[PATCH 16/18] conf: Allow user-specified auto-scanned port forwarding ranges

David Gibson
2026-04-07T03:16:43Z

The forwarding table now allows for arbitrary port ranges to be marked as
FWD_SCAN, meaning we don't open sockets for every port, but only those we
scan as listening on the target side. However, there's currently no way
to create such rules, except -[tTuU] auto which always scans every port
with an unspecified listening address and interface.

Allow user-specified "auto" ranges by moving the parsing of the "auto"
keyword from conf_ports(), to conf_ports_spec() as part of the port
specified. "auto" can be combined freely with other port ranges, e.g.
-t 127.0.0.1/auto
-u %lo/5000-7000,auto
-T auto,12345
-U auto,~1-9000

Note that any address and interface given only affects where the automatic
forwards listen, not what addresses we consider when scanning. That is,
if the target side is listening on *any* address, we will create a forward
on the specified address.

Link: https://bugs.passt.top/show_bug.cgi?id=180

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
conf.c | 66 ++++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 48 insertions(+), 18 deletions(-)

diff --git a/conf.c b/conf.c
index fcc75d25..86c30c7f 100644
--- a/conf.c
+++ b/conf.c
@@ -13,6 +13,7 @@
*/

#include <arpa/inet.h>
+#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <getopt.h>
@@ -112,6 +113,28 @@ static int parse_port_range(const char *s, const char **endptr,
return 0;
}

+/**
+ * parse_keyword() - Parse a literal keyword
+ * @s: String to parse
+ * @endptr: Update to the character after the keyword
+ * @kw: Keyword to accept
+ *
+ * Return: 0, if @s starts with @kw, -EINVAL if it does not
+ */
+static int parse_keyword(const char *s, const char **endptr, const char *kw)
+{
+ size_t len = strlen(kw);
+
+ if (strlen(s) < len)
+ return -EINVAL;
+
+ if (memcmp(s, kw, len))
+ return -EINVAL;
+
+ *endptr = s + len;
+ return 0;
+}
+
/**
* conf_ports_range_except() - Set up forwarding for a range of ports minus a
* bitmap of exclusions
@@ -249,6 +272,7 @@ static void conf_ports_spec(const struct ctx *c,
uint8_t exclude[PORT_BITMAP_SIZE] = { 0 };
bool exclude_only = true;
const char *p, *ep;
+ uint8_t flags = 0;
unsigned i;

if (!strcmp(spec, "all")) {
@@ -256,15 +280,32 @@ static void conf_ports_spec(const struct ctx *c,
spec = "";
}

- /* Mark all exclusions first, they might be given after base ranges */
+ /* Parse excluded ranges and "auto" in the first pass */
for_each_chunk(p, ep, spec, ",") {
struct port_range xrange;

- if (*p != '~') {
- /* Not an exclude range, parse later */
+ if (isdigit(*p)) {
+ /* Include range, parse later */
exclude_only = false;
continue;
}
+
+ if (parse_keyword(p, &p, "auto") == 0) {
+ if (p != ep) /* Garbage after the keyword */
+ goto bad;
+
+ if (c->mode != MODE_PASTA) {
+ die(
+"'auto' port forwarding is only allowed for pasta");
+ }
+
+ flags |= FWD_SCAN;
+ continue;
+ }
+
+ /* Should be an exclude range */
+ if (*p != '~')
+ goto bad;
p++;

if (parse_port_range(p, &p, &xrange))
@@ -283,7 +324,7 @@ static void conf_ports_spec(const struct ctx *c,
conf_ports_range_except(c, optname, optarg, fwd,
proto, addr, ifname,
1, NUM_PORTS - 1, exclude,
- 1, FWD_WEAK);
+ 1, flags | FWD_WEAK);
return;
}

@@ -291,8 +332,8 @@ static void conf_ports_spec(const struct ctx *c,
for_each_chunk(p, ep, spec, ",") {
struct port_range orig_range, mapped_range;

- if (*p == '~')
- /* Exclude range, already parsed */
+ if (!isdigit(*p))
+ /* Already parsed */
continue;

if (parse_port_range(p, &p, &orig_range))
@@ -320,7 +361,7 @@ static void conf_ports_spec(const struct ctx *c,
proto, addr, ifname,
orig_range.first, orig_range.last,
exclude,
- mapped_range.first, 0);
+ mapped_range.first, flags);
}

return;
@@ -366,17 +407,6 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
if (proto == IPPROTO_UDP && c->no_udp)
die("UDP port forwarding requested but UDP is disabled");

- if (!strcmp(optarg, "auto")) {
- if (c->mode != MODE_PASTA)
- die("'auto' port forwarding is only allowed for pasta");
-
- conf_ports_range_except(c, optname, optarg, fwd,
- proto, NULL, NULL,
- 1, NUM_PORTS - 1, NULL, 1, FWD_SCAN);
-
- return;
- }
-
strncpy(buf, optarg, sizeof(buf) - 1);

if ((spec = strchr(buf, '/'))) {
--
2.53.0