Taken from: https://groups.google.com/g/linux.debian.bugs.dist/c/mX4GpUJih4k.

--- a/configure.ac
+++ b/configure.ac
@@ -132,6 +132,16 @@ if test "$os_unix" = "yes"; then
 	AC_CHECK_FUNCS([getpeereid])
 	AC_CHECK_FUNCS([getpeerucred])
 	AC_CHECK_FUNCS([issetugid])
+	case "$host_os" in
+	kfreebsd*-gnu | gnu*)
+		have_getpeereid=no
+		AC_CHECK_LIB(bsd, getpeereid, have_getpeereid=yes)
+		if test "x$have_getpeereid" = "xyes"; then
+			AC_DEFINE([HAVE_GETPEEREID], [1], [have getpeereid])
+			AC_SEARCH_LIBS([getpeereid], [bsd])
+		fi
+	;;
+	esac
 
 	AC_CACHE_CHECK([for thread-local storage class],
 		[ac_cv_tls_keyword],

--- a/common/unix-peer.c
+++ b/common/unix-peer.c
@@ -47,6 +47,11 @@
 #  include <ucred.h>
 #endif
 
+#ifdef HAVE_GETPEEREID
+/* Declare getpeereid from /usr/include/bsd/unistd.h */
+extern int getpeereid(int s, uid_t *euid, gid_t *egid);
+#endif
+
 /* Returns the unix domain socket peer information.
  * Returns zero on success.
  */
@@ -73,7 +78,8 @@ p11_get_upeer_id (int cfd, uid_t *uid, u
 		*pid = cr.pid;
 
 #elif defined(HAVE_GETPEEREID)
-	/* *BSD/MacOSX */
+	/* *BSD/MacOSX/kFreeBSD/Hurd */
+
 	uid_t euid;
 	gid_t egid;
 94'>refs</a><a class='active' href='/guix/log/tests/offload.scm'>log</a><a href='/guix/tree/tests/offload.scm?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>tree</a><a href='/guix/commit/tests/offload.scm?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>commit</a><a href='/guix/diff/tests/offload.scm?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/offload.scm'>
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<div class='path'>path: <a href='/guix/log/?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>root</a>/<a href='/guix/log/tests?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>tests</a>/<a href='/guix/log/tests/offload.scm?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294'>offload.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/offload.scm?id=fc61ff1d792773c3e85abc229a9f8cc4b9e8c294&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-08-25 13:45:27 -0400'>2020-08-25</span></td><td><a href='/guix/commit/tests/offload.scm?id=4b5a6fbc9b754c0ca70d033dd99f17c4f028733a'>offload: Modify the build-machine record to accept multiple systems.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/offload.scm (&lt;build-machine&gt;)[systems]: New field.
[system]: Accessor changed to %build-machine-system.  Default to #f.
* guix/scripts/offload.scm (build-machine-system): Wrap %build-machine-system
with a deprecation warning.
(build-machine-systems): Access the new systems field or fallback to use
build-machine-system, for backward compatibility.
(machine-matches?): Adjust.
* tests/offload.scm: Add tests...
* Makefile.am (SCM_TESTS): ...and register them.
* doc/guix.texi (Daemon Offload Setup): Update doc.
</span></span></td><td>Maxim Cournoyer</td></tr>