[PATCH v4 0/2] [PATCH 0/2] dhcpv6: Add --dhcpv6-opt for custom DHCPv6 options

[PATCH v4 0/2] [PATCH 0/2] dhcpv6: Add --dhcpv6-opt for custom DHCPv6 options

[Anshu Kumari]

This series adds a --dhcpv6-opt CODE,VALUE command-line option to
inject custom options into DHCPv6 replies, complementing the existing
--dhcp-opt support for DHCPv4.

The primary use case is UEFI HTTP Boot, which requires Vendor Class
(option 16) with the correct enterprise-number + length-prefixed wire
encoding per RFC 8415 Section 21.16, and Boot File URL (option 59).

Value formats are determined automatically from the option code via a
type table. Supported types include plain strings, IPv6
addresses (single and list), 8/16/32-bit integers, vendor class
(ENTERPRISE:DATA), and length-prefixed string lists.

Anshu Kumari (2):
  dhcpv6: Add --dhcpv6-opt with option type table and value parser
  dhcpv6: Inject custom options into DHCPv6 replies

 conf.c   |  26 +++-
 dhcpv6.c | 375 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 dhcpv6.h |   2 +
 passt.1  |  31 +++++
 4 files changed, 433 insertions(+), 1 deletion(-)

-- 
2.55.0

[PATCH v4 1/2] dhcpv6: Add --dhcpv6-opt with option type table and value parser

[Anshu Kumari]

Introduce the --dhcpv6-opt flag that allows setting arbitrary DHCPv6
options from command-line in the form [--dhcpv6-opt CODE,VALUE].

Add a type lookup table mapping option codes to value types (IPv6,
IPv6 list, integer, string, vendor class, length-prefixed string
list) and dhcpv6_opt_parse() to convert CLI strings to binary wire
format. If the same option code is given more than once, the
last value wins.

Link: https://bugs.passt.top/show_bug.cgi?id=192
Signed-off-by: Anshu Kumari <anskuma@redhat.com>
---
v4:
  - The dhcpv6_opt_code < 1 check was replaced with proper validation:
    dhcpv6_opt_code == 0 || dhcpv6_opt_code > UINT16_MAX in conf.c
  - All uint16_t/uint32_t writes to byte buffers now use intermediate
    variables + memcpy() instead of direct pointer casts.
  - The DHCPV6_OPT_STR case now uses slen > buf_len instead of slen >= buf_len.
  - Option 17 (Vendor-specific Info) excluded.
  - A separate char optbuf[512] was added in conf_print() instead of
    reusing the small INANY_ADDRSTRLEN-sized buf for option rendering.
  - inet_ntop() return value checked.
  - Uses off += strlen(buf + off) instead of off = strlen(buf) in
    the IPv6 list rendering loop.
  - Iterator macro added — for_each_in6_off(off, len).
  - The addr and af variables are declared at function scope
    in dhcpv6_opt_parse() rather than inside block wrappers.
  - snprintf() alignment and line formatting.

v3:
  - Use parse_unsigned(), parse_literal(), parse_eoi() from parse.c
    instead of manual strtoul()/errno/strchr() in dhcpv6_opt_parse()
    for UINT and VENDOR_CLASS cases.
  - Store options as pre-parsed binary in a static array indexed by
    option code, instead of storing raw strings.
  - Add dhcpv6_opt_to_str() to render binary option values back to
    printable strings for startup logging in conf_print().

v2:
  - Renamed custom_v6opts to dhcpv6_opts, MAX_CUSTOM_DHCPV6_OPTS
    to MAX_DHCPV6_OPTS.
  - Dropped val/len from ctx struct.
  - Moved dhcpv6_add_option() to conf.c as static
    conf_dhcpv6_option().
  - Made dhcpv6_opt_parse() non-static, declared in dhcpv6.h
  - Omitted explicit [256] from dhcpv6_opt_types[].
  - Moved chunk declaration into while block.
  - Removed redundant !slen check in DHCPV6_OPT_STR case.
  - All errors in dhcpv6_opt_parse() return -1, removed die()
    calls.
---
 conf.c   |  26 ++++-
 dhcpv6.c | 339 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 dhcpv6.h |   2 +
 passt.1  |  31 +++++
 4 files changed, 397 insertions(+), 1 deletion(-)

diff --git a/conf.c b/conf.c
index faf26814..dba62bf8 100644
--- a/conf.c
+++ b/conf.c
@@ -47,6 +47,7 @@
 #include "lineread.h"
 #include "isolation.h"
 #include "log.h"
+#include "dhcpv6.h"
 #include "vhost_user.h"
 #include "epoll_ctl.h"
 #include "conf.h"
@@ -632,7 +633,8 @@ static void usage(const char *name, FILE *f, int status)
 		"  -S, --search LIST	Space-separated list, search domains\n"
 		"    a single, empty option disables the DNS search list\n"
 		"  -H, --hostname NAME 	Hostname to configure client with\n"
-		"  --fqdn NAME		FQDN to configure client with\n");
+		"  --fqdn NAME		FQDN to configure client with\n"
+		"  --dhcpv6-opt CODE,VAL	Set DHCPv6 option CODE to VAL\n");
 	if (strstr(name, "pasta"))
 		FPRINTF(f, "    default: don't use any search list\n");
 	else
@@ -800,6 +802,7 @@ enum passt_modes conf_mode(int argc, char *argv[])
 static void conf_print(const struct ctx *c)
 {
 	char buf[INANY_ADDRSTRLEN];
+	char optbuf[512];
 	int i;
 
 	if (c->fd_control_listen >= 0)
@@ -900,6 +903,10 @@ static void conf_print(const struct ctx *c)
 		info("    our link-local: %s",
 		     inet_ntop(AF_INET6, &c->ip6.our_tap_ll,
 			       buf, sizeof(buf)));
+		for (i = 1; i < UINT16_MAX; i++) {
+			if (dhcpv6_opt_to_str(i, optbuf, sizeof(optbuf)))
+				info("    option %u: %s", i, optbuf);
+		}
 
 dns6:
 		for (i = 0; i < ARRAY_SIZE(c->ip6.dns); i++) {
@@ -1348,6 +1355,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"stats", required_argument,		NULL,		31 },
 		{"conf-path",	required_argument,	NULL,		'c' },
 		{"chroot-fallback", no_argument,	NULL, 		32 },
+		{"dhcpv6-opt", required_argument,	NULL,		35 },
 		{ 0 },
 	};
 	const char *optstring = "+dqfel:hs:c:F:I:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:T:U:";
@@ -1589,6 +1597,22 @@ void conf(struct ctx *c, int argc, char **argv)
 		case 32:
 			c->chroot_fallback = true;
 			break;
+		case 35: {
+			unsigned long dhcpv6_opt_code;
+
+			p = optarg;
+
+			if (!parse_unsigned(&p, 0, &dhcpv6_opt_code) ||
+			    !parse_literal(&p, ","))
+				die("--dhcpv6-opt requires CODE,VALUE format");
+
+			if (dhcpv6_opt_code == 0 ||
+			    dhcpv6_opt_code > UINT16_MAX)
+				die("Invalid DHCPv6 option code: %s", optarg);
+
+			dhcpv6_set_opt(dhcpv6_opt_code, p);
+			break;
+		}
 		case 'd':
 			c->debug = 1;
 			c->quiet = 0;
diff --git a/dhcpv6.c b/dhcpv6.c
index 29c7e320..abc45813 100644
--- a/dhcpv6.c
+++ b/dhcpv6.c
@@ -31,6 +31,8 @@
 #include "passt.h"
 #include "tap.h"
 #include "log.h"
+#include "inany.h"
+#include "parse.h"
 
 /**
  * struct opt_hdr - DHCPv6 option header
@@ -63,6 +65,11 @@ struct opt_hdr {
 #define OPT_MAX_SIZE		IPV6_MIN_MTU - (sizeof(struct ipv6hdr) + \
 						UDP_MSG_HDR_SIZE)
 
+#define for_each_in6_off(off, len)					\
+	for ((off) = 0;							\
+	     (off) + sizeof(struct in6_addr) <= (unsigned int)(len);	\
+	     (off) += sizeof(struct in6_addr))
+
 /**
  * struct opt_client_id - DHCPv6 Client Identifier option
  * @hdr:		Option header
@@ -278,6 +285,338 @@ static struct resp_not_on_link_t {
 	{ 0, },
 };
 
+/**
+ * enum dhcpv6_opt_type - DHCPv6 option value types
+ * @DHCPV6_OPT_NONE:		Unsupported or unknown option
+ * @DHCPV6_OPT_STR:		Variable-length string
+ * @DHCPV6_OPT_IPV6:		Single IPv6 address
+ * @DHCPV6_OPT_IPV6_LIST:	Multiple IPv6 addresses, comma-separated
+ * @DHCPV6_OPT_UINT8:		Unsigned 8-bit integer
+ * @DHCPV6_OPT_UINT16:		Unsigned 16-bit integer
+ * @DHCPV6_OPT_UINT32:		Unsigned 32-bit integer
+ * @DHCPV6_OPT_VENDOR_CLASS:	Enterprise number + length-prefixed data
+ * @DHCPV6_OPT_LEN_STR_LIST:	Length-prefixed string list
+ */
+enum dhcpv6_opt_type {
+	DHCPV6_OPT_NONE,
+	DHCPV6_OPT_STR,
+	DHCPV6_OPT_IPV6,
+	DHCPV6_OPT_IPV6_LIST,
+	DHCPV6_OPT_UINT8,
+	DHCPV6_OPT_UINT16,
+	DHCPV6_OPT_UINT32,
+	DHCPV6_OPT_VENDOR_CLASS,
+	DHCPV6_OPT_LEN_STR_LIST,
+};
+
+/**
+ * dhcpv6_opt_types - Maps DHCPv6 option code to value type, indexed by code
+ * RFC 8415 Options: 7, 15, 16, 32, 82, 83
+ * RFC 5970 Options: 59, 60
+ * RFC 4075 Options: 31
+ */
+static const enum dhcpv6_opt_type dhcpv6_opt_types[] = {
+	[7]   = DHCPV6_OPT_UINT8,		/* Preference */
+	[15]  = DHCPV6_OPT_LEN_STR_LIST,	/* User Class */
+	[16]  = DHCPV6_OPT_VENDOR_CLASS,	/* Vendor Class */
+	[31]  = DHCPV6_OPT_IPV6_LIST,		/* SNTP Servers */
+	[32]  = DHCPV6_OPT_UINT32,		/* Information Refresh Time */
+	[59]  = DHCPV6_OPT_STR,			/* Boot File URL */
+	[60]  = DHCPV6_OPT_LEN_STR_LIST,	/* Boot File Params */
+	[82]  = DHCPV6_OPT_UINT32,		/* SOL_MAX_RT */
+	[83]  = DHCPV6_OPT_UINT32,		/* INF_MAX_RT */
+};
+
+/**
+ * dhcpv6_opt_parse() - Parse a DHCPv6 option value string into binary
+ * @code:	DHCPv6 option code
+ * @str:	Value string from command line
+ * @buf:	Output buffer for binary value
+ * @buf_len:	Size of output buffer
+ *
+ * Return: number of bytes written to @buf, or -1 on error
+ */
+static int dhcpv6_opt_parse(uint16_t code, const char *str,
+			    uint8_t *buf, size_t buf_len)
+{
+	enum dhcpv6_opt_type type;
+	union inany_addr addr;
+	unsigned long val;
+	uint16_t slen_net;
+	sa_family_t af;
+	uint8_t width;
+	uint32_t ent;
+	size_t slen;
+	int len;
+
+	if (!*str)
+		return -1;
+
+	if (code >= ARRAY_SIZE(dhcpv6_opt_types))
+		return -1;
+
+	type = dhcpv6_opt_types[code];
+
+	switch (type) {
+	case DHCPV6_OPT_NONE:
+		return -1;
+	case DHCPV6_OPT_IPV6:
+	case DHCPV6_OPT_IPV6_LIST:
+		len = 0;
+
+		do {
+			if (len + sizeof(struct in6_addr) > buf_len)
+				return -1;
+
+			if (!parse_inany_(&str, &addr, &af) || af != AF_INET6)
+				return -1;
+
+			memcpy(buf + len, &addr.a6, sizeof(struct in6_addr));
+			len += sizeof(struct in6_addr);
+
+			if (type == DHCPV6_OPT_IPV6)
+				break;
+		} while (parse_literal(&str, ","));
+
+		if (!len || !parse_eoi(str))
+			return -1;
+
+		return len;
+	case DHCPV6_OPT_UINT8:
+	case DHCPV6_OPT_UINT16:
+	case DHCPV6_OPT_UINT32:
+		if (type == DHCPV6_OPT_UINT8)
+			width = 1;
+		else if (type == DHCPV6_OPT_UINT16)
+			width = 2;
+		else
+			width = 4;
+
+		if (buf_len < width)
+			return -1;
+
+		if (!parse_unsigned(&str, 0, &val) || !parse_eoi(str) ||
+		    val >= (1ULL << (width * 8)))
+			return -1;
+
+		if (type == DHCPV6_OPT_UINT16) {
+			uint16_t v16 = htons(val);
+			memcpy(buf, &v16, sizeof(v16));
+		} else if (type == DHCPV6_OPT_UINT32) {
+			uint32_t v32 = htonl(val);
+			memcpy(buf, &v32, sizeof(v32));
+		} else {
+			buf[0] = val;
+		}
+
+		return width;
+	case DHCPV6_OPT_STR:
+		slen = strlen(str);
+
+		if (slen > buf_len)
+			return -1;
+
+		memcpy(buf, str, slen);
+
+		return slen;
+	case DHCPV6_OPT_VENDOR_CLASS:
+		if (!parse_unsigned(&str, 0, &val) ||
+		    !parse_literal(&str, ":") || val > UINT32_MAX)
+			return -1;
+
+		slen = strlen(str);
+		if (!slen)
+			return -1;
+
+		len = sizeof(uint32_t) + sizeof(uint16_t) + slen;
+		if ((size_t)len > buf_len)
+			return -1;
+
+		ent = htonl(val);
+		memcpy(buf, &ent, sizeof(ent));
+
+		slen_net = htons(slen);
+		memcpy(buf + sizeof(uint32_t), &slen_net, sizeof(slen_net));
+
+		memcpy(buf + sizeof(uint32_t) + sizeof(uint16_t), str, slen);
+
+		return len;
+	case DHCPV6_OPT_LEN_STR_LIST:
+		len = 0;
+
+		while (*str) {
+			slen = strcspn(str, ",");
+			if (!slen)
+				return -1;
+
+			if (len + (int)(sizeof(uint16_t) + slen) > (int)buf_len)
+				return -1;
+
+			slen_net = htons(slen);
+			memcpy(buf + len, &slen_net, sizeof(slen_net));
+			len += sizeof(uint16_t);
+
+			memcpy(buf + len, str, slen);
+			len += slen;
+
+			str += slen;
+			if (*str == ',')
+				str++;
+		}
+
+		if (!len)
+			return -1;
+
+		return len;
+	}
+
+	return -1;
+}
+
+/**
+ * struct dhcpv6_user_opts - User-specified DHCPv6 options from --dhcpv6-opt
+ * @val:	Binary option value in wire format
+ * @len:	Length of @val in bytes, 0 if not set
+ */
+static struct {
+	uint8_t val[UINT16_MAX];
+	int len;
+} dhcpv6_user_opts[ARRAY_SIZE(dhcpv6_opt_types)];
+
+/**
+ * dhcpv6_opt_to_str() - Render a binary DHCPv6 option value to printable string
+ * @code:	DHCPv6 option code
+ * @buf:	Output string buffer
+ * @buf_len:	Size of output buffer
+ *
+ * Return: pointer to @buf if option is set, NULL otherwise
+ */
+const char *dhcpv6_opt_to_str(uint16_t code, char *buf, size_t buf_len)
+{
+	enum dhcpv6_opt_type type;
+	unsigned int i;
+	uint16_t slen;
+	int off = 0;
+
+	if (code >= ARRAY_SIZE(dhcpv6_user_opts) ||
+	    !dhcpv6_user_opts[code].len)
+		return NULL;
+
+	type = dhcpv6_opt_types[code];
+
+	switch (type) {
+	case DHCPV6_OPT_IPV6:
+	case DHCPV6_OPT_IPV6_LIST:
+		for_each_in6_off(i, dhcpv6_user_opts[code].len) {
+			if (off) {
+				if (off + 1 >= (int)buf_len)
+					return NULL;
+				buf[off++] = ',';
+			}
+			if (!inet_ntop(AF_INET6,
+				       dhcpv6_user_opts[code].val + i,
+				       buf + off, buf_len - off))
+				return NULL;
+			off += strlen(buf + off);
+		}
+		return buf;
+	case DHCPV6_OPT_UINT8:
+	case DHCPV6_OPT_UINT16:
+	case DHCPV6_OPT_UINT32: {
+		uint32_t val = 0;
+
+		if (dhcpv6_user_opts[code].len == 1) {
+			val = dhcpv6_user_opts[code].val[0];
+		} else if (dhcpv6_user_opts[code].len == 2) {
+			uint16_t v16;
+			memcpy(&v16, dhcpv6_user_opts[code].val, sizeof(v16));
+			val = ntohs(v16);
+		} else if (dhcpv6_user_opts[code].len == 4) {
+			memcpy(&val, dhcpv6_user_opts[code].val, sizeof(val));
+			val = ntohl(val);
+		}
+
+		(void)snprintf(buf, buf_len, "%u", val);
+		return buf;
+	}
+	case DHCPV6_OPT_STR:
+		(void)snprintf(buf, buf_len, "%.*s",
+		               dhcpv6_user_opts[code].len,
+		               dhcpv6_user_opts[code].val);
+		return buf;
+	case DHCPV6_OPT_VENDOR_CLASS: {
+		uint32_t ent;
+		int doff;
+
+		if (dhcpv6_user_opts[code].len <
+		    (int)(sizeof(uint32_t) + sizeof(uint16_t)))
+			return NULL;
+
+		memcpy(&ent, dhcpv6_user_opts[code].val, sizeof(ent));
+		memcpy(&slen, dhcpv6_user_opts[code].val + sizeof(uint32_t),
+		       sizeof(slen));
+		doff = sizeof(uint32_t) + sizeof(uint16_t);
+
+		(void)snprintf(buf, buf_len, "%u:%.*s",
+		               ntohl(ent), ntohs(slen),
+		               dhcpv6_user_opts[code].val + doff);
+		return buf;
+	}
+	case DHCPV6_OPT_LEN_STR_LIST:
+		off = 0;
+		i = 0;
+
+		while (i + sizeof(uint16_t) <=
+		       (unsigned int)dhcpv6_user_opts[code].len) {
+			memcpy(&slen, dhcpv6_user_opts[code].val + i,
+			       sizeof(slen));
+			slen = ntohs(slen);
+			i += sizeof(uint16_t);
+
+			if (i + slen > (unsigned int)dhcpv6_user_opts[code].len)
+				break;
+
+			if (off) {
+				if (off + 1 >= (int)buf_len)
+					return NULL;
+				buf[off++] = ',';
+			}
+			if (off + slen >= (int)buf_len)
+				return NULL;
+			memcpy(buf + off, dhcpv6_user_opts[code].val + i,
+			       slen);
+			off += slen;
+			i += slen;
+		}
+		buf[off] = '\0';
+		return buf;
+	default:
+		return NULL;
+	}
+}
+
+/**
+ * dhcpv6_set_opt() - Parse and store a user-specified DHCPv6 option
+ * @code:	DHCPv6 option code
+ * @val_str:	Value string from command line
+ */
+void dhcpv6_set_opt(uint16_t code, const char *val_str)
+{
+	int ret;
+
+	if (code >= ARRAY_SIZE(dhcpv6_opt_types) ||
+	    dhcpv6_opt_types[code] == DHCPV6_OPT_NONE)
+		die("DHCPv6 option %u is not supported", code);
+
+	ret = dhcpv6_opt_parse(code, val_str,
+			       dhcpv6_user_opts[code].val,
+			       sizeof(dhcpv6_user_opts[code].val));
+	if (ret < 0)
+		die("Invalid value for DHCPv6 option %u: %s", code, val_str);
+
+	dhcpv6_user_opts[code].len = ret;
+}
+
 /**
  * dhcpv6_opt() - Get option from DHCPv6 message
  * @data:	Buffer with options, set to matching option on return
diff --git a/dhcpv6.h b/dhcpv6.h
index 1015a1a7..973c477c 100644
--- a/dhcpv6.h
+++ b/dhcpv6.h
@@ -9,5 +9,7 @@
 int dhcpv6(struct ctx *c, struct iov_tail *data,
 	   const struct in6_addr *saddr, const struct in6_addr *daddr);
 void dhcpv6_init(const struct ctx *c);
+void dhcpv6_set_opt(uint16_t code, const char *val_str);
+const char *dhcpv6_opt_to_str(uint16_t code, char *buf, size_t buf_len);
 
 #endif /* DHCPV6_H */
diff --git a/passt.1 b/passt.1
index 53e072a9..249ed5cf 100644
--- a/passt.1
+++ b/passt.1
@@ -440,6 +440,37 @@ Send \fIname\fR as DHCP option 12 (hostname).
 FQDN to configure the client with.
 Send \fIname\fR as Client FQDN: DHCP option 81 and DHCPv6 option 39.
 
+.TP
+.BR \-\-dhcpv6-opt " " \fICODE\fR,\fIVALUE\fR
+Set DHCPv6 option \fICODE\fR to \fIVALUE\fR.  The value format depends
+on the option type and is determined automatically from the option code.
+Multiple IPv6 addresses are comma-separated.
+This option can be specified multiple times.  If the same option code is
+given more than once, the last value wins.
+.RS
+.TP
+.B String options
+59 (Boot File URL, RFC 5970)
+.TP
+.B Length-prefixed string list options (comma-separated entries)
+15 (User Class, RFC 8415), 60 (Boot File Params, RFC 5970).
+Each comma-separated entry is encoded with a 2-byte length prefix.
+Example: \fB\-\-dhcpv6-opt 15,class1,class2\fR.
+.TP
+.B Vendor class options (ENTERPRISE:DATA format)
+16 (Vendor Class, RFC 8415).
+VALUE is \fIENTERPRISE\fR:\fIDATA\fR where \fIENTERPRISE\fR is the IANA
+Private Enterprise Number and \fIDATA\fR is the vendor class string.
+Example: \fB\-\-dhcpv6-opt 16,0:HTTPClient\fR for UEFI HTTP Boot.
+.TP
+.B IPv6 address list options (comma-separated)
+31 (SNTP Servers)
+.TP
+.B Integer options
+7 (Preference, 8-bit), 32 (Information Refresh Time, 32-bit),
+82 (SOL_MAX_RT, 32-bit), 83 (INF_MAX_RT, 32-bit)
+.RE
+
 .TP
 .BR \-t ", " \-\-tcp-ports " " \fIspec
 Configure TCP port forwarding to guest or namespace. \fIspec\fR can be either:
-- 
2.55.0

[PATCH v4 2/2] dhcpv6: Inject custom options into DHCPv6 replies

[Anshu Kumari]

Append user-specified options from --dhcpv6-opt to DHCPv6 reply
messages.  Options that exceed available buffer space are skipped
with a warning.

Link: https://bugs.passt.top/show_bug.cgi?id=192
Signed-off-by: Anshu Kumari <anskuma@redhat.com>
---
v4:
  - warn() instead of debug() when options don't fit in the reply.
  - doc comment for dhcpv6_user_opts_fill() updated.

v3:
  - rename dhcpv6_custom_opts_fill() to dhcpv6_user_opts_fill().
  - Iterate directly over the pre-parsed binary dhcpv6_user_opts[]
    array instead of re-parsing string values at reply time.

v2:
  - Updated dhcpv6_custom_opts_fill() to parse str at reply time
    using dhcpv6_opt_parse() instead of copying cached val/len.
---
 dhcpv6.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/dhcpv6.c b/dhcpv6.c
index abc45813..a0545406 100644
--- a/dhcpv6.c
+++ b/dhcpv6.c
@@ -879,6 +879,41 @@ static size_t dhcpv6_client_fqdn_fill(const struct iov_tail *data,
 	return offset + sizeof(struct opt_hdr) + opt_len;
 }
 
+/**
+ * dhcpv6_user_opts_fill() - Append user-specified options to reply
+ * @buf:	Response message buffer
+ * @offset:	Current offset in buffer
+ *
+ * Return: offset after appended user options
+ */
+static size_t dhcpv6_user_opts_fill(char *buf, size_t offset)
+{
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(dhcpv6_user_opts); i++) {
+		int vlen = dhcpv6_user_opts[i].len;
+		struct opt_hdr *hdr;
+
+		if (!vlen)
+			continue;
+
+		if (offset + sizeof(struct opt_hdr) + vlen > OPT_MAX_SIZE) {
+			warn("DHCPv6: option %u doesn't fit, skipping", i);
+			continue;
+		}
+
+		hdr = (struct opt_hdr *)(buf + offset);
+		hdr->t = htons(i);
+		hdr->l = htons(vlen);
+		offset += sizeof(struct opt_hdr);
+
+		memcpy(buf + offset, dhcpv6_user_opts[i].val, vlen);
+		offset += vlen;
+	}
+
+	return offset;
+}
+
 /**
  * dhcpv6() - Check if this is a DHCPv6 message, reply as needed
  * @c:		Execution context
@@ -1017,6 +1052,7 @@ int dhcpv6(struct ctx *c, struct iov_tail *data,
 	    sizeof(struct opt_hdr) + ntohs(client_id->l);
 	n = dhcpv6_dns_fill(c, (char *)&resp, n);
 	n = dhcpv6_client_fqdn_fill(data, c, (char *)&resp, n);
+	n = dhcpv6_user_opts_fill((char *)&resp, n);
 
 	resp.hdr.xid = mh->xid;
 
-- 
2.55.0