Use getpeerucred if available (not tested).

compat/getpeereid.c

@@ -19,6 +19,10 @@
 
 #include <stdio.h>
 
+#ifdef HAVE_UCRED_H
+#include <ucred.h>
+#endif
+
 #include "compat.h"
 
 int
@@ -33,6 +37,21 @@ getpeereid(int s, uid_t *uid, gid_t *gid)
 	*uid = uc.uid;
 	*gid = uc.gid;
 	return (0);
+#elif defined(HAVE_GETPEERUCRED)
+int
+getpeereid(int s, uid_t *uid, gid_t *gid)
+{
+        ucred_t *ucred = NULL;
+
+        if (getpeerucred(s, &ucred) == -1)
+                return (-1);
+        if ((*uid = ucred_geteuid(ucred)) == -1)
+                return (-1);
+        if ((*gid = ucred_getrgid(ucred)) == -1)
+                return (-1);
+        ucred_free(ucred);
+        return (0);
+}
 #else
 	errno = EOPNOTSUPP;
 	return (-1);

configure.ac

@@ -128,6 +128,7 @@ AC_CHECK_HEADERS([ \
 	sys/dir.h \
 	sys/ndir.h \
 	sys/tree.h \
+	ucred.h \
 	util.h \
 ])
 
@@ -146,7 +147,8 @@ AC_CHECK_FUNCS([ \
 	flock \
 	prctl \
 	proc_pidinfo \
-	sysconf
+	getpeerucred \
+	sysconf \
 ])
 
 # Check for functions with a compatibility implementation.
@@ -170,7 +172,7 @@ AC_REPLACE_FUNCS([ \
 	strlcat \
 	strlcpy \
 	strndup \
-	strsep
+	strsep \
 ])
 AC_FUNC_STRNLEN