services: wireguard: Implement a dynamic IP monitoring feature.

* gnu/services/vpn.scm (<wireguard-configuration>)
[monitor-ips?, monitor-ips-internal]: New fields.
* gnu/services/vpn.scm (define-with-source): New syntax.
(wireguard-service-name, strip-port/maybe)
(ipv4-address?, ipv6-address?, host-name?)
(endpoint-host-names): New procedure.
(wireguard-monitoring-jobs): Likewise.
(wireguard-service-type): Register it.
* tests/services/vpn.scm: New file.
* Makefile.am (SCM_TESTS): Register it.
* doc/guix.texi (VPN Services): Update doc.

Reviewed-by: Bruno Victal <mirai@makinata.eu>

1 files changed, 146 insertions, 4 deletions

diff --git a/gnu/services/vpn.scm b/gnu/services/vpn.scm
index a884d71eb2..17aed8b3b2 100644
--- a/gnu/services/vpn.scm
+++ b/gnu/services/vpn.scm
@@ -11,6 +11,7 @@
 ;;; Copyright © 2021 Nathan Dehnel <ncdehnel@gmail.com>
 ;;; Copyright © 2022 Cameron V Chaparro <cameron@cameronchaparro.com>
 ;;; Copyright © 2022 Timo Wilken <guix@twilken.net>
+;;; Copyright © 2023 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -31,10 +32,12 @@
   #:use-module (gnu services)
   #:use-module (gnu services configuration)
   #:use-module (gnu services dbus)
+  #:use-module (gnu services mcron)
   #:use-module (gnu services shepherd)
   #:use-module (gnu system shadow)
   #:use-module (gnu packages admin)
   #:use-module (gnu packages vpn)
+  #:use-module (guix modules)
   #:use-module (guix packages)
   #:use-module (guix records)
   #:use-module (guix gexp)
@@ -73,6 +76,8 @@
             wireguard-configuration-addresses
             wireguard-configuration-port
             wireguard-configuration-dns
+            wireguard-configuration-monitor-ips?
+            wireguard-configuration-monitor-ips-interval
             wireguard-configuration-private-key
             wireguard-configuration-peers
             wireguard-configuration-pre-up
@@ -741,6 +746,10 @@ strongSwan.")))
                       (default '()))
   (dns                wireguard-configuration-dns ;list of strings
                       (default #f))
+  (monitor-ips?       wireguard-configuration-monitor-ips? ;boolean
+                      (default #f))
+  (monitor-ips-interval wireguard-configuration-monitor-ips-interval
+                        (default '(next-minute (range 0 60 5)))) ;string | list
   (pre-up             wireguard-configuration-pre-up ;list of strings
                       (default '()))
   (post-up            wireguard-configuration-post-up ;list of strings
@@ -871,6 +880,58 @@ PostUp = ~a set %i private-key ~a~{ peer ~a preshared-key ~a~}
             (chmod #$private-key #o400)
             (close-pipe pipe))))))
 
+;;; XXX: Copied from (guix scripts pack), changing define to define*.
+(define-syntax-rule (define-with-source (variable args ...) body body* ...)
+  "Bind VARIABLE to a procedure accepting ARGS defined as BODY, also setting
+its source property."
+  (begin
+    (define* (variable args ...)
+      body body* ...)
+    (eval-when (load eval)
+      (set-procedure-property! variable 'source
+                               '(define* (variable args ...) body body* ...)))))
+
+(define (wireguard-service-name interface)
+  "Return the WireGuard service name (a symbol) configured to use INTERFACE."
+  (symbol-append 'wireguard- (string->symbol interface)))
+
+(define-with-source (strip-port/maybe endpoint #:key ipv6?)
+  "Strip the colon and port, if present in ENDPOINT, a string."
+  (if ipv6?
+      (if (string-prefix? "[" endpoint)
+          (first (string-split (string-drop endpoint 1) #\])) ;ipv6
+          endpoint)
+      (first (string-split endpoint #\:)))) ;ipv4
+
+(define* (ipv4-address? address)
+  "Predicate to check whether ADDRESS is a valid IPv4 address."
+  (let ((address (strip-port/maybe address)))
+    (false-if-exception
+     (->bool (getaddrinfo address #f AI_NUMERICHOST AF_INET)))))
+
+(define* (ipv6-address? address)
+  "Predicate to check whether ADDRESS is a valid IPv6 address."
+  (let ((address (strip-port/maybe address #:ipv6? #t)))
+    (false-if-exception
+     (->bool (getaddrinfo address #f AI_NUMERICHOST AF_INET6)))))
+
+(define (host-name? name)
+  "Predicate to check whether NAME is a host name, i.e. not an IP address."
+  (not (or (ipv6-address? name) (ipv4-address? name))))
+
+(define (endpoint-host-names peers)
+  "Return an association list of endpoint host names keyed by their peer
+public key, if any."
+  (reverse
+   (fold (lambda (peer host-names)
+           (let ((public-key (wireguard-peer-public-key peer))
+                 (endpoint (wireguard-peer-endpoint peer)))
+             (if (and endpoint (host-name? endpoint))
+                 (cons (cons public-key endpoint) host-names)
+                 host-names)))
+         '()
+         peers)))
+
 (define (wireguard-shepherd-service config)
   (match-record config <wireguard-configuration>
     (wireguard interface)
@@ -878,9 +939,7 @@ PostUp = ~a set %i private-key ~a~{ peer ~a preshared-key ~a~}
           (config (wireguard-configuration-file config)))
       (list (shepherd-service
              (requirement '(networking))
-             (provision (list
-                         (symbol-append 'wireguard-
-                                        (string->symbol interface))))
+             (provision (list (wireguard-service-name interface)))
              (start #~(lambda _
                        (invoke #$wg-quick "up" #$config)))
              (stop #~(lambda _
@@ -888,6 +947,87 @@ PostUp = ~a set %i private-key ~a~{ peer ~a preshared-key ~a~}
                        #f))                       ;stopped!
              (documentation "Run the Wireguard VPN tunnel"))))))
 
+(define (wireguard-monitoring-jobs config)
+  ;; Loosely based on WireGuard's own 'reresolve-dns.sh' shell script (see:
+  ;; https://raw.githubusercontent.com/WireGuard/wireguard-tools/
+  ;; master/contrib/reresolve-dns/reresolve-dns.sh).
+  (match-record config <wireguard-configuration>
+    (interface monitor-ips? monitor-ips-interval peers)
+    (let ((host-names (endpoint-host-names peers)))
+      (if monitor-ips?
+          (if (null? host-names)
+              (begin
+                (warn "monitor-ips? is #t but no host name to monitor")
+                '())
+              ;; The mcron monitor job may be a string or a list; ungexp strips
+              ;; one quote level, which must be added back when a list is
+              ;; provided.
+              (list
+               #~(job
+                  (if (string? #$monitor-ips-interval)
+                      #$monitor-ips-interval
+                      '#$monitor-ips-interval)
+                  #$(program-file
+                     (format #f "wireguard-~a-monitoring" interface)
+                     (with-imported-modules (source-module-closure
+                                             '((gnu services herd)
+                                               (guix build utils)))
+                       #~(begin
+                           (use-modules (gnu services herd)
+                                        (guix build utils)
+                                        (ice-9 popen)
+                                        (ice-9 match)
+                                        (ice-9 textual-ports)
+                                        (srfi srfi-1)
+                                        (srfi srfi-26))
+
+                           (define (resolve-host name)
+                             "Return the IP address resolved from NAME."
+                             (let* ((ai (car (getaddrinfo name)))
+                                    (sa (addrinfo:addr ai)))
+                               (inet-ntop (sockaddr:fam sa)
+                                          (sockaddr:addr sa))))
+
+                           (define wg #$(file-append wireguard-tools "/bin/wg"))
+
+                           #$(procedure-source strip-port/maybe)
+
+                           (define service-name '#$(wireguard-service-name
+                                                    interface))
+
+                           (when (live-service-running
+                                  (current-service service-name))
+                             (let* ((pipe (open-pipe* OPEN_READ wg "show"
+                                                      #$interface "endpoints"))
+                                    (lines (string-split (get-string-all pipe)
+                                                         #\newline))
+                                    ;; IPS is an association list mapping
+                                    ;; public keys to IP addresses.
+                                    (ips (map (match-lambda
+                                                ((public-key ip)
+                                                 (cons public-key
+                                                       (strip-port/maybe ip))))
+                                              (map (cut string-split <> #\tab)
+                                                   (remove string-null?
+                                                           lines)))))
+                               (close-pipe pipe)
+                               (for-each
+                                (match-lambda
+                                  ((key . host-name)
+                                   (let ((resolved-ip (resolve-host
+                                                       (strip-port/maybe
+                                                        host-name)))
+                                         (current-ip (assoc-ref ips key)))
+                                     (unless (string=? resolved-ip current-ip)
+                                       (format #t "resetting `~a' peer \
+endpoint to `~a' due to stale IP (`~a' instead of `~a')~%"
+                                               key host-name
+                                               current-ip resolved-ip)
+                                       (invoke wg "set" #$interface "peer" key
+                                               "endpoint" host-name)))))
+                                '#$host-names)))))))))
+          '()))))                     ;monitor-ips? is #f
+
 (define wireguard-service-type
   (service-type
    (name 'wireguard)
@@ -898,6 +1038,8 @@ PostUp = ~a set %i private-key ~a~{ peer ~a preshared-key ~a~}
                              wireguard-activation)
           (service-extension profile-service-type
                              (compose list
-                                      wireguard-configuration-wireguard))))
+                                      wireguard-configuration-wireguard))
+          (service-extension mcron-service-type
+                             wireguard-monitoring-jobs)))
    (description "Set up Wireguard @acronym{VPN, Virtual Private Network}
 tunnels.")))