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157 lines
9.2 KiB
ReStructuredText
===========================
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Network Address Translation
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===========================
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Network Address Translation (abbreviated to NAT) is a way to separate external and internal networks (WANs and LANs),
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and to share an external IP between clients on the interal network. NAT can be used on IPv4 and IPv6. For IPv6,
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:doc:`Network Prefix Translation <nptv6>` is also available.
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Most of the options below use three different addresses: the source, destination and redirect address. These
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addresses are used for the following:
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============= ===========================================================================================================
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Source Where the traffic comes from. This can often be left on “any”.
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Destination Where the traffic is headed. For incoming traffic from outside, this is usually your external IP address.
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Redirect Where the traffic should be redirected.
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============= ===========================================================================================================
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.. warning::
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- Network Address Translation should not be relied upon as a security measure.
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- Disabling pf will also disable NAT.
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--------------------
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Some terms explained
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--------------------
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**BINAT**: NAT generally works in one direction. However, if you have networks of equal size, you can also use BINAT, which is
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bidirectional. This can simplify your set-up. If you don't have networks of equal size, you can only use regular NAT.
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**NAT reflection**: When a client on the internal network tries to access another client, but using the *external* IP
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instead of the internal one (which would the most logical), NAT reflection can rewrite this request so that it uses
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the internal IP, in order to avoid taking a detour and applying rules meant for actual outside traffic.
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**Pool options**: When there are multiple IPs to choose from, this option will allow regulating which IP gets used.
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The default, Round Robin, will simply distribute packets to one server after the other. If you only have one external
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IP, this option has no effect.
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---------------
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Port forwarding
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---------------
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When multiple clients share an external IP address, any connection not initiated by one of the clients will not
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succeed since the firewall will not know where to send the traffic. This can be addressed by creating port
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forwarding rules. For example, for a web server behind the firewall to be accessible, ports 80 and 443 need to
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be redirected to it.
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Port forwarding is also referred to as “Destination NAT” or “DNAT”.
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In OPNsense, port forwarding can be set up by navigating to :menuselection:`Firewall --> NAT --> Port Forward`. Here, you will see
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an overview of port forwarding rules. New rules can be added by clicking **Add** in the upper right corner.
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When adding a rule, the following fields are available:
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========================= =========================================================================================================
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Disabled Disable this rule without removing it.
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No RDR (NOT) Do not create a redirect rule. Leave this disabled unless you know what you are doing.
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Interface Which interface this rule should apply to. Most of the time, this will be WAN.
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TCP/IP version IPv4, IPv6 or both.
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Protocol In typical scenarios, this will be TCP.
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Source Where the traffic comes from. Click “Advanced” to see the other source settings.
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Source / Invert Invert match in “Source” field.
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Source port range
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Destination / Invert Invert match in “Destination” field.
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Destination Where the traffic is headed.
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Destination port range
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Redirect target IP Where to redirect the traffic to.
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Redirect target port
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Pool Options See “Some terms explained”. The default is to use Round robin.
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Description A description to easily find the rule in the overview.
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Set local tag Set a tag that other NAT rules and filters can check for.
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Match local tag Check for a tag set by another rule.
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No XMLRPC sync Prevent this rule from being synced to a backup host. (Checking this on the backup host has no effect.)
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NAT reflection See “Some terms explained”. Leave this on the default unless you have a good reason not to.
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Filter rule association Associate this with a regular firewall rule.
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========================= =========================================================================================================
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.. Note:
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In OPNsense, this feature is also used to implement transparent proxies. A connection needs to be forwarded to a
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daemon (listening on localhost), which then tries to get the original destination IP from the `/dev/pf` device.
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For example, a transparent proxy that handles HTTP traffic needs a rule that forwards traffic from TCP port 80,
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IPv4 to 127.0.0.1:3128 (in the default configuration).
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----------
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One-to-one
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----------
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One-to-one NAT will, as the name implies, translate two IPs one-to-one, rather than one-to-many as is most common.
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In this respect, it is similar to what NPT does for IPv6.
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In OPNsense, one-to-one NAT can be set up by navigating to :menuselection:`Firewall --> NAT --> One-to-one`. Here, you will see an
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overview of one-to-one rules. New rules can be added by clicking **Add** in the upper right corner.
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When adding a rule, the following fields are available:
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====================== =================================================================================================
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Disabled Disable this rule without removing it.
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Interface Which interface this rule should apply to. Most of the time, this will be WAN.
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Type BINAT (default) or NAT. See “Some terms explained”.
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External network Starting address of external network.
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Source / invert Invert match in “Source” field.
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Source
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Destination / invert Invert match in “Destination” field.
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Destination
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Description A description to easily find the rule in the overview.
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NAT reflection See “Some terms explained”. Leave this on the default unless you have a good reason not to.
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====================== =================================================================================================
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--------
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Outbound
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--------
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When a client on an internal network makes an outbound request, the gateway will have to change the source IP to
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the external IP of the gateway, since the outside server will not be able to send an answer back otherwise.
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Outbound NAT is also referred to as “Source NAT” or “SNAT”.
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If you only have one external IP, then you leave the Outbound NAT options on automatic. However, if you have
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multiple IP addresses, you might want to change the settings and add some custom rules.
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The main settings for outbound are as follows:
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======================================== =====================================================================================================
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Automatic outbound NAT rule generation The default. Follows the behaviour described above, and is good for most scenarios.
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Manual outbound NAT rule generation No automatic rules are generated. They can be added manually.
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Hybrid outbound NAT rule generation Automatic rules are added, but additional manual rules can be added as well.
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Disable outbound NAT rule generation Disables outbound NAT. This is used for :doc:`transparent bridges <how-tos/transparent_bridge>`, for example.
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======================================== =====================================================================================================
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New rules can be added by clicking **Add** in the upper right corner.
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When adding a rule, the following fields are available:
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===================== ==========================================================================================================
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Disabled Disable this rule without removing it.
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Do not NAT Disable NAT for all traffic matching this rule. Leave this disabled unless you know what you are doing.
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Interface Which interface this rule should apply to. Most of the time, this will be WAN.
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TCP/IP version IPv4 or IPv6
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Protocol In typical scenarios, this will be TCP.
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Source invert Invert match in “Source” field.
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Source
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Source port
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Destination invert Invert match in “Destination” field.
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Destination
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Destination port
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Translation / target What to translate matching packets to.
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Log Put packets matching this rule in the logs. Use this sparingly to avoid overflowing the logs.
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Translation / port
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Static-port Prevents pf(4) from modifying the source port on TCP and UDP packets.
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Pool options See “Some terms explained”. The default is to use Round robin.
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Set local tag Set a tag that other NAT rules and filters can check for.
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Match local tag Check for a tag set by another rule.
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No XMLRPC sync Prevent this rule from being synced to a backup host. (Checking this on the backup host has no effect.)
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Description A description to easily find the rule in the overview.
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===================== ==========================================================================================================
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