Thanks to John Cummings.

src/cmd/upas/smtp/greylist.c

@@ -0,0 +1,274 @@
+#include "common.h"
+#include "smtpd.h"
+#include "smtp.h"
+#include <ctype.h>
+#include <ip.h>
+#include <ndb.h>
+
+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;
+
+/*
+ * There's a bit of a problem with yahoo; they apparently 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.  Various other goofballs,
+ * notably the IEEE, try to send mail just before 9 AM, then refuse to try
+ * again until after 5 PM.  Doh!
+ */
+enum {
+	Nonspammax = 14*60*60,  /* must call back within this time if real */
+};
+static char whitelist[] = "/mail/lib/whitelist";
+
+/*
+ * matches ip addresses or subnets in whitelist against nci->rsys.
+ * ignores comments and blank lines in /mail/lib/whitelist.
+ */
+static int
+onwhitelist(void)
+{
+	int lnlen;
+	char *line, *parse;
+	char input[128];
+	uchar ip[IPaddrlen], ipmasked[IPaddrlen];
+	uchar mask4[IPaddrlen], addr4[IPaddrlen];
+	uchar mask[IPaddrlen], addr[IPaddrlen], addrmasked[IPaddrlen];
+	Biobuf *wl;
+	static int beenhere;
+	static allzero[IPaddrlen];
+
+	if (!beenhere) {
+		beenhere = 1;
+		fmtinstall('I', eipfmt);
+	}
+
+	parseip(ip, nci->rsys);
+	wl = Bopen(whitelist, OREAD);
+	if (wl == nil)
+		return 1;
+	while ((line = Brdline(wl, '\n')) != nil) {
+		if (line[0] == '#' || line[0] == '\n')
+			continue;
+		lnlen = Blinelen(wl);
+		line[lnlen-1] = '\0';		/* clobber newline */
+
+		/* default mask is /32 (v4) or /128 (v6) for bare IP */
+		parse = line;
+		if (strchr(line, '/') == nil) {
+			strncpy(input, line, sizeof input - 5);
+			if (strchr(line, '.') != nil)
+				strcat(input, "/32");
+			else
+				strcat(input, "/128");
+			parse = input;
+		}
+		/* sorry, dave; where's parsecidr for v4 or v6? */
+		v4parsecidr(addr4, mask4, parse);
+		v4tov6(addr, addr4);
+		v4tov6(mask, mask4);
+
+		maskip(addr, mask, addrmasked);
+		maskip(ip, mask, ipmasked);
+		if (memcmp(ipmasked, addrmasked, IPaddrlen) == 0)
+			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)
+{
+	long mtime = -1;
+	Dir *ds = dirstat(file);
+
+	if (ds != nil) {
+		mtime = ds->mtime;
+		free(ds);
+	}
+	return mtime;
+}
+
+static void
+tryaddgrey(char *file, Greysts *gsp)
+{
+	int fd = create(file, OWRITE|OEXCL, 0444|DMEXCL);
+
+	gsp->created = (fd >= 0);
+	if (fd >= 0) {
+		close(fd);
+		gsp->existed = 0;  /* just created; couldn't have existed */
+	} 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 >= 30 && 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/%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 seconds 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 */
+			if ((fqdn = csgetvalue(nil, "ip", nci->rsys, "dom", nil)) != nil)
+				fprint(fd, "# %s\n%s\n\n", fqdn, nci->rsys);
+			else
+				fprint(fd, "# unknown\n%s\n\n", nci->rsys);
+			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");
+	}
+}