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;
 530884'>refs</a><a class='active' href='/guix/log/nix/libutil/util.hh'>log</a><a href='/guix/tree/nix/libutil/util.hh?id=e786bef04825a5b8bd109471259d9c81ea530884'>tree</a><a href='/guix/commit/nix/libutil/util.hh?id=e786bef04825a5b8bd109471259d9c81ea530884'>commit</a><a href='/guix/diff/nix/libutil/util.hh?id=e786bef04825a5b8bd109471259d9c81ea530884'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/nix/libutil/util.hh'>
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<div class='path'>path: <a href='/guix/log/?id=e786bef04825a5b8bd109471259d9c81ea530884'>root</a>/<a href='/guix/log/nix?id=e786bef04825a5b8bd109471259d9c81ea530884'>nix</a>/<a href='/guix/log/nix/libutil?id=e786bef04825a5b8bd109471259d9c81ea530884'>libutil</a>/<a href='/guix/log/nix/libutil/util.hh?id=e786bef04825a5b8bd109471259d9c81ea530884'>util.hh</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/nix/libutil/util.hh?id=e786bef04825a5b8bd109471259d9c81ea530884&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-08 22:30:07 +0100'>2020-12-08</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=2e308238ad09e9527f9b8c3ec4a90b6b05a93367'>daemon: 'Agent' constructor takes a list of environment variables.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/libutil/util.hh (struct Agent)[Agent]: Add 'env' parameter.
* nix/libutil/util.cc (Agent::Agent): Honor it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-14 15:42:55 +0200'>2020-09-14</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=ee9dff34f9317509cb2b833d07a0d5e01a36a4ae'>daemon: Move 'Agent' to libutil.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/libstore/build.cc (DerivationGoal::tryBuildHook): Add "offload" to
'args' and pass settings.guixProgram as the first argument to
Agent::Agent.
(pathNullDevice, commonChildInit, Agent, Agent::Agent)
(Agent::~Agent): Move to...
* nix/libutil/util.cc: ... here.
* nix/libutil/util.hh (struct Agent, commonChildInit): New
declarations.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-12 00:19:09 +0100'>2020-01-12</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=7033c7692ccbbbad8f7b9952015de071a5588e87'>daemon: Account for deleted store files when deduplication is on.</a><span class='msg-avail'>...<span class='msg-tooltip'>Previously, a store item that is a regular file would not be accounted
for in the 'bytesFreed' value computed by 'deletePath' because its
'st_nlink' count would always be &gt;= 2.  This commit fixes that.

* nix/libutil/util.hh (deletePath): Add optional 'linkThreshold' argument.
* nix/libutil/util.cc (_deletePath): Add 'linkThreshold' argument and
honor it.  Pass it down in recursive call.
(deletePath): Add 'linkThreshold' and honor it.
* nix/libstore/gc.cc (LocalStore::deleteGarbage): Pass 'linkThreshold'
argument to 'deletePath', with a value of 2 when PATH is a store item
and deduplication is on.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-18 11:31:28 +0200'>2017-08-18</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=ed464f48efd80e6439441d7a9f6d0e7988fb1272'>nix: Remove unused function.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/libutil/util.hh:
* nix/libutil/util.cc (decodeOctalEscaped): Remove unused and buggy function.
</span></span></td><td>Andy Wingo</td></tr>