[PATCH v3 01/11] conf: Support CIDR notation for -a/--address option

[Jon Maloy <jmaloy@redhat.com>]

Extend the -a/--address option to accept addresses in CIDR notation
(e.g., 192.168.1.1/24 or 2001:db8::1/64) as an alternative to using
separate -a and -n options.

We add a new inany_prefix_pton() helper function that:
- Parses address strings with optional /prefix_len suffix
- Validates prefix length based on address family (0-32 for IPv4,
  0-128 for IPv6), including handling of IPv4-to-IPv6 mapping case.
- Returns identified address family, if any.

For IPv4, the prefix length is stored in ip4.prefix_len when provided.
Mixing -n and CIDR notation results in an error to catch likely user
mistakes.

Also fix a bug in conf_ip4_prefix() that was incorrectly using the
global 'optarg' instead of its 'arg' parameter.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

---
v3: Fixes after feedback from Laurent, David and Stefano
    Notably, updated man page for the -a option

v4: Fixes based on feedback from David G:
  - Handling prefix length adjustment when IPv4-to-IPv6 mapping
  - Removed redundant !IN6_IS_ADDR_V4MAPPED(&addr.a6) test
  - Simplified tests of acceptable address types
  - Merged documentation and code commits
  - Some documentation text clarifications

v5: - Moved address/prefix parsing into a refactored
      inany_prefix_pton() function.
    - inany_prefix_pton() now only caluclates IPv6 style
      prefix lengths
    - Stricter distinction between error causes.
    - Some refactoring of the 'case a:' branch in conf()
    - Some small fixes in passt.1
---
 conf.c  | 58 +++++++++++++++++++++++++++++-------------------
 inany.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 inany.h |  1 +
 ip.c    | 21 ++++++++++++++++++
 ip.h    |  2 ++
 passt.1 | 17 ++++++++++-----
 6 files changed, 139 insertions(+), 28 deletions(-)

diff --git a/conf.c b/conf.c
index 2942c8c..98d5d17 100644
--- a/conf.c
+++ b/conf.c
@@ -682,7 +682,7 @@ static int conf_ip4_prefix(const char *arg)
 			return -1;
 	} else {
 		errno = 0;
-		len = strtoul(optarg, NULL, 0);
+		len = strtoul(arg, NULL, 0);
 		if (len > 32 || errno)
 			return -1;
 	}
@@ -896,7 +896,7 @@ static void usage(const char *name, FILE *f, int status)
 		"    a zero value disables assignment\n"
 		"    default: 65520: maximum 802.3 MTU minus 802.3 header\n"
 		"                    length, rounded to 32 bits (IPv4 words)\n"
-		"  -a, --address ADDR	Assign IPv4 or IPv6 address ADDR\n"
+		"  -a, --address ADDR	Assign IPv4 or IPv6 address ADDR[/PREFIXLEN]\n"
 		"    can be specified zero to two times (for IPv4 and IPv6)\n"
 		"    default: use addresses from interface with default route\n"
 		"  -n, --netmask MASK	Assign IPv4 MASK, dot-decimal or bits\n"
@@ -1498,6 +1498,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	const char *optstring = "+dqfel:hs:F:I:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:T:U:";
 	const char *logname = (c->mode == MODE_PASTA) ? "pasta" : "passt";
 	char userns[PATH_MAX] = { 0 }, netns[PATH_MAX] = { 0 };
+	bool prefix_from_cidr = false, prefix_from_opt = false;
 	bool copy_addrs_opt = false, copy_routes_opt = false;
 	enum fwd_ports_mode fwd_default = FWD_NONE;
 	bool v4_only = false, v6_only = false;
@@ -1808,35 +1809,46 @@ void conf(struct ctx *c, int argc, char **argv)
 			c->mtu = mtu;
 			break;
 		}
-		case 'a':
-			if (inet_pton(AF_INET6, optarg, &c->ip6.addr)	&&
-			    !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr)	&&
-			    !IN6_IS_ADDR_LOOPBACK(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_V4MAPPED(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_V4COMPAT(&c->ip6.addr)		&&
-			    !IN6_IS_ADDR_MULTICAST(&c->ip6.addr)) {
-				if (c->mode == MODE_PASTA)
-					c->ip6.no_copy_addrs = true;
-				break;
-			}
-
-			if (inet_pton(AF_INET, optarg, &c->ip4.addr)	&&
-			    !IN4_IS_ADDR_UNSPECIFIED(&c->ip4.addr)	&&
-			    !IN4_IS_ADDR_BROADCAST(&c->ip4.addr)	&&
-			    !IN4_IS_ADDR_LOOPBACK(&c->ip4.addr)		&&
-			    !IN4_IS_ADDR_MULTICAST(&c->ip4.addr)) {
+		case 'a': {
+			union inany_addr addr = { 0 };
+			int prefix_len = 0;
+			int af;
+
+			af = inany_prefix_pton(optarg, &addr, &prefix_len);
+
+			if (inany_is_unspecified(&addr) ||
+			    inany_is_multicast(&addr) ||
+			    inany_is_loopback(&addr) ||
+			    IN6_IS_ADDR_V4COMPAT(&addr.a6))
+				die("Invalid address: %s", optarg);
+
+			if (prefix_len && !prefix_from_opt)
+				prefix_from_cidr = true;
+			else if (prefix_len)
+				die("Can't mix CIDR with -n");
+
+			if (af == AF_INET) {
+				c->ip4.addr = *inany_v4(&addr);
+				c->ip4.prefix_len = prefix_len ? prefix_len - 96 :
+					ip4_class_prefix_len(&c->ip4.addr);
 				if (c->mode == MODE_PASTA)
 					c->ip4.no_copy_addrs = true;
-				break;
+			} else if (af == AF_INET6) {
+				c->ip6.addr = addr.a6;
+				if (c->mode == MODE_PASTA)
+					c->ip6.no_copy_addrs = true;
+			} else {
+				die("Invalid prefix length: %d", prefix_len);
 			}
-
-			die("Invalid address: %s", optarg);
 			break;
+		}
 		case 'n':
+			if (prefix_from_cidr)
+				die("Can't use both -n and CIDR prefix length");
 			c->ip4.prefix_len = conf_ip4_prefix(optarg);
 			if (c->ip4.prefix_len < 0)
 				die("Invalid netmask: %s", optarg);
-
+			prefix_from_opt = true;
 			break;
 		case 'M':
 			parse_mac(c->our_tap_mac, optarg);
diff --git a/inany.c b/inany.c
index 7680439..00d44e0 100644
--- a/inany.c
+++ b/inany.c
@@ -11,6 +11,7 @@
 #include <assert.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <errno.h>
 
 #include "util.h"
 #include "ip.h"
@@ -57,3 +58,70 @@ int inany_pton(const char *src, union inany_addr *dst)
 
 	return 0;
 }
+
+/** inany_prefix_pton - Parse an IPv[46] address with prefix length adjustment
+ * @src:	IPv[46] address string
+ * @dst:	output buffer, filled with parsed address
+ * @prefix_len:	pointer to prefix length
+ *
+ * Return: AF_INET for IPv4, AF_INET6 for IPv6, -1 on error
+ */
+int inany_prefix_pton(char *src, union inany_addr *dst, int *prefix_len)
+{
+	bool mapped = false;
+	struct in6_addr a6;
+	struct in_addr a4;
+	char *slash;
+	char *end;
+	int af;
+
+	*prefix_len = 0;
+
+	/* Check for presence of /prefix_len suffix */
+	slash = strchr(src, '/');
+	if (slash)
+		*slash = '\0';
+
+	/* Read address */
+	if (inet_pton(AF_INET, src, &a4)) {
+		inany_from_af(dst, AF_INET, &a4);
+		af = AF_INET;
+	} else if (inet_pton(AF_INET6, src, &a6)) {
+		inany_from_af(dst, AF_INET6, &a6);
+		af = AF_INET6;
+		if (inany_v4(dst))
+			mapped = true;
+	} else {
+		memset(dst, 0, sizeof(*dst));
+		return -1;
+	}
+
+	if (!slash)
+		return mapped ? AF_INET : af;
+
+	/* Read prefix_len - /0 is not allowed */
+	errno = 0;
+	*prefix_len = strtoul(slash + 1, &end, 10);
+	if (errno || *end || *prefix_len == 0)
+		return -1;
+
+	if (mapped) {
+		/* IPv4-mapped: prefix already in IPv6 format, must be 96-128 */
+		if (*prefix_len < 96 || *prefix_len > 128)
+			return -1;
+		return AF_INET;
+	}
+
+	if (af == AF_INET) {
+		/* Native IPv4: convert to IPv6 format */
+		if (*prefix_len > 32)
+			return -1;
+		*prefix_len += 96;
+		return AF_INET;
+	}
+
+	/* Native IPv6: keep as-is */
+	if (*prefix_len > 128)
+		return -1;
+	return AF_INET6;
+}
diff --git a/inany.h b/inany.h
index 61b36fb..316ee44 100644
--- a/inany.h
+++ b/inany.h
@@ -295,5 +295,6 @@ static inline void inany_siphash_feed(struct siphash_state *state,
 
 const char *inany_ntop(const union inany_addr *src, char *dst, socklen_t size);
 int inany_pton(const char *src, union inany_addr *dst);
+int inany_prefix_pton(char *src, union inany_addr *dst, int *prefix_len);
 
 #endif /* INANY_H */
diff --git a/ip.c b/ip.c
index 9a7f4c5..40dc24e 100644
--- a/ip.c
+++ b/ip.c
@@ -13,6 +13,8 @@
  */
 
 #include <stddef.h>
+#include <netinet/in.h>
+
 #include "util.h"
 #include "ip.h"
 
@@ -67,3 +69,22 @@ found:
 	*proto = nh;
 	return true;
 }
+
+/**
+ * ip4_class_prefix_len() - Get class based prefix length for IPv4 address
+ * @addr:	IPv4 address
+ *
+ * Return: prefix length based on address class, or 32 for other
+ */
+int ip4_class_prefix_len(const struct in_addr *addr)
+{
+	in_addr_t a = ntohl(addr->s_addr);
+
+	if (IN_CLASSA(a))
+		return 32 - IN_CLASSA_NSHIFT;
+	if (IN_CLASSB(a))
+		return 32 - IN_CLASSB_NSHIFT;
+	if (IN_CLASSC(a))
+		return 32 - IN_CLASSC_NSHIFT;
+	return 32;
+}
diff --git a/ip.h b/ip.h
index 5830b92..bd28640 100644
--- a/ip.h
+++ b/ip.h
@@ -135,4 +135,6 @@ static const struct in_addr in4addr_broadcast = { 0xffffffff };
 #define IPV6_MIN_MTU		1280
 #endif
 
+int ip4_class_prefix_len(const struct in_addr *addr);
+
 #endif /* IP_H */
diff --git a/passt.1 b/passt.1
index db0d662..53537c4 100644
--- a/passt.1
+++ b/passt.1
@@ -156,10 +156,14 @@ By default, the advertised MTU is 65520 bytes, that is, the maximum 802.3 MTU
 minus the length of a 802.3 header, rounded to 32 bits (IPv4 words).
 
 .TP
-.BR \-a ", " \-\-address " " \fIaddr
+.BR \-a ", " \-\-address " " \fIaddr\fR[/\fIprefix_len\fR]
 Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option
 5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3)
 for an IPv6 \fIaddr\fR.
+An optional /\fIprefix_len\fR (1-32 for IPv4, 1-128 for IPv6) can be
+appended in CIDR notation (e.g. 192.0.2.1/24). This is an alternative to
+using the \fB-n\fR, \fB--netmask\fR option. Mixing CIDR notation with
+\fB-n\fR results in an error.
 This option can be specified zero (for defaults) to two times (once for IPv4,
 once for IPv6).
 By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
@@ -172,10 +176,13 @@ is assigned for IPv4, and no additional address will be assigned for IPv6.
 .TP
 .BR \-n ", " \-\-netmask " " \fImask
 Assign IPv4 netmask \fImask\fR, expressed as dot-decimal or number of bits, via
-DHCP (option 1).
-By default, the netmask associated to the host address matching the assigned one
-is used. If there's no matching address on the host, the netmask is determined
-according to the CIDR block of the assigned address (RFC 4632).
+DHCP (option 1). Alternatively, the prefix length can be specified using CIDR
+notation with the \fB-a\fR, \fB--address\fR option (e.g. \fB-a\fR 192.0.2.1/24).
+Mixing \fB-n\fR with CIDR notation results in an error.
+If no address is indicated, the netmask associated with the adopted host address,
+if any, is used. If an address is indicated, but without a prefix length, the
+netmask is determined based on the corresponding network class. In all other
+cases, the netmask is determined by using the indicated prefix length.
 
 .TP
 .BR \-M ", " \-\-mac-addr " " \fIaddr
-- 
2.52.0