Note that a newer, better version of this is available from
http://www.palomine.net/qmail/checkcdb.tar.gz. It's functionally identical to
this version, but the alternate user file is stored in a cdb database, rather
than a flat text file, for *much* faster lookups.

Apply this patch to checkpassword.c from DJB's checkpassword-0.81 package.

By Chris Johnson <cjohnson-qmail@palomine.net>, based on a similar
implementation by Jedi/Sector One <j@4u.net>.

This version of checkpassword allows an alternate pop user password file,
consisting of lines of the format: pop_login:crypted_password:real_login:path

For example:

joe:I5zMOM3LREGQA:popuser:/var/qmail/popboxes/joe

It looks for the user first in the system password file. If the user isn't
found there, it looks in /var/qmail/users/poppasswd.

--- checkpassword.c.orig	Sat Nov 21 15:29:52 1998
+++ checkpassword.c	Sun Mar 28 17:11:13 1999
@@ -1,3 +1,12 @@
+#define CONFIG_FILE "/var/qmail/users/poppasswd"
+
+#ifndef LINE_MAX
+#define LINE_MAX 2048
+#endif
+
+#include <string.h>
+#include <stdio.h>
+
 #include <errno.h>
 extern int errno;
 extern char *crypt();
@@ -15,6 +24,7 @@
 #include <pwd.h>
 static struct passwd *pw;
 static char *stored;
+static char *userdir;
 
 #include "hasspnam.h"
 #ifdef HASGETSPNAM
@@ -31,29 +41,56 @@
 void doit(login)
 char *login;
 {
+  static char	line[LINE_MAX + 1];
+  FILE		*fp;
+  char		*lineptr, *newlogin = NULL;
+
   pw = getpwnam(login);
-  if (pw)
+  if (pw) {
     stored = pw->pw_passwd;
-  else {
-    if (errno == error_txtbsy) _exit(111);
-    _exit(1);
-  }
+    userdir = pw->pw_dir;
 
 #ifdef HASUSERPW
-  upw = getuserpw(login);
-  if (upw)
-    stored = upw->upw_passwd;
-  else
-    if (errno == error_txtbsy) _exit(111);
+    upw = getuserpw(login);
+    if (upw)
+      stored = upw->upw_passwd;
+    else
+      if (errno == error_txtbsy) _exit(111);
 #endif
 
 #ifdef HASGETSPNAM
-  spw = getspnam(login);
-  if (spw)
-    stored = spw->sp_pwdp;
-  else
+    spw = getspnam(login);
+    if (spw)
+      stored = spw->sp_pwdp;
+    else
+      if (errno == error_txtbsy) _exit(111);
+#endif 
+
+    return;
+  }
+  if (errno == error_txtbsy) _exit(111);
+
+  if (!(fp = fopen(CONFIG_FILE, "r"))) _exit(2);
+  while (fgets(line, LINE_MAX, fp)) { 
+    if (!(lineptr = strtok(line, ":"))) _exit(2);
+    if (strcmp(lineptr, login) == 0) {
+      if (!(lineptr = strtok(NULL, ":"))) _exit(2);
+      stored = lineptr;
+      if (!(lineptr = strtok(NULL, ":"))) _exit(2);
+      newlogin = lineptr;
+      if (!(lineptr = strtok(NULL, ":"))) _exit(2);
+      userdir = lineptr;
+      if (lineptr = strchr(userdir, '\n')) *lineptr = '\0';  
+      break;
+    }
+  }
+  fclose(fp);
+  if (!newlogin) _exit(1);
+  pw = getpwnam(newlogin);
+  if (!pw) {
     if (errno == error_txtbsy) _exit(111);
-#endif
+    _exit(1);
+  }
 }
 
 char *str1e2(name,value) char *name; char *value;
@@ -114,7 +151,7 @@
  
   if (prot_gid((int) pw->pw_gid) == -1) _exit(1);
   if (prot_uid((int) pw->pw_uid) == -1) _exit(1);
-  if (chdir(pw->pw_dir) == -1) _exit(111);
+  if (chdir(userdir) == -1) _exit(111);
  
   numenv = 0;
   while (environ[numenv]) ++numenv;
@@ -122,7 +159,7 @@
   if (!newenv) _exit(111);
   for (i = 0;i < numenv;++i) newenv[i] = environ[i];
   newenv[numenv++] = str1e2("USER",pw->pw_name);
-  newenv[numenv++] = str1e2("HOME",pw->pw_dir);
+  newenv[numenv++] = str1e2("HOME",userdir);
   newenv[numenv++] = str1e2("SHELL",pw->pw_shell);
   newenv[numenv] = 0;
   environ = newenv;