[bug#48975] [PATCH] gnu: simple-firewall-service: Add a simple service wrapping iptables

[antlers]

From: antlers <antlers@luris.net>

 * gnu/services/networking.scm (simple-firewall-service): Add.
   (iptables-service): Allow a crude sort of service extension.

I tried out a keyword-based syntax:
```
(simple-firewall-configuration
  (allow-forwarding? #t)
  (allowed-ports '(#:both 51234
                   #:tcp  80 443
                   #:udp  4444))
```
But kept the more verbose tcp and udp fields because I don't want
people to have to use quasiquotes to splice in evaluated port-numbers
after the keywords.

I like the suggestion that there should be a field for redirecting
packets, whether to loopback or another box, as it took me a while to
learn about eg. masquerading last time I needed to set something like
that up. Not sure what command would be equivalent to the NAT
suggestion?

I guess nftables has superseded iptables, but I'm not as familiar with
it? Perhaps I can add it as a second back-end in the future. My
primary concern right now is a pure Scheme interface for networking
configuration; most notably via service inheritance! Simple-firewall
now lets you open ports via extensions in other services; in order for
this option to be widely available, perhaps it's the
{nf,ip}tables-services that should be extensible? It's a tricky
problem atm because we don't really want services that need ports
depending on a specific backend, there are existing API's, they use
plain-file's over structs or strings, and rule orders need to be
really specific/coordinated. Idk, maybe that isn't something we really
want in the first place, but it sure feels good from a configuration /
organizational point-of-view. Happy to tweak this again if anyone has
ideas.
---
 gnu/services/networking.scm | 79 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 2 deletions(-)

diff --git a/gnu/services/networking.scm b/gnu/services/networking.scm
index 19aba8c266..0866c10b34 100644
--- a/gnu/services/networking.scm
+++ b/gnu/services/networking.scm
@@ -18,6 +18,8 @@
 ;;; Copyright © 2021 Christine Lemmer-Webber <cwebber@dustycloud.org>
 ;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2021 Guillaume Le Vaillant <glv@posteo.net>
+;;; Copyright © 2021 Solene Rapenne
+;;; Copyright © 2022 antlers <autumnalantlers@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -225,7 +227,11 @@ (define-module (gnu services networking)
 
             keepalived-configuration
             keepalived-configuration?
-            keepalived-service-type))
+            keepalived-service-type
+
+            simple-firewall-service-type
+            simple-firewall-configuration
+            simple-firewall-configuration?))
 
 ;;; Commentary:
 ;;;
@@ -1721,7 +1727,13 @@ (define iptables-service-type
     "Run @command{iptables-restore}, setting up the specified rules.")
    (extensions
     (list (service-extension shepherd-root-service-type
-                             (compose list iptables-shepherd-service))))))
+                             (compose list iptables-shepherd-service))))
+   ;; Some services extend iptables, but such services are mutually exclusive,
+   ;; and should be either extended directly or superseded entirely depending
+   ;; the complexity of your desired configuration.
+   (compose identity)
+   (extend (lambda (config entries)
+             (last entries)))))
 
 ;;;
 ;;; nftables
@@ -2186,4 +2198,67 @@ (define keepalived-service-type
                  "Run @uref{https://www.keepalived.org/, Keepalived}
 routing software.")))
 
+
+;;;
+;;; Simple Firewall
+;;;
+
+(define-record-type* <simple-firewall-configuration>
+  simple-firewall-configuration make-simple-firewall-configuration
+  simple-firewall-configuration?
+  (allow-icmp? simple-firewall-configuration-allow-icmp?
+               (default #f))
+  (allow-forwarding? simple-firewall-configuration-allow-forwarding?
+                     (default #f))
+
+  (open-tcp-ports simple-firewall-configuration-open-tcp-ports
+                 (default '()))
+  (open-udp-ports simple-firewall-configuration-open-udp-ports
+                 (default '())))
+
+(define simple-firewall-configuration->iptables-rules
+  (match-lambda
+    (($ <simple-firewall-configuration>
+        allow-icmp? allow-forwarding?
+        open-tcp-ports open-udp-ports)
+     (string-join
+       `("*filter"
+         ":INPUT DROP"
+         ,(string-append ":FORWARD " (if allow-forwarding? "ACCEPT" "DROP"))
+         ":OUTPUT ACCEPT"
+         "-A INPUT -i lo -j ACCEPT"
+         "-A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT"
+         ,@(unless allow-icmp? '("-A INPUT -p icmp   -j DROP"
+                                 "-A INPUT -p icmpv6 -j DROP"))
+         ,@(map (cut string-append "-A INPUT -p tcp --dport " <>  " -j 
ACCEPT") (map number->string open-tcp-ports))
+         ,@(map (cut string-append "-A INPUT -p udp --dport " <>  " -j 
ACCEPT") (map number->string open-udp-ports))
+         "-A INPUT -j REJECT --reject-with icmp-port-unreachable"
+         "COMMIT")
+       "\n" 'suffix))))
+
+(define (simple-firewall-configuration->iptables-configuration config)
+  (let ((rules (simple-firewall-configuration->iptables-rules config)))
+    (iptables-configuration
+      (ipv4-rules (plain-file "iptables.rules" rules))
+      (ipv6-rules (plain-file "ip6tables.rules" rules)))))
+
+(define simple-firewall-service-type
+  (service-type
+   (name 'simple-firewall)
+   (description
+    "Run @command{iptables-restore}, setting up the specified rules.")
+   (extensions
+     (list (service-extension iptables-service-type
+                              
simple-firewall-configuration->iptables-configuration)))
+   (compose concatenate)
+   (extend (lambda (config entries)
+             (simple-firewall-configuration
+              (inherit config)
+              (open-tcp-ports
+                (concatenate (map simple-firewall-configuration-open-tcp-ports
+                                  (cons config entries))))
+              (open-udp-ports
+                (concatenate (map simple-firewall-configuration-open-udp-ports
+                                  (cons config entries)))))))))
+
 ;;; networking.scm ends here
-- 
2.38.0