diff --git a/ChangeLog b/ChangeLog
index 4fd317f4a9..3b7eebe0ed 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,13 @@ Changes in version 0.2.1.7-alpha - 2008-11-xx
       exit policy doesn't allow it, we would remember what IP address
       the relay said the destination address resolves to, even if it's
       an internal IP address. Bugfix on 0.2.0.7-alpha; patch by rovv.
+    - The "User" and "Group" config options did not clear the
+      supplementary group entries for the process. The "User" option
+      has been made more robust, and also now also sets the groups to
+      the specified user's primary group. The "Group" option is now
+      ignored. For more detailed logging on credential switching, set
+      CREDENTIAL_LOG_LEVEL in common/compat.c to LOG_NOTICE or higher;
+      patch by Jacob Appelbaum and Steven Murdoch.
 
   o Minor features:
     - Now NodeFamily and MyFamily config options allow spaces in
diff --git a/configure.in b/configure.in
index d383a627a1..878a8da97f 100644
--- a/configure.in
+++ b/configure.in
@@ -630,6 +630,9 @@ syslog_facility="$withval", syslog_facility="LOG_DAEMON")
 AC_DEFINE_UNQUOTED(LOGFACILITY,$syslog_facility,[name of the syslog facility])
 AC_SUBST(LOGFACILITY)
 
+# Check if we have getresuid and getresgid
+AC_CHECK_FUNCS(getresuid getresgid)
+
 # Check for gethostbyname_r in all its glorious incompatible versions.
 #   (This logic is based on that in Python's configure.in)
 AH_TEMPLATE(HAVE_GETHOSTBYNAME_R,
diff --git a/doc/tor.1.in b/doc/tor.1.in
index 67eb17e2a6..67a7db4f9f 100644
--- a/doc/tor.1.in
+++ b/doc/tor.1.in
@@ -264,10 +264,6 @@ script to enumerate Tor nodes that exit to certain addresses.
 (Default: 0)
 .LP
 .TP
-\fBGroup \fR\fIGID\fP
-On startup, setgid to this group.
-.LP
-.TP
 \fBHttpProxy\fR \fIhost\fR[:\fIport\fR]\fP
 Tor will make all its directory requests through this host:port
 (or host:80 if port is not specified),
@@ -350,7 +346,7 @@ about what sites a user might have visited. (Default: 1)
 .LP
 .TP
 \fBUser \fR\fIUID\fP
-On startup, setuid to this user.
+On startup, setuid to this user and setgid to their primary group.
 .LP
 .TP
 \fBHardwareAccel \fR\fB0\fR|\fB1\fP
diff --git a/src/common/compat.c b/src/common/compat.c
index 31029844d1..3678913a08 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -920,61 +920,215 @@ set_max_file_descriptors(rlim_t limit, int *max_out)
   return 0;
 }
 
-/** Call setuid and setgid to run as <b>user</b>:<b>group</b>.  Return 0 on
- * success.  On failure, log and return -1.
+/** Log details of current user and group credentials. Return 0 on
+ * success. Logs and return -1 on failure.
  */
 int
-switch_id(const char *user, const char *group)
+log_credential_status()
+{
+#define CREDENTIAL_LOG_LEVEL LOG_INFO
+#ifndef MS_WINDOWS
+  /* Real, effective and saved UIDs */
+  uid_t ruid, euid, suid;
+  /* Read, effective and saved GIDs */
+  gid_t rgid, egid, sgid;
+  /* Supplementary groups */
+  gid_t sup_gids[NGROUPS_MAX + 1];
+  /* Number of supplementary groups */
+  int ngids;
+
+  /* log UIDs */
+#ifdef HAVE_GETRESUID
+  if (getresuid(&ruid, &euid, &suid) != 0 ) {
+    log_warn(LD_GENERAL, "Error getting changed UIDs: %s", strerror(errno));
+    return -1;
+  } else {
+    log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "UID is %u (real), %u (effective), %u (saved)", ruid, euid, suid);
+  }
+#else
+  /* getresuid is not present on MacOS X, so we can't get the saved (E)UID */
+  ruid = getuid();
+  euid = geteuid();
+  (void)suid;
+
+  log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "UID is %u (real), %u (effective), unknown (saved)", ruid, euid);
+#endif
+
+  /* log GIDs */
+#ifdef HAVE_GETRESGID
+  if (getresgid(&rgid, &egid, &sgid) != 0 ) {
+    log_warn(LD_GENERAL, "Error getting changed GIDs: %s", strerror(errno));
+    return -1;
+  } else {
+    log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "GID is %u (real), %u (effective), %u (saved)", rgid, egid, sgid);
+  }
+#else
+  /* getresgid is not present on MacOS X, so we can't get the saved (E)GID */
+  rgid = getgid();
+  egid = getegid();
+  (void)sgid;
+  log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "GID is %u (real), %u (effective), unknown (saved)", rgid, egid);
+#endif
+
+  /* log supplementary groups */
+  if((ngids = getgroups(NGROUPS_MAX + 1, sup_gids)) < 0) {
+    log_warn(LD_GENERAL, "Error getting supplementary GIDs: %s", strerror(errno));
+    return -1;
+  } else {
+    int i;
+    char *strgid;
+    char *s = NULL;
+    int formatting_error = 0;
+    smartlist_t *elts = smartlist_create();
+
+    for (i = 0; i<ngids; i++) {
+      strgid = tor_malloc(11);
+      if (tor_snprintf(strgid, 11, "%u", (unsigned)sup_gids[i]) == -1) {
+        log_warn(LD_GENERAL, "Error printing supplementary GIDs");
+        formatting_error = 1;
+        goto error;
+      }
+      smartlist_add(elts, strgid);
+    }
+
+    s = smartlist_join_strings(elts, " ", 0, NULL);
+
+    log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "Supplementary groups are: %s", s);
+
+   error:
+    tor_free(s);
+    SMARTLIST_FOREACH(elts, char *, cp,
+    {
+      tor_free(cp);
+    });
+    smartlist_free(elts);
+
+    if (formatting_error)
+      return -1;
+  }
+#endif
+
+  return 0;
+}
+
+/** Call setuid and setgid to run as <b>user</b> and only switch to their
+ * primary group.  Return 0 on success.  On failure, log and return -1.
+ */
+int
+switch_id(const char *user)
 {
 #ifndef MS_WINDOWS
   struct passwd *pw = NULL;
-  struct group *gr = NULL;
+  uid_t old_uid;
+  gid_t old_gid;
+  
+  tor_assert(user);
 
+  /* Log the initial credential state */ 
+  if (user) {
+    if (log_credential_status()) {
+      return -1;
+    }
+  }
+  log_fn(CREDENTIAL_LOG_LEVEL, LD_GENERAL, "Changing user and groups");
+
+  /* Get old UID/GID to check if we changed correctly */
+  old_uid = getuid();
+  old_gid = getgid();
+
+  /* Lookup the user and group information, if we have a problem, bail out. */
   if (user) {
     pw = getpwnam(user);
     if (pw == NULL) {
-      log_warn(LD_CONFIG,"User '%s' not found.", user);
+      log_warn(LD_CONFIG, "Error setting configured user: "
+      "'%s' not found.", user);
+      return -1;
+    }
+  } else {
+    /* We have no user supplied and so we'll bail out. */
+    log_warn(LD_CONFIG, "Error setting configured user: No user supplied.");
+    return -1;
+  }
+
+  /* Properly switch egid,gid,euid,uid here or bail out */
+  if (setgroups(1, &pw->pw_gid)) {
+    log_warn(LD_GENERAL, "Error setting configured groups: %s",
+    strerror(errno));
+    return -1;
+  }
+
+  if (setegid(pw->pw_gid)) {
+    log_warn(LD_GENERAL, "Error setting configured egid: %s",
+    strerror(errno));
+    return -1;
+  }
+
+  if (setgid(pw->pw_gid)) {
+    log_warn(LD_GENERAL, "Error setting configured gid: %s",
+    strerror(errno));
+    return -1;
+  }
+
+  if (setuid(pw->pw_uid)) {
+    log_warn(LD_GENERAL, "Error setting configured uid: %s",
+    strerror(errno));
+    return -1;
+  }
+
+  if (seteuid(pw->pw_uid)){
+    log_warn(LD_GENERAL, "Error setting configured euid: %s",
+    strerror(errno));
+    return -1;
+  }
+
+  /* This is how OpenBSD rolls:
+  if (setgroups(1, &pw->pw_gid) || setegid(pw->pw_gid) ||
+      setgid(pw->pw_gid) || setuid(pw->pw_uid) || seteuid(pw->pw_uid)) {
+      setgid(pw->pw_gid) || seteuid(pw->pw_uid) || setuid(pw->pw_uid)) {
+    log_warn(LD_GENERAL, "Error setting configured UID/GID: %s",
+    strerror(errno));
+    return -1;
+  }
+  */
+
+  /* We've properly switched egid, gid, euid, uid, and supplementary groups if
+   * we're here. */
+
+#if !defined(CYGWIN) && !defined(__CYGWIN__)
+  /* If we tried to drop privilege to a group/user other than root, attempt to
+   * restore root (E)(U|G)ID, and abort if the operation succeeds */
+
+  /* Only check for privilege dropping if we were asked to be non-root */
+  if (pw->pw_uid) {
+    /* Try changing GID/EGID */
+    if (pw->pw_gid != old_gid && (setgid(old_gid) != -1 || setegid(old_gid) != -1)) {
+      log_warn(LD_GENERAL, "Was able to restore group credentials");
+      return -1;
+    }
+  
+    /* Try changing UID/EUID */
+    if (pw->pw_uid != old_uid && (setuid(old_uid) != -1 || seteuid(old_uid) != -1)) {
+      log_warn(LD_GENERAL, "Was able to restore user credentials");
       return -1;
     }
   }
+#endif
 
-  /* switch the group first, while we still have the privileges to do so */
-  if (group) {
-    gr = getgrnam(group);
-    if (gr == NULL) {
-      log_warn(LD_CONFIG,"Group '%s' not found.", group);
-      return -1;
-    }
-
-    if (setgid(gr->gr_gid) != 0) {
-      log_warn(LD_GENERAL,"Error setting to configured GID: %s",
-               strerror(errno));
-      return -1;
-    }
-  } else if (user) {
-    if (setgid(pw->pw_gid) != 0) {
-      log_warn(LD_GENERAL,"Error setting to user GID: %s", strerror(errno));
-      return -1;
-    }
-  }
-
-  /* now that the group is switched, we can switch users and lose
-     privileges */
+  /* Check what really happened */
   if (user) {
-    if (setuid(pw->pw_uid) != 0) {
-      log_warn(LD_GENERAL,"Error setting UID: %s", strerror(errno));
+    if (log_credential_status()) {
       return -1;
     }
   }
 
   return 0;
+
 #else
   (void)user;
-  (void)group;
 #endif
 
   log_warn(LD_CONFIG,
-           "User or group specified, but switching users is not supported.");
+           "User specified but switching users is unsupported on your OS.");
   return -1;
 }
 
diff --git a/src/common/compat.h b/src/common/compat.h
index f0036d7764..748b4e6538 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -447,7 +447,7 @@ void set_uint32(char *cp, uint32_t v) ATTR_NONNULL((1));
 typedef unsigned long rlim_t;
 #endif
 int set_max_file_descriptors(rlim_t limit, int *max);
-int switch_id(const char *user, const char *group);
+int switch_id(const char *user);
 #ifdef HAVE_PWD_H
 char *get_user_homedir(const char *username);
 #endif
diff --git a/src/or/config.c b/src/or/config.c
index bd42860011..66ee6cd52b 100644
--- a/src/or/config.c
+++ b/src/or/config.c
@@ -219,7 +219,7 @@ static config_var_t _option_vars[] = {
   V(GeoIPFile,                   FILENAME,
     SHARE_DATADIR PATH_SEPARATOR "tor" PATH_SEPARATOR "geoip"),
 #endif
-  V(Group,                       STRING,   NULL),
+  OBSOLETE("Group"),
   V(HardwareAccel,               BOOL,     "0"),
   V(HashedControlPassword,       LINELIST, NULL),
   V(HidServDirectoryV2,          BOOL,     "1"),
@@ -434,7 +434,6 @@ static config_var_description_t options_description[] = {
   /* { "FastFirstHopPK", "" }, */
   /* FetchServerDescriptors, FetchHidServDescriptors,
    * FetchUselessDescriptors */
-  { "Group", "On startup, setgid to this group." },
   { "HardwareAccel", "If set, Tor tries to use hardware crypto accelerators "
     "when it can." },
   /* HashedControlPassword */
@@ -1084,13 +1083,12 @@ options_act_reversible(or_options_t *old_options, char **msg)
 #endif
 
   /* Setuid/setgid as appropriate */
-  if (options->User || options->Group) {
+  if (options->User) {
     /* XXXX021 We should only do this the first time through, not on
      * every setconf. */
-    if (switch_id(options->User, options->Group) != 0) {
+    if (switch_id(options->User) != 0) {
       /* No need to roll back, since you can't change the value. */
-      *msg = tor_strdup("Problem with User or Group value. "
-                        "See logs for details.");
+      *msg = tor_strdup("Problem with User value. See logs for details.");
       goto done;
     }
   }