Add truncate to defintions

This indicates an option might be truncated from it's natural
length and will be zero padded on expansion.
Only supported for the ip6address option.

While here, support 1 as a bitflag to just print the bit.

Fixes #508.

Author: rsmarples

src/dhcp-common.c

@@ -135,6 +135,8 @@ dhcp_print_option_encoding(const struct dhcp_opt *opt, int cols)
 		printf(" request");
 	if (opt->type & OT_NOREQ)
 		printf(" norequest");
+	if (opt->type & OT_TRUNCATED)
+		printf(" truncated");
 	putchar('\n');
 	fflush(stdout);
 }
@@ -614,6 +616,8 @@ dhcp_optlen(const struct dhcp_opt *opt, size_t dl)
 		return (ssize_t)dl;
 	}
 	if (dl < sz) {
+		if (opt->type & OT_TRUNCATED)
+			return (ssize_t)dl;
 		errno = EOVERFLOW;
 		return -1;
 	}
@@ -751,6 +755,11 @@ print_option(FILE *fp, const char *prefix, const struct dhcp_opt *opt,
 		    l < sizeof(opt->bitflags);
 		    l++, sl--)
 		{
+			if (opt->bitflags[l] == '1') {
+				if (fprintf(fp, "%d", *data & (1 << sl)) == -1)
+					goto err;
+				continue;
+			}
 			/* Don't print NULL or 0 flags */
 			if (opt->bitflags[l] != '\0' &&
 			    opt->bitflags[l] != '0' &&
@@ -808,9 +817,14 @@ print_option(FILE *fp, const char *prefix, const struct dhcp_opt *opt,
 				goto err;
 			data += sizeof(addr.s_addr);
 		} else if (opt->type & OT_ADDRIPV6) {
+			uint8_t databuf[sizeof(struct in6_addr)] = { 0 };
+			size_t datalen = e - data >= 16 ? 16 : (size_t)(e - data);
 			char buf[INET6_ADDRSTRLEN];
 
-			if (inet_ntop(AF_INET6, data, buf, sizeof(buf)) == NULL)
+			/* avoid inet_ntop going beyond our option space by
+			 * copying out into a temporary buffer. */
+			memcpy(databuf, data, datalen);
+			if (inet_ntop(AF_INET6, databuf, buf, sizeof(buf)) == NULL)
 				goto err;
 			if (fprintf(fp, "%s", buf) == -1)
 				goto err;

src/dhcp-common.h

@@ -76,6 +76,7 @@
 #define	OT_BITFLAG		(1 << 27)
 #define	OT_RESERVED		(1 << 28)
 #define	OT_URI			(1 << 29)
+#define	OT_TRUNCATED		(1 << 30)
 
 #define	DHC_REQ(r, n, o) \
 	(has_option_mask((r), (o)) && !has_option_mask((n), (o)))

src/dhcpcd-definitions-small.conf

@@ -71,10 +71,10 @@ embed		uint32			mtu
 
 definend 24	index embed		route_information
 embed		byte			length
-# bits 4 and 5 are route preference, but we can't express this
-embed		byte			reserved
+# bits 4 and 5 are route preference
+embed		bitflags=00011		prf
 embed		uint32			lifetime
-embed		ip6address		prefix
+embed		truncated ip6address	prefix
 
 # ND6 options, RFC6101
 definend 25	index embed		rdnss

src/dhcpcd-definitions.conf

@@ -414,10 +414,10 @@ embed		uint16			lifetime
 
 definend 24	index embed		route_information
 embed		byte			length
-# bits 4 and 5 are route preference, but we can't express this
-embed		byte			reserved
+# bits 4 and 5 are route preference
+embed		bitflags=00011		prf
 embed		uint32			lifetime
-embed		ip6address		prefix
+embed		truncated ip6address	prefix
 
 # ND6 options, RFC6101
 definend 25	index embed		rdnss

src/dhcpcd.conf.5.in

@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 15, 2025
+.Dd June 12, 2025
 .Dt DHCPCD.CONF 5
 .Os
 .Sh NAME
@@ -957,6 +957,10 @@ The option can appear more than once and will be indexed.
 .It Ic array
 The option data is split into a space separated array, each element being
 the same type.
+.It Ic truncated
+The option might truncated from its normal length.
+The end of the normal size is zero padded on expansion.
+Currently this is only supported for ip6address where it's a prefix.
 .El
 .Ss Types to define
 The type directly affects the length of data consumed inside the option.
@@ -979,6 +983,7 @@ C 00100000, etc.
 If the bit is not set, the flag is not printed.
 A flag of 0 is not printed even if the bit position is set.
 This is to allow reservation of the first bits while assigning the last bits.
+A flag of 1 prints the bit set or unset.
 .It Ic int16
 A signed 16bit integer, 2 bytes.
 .It Ic uint16

src/if-options.c

@@ -1995,6 +1995,15 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 				return -1;
 			}
 			*fp++ = '\0';
+		} else if (strcasecmp(arg, "truncated") == 0) {
+			t |= OT_TRUNCATED;
+			arg = strskipwhite(fp);
+			fp = strwhite(arg);
+			if (fp == NULL) {
+				logerrx("incomplete truncated type");
+				return -1;
+			}
+			*fp++ = '\0';
 		}
 		if (strcasecmp(arg, "ipaddress") == 0)
 			t |= OT_ADDRIPV4;
@@ -2084,6 +2093,10 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 				t |= OT_RESERVED;
 			}
 		}
+		if (t & OT_TRUNCATED && t != (OT_ADDRIPV6 | OT_TRUNCATED)) {
+			logerrx("truncated only works for ip6address");
+			return -1;
+		}
 		if (opt != O_EMBED) {
 			for (dl = 0, ndop = *dop; dl < *dop_len; dl++, ndop++)
 			{