurations that may have changed unintentionally. To audit network services that are running on your system, use the ss command to list all the TCP and UDP ports that are in use on a server.
The Windows Firewall will block the trusted programs to access the Internet. But now, it is easy for you to allow a program or feature through Windows Firewall not clickable. This post will show you how to allow a program through Firewall Windows Besides, you can also use MiniTool software to keep computer safe. Windows Firewall is a Windows built-in application that filters information coming to your system from the Internet and blocking potentially harmful programs.
It was called Internet Connection Firewall in the past. With the release of Windows 10 version , it was renamed as Windows Defender Firewall. And what does roll the Windows Defender play in? As a matter of fact, the Windows Firewall can prevent some programs on your computer from access the Internet. If a program is suspicious, the Windows Firewall will not allow it to access the Internet.
In addition, you may get a notification during the installation process, which asks whether the application should be added as an exception to Windows Firewall or not.
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