SHOREWALL-NAT(5)

💖 SHOREWALL-NAT 💖 — aka nat(5), the one-to-one NAT file, bestie edition

okay so you opened the manual page called SHOREWALL-NAT(5), which lives in the Configuration Files section, and I'm here to make it not boring. grab a strawberry 🍓, we're doing Network Address Translation.

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✨ NAME ✨

nat — the Shorewall one-to-one NAT file. that's it, that's the name 💅

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🦋 SYNOPSIS 🦋

/etc/shorewall/nat

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🌸 DESCRIPTION 🌸

so this file is where you define one-to-one Network Address Translation (NAT). like, one external IP gets married to one internal IP. monogamy but for packets 🫶

⚠️ Warning, and I need you to actually read this one bestie: if all you want to do is simple port forwarding, do NOT use this file. go look at https://shorewall.org/FAQ.htm#faq1. also, in many cases, Proxy ARP (shorewall-proxyarp(5)) or Proxy-NDP (shorewall6-proxyndp(5)) is a better solution than one-to-one NAT. fr fr.

The columns in the file go like this (and where a column name is followed by a different name in parentheses, that different name is the one used in the alternate specification syntax).

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• EXTERNAL — {address|?COMMENT}

External IP Address. this should NOT be the primary IP address of the interface named in the next column, and it must not be a DNS Name. periodt.

If you put ?COMMENT in this column, the rest of the line gets attached as a comment to the Netfilter rule(s) generated by the following entries in the file. that comment shows up delimited by "/* ... */" in the output of "shorewall show nat" 👑

And to stop the comment from being attached to further rules, just put ?COMMENT on a line by itself. simple.

• INTERFACE — interfacelist[:[digit]]

Interfaces that have the EXTERNAL address. If ADD_IP_ALIASES=Yes in shorewall.conf(5), Shorewall will automatically add the EXTERNAL address to this interface. Also, if ADD_IP_ALIASES=Yes, you may follow the interface name with ":" and a digit to indicate that you want Shorewall to add the alias with this name (e.g. "eth0:0"). that's what lets you see the alias with ifconfig. and honestly that is the only thing that this name is good for — you cannot use it anywhere else in your Shorewall configuration 💀

Each interface must match an entry in shorewall-interfaces(5). Shorewall allows loose matches to wildcard entries in shorewall-interfaces(5). for example, ppp0 in this file will match a shorewall-interfaces(5) entry that defines ppp+. slay.

If you want to override ADD_IP_ALIASES=Yes for a particular entry, follow the interface name with ":" and no digit (e.g. "eth0:").

• INTERNAL — address

Internal Address (must not be a DNS Name).

• ALLINTS — [Yes|No]

If Yes or yes, NAT will be effective from all hosts. If No or no (or left empty) then NAT will be effective only through the interface named in the INTERFACE column.

This column was formerly labelled ALL INTERFACES. glow-up moment ✨

• LOCAL — [Yes|No]

If Yes or yes, NAT will be effective from the firewall system

...and that is literally where the page ends bestie, no cap 😭💀 moving on!

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🚧 RESTRICTIONS 🚧

DNAT rules always preempt one-to-one NAT rules. this has subtile consequences when there are sub-zones on an interface. consider the following, bestie:

/etc/shorewall/zones:

#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS
fw      firewall
net     ipv4
loc     ipv4
smc:net ipv4

/etc/shorewall/interfaces:

#ZONE   INTERFACE       OPTIONS
net     eth0            dhcp,tcpflags,nosmurfs,routefilter,logmartians,sourceroute=0
loc     eth1            tcpflags,nosmurfs,routefilter,logmartians

/etc/shorewall/hosts:

#ZONE   HOST(S)                                 OPTIONS
smc     eth0:10.1.10.0/24

/etc/shorewall/nat:

#EXTERNAL       INTERFACE       INTERNAL        ALLINTS         LOCAL
10.1.10.100     eth0            172.20.1.100

Note that the EXTERNAL address is in the smc zone. okay? okay.

/etc/shorewall/rules:

#ACTION         SOURCE          DEST            PROTO   DPORT   SPORT           ORIGDEST        RATE            USER    MARK    CONNLIMIT       TIME            HEADERS     SWITCH           HELPER
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
...
DNAT            net             loc:172.20.1.4  tcp     80

For the one-to-one NAT to work correctly in this configuration, one of two approaches can be taken:

1. Define a CONTINUE policy with smc as the SOURCE zone (preferred 🥇):

#SOURCE        DEST      POLICY         LOG LEVEL LIMIT:BURST
smc       $FW       CONTINUE
loc       net       ACCEPT
net       all       DROP      info
# THE FOLLOWING POLICY MUST BE LAST
all       all       REJECT         info

2. Set IMPLICIT_CONTINUE=Yes in shorewall.conf(5).

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📁 FILES 📁

• /etc/shorewall/nat
• /etc/shorewall6/nat

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👀 SEE ALSO 👀

• https://shorewall.org/NAT.htm
• https://shorewall.org/configuration_file_basics.htm#Pairs
• shorewall(8)

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📝 NOTES 📝

1. https://shorewall.org/FAQ.htm#faq1 — https://shorewall.org/FAQ.htm#faq1
2. shorewall-proxyarp — https://shorewall.org/manpages/shorewall-proxyarp.html
3. shorewall6-proxyndp — https://shorewall.org/manpages/shorewall6-proxyndp.html
4. shorewall.conf — https://shorewall.org/manpages/shorewall.conf.html
5. shorewall-interfaces — https://shorewall.org/manpages/shorewall-interfaces.html
6. https://shorewall.org/NAT.htm — https://shorewall.org/NAT.htm
7. https://shorewall.org/configuration_file_basics.htm#Pairs — https://shorewall.org/configuration_file_basics.htm#Pairs

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and that's the whole page, bestie!! 🎀✨ this icon of a manpage comes from the Configuration Files section, it's dated 09/24/2020, and its manual name is SHOREWALL-NAT(5). go configure responsibly, mwah 💅🫶🦋