ref: a1c3c34c70431447a89fbe08c3475dbfaee7b672
dir: /sys/src/cmd/upas/smtp/greylist.c/
/*
* greylisting is the practice of making unknown callers call twice, with
* a pause between them, before accepting their mail and adding them to a
* whitelist of known callers.
*
* There's a bit of a problem with yahoo and other large sources of mail;
* they have a vast pool of machines that all run the same queue(s), so a
* 451 retry can come from a different IP address for many, many retries,
* and it can take ~5 hours for the same IP to call us back. To cope
* better with this, we immediately accept mail from any system on the
* same class C subnet (IPv4 /24) as anybody on our whitelist, since the
* mail-sending machines tend to be clustered within a class C subnet.
*
* Various other goofballs, notably the IEEE, try to send mail just
* before 9 AM, then refuse to try again until after 5 PM. D'oh!
*/
#include "common.h"
#include "smtpd.h"
#include "smtp.h"
#include <ctype.h>
#include <ip.h>
#include <ndb.h>
enum {
Nonspammax = 14*60*60, /* must call back within this time if real */
Nonspammin = 5*60, /* must wait this long to retry */
};
typedef struct {
int existed; /* these two are distinct to cope with errors */
int created;
int noperm;
long mtime; /* mod time, iff it already existed */
} Greysts;
static char whitelist[] = "/mail/grey/whitelist";
/*
* matches ip addresses or subnets in whitelist against nci->rsys.
* ignores comments and blank lines in /mail/grey/whitelist.
*/
static int
onwhitelist(void)
{
char *line, *p;
Biobuf *wl;
wl = Bopen(whitelist, OREAD);
if (wl == nil)
return 0;
while ((line = Brdline(wl, '\n')) != nil) {
line[Blinelen(wl)-1] = '\0'; /* clobber newline */
p = strpbrk(line, " \t");
if (p)
*p = 0;
if (line[0] == '#' || line[0] == 0)
continue;
if(ipcheck(line))
break;
}
Bterm(wl);
return line != nil;
}
static int mkdirs(char *);
/*
* if any directories leading up to path don't exist, create them.
* modifies but restores path.
*/
static int
mkpdirs(char *path)
{
int rv = 0;
char *sl = strrchr(path, '/');
if (sl != nil) {
*sl = '\0';
rv = mkdirs(path);
*sl = '/';
}
return rv;
}
/*
* if path or any directories leading up to it don't exist, create them.
* modifies but restores path.
*/
static int
mkdirs(char *path)
{
int fd;
if (access(path, AEXIST) >= 0)
return 0;
/* make presumed-missing intermediate directories */
if (mkpdirs(path) < 0)
return -1;
/* make final directory */
fd = create(path, OREAD, 0777|DMDIR);
if (fd < 0)
/*
* we may have lost a race; if the directory now exists,
* it's okay.
*/
return access(path, AEXIST) < 0? -1: 0;
close(fd);
return 0;
}
static long
getmtime(char *file)
{
int fd;
long mtime = -1;
Dir *ds;
fd = open(file, ORDWR);
if (fd < 0)
return mtime;
ds = dirfstat(fd);
if (ds != nil) {
mtime = ds->mtime;
/*
* new twist: update file's mtime after reading it,
* so each call resets the future time after which
* we'll accept calls. thus spammers who keep pounding
* us lose, but just pausing for a few minutes and retrying
* will succeed.
*/
if (0) {
/*
* apparently none can't do this wstat
* (permission denied);
* more undocumented whacky none behaviour.
*/
ds->mtime = time(0);
if (dirfwstat(fd, ds) < 0)
syslog(0, "smtpd", "dirfwstat %s: %r", file);
}
free(ds);
write(fd, "x", 1);
}
close(fd);
return mtime;
}
static void
tryaddgrey(char *file, Greysts *gsp)
{
int fd = create(file, OWRITE|OEXCL, 0666);
gsp->created = (fd >= 0);
if (fd >= 0) {
close(fd);
gsp->existed = 0; /* just created; couldn't have existed */
gsp->mtime = time(0);
} else {
/*
* why couldn't we create file? it must have existed
* (or we were denied perm on parent dir.).
* if it existed, fill in gsp->mtime; otherwise
* make presumed-missing intermediate directories.
*/
gsp->existed = access(file, AEXIST) >= 0;
if (gsp->existed)
gsp->mtime = getmtime(file);
else if (mkpdirs(file) < 0)
gsp->noperm = 1;
}
}
static void
addgreylist(char *file, Greysts *gsp)
{
tryaddgrey(file, gsp);
if (!gsp->created && !gsp->existed && !gsp->noperm)
/* retry the greylist entry with parent dirs created */
tryaddgrey(file, gsp);
}
static int
recentcall(Greysts *gsp)
{
long delay = time(0) - gsp->mtime;
if (!gsp->existed)
return 0;
/* reject immediate call-back; spammers are doing that now */
return delay >= Nonspammin && delay <= Nonspammax;
}
/*
* policy: if (caller-IP, my-IP, rcpt) is not on the greylist,
* reject this message as "451 temporary failure". if the caller is real,
* he'll retry soon, otherwise he's a spammer.
* at the first rejection, create a greylist entry for (my-ip, caller-ip,
* rcpt, time), where time is the file's mtime. if they call back and there's
* already a greylist entry, and it's within the allowed interval,
* add their IP to the append-only whitelist.
*
* greylist files can be removed at will; at worst they'll cause a few
* extra retries.
*/
static int
isrcptrecent(char *rcpt)
{
char *user;
char file[256];
Greysts gs;
Greysts *gsp = &gs;
if (rcpt[0] == '\0' || strchr(rcpt, '/') != nil ||
strcmp(rcpt, ".") == 0 || strcmp(rcpt, "..") == 0)
return 0;
/* shorten names to fit pre-fossil or pre-9p2000 file servers */
user = strrchr(rcpt, '!');
if (user == nil)
user = rcpt;
else
user++;
/* check & try to update the grey list entry */
snprint(file, sizeof file, "/mail/grey/tmp/%s/%s/%s",
nci->lsys, nci->rsys, user);
memset(gsp, 0, sizeof *gsp);
addgreylist(file, gsp);
/* if on greylist already and prior call was recent, add to whitelist */
if (gsp->existed && recentcall(gsp)) {
syslog(0, "smtpd",
"%s/%s was grey; adding IP to white", nci->rsys, rcpt);
return 1;
} else if (gsp->existed)
syslog(0, "smtpd", "call for %s/%s was just minutes ago "
"or long ago", nci->rsys, rcpt);
else
syslog(0, "smtpd", "no call registered for %s/%s; registering",
nci->rsys, rcpt);
return 0;
}
void
vfysenderhostok(void)
{
char *fqdn;
int recent = 0;
Link *l;
if (onwhitelist())
return;
for (l = rcvers.first; l; l = l->next)
if (isrcptrecent(s_to_c(l->p)))
recent = 1;
/* if on greylist already and prior call was recent, add to whitelist */
if (recent) {
int fd = create(whitelist, OWRITE, 0666|DMAPPEND);
if (fd >= 0) {
seek(fd, 0, 2); /* paranoia */
fqdn = csgetvalue(nci->root, "ip", nci->rsys, "dom",
nil);
if (fqdn != nil)
fprint(fd, "%s %s\n", nci->rsys, fqdn);
else
fprint(fd, "%s\n", nci->rsys);
free(fqdn);
close(fd);
}
} else {
syslog(0, "smtpd",
"no recent call from %s for a rcpt; rejecting with temporary failure",
nci->rsys);
reply("451 please try again soon from the same IP.\r\n");
exits("no recent call for a rcpt");
}
}