Subject: Re: [PATCH] gnu: simple-firewall-service: Add a simple service wrapping iptables

From: antlers <[email-masked]>
Date: Sun, 6 Nov 2022 12:39:26 -0800

After googling around a bit it looks like the `filter*` and `COMMIT`
commands in iptables configurations do in fact form a transactional block
that would allow us to accept additional plain-files via extensions and
just concatenate them, it's that's a road we want to go down

On Fri, Nov 4, 2022 at 12:26 AM antlers <[email-masked]> wrote:

> From: antlers <[email-masked]>
>
>  * 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 I guess nftables has superseded iptables, but I'm not as familiar with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 <[email-masked]>
>  ;;; Copyright © 2021 Maxime Devos <[email-masked]>
>  ;;; Copyright © 2021 Guillaume Le Vaillant <[email-masked]>
> +;;; Copyright © 2021 Solene Rapenne
> +;;; Copyright © 2022 antlers <[email-masked]>
>  ;;;
>  ;;; 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
>
>