Introduction

ASL has two different ways you can configure the firewall on your system:

1) Fast/Easy Mode

This configuration system should be the one most new users are familiar with. This system lets you quickly define a list of ports to allow or deny in a list. For most uses the Fast/Easy mode is what you will want to use.

2) Advanced Firewall Rule Manager

This is a full featured netfilter GUI. This will allow you to define arbitrary chains, actions, modules and other advanced iptables features. This is recommended for advanced users.

How the firewall works

The ASL stateful packet inspection firewall works much like other firewalls. It inserts rules into the Linux kernel which govern how the kernel will handle network traffic. All firewalls work in a "first match wins" manner. That means rules are checked in order, and the first rule to apply "wins". For example, if you have two firewall rules in this order:

Allow traffic on port 80
Block all traffic

Lets say you have a connection comes in on port 80. The firewall will run through the rules in the order you configure. In the example above, the first rule says to allow traffic on port 80. This connection is to port 80, so the rule will match and the traffic will be allowed and processing will stop. Theres nothing else for the firewall to do, its allowed the traffic so its done.

However, lets say in the example above if a packet comes in on port 23. The firewall will run through the rules again and this time rule 1 will not match. This is a request to access port 23, not port 80. So processing will continue. Moving on to rule 2, this rule says to block all traffic. So, this rule will match, because all traffic means all traffic, including this request to port 23. Therefore this rule will match, the firewall will block the request, and processing will stop because theres nothing else to do, the connection was blocked.

Heres another important example. Lets say your firewall rules were in this order. This is a very common mistake made by those new to firewalls:

Block all traffic
Allow traffic on port 80

In this case, if a connection comes in on port 80 it will be blocked. Even though rule #2 says to allow it, it will be blocked. Lets review why.

This is because rule 1 says to block all traffic. When the request comes in, the firewall looks at its rule. Rule #1 says to block all traffic, so it blocks all traffic. And as the traffic has been blocked, theres nothing else to do. This is an important mistake, because even though there is a later rule that says to allow traffic on port 80, rule #2 will never match anything, because rule 1 matched first, blocked the traffic and processing stopped. Rules process in order, the first match always wins.

This is why firewall rules are called rules. They arent configuration options, they are strict rules and order always matter. If a request matches a rule, that rule applies as long as no other rule before it applies. Remember, rules are processed in order, 1, 2, 3, 4, etc. When a match occurs, action happens and if that action is to block or allow something, it doesnt matter what any later rules may be, the currently matching rule always "wins".

This is normal behaviour for all firewalls. All firewalls behave this way. Rules are processed in order. Therefore, the order of rules is the most important thing to how a firewall will behave. Make sure you understand the order of your rules to ensure that they are performing as you expect. Most firewall rule problems are due to order.

Stateful packet inspection

When used, the ASL firewall keeps track of the state of network connections. The ASL firewall is designed to distinguish legitimate packets for different types of connections, and to only allow packets matching a known active connection.

This behaviour can be disabled through the rule manager.

Console firewall messages

Note that if you get undesired logging of firewall rule matches to your console, this is not an ASL problem. ASL will not and can not configure your system to do this. If this is happening on your system, this is generally caused by a nonstandard syslogd or rsyslog configuration.

No known Linux distribution will configure itself to do this by default, so if your system is doing this its either because a third party product has configured your system to do this, or a third party, such as your hosting company or sysadmin, has altered the configuration of your system to do this. The following operating system command, run as root, will disable console messages until your next reboot:

dmesg -n 1

This will not prevent this from occuring the next time you reboot your system. Please contact the third party that has made this modification to your system for assistance, or contact sales and we can schedule some time with our professional services team to help you.

Fast/Easy Mode

To access the Fast and Easy firewall configuration mode:

1) Log into the ASL web console

This is on your server on port 30000.

2) Click on Settings Tab in the ASL GUI

3) then select the "ASL Configuration" menu option.

4) From there scroll down to the Firewall option on the left and click on that. Below are the settings you can configure and what they do:

Inbound/Outbound ports

ASL makes it easy to implement a fast global policy of network ports that your system will allow users to connect to, and ports that you will allow your users to connect to other servers, from your server.

FW_INBOUND_TCP_SERVICES

List of user configured allowed inbound TCP services, comma delimited. Just provide a list of the ports you want to allow in. Anything not on the list will be denied by default.

Default: ASL does not block any ports by default. By default all ports are open, ports are only closed if the user defines them via this setting.

Example: 22,21,25,53,80,443,465,110,143,993,995,587,8443,30000

Additional options:

no - Setting this to "no" will remove all blocks, and will allow all TCP ports to be accessed. Setting this to "" (blank) will block all inbound access to TCP ports.

(blank) - removing everything from this field and leaving it blank will all inbound access to TCP ports.

Ranges are supported with the : delimiter. Example of allowing range 1024-1030

22,21,25,53,80,443,465,110,143,993,995,587,8443,30000,1024:1030

If you use a control panel, some vendors provide a list of recommended ports you should have open for your control panel. The following links are to those vendors websites. Any questions about these recommended ports should be directed to the respective vendor.

Cpanel Ports

Webmin Ports

Parallels Plesk Ports

DirectAdmin Ports

If you want to restrict access to a port for specific IPs, please see this article:

https://www.atomicorp.com/wiki/index.php/ASL_firewall#Per_Port_ACLs

FW_INBOUND_UDP_SERVICES

Please note: This is for the Fast/Easy Mode method. You can find directions on accessing Fast/Easy Mode here: https://www.atomicorp.com/wiki-admin//index.php/ASL_firewall#Fast.2FEasy_Mode

List of user defined allowed inbound UDP services, comma delimited. Just provide a list of the ports you want to allow in. Anything not on the list will be denied by default.

Default: ASL does not block any ports by default. By default all ports are open, ports are only closed if the user defines them via this setting.

Example: 53,123

Setting this to "no" will remove all blocks, and will allow all UDP ports to be accessed. Setting this to "" (blank) will block all inbound access to UDP ports.

If you use a control panel, some vendors provide a list of recommended ports you should have open for your control panel. The following links are to those vendors websites. Any questions about these recommended ports should be directed to the respective vendor.

Cpanel Ports

Webmin Ports

Parallels Plesk Ports

DirectAdmin Ports

FW_OUTPUT_TCP_SERVICES

List of user defined allowed outbound TCP connections, in a comma delimited format. Everything else is blocked, and logged along with the uid for the process that attempted to connect outbound and was denied.

Default: ASL does not block any ports by default. By default all ports are open, ports are only closed if the user defines them via this setting.

To configure, just provide a list of the ports you want to allow out. Anything not on the list will be denied by default.

You do not need to add any rules for services that are listening. For example, if you allow port 80 in, you do not need to allow port 80 out. ASL uses a stateful firewall and will allow listening services to communicate out. This list is for client initiated requests outbound from the server. For example, if a user wanted to ssh from the server to another server, you would need to allow port 22 out. You do not need to allow port 22 out to allow connections to the ssh service running on the server.

Example: 25,80,443,465,123,53

Remember to allow out other services your system uses such as DNS (port 53). If you do not allow out DNS queries, your system will not be able to resolve any domain names, and things like yum updates, ASL updates, and outbound email will fail.

Setting this to "no" will remove all blocks, and will allow all outbound TCP ports to be accessed. Setting this to "" (blank) will block all outbound access to TCP ports.

FW_OUTPUT_UDP_SERVICES

List of user defined allowed outbound UDP connections, in a comma delimited format.

Default: ASL does not block any ports by default. By default all ports are open, ports are only closed if the user defines them via this setting.

Just provide a list of the ports you want to allow out. Anything not on the list will be denied by default. You do not need to add any rules for services that are listening. For example, if you allow port 53 in, you do not need to allow port 53 out. ASL uses a stateful firewall and will allow listening services to communicate out. This list is for client initiated requests outbound from the server. However, if you had a service that needed to open connections outbound on its own, for example a DNS server would open connections on its own to look up domain names, you would need to allow port 53 out. You do not need to allow port 53 out to allow connections to the dns service running on the server, only if the DNS server needed to make its own independent connections outbound.

Example: 53,123

Also, remember to allow out other services your system uses such as DNS (port 53). If you do not allow out DNS queries, your system will not be able to resolve any domain names, and things like yum updates, ASL updates, and outbound email will fail.

Setting this to "no" will remove all blocks, and will allow all outbound UDP ports to be accessed. Setting this to "" (blank) will block all outbound access to UDP ports.

Recommended ports

At a minimum, to use ASL you will need to ensure that port 30000/tcp is open inbound to your system.

The sections below provide a list of URLs to vendors that have documented additional ports they require to be open for their respective products to work correctly. If you have questions about what ports to open for these products, please contact the vendors for those products.

Engine settings

The Fast/Easy Mode also allows you to configure the core functionality of the netfilter engine. The following is a list of options you can set and what they do.

FW_WHITELIST

This allows you to apply the global whitelist to the firewall. This means any host on the global whitelist will be able to connect to any port, at any rate and as many times as the host wants.

Note: Whitelisted hosts are already automatically not shunned by ASL. There is no need to activate this to prevent shunning of whitelisted hosts. Enabling this option simply disables the firewall for whitelisted systems.

Default:[no]

Note: This option is available in ASL 4.x and up.

FW_WHITELIST_LOG

This logs if connections or packets are accepted if they are on the whitelist.

Default:[yes]

Note: This option is available in ASL 4.x and up.

FW_RATE_LIMIT

This allows you to limit the rate of connections to TCP and UDP ports, as well as the maximum number of connections a single IP can have to a port. This is configured in the /etc/asl/firewall/rate-limit-tcp file for TCP, and /etc/asl/firewall/rate-limit-udp for UDP.

Default:[no]

Notice: Please see the documentation on the Ratelimit page to configure this for your system.

Note: This option is available in ASL 4.x and up.

FW_RATE_LIMIT_LOG

This logs if connections or packets are dropped by the Rate Limit engine.

Default:[yes]

Notice: Please see the documentation on the Ratelimit page to configure this for your system.

Note: This option is available in ASL 4.x and up.

FW_OUTPUT_MTA

Outbound MTA policy, enabling this rule class will only allow outbound tcp/25 connections for users in the access list /etc/asl/firewall/mta-output-acl. This allows you to restrict outbound SMTP connections to specific users, such as the Mail Servers (MTA) user, which would require your users to use your local mail server for outbound connections. This allows you to filter all outbound email for spam, and prevents your users from setting up spam bots to send email out directly.

Simply put a list of the user names you wish to allow outbound SMTP connections from in the /etc/asl/firewall/mta-output-acl file. The format is one username per line.

In general, you will only want to use the username of the MTA, not your actually users. For example, if qmail-send ran as the user "qmail", you would add that. If postfix ran as the user "postfix", add that. If you had a user "jdoe" that sent mail through your mail server, you would not add jdoe to the /etc/asl/firewall/mta-output-acl file.

So the most important step is to determine what user(s) your mail server uses to send mail. The table below contains a list of the users typically used by the most popular mail servers. Please confirm that this is correct for your mail server configuration, as some vendors change these names:

Mail Server	Typical Usernames
Postfix	postfix
QMail	qmailr
Exim	mailnull
Sendmail	smmsp

Please check with your mail server vendor to confirm the username that the mail server will use to send mail.

FW_PLESK_UPDATES

Enable/Disable Plesk keyserver update firewall policy.

Default:[no]

FW_SPAMASSASSIN_UPDATES

Enable/Disable Spamassassin update firewall policy for support packages DCC, Razor, and Pyzor.

Default:[no]

FW_PORTKNOCK

Note: This option is available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

Enable/Disable the portknocking protection engine.

Default:[no]

FW_PORTKNOCK_SIMPLE

Enable/Disable the use of simple portknocking protection.

Default:[no]

Warning: Please read the portknocking documentation on this feature in full before using it. This will require the use of a special client or telnet sequence to open the port.

Note: This option is available in ASL 4.x and up.

FW_PORTKNOCK_SIMPLE_LOG_KNOCK

Enable/Disable logging of the knock packet.

Default:[yes]

Note: This option is available in ASL 4.x and up.

FW_PORTKNOCK_SIMPLE_LOG_ALLOWED

Enable/Disable logging of the allowed knock connection.

Default:[yes]

Note: This option is available in ASL 4.x and up.

FW_PORTKNOCK_ADVANCED

Enable/Disable the use of advanced Simple Packet Authentication (SPA) portknocking protection. This uses One Time Passwords, which limits the knock to one per minute (because of the epoch_min).

Default:[no]

Warning: Please read the portknocking documentation on this feature in full before using it. This will require the use of a special client or telnet sequence to open the port.

Note: This option is available in ASL 4.x and up.

FW_PORTKNOCK_ADVANCED_LOG_KNOCK

Enable/Disable logging of the knock packet.

Default:[yes]

Note: This option is available in ASL 4.x and up.

FW_PORTKNOCK_ADVANCED_LOG_ALLOWED

Enable/Disable logging of the allowed knock connection.

Default:[yes]

Note: This option is available in ASL 4.x and up.

FW_LASSO

Enable/Disable the Spamhaus Lasso blacklist. Enabling this option will block all hosts on the Spamhaus Lasso blacklist.

Default: No

Note: This is a third party list not operated by Atomicorp.

FW_LASSO_LOG

Enable/Disable the logging of any blocks from the Spamhaus Lasso blacklist.

Note: This requires that FW_LASSO be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

Default: Yes

FW_ELASSO

Enable/Disable the Spamhaus ELasso blacklist, an extension of the LASSO list, this includes subnet blocks allegedly controlled by cyber criminals (according to Spamhaus).

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_ELASSO_LOG

Enable/Disable the logging of Spamhaus Lasso Blacklist IPs that are blocked.

Default:[yes]

Note: This requires that FW_ELASSO be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

FW_CIARMY

Enable/Disable the C.I. Army blacklist. The Collective Intelligence blacklist is a community blacklist of known malicious actors. It is updated weekly.

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_CIARMY_LOG

Enable/Disable the logging of C.I. Army Blacklist IPs that are blocked. Default:[yes]

Note: This requires that FW_CIARMY be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

FW_EMERGING_THREATS

Enable/Disable the Emerging Threats blacklist.

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_EMERGING_THREATS_LOG

Enable/Disable logging of Emerging Threats RBN IPs that are blocked.

Note: This requires that FW_EMERGING_THREATS be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

Default:[yes]

FW_OPENBL

Enable/Disable the OpenBL blacklist. OpenBL tracks multiple service abuse, including ssh, ftp, smtp, and http.

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_OPENBL_LOG

Enable/Disable logging of IP's blocked by the OpenBL blacklist. Default:[yes]

Note: This requires that FW_OPENBL be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

FW_AUTOSHUN

Enable/Disable the autoshun.org blacklist. AutoShun is a community sourced blacklist from the Snort IDS.

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_AUTOSHUN_LOG

Enable/Disable logging of IP's blocked by the AutoShun blacklist.

Default:[yes]

Note: This requires that FW_AUTOSHUN be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

FW_OPENPROXIES

Enable/Disable the Open Proxy blacklist.

Default:[no]

Note: This is a third party list not operated by Atomicorp.

FW_OPENPROXIES_LOG

Enable/Disable logging of IP's blocked by the Open Proxy blacklist.

Default:[yes]

Note: This requires that FW_OPENPROXIES be enabled to actually block any IPs on the Spamhaus Lasso blacklist.

FW_DSHIELD

Enable/Disable the DShield blacklist. Enabling this option will block all hosts on the DShield blacklist.

Default: No

Note: This is a third party list not operated by Atomicorp.

FW_DSHIELD_LOG

Enable/Disable the logging of any blocks from the DShield blacklist.

Note: This requires that FW_DSHIELD be enabled to actually block any IPs on the DShield blacklist.

Default: Yes

FW_TOR

Enable/Disable the TOR exit node list. Enabling this option will block all current TOR exit nodes.

Default: No

Note: This is a third party list not operated by Atomicorp.

FW_TOR_LOG

Enable/Disable the logging of any blocks from FW_TOR, which blocks any IPs on the TOR exit node list.

Note: This requires that FW_TOR be enabled to actually block any IPs.

Default: Yes

FW_PORTSCAN

This will detect portscans against the system and will drop these packets. This will confuse the port scanner, and will cause it to not be able to identify open and closed ports on the system.

Default: yes

ASL 3 currently blocks these types of port scans:

SYN/FIN
SYN,RST
XMAS
NULL
FIN/URG/PSH
FIN/RST/PSH
ACK/PSH/URG RST scans
All flag scans

If you have ASL configured to only allow connections to specific ports, via the Fast Mode, then ASL will also disrupt portscans on closed ports.

Note: ASL 4 blocks the following additional types of port scans:

Syn scans
Additional stealth scans
"Connect" scans
"Grab" scans

These are enabled via the additional firewall options below.

FW_LOWLEVEL_PORTSCAN

These are available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

Enable/Disable detection of low level portscans.

Default:[no]

Note: This uses a different method from the other portscan detectors offered in ASL. You can use them all together, or separately. This method will detect the portscan after it has been carried out, but this information can be useful to block the remote host's future connections. This method will match on the probable last packet the remote side sent to the system.

FW_DROP_SYNSCAN

Enable/Disable detection of syn scans.

Default:[no]

Warning: Some clients with buggy IP stacks (e.g. Windows XP) may generate traffic thats is identical to a SYN scan and therefore cause false alarms with this type of scan detection, use with caution.

Note: This option is available in ASL 4.x and up.

FW_DROP_STEALTH_SCAN

Enable/Disable detection of stealthy port scans.

Default:[no]

Note: This option is available in ASL 4.x and up.

FW_DROP_CONNECT_SCAN

Enable/Disable detection of connect() port scans. This will detect, and block IPs that have made a full connect() to the system, but then tore down the connection after completion of the 3-way handshake. Most network applications already log these types of connections, so this type of port scan detection is generally not necessary at the kernel level.

Default:[no]

Note: This option is available in ASL 4.x and up.

Warning: Some network testing tools use connect() scans (connecting to the port, and then tearing down the connection without sending any data) to test if services are up. Enabling this may cause those tools to fail or to be blocked.

FW_DROP_GRAB_SCAN

Enable/Disable detection of grab scans. This detects when a client connects to port, gets a response and drops the connect. For example, banner grabbing scans.

This will trigger if data in the connection only was sent in one direction, where a connection was terminated after a locally running daemon sent its identification. (E.g. openssh, smtp, ftpd.)

Default:[no]

Warning: This may falsely trigger on warranted single-direction data flows, usually bulk data transfers such as FTP DATA connections or IRC DCC. ASL requires that you define a set of ports for this rule in the file /etc/asl/firewall/grscan-ports (This link includes an initial set of ports that you will need to test for your system). If you get false positives, that means the port you configured operates this way normally (sometimes the connection is just one way from the server to the client)

You must restart the firewall, if you change this list of ports, for any changes to take effect. Run this command as root:

service asl-firewall restart

Note: This option is available in ASL 4.x and up.

FW_ADVANCED_PORTSCAN

These are available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

Enable/Disable the advanced port scan detector.

Default:[no]

PSD_W_THRSH

Advanced Portscan detection weight threshold.

Default: 21

Warning: This is an advanced setting, do not change this setting unless you know what you are doing.

Note: This option is available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

PSD_DELAY

Portscan detection delay threshold

Default: 300

Warning: This is an advanced setting, do not change this setting unless you know what you are doing.

Note: This option is available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

PSD_LO

Privileged ports weight (1-1023).

Default: 3

Warning: This is an advanced setting, do not change this setting unless you know what you are doing.

Note: This option is available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

PSD_HI

High ports weight (1024-65535).

Default: 1

Warning: This is an advanced setting, do not change this setting unless you know what you are doing.

Note: This option is available in ASL 4.x and up, and only with the ASL kernel. These options are not available in non-ASL kernels.

FW_LOG_AR_DROP

Enable/Disable the logging of IPs that are blocked by the ASL active response system.

Default:[yes]

Note: This option is available in ASL 4.0 and up.

FW_BAD_PACKETS

This will detect and drop packets that use bad or invalid options.

Default: yes

FW_MSS_DROP

Note: This option is available in ASL 4.x and up.

This will detect and drop packets that have an invalid MSS.

Default: no

FW_MSS_DROP_LOG

Note: This option is available in ASL 4.x and up.

This will log if packets are dropped by the FW_MSS_DROP option.

Default: yes

FW_SMALL_PACKETS

This will detect packets that are too small to be valid. Each protocol has a minimum size, and this will detect if a packet is too small to be valid.

Default: yes

FW_FRAGMENTS

This will detect fragmented packets. ASL uses a stateful firewall and will automatically handle fragmented packets. This detects if someone is maliciously attempting to bypass the state engine. It is recommend you always have this enabled.

Default: yes

FW_DROP_INVALID

This will detect, and drop invalid packets. Invalid packets include "orphan" packets that do not belong to a stateful connection, as well as all packets with invalid TCP flags, headers or checksums, out of sequence packets which can be caused by sequence prediction or other similar attacks, drop invalid ICMP messages generated by other types of network attacks (Example: recieving a port unreachable ICMP message from a host that the system is not expecting any traffic from).

This setting is important to both stop attacks, and also to prevent false positive shuns under rule 4151. If your systems users connections generate a lot of orphaned packets, you will need to enable this so they are not treated as possible attacks. Some operating systems have poorly implemented network stacks, or bugs that will generate lots of packets that are not part of an established connection and this setting will prevent them from being treated as attacks. These poorly implemented stacks are not caused by ASL, this setting exists to compensate for these buggy and poorly designed network stacks and to prevent false positives with them.

Default: yes

Note: Do not change the default setting to "no" unless you know what you are doing. This may result in valid connections being shunned. Some poorly configured kernels may not support this capability. All major vendors kernels do this support this capability (Redhat, Centos, ASL), but some vendors have poorly configured their network stack and firewalling capabilities. If you are stuck with one of these inferior kernels, you will need to disable HIDS_4151. We recommend you instead use a more robust kernel, such as the ASL kernel.

FW_DROP_INVALID_LOG

If FW_DROP_INVALID is set to yes, and this option is set to yes, ASL will log all invalid packets it drops.

Default: no

Please see the link below for information about log messages you may see this enabled:

https://www.atomicorp.com/wiki/index.php/ASL_FAQ#kernel:_ASL_INVALID_INPUT

FW_LOG_BLACKLIST_DROP

If FW_LOG_BLACKLIST_DROP is set to yes ASL will log all blacklist IP packets it drops.

Default: yes

FW_LOG_GEOBLOCK_DROP

Setting this option to yes ASL will log all Geoblocked IP packets ASL drops.

Default: yes

FW_IGNORE_BROADCASTS

If FW_IGNORE_BROADCASTS is set to yes, ASL will silently drop most broadcast packets. ASL will drop the following broadcast packets:

TCP and UDP: 255.255.255.255
Multicast: 224.0.0.0/24
Broadcasts to the subnets broadcast address (ASL kernel only)

Note: If you are using the ASL kernel, the ASL kernel can also intelligently detect and drop broadcast packets to the subnets broadcast address. Other kernels will not do this, so you may still see some broadcasts if you are not using the ASL kernel. Please contact the vendors for those kernels for support with detecting broadcasts.

Default: no

FW_ACCEPT_REDIRECTS

This option controls how the system will respond to redirects. The default setting of "no" is recommended and is also the default setting in Linux. You should not enable this setting unless you know what you are doing. Enabling this can make it possible for an attacker to bypass your firewall rules.

Default: No

FW_ACCEPT_SOURCE_ROUTE

This option controls how the system will respond to a Source Route packet. It is highly recommend you not enable this, and leave the default setting of "no". Source Routed packets can bypass all your firewall rules, are not necessary for proper network functioning and are a relic of IPv4 that should never be used for secure networking.

FW_ICMP_IGNORE_ALL

This option controls how the system will respond to any ICMP broadcast request, such as a ping. It is recommended you not enable this option if you want your system to respond to pings and other ICMP queries.

FW_ICMP_IGNORE_BROADCASTS

This option controls how the system will respond to bogus ICMP broadcast request (so called smurf attacks). It is recommended you enable this option.

FW_IGNORE_ICMP_BOGUS

This option controls how the system will respond to bogus ICMP requests. It is recommended you enable this option.

FW_IPV4_FORWARD

This option enables and disable IPv4 forwarding. If your system is not forwarding traffic to another host, then it is highly recommended you disable forwarding. The default is to disable forwarding.

FW_IPV6_FORWARD

This option enables and disable IPv6 forwarding. If your system is not forwarding traffic to another host, then it is highly recommended you disable forwarding. The default is to disable forwarding.

FW_PROXY_ARP

Enable/Disable proxy arp. It is recommend you not enable this, and leave the default setting of "no". Unless your system is acting as a layer two bridge, you will not need to proxy ARP packets. Enabling this if you are not bridging may result in unexpected networking side effects.

FW_RP_FILTER

Enable/Disable rp_filter. This option controls the Reverse Path Filtering capability in netfilter. This controls whether a response packet is allowed to go out an interface that the request did not originate from. For example, if a system has two interfaces eth0 and eth1, and a packet comes in on eth0. If the system responded, and tried to send the request out eth1 if RP_FILTER was enabled this would be denied. This is to help prevent specific types of routing attacks as well as to restrict egress packets from the system. On systems with complex routing tables, and virtual interfaces this may cause problems as packets will not always exit the same interface they came in on. Therefore, the default is not to enable this. If you do enable this, you should test to make sure your systems configuration will work correctly with this type of restriction.

FW_SYN_COOKIES

Enable/Disable SYN cookies. This option controls the use of SYN cookies. This is a Denial of Service protection feature of netfilter, and we recommend you enable this option.

FW_TCP_ECN

Enable/Disable Excplicit Congestion Notification (ECN). This will allow you to enable/disable ECN.

There are three options:

Yes: Enable ECN

No: Disable ECN

Requested: Only use ECN if the client requests it. (Available in ASL 3.0.26)

For most systems, you should set this to Yes or Requested. We recommend you not disable this unless you know how this will effect your system.

FW_TCP_TIMESTAMPS

Enable/Disable TCP timestamps. This will allow you to enable TCP timestamps which may help with the network performance and quality of some protocols.

For most systems, you should set this to Yes. We recommend you not disable this unless you know how this will effect your system.

FW_TCP_WINDOW_SCALING

Enable/Disable TCP Window Scaling. This will allow you to enable TCP Window scaling which may help with the network performance and quality of some protocols.

For most systems, you should set this to Yes. We recommend you not disable this unless you know how this will effect your system.

Advanced Firewall Rule Manager

The Advanced Rule Manager is designed to let you directly control the netfilter engine in Linux through an advanced GUI system. If you are not familiar with how advanced Linux firewalling works please use the Fast/Easy Mode for simple firewall changes.

Access the firewall rule manager

Step 1: Log into the ASL GUI

Step 2: Click on the Firewall Tab.

Step 3: Click on the firewall rules menu item.

This will open the firewall rules window.

Rule Types

Linux supports three types of firewall rules:

NAT

Network Address Translation Rules: These rules are used to change the source and/or destination of packets.

Mangle

This types of rules are used to modify or "mangle" packets before they are processed

Filter

These types of rules are used to filter or "block" packets. These are the most common rules and are probably the type of rule you want if you want to allow or block traffic to specific port.

Filter subtypes

INPUT

These are rules for traffic going to the server itself. These are the most common subtype, and are what you want if you are looking to allow traffic to a port, or to block traffic to a port.

OUTPUT

These are rules for traffic coming from the server itself. If you have created input rules that are stateful (checking the "stateful" checkbox), then you do not need to create OUTPUT rules for that traffic. The state engine will automatically handle outbound traffic from the server, and will automatically allow this traffic when needed.

These are also different from FORWARD rules. If you are forwarding traffic through your server to a different server, you do not need to create OUTPUT rules for that traffic. OUTPUT rules are only for traffic coming from the server itself. For example, if the server wants to initiate a connection to a mail server on a different server. That would require an OUTPUT rules. If the server was responding to traffic initiated from a different server, and there were stateful INPUT rules to allow that different server to talk to your server on that port, you would not need OUTPUT rules for that traffic.

FORWARD

This subtype is for all all other traffic that is not INPUT or OUTPUT. Specifically, this would be traffic that is passing through the server if the server were acting as a classic firewall for another system. FORWARD rules have nothing to do with the server itself.

Jump Targets

In Linux these tell the rule to jump to an action. This is a way of telling Linux what to do, such as to allow, deny or do something else if the rule is triggered.

ACCEPT

Accept the traffic. This will allow the traffic.

DROP

Silently drop the packet. The client will get no response from the server.

REJECT

Reject the packet. The client will get a reject at the TCP or UDP layer from the server.

Rule options

Using the firewall manager

Do not modify any ASL chains

Do not modify any chains with "ASL" in the name. For example, do not modify the "ASL-Firewall-INPUT" chain. This, and all other ASL-* chains are dynamically modified by ASL on the fly, and changes to these chains may have unpredictable effects and will not be saved by ASL. Your changes will likely be lost, at best, and at worst may cause your system to be unreachable.

If you want to make changes to your firewall, you may modify other chains, or add your own in whatever order you wish.

Adding a firewall rule

Step 1: Select the type of rule you want to add from the list above

Step 2: Select the subtype of the rule you want to add from the list above

Step 3: From the firewall rule manager, select the subtype. For example, to add a filter -> INPUT rule select "INPUT" under the filter tree branch.

this will pull up any INPUT rules you have on the system. To add a new rule select "Add Rule".

Step 4: Select the subtype by selecting it under "Chain". For example, if you want to add an INPUT rule select Table "filter", this should autoselect "INPUT". If it does not just select INPUT as the "Chain".

Step 5: Select the command, Append or Insert. In the Linux firewall, Append means to add the rule after all other rules. Linux firewalling is linear, that means it processes the rules in order "first come first served". Appending places it last. Insert places the rule first. With insert you can also set the rule number in case you prefer that the rule run second, third, etc. Be very careful with Insert as you can end up setting you rules up in the opposite order of what you want (for example, putting your drop rules before your allow rules).

Step 6: Select the protocol by clicking on the "n/a" drop down. The drop down before this allows you to add a "negative", for example if you want to add a rule to say "not tcp". If you want this rule to be for TCP, just select tcp in the drop down, and leave the first drop down as "-".

Step 7: Type in the source IP/Network for this rule. If you want to allow any network, just leave this blank.

Step 8: Type in the destination for this rule. If you are creating a rule for the server itself, for example to allow connections to a specific port, just leave this blank.

Step 9: Type in the source port. If you are a creating a rule to allow connections to the server from another server, leave this blank.

Step 10: Type in the Destination port. For example, if you want to allow connections to port 22, type in 22.

Step 11: Set the interface. If you want to set this rule for all interfaces, leave the default of "n/a".

Step 12: Set the "Jump Target". In the link this is sort of like the "action". You will see a number of options described above, such as ACCEPT, REJECT. Please see above for more information about these. In general, if you want to allow a connection, select "ACCEPT".

Step 13: Select Add Rule.

Your new rule is now implemented.

Per Port ACLs

These allow you to configure specific IP addresses and networks to be allowed access to specific ports, in a simple manner. For example you could create an access list for SSH that would only allow 3 IPs to connect. Or an access list for SMTP that only allows certain networks to connect, or any combination of these.

Access Control Lists (ACLs) are stored in this directory:

/etc/asl/firewall

To create a new access control system for a port:

Step 1) create a file in /etc/asl/firewall directory using the format in #2 below

Step 2) Naming convention for file is: INPUT-<name>-<protocol>-<port>-any-acl

example: INPUT-sshd-tcp-22-any-acl

Fields in filename you can change:

INPUT - this defines the chain it goes into, INPUT is the only supported chain right now. In the future we will add support for other chains.

name - an arbitrary alpha numeric name (a-z 0-9), this must be unique as this will be used to name the chain and you cant have duplicate chain names. Example: smtp1

protocol - any supported iptables protocol on the system that takes a port as an argument. Examples: tcp, udp

port - 1-65535

Step 3) list IP's, one per line in the file you create in #2 above.

1.2.3.4 5.6.7.8

Step 4) If you have defined it, remove the port number for the service from FW_INBOUND_SERVICES. If you allow a port in FW_INBOUND_SERVICES, that will override the ACL and make the allow global (anyone can connect to the port). If you want to use an ACL for a port, do NOT list the port in FW_INBOUND_TCP_SERVICES or FW_INBOUND_UDP_SERVICES.

Step 5) reload the firewall policy with:

/etc/init.d/asl-firewall restart

Examples

Using the Fast/Easy mode

How to block all incoming TCP connections except to certain ports

Step 1: Log into the ASL GUI

Step 2: Click the Configuration Tab

Step 3: Select the ASL Configuration menu option.

Step 4: Scroll down to Firewall configuration

Step 5: Enter the list of TCP ports you want to allow into the FW_INBOUND_TCP_SERVICES list.

Example:

21,22,25,80,110,143,443,993,995,8443,30000

Step 6: Click Update

Your new rule is now implemented.

Using the Advanced Firewall Manager

How to block all incoming connections to a TCP port

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "INPUT"

Step 7: For command select "Insert"

Step 6: For protocol select tcp in the drop down, and leave the first drop down as "-".

Step 7: Type in the source IP/Network for this rule. If you want to block all sources, just leave this blank.

Step 8: In the Destination port window type in the port number you wish to block. For example, if you want to block connections to port 22, type in 22. Leave the first drop down as "-".

Step 9: Set the "Jump Target" to "DROP" or "REJECT". DROP will silently block the attempt so the client is not sure why it was blocked, REJECT will send ICMP destination denied packets to the client applications network stack telling the stack that the connection was dropped at the network level.

Step 10: Then press the Add Rule button.

Your new rule is now implemented.

How to restrict all incoming connections to a TCP port except from a specific IP or network

This procedure will allow you to create a ruleset to restrict access to a port, except from specific IPs/networks.

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "INPUT"

Step 7: For command select "Insert"

Step 6: For protocol select tcp in the drop down, and leave the first drop down as "-".

Step 7: In the "Source:" field, type in the source IP/Network that you wish to allow access to this port.

Step 8: In the Destination port window type in the port number you wish to restrict access. For example, if you want to restrict connections to port 22, type in 22. Leave the first drop down as "-".

Step 9: Set the "Jump Target" to "ACCEPT", this will allow this IP and/or network to connect to this point.

Step 10: Then press the Add Rule button.

Step 11: Continue adding ACCEPT rules for all the IPs and/or networks you want to allow by repeating Steps 1-10.

Step 12: If you are done adding IPs and/or networks to allow, now you will add in your rule to block all other IPs/Networks. Click the "Add Rule" button.

Step 13: For table, select filter.

Step 14: For Chain select "INPUT"

Step 15: For command select "Append"

Step 16: For protocol select tcp in the drop down, and leave the first drop down as "-".

Step 17: Type in the source IP/Network for this rule. If you want to block all sources, just leave this blank.

Step 18: In the Destination port window type in the port number you wish to block. For example, if you want to block connections to port 22, type in 22. Leave the first drop down as "-".

Step 19: Set the "Jump Target" to "DROP" or "REJECT". DROP will silently block the attempt so the client is not sure why it was blocked, REJECT will send ICMP destination denied packets to the client applications network stack telling the stack that the connection was dropped at the network level.

Step 20: Then press the Add Rule button.

Your new ruleset to restrict access to a port, except from certain IPs/Networks is now implemented.

Your new rule is now implemented.

How to allow access to all ports from a single IP or network

This procedure will allow you to create a ruleset to restrict access to a port, except from specific IPs/networks.

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "INPUT"

Step 7: For command select "Insert"

Step 6: For protocol select all in the drop down, and leave the first drop down as "-".

Step 7: In the "Source:" field, type in the source IP/Network that you wish to allow access to all ports on the server.

Step 8: Set the "Jump Target" to "ACCEPT", this will allow this IP and/or network to connect to this point.

Step 9: Then press the Add Rule button.

Step 10: Continue adding ACCEPT rules for all the IPs and/or networks you want to allow by repeating Steps 1-9.

Your new rule is now implemented.

How to allow connections to a port

Fast Mode Method

To add TCP ports to the allowed inbound list, please see the FW_INBOUND_TCP_SERVICES option.

To add UDP ports to the allowed inbound list, please see the FW_INBOUND_UDP_SERVICES option.

Advanced Firewall Management Method

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "INPUT"

Step 7: For command select "Insert"

Step 6: Select the protocol. For example, for TCP select tcp in the drop down, and leave the first drop down as "-".

Step 7: Type in the source IP/Network for this rule. If you want to allow any network, just leave this blank.

Step 8: In the Destination port window type in the port number you wish to allow. For example, if you want to allow connections to port 22, type in 22. Leave the first drop down as "-".

Step 9: Set the "Jump Target" to "ACCEPT".

How to allow outbound connections to a port

Fast Mode Method

To add TCP ports to the allowed outbound list, please see the FW_OUTPUT_TCP_SERVICES option.

To add UDP ports to the allowed outbound list, please see the FW_OUTPUT_UDP_SERVICES option.

Advanced Firewall Management Method

Note: This is only necessary if you have setup either ASL or your firewall rules to only allow connections outbound to certain ports. By default ASL does not block any outbound ports.

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "OUTPUT"

Step 7: For command select "Insert"

Step 6: For protocol select tcp in the drop down, and leave the first drop down as "-".

Step 7: Type in the destination IP/Network for this rule. If you want to allow outbound connections on the port to any network or destination, just leave this blank.

Step 8: In the Destination port window type in the port number you wish to allow. For example, if you want to allow connections outbound port 25, type in 25. Leave the first drop down as "-".

Step 9: Set the "Jump Target" to "ACCEPT".

Step 10: Then press the Add Rule button.

Your new rule is now implemented.

Using the Advanced Firewall Manager and Fast/Easy Mode together

Restrict access to a specific port, except from specific IPs/networks

This procedure will allow you to create a ruleset to restrict access to a port, except from specific IPs/networks when you have blocked the port via Fast/Easy mode.

Step 1: Log into the ASL GUI

Step 2: Click the Firewall Tab

Step 3: Select the Firewall Rules menu option.

Step 4: Click the "Add Rule" button.

Step 5: For table, select filter.

Step 6: For Chain select "INPUT"

Step 7: For command select "Insert"

Step 6: For protocol select tcp in the drop down, and leave the first drop down as "-".

Step 7: In the "Source:" field, type in the source IP/Network that you wish to allow access to this port.

Step 8: In the Destination port window type in the port number you wish to restrict access. For example, if you want to restrict connections to port 22, type in 22. Leave the first drop down as "-".

Step 9: Set the "Jump Target" to "ACCEPT", this will allow this IP and/or network to connect to this point.

Step 10: Then press the Add Rule button.

Step 11: Continue adding ACCEPT rules for all the IPs and/or networks you want to allow by repeating Steps 1-10.

Step 12: If you are done adding IPs and/or networks to allow, and you used Fast/Easy mode to block the port, then your new ruleset to restrict access to a port, except from certain IPs/Networks is now implemented.

Your new rule is now implemented.

Frequently Asked Questions

How can I change the ASL-Firewall-INPUT rules?

These are generated based on your configuration of these settings:

https://www.atomicorp.com/wiki/index.php/ASL_firewall#Fast.2FEasy_Mode

How can I change the ASL-Firewall-OUTPUT rules?

These are generated based on your configuration of these settings:

https://www.atomicorp.com/wiki/index.php/ASL_firewall#Fast.2FEasy_Mode

Can I use a range of ports?

Yes, ranges are delimited with the colon, ":" symbol. The format for ranges is x:y, where x is the beginning of the range and y is the end. For example:

1024:65535

Would create a range of 1024 to 65535.

Will changes survive a reboot?

Yes, if you configure your firewall through the ASL web console your changes are saved by ASL.

Will my ASL firewall configuration survive an upgrade of ASL?

Check the release notes for the upgrade. If the notes do not mention an issue with retaining your firewall configuration between upgrades, then the configuration will survive an upgrade.

If I create an INPUT rule do I need to create an OUTPUT rule?

No, as long as you make the INPUT rule stateful (which is the default). You'll see a checkbox that says stateful, leave it checked and ASL will automatically manage whatever outbound packets that need to be let out. OUTPUT rules are not necessary with a stateful INPUT rule.

For example, if you allow traffic in on port 22 via an INPUT rule, you do not need to create an OUTPUT rule for port 22 if you have checked the stateful box (which is checked by default) when creating your rule.

How can the firewall be reset to its defaults?

You can reset your firewall rules to the defaults by running these commands as the root user (not via sudo):

cp /etc/asl/firewall/running.fw /root

rm /etc/asl/firewall/running.fw

service asl-firewall restart

NOTE: You can restore your custom firewall rules, only if you used the commands above to clear your firewall rules by running these commands as the root user:

cp /root/running.fw /etc/asl/firewall/running.fw

service asl-firewall restart

Logs

Please see the Firewall Logs article.

ASL firewall

Contents

Introduction

How the firewall works

Stateful packet inspection

Console firewall messages

Fast/Easy Mode

Inbound/Outbound ports

FW_INBOUND_TCP_SERVICES

FW_INBOUND_UDP_SERVICES

FW_OUTPUT_TCP_SERVICES

FW_OUTPUT_UDP_SERVICES

Recommended ports

Cpanel

Parallels Plesk

Directadmin

Webmin

Engine settings

FW_WHITELIST

FW_WHITELIST_LOG

FW_RATE_LIMIT

FW_RATE_LIMIT_LOG

FW_OUTPUT_MTA

FW_PLESK_UPDATES

FW_SPAMASSASSIN_UPDATES

FW_PORTKNOCK

FW_PORTKNOCK_SIMPLE

FW_PORTKNOCK_SIMPLE_LOG_KNOCK

FW_PORTKNOCK_SIMPLE_LOG_ALLOWED

FW_PORTKNOCK_ADVANCED

FW_PORTKNOCK_ADVANCED_LOG_KNOCK

FW_PORTKNOCK_ADVANCED_LOG_ALLOWED

FW_LASSO

FW_LASSO_LOG

FW_ELASSO

FW_ELASSO_LOG

FW_CIARMY

FW_CIARMY_LOG

FW_EMERGING_THREATS

FW_EMERGING_THREATS_LOG

FW_OPENBL

FW_OPENBL_LOG

FW_AUTOSHUN

FW_AUTOSHUN_LOG

FW_OPENPROXIES

FW_OPENPROXIES_LOG

FW_DSHIELD

FW_DSHIELD_LOG

FW_TOR

FW_TOR_LOG

FW_PORTSCAN

FW_LOWLEVEL_PORTSCAN

FW_DROP_SYNSCAN

FW_DROP_STEALTH_SCAN

FW_DROP_CONNECT_SCAN

FW_DROP_GRAB_SCAN

FW_ADVANCED_PORTSCAN

PSD_W_THRSH

PSD_DELAY

PSD_LO

PSD_HI

FW_LOG_AR_DROP

FW_BAD_PACKETS

FW_MSS_DROP

FW_MSS_DROP_LOG

FW_SMALL_PACKETS

FW_FRAGMENTS

FW_DROP_INVALID

FW_DROP_INVALID_LOG

FW_LOG_BLACKLIST_DROP

FW_LOG_GEOBLOCK_DROP

FW_IGNORE_BROADCASTS

FW_ACCEPT_REDIRECTS

FW_ACCEPT_SOURCE_ROUTE

FW_ICMP_IGNORE_ALL

FW_ICMP_IGNORE_BROADCASTS

FW_IGNORE_ICMP_BOGUS

FW_IPV4_FORWARD

FW_IPV6_FORWARD

FW_PROXY_ARP