[PATCH v5 4/7] dhcp: Add --dhcp-opt with option table and value parser

[Anshu Kumari <anskuma@redhat.com>]

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

Add a type lookup table mapping option codes to RFC 2132 value types
(IPv4, IPv4 list, integer, string) and dhcp_opt_parse() to convert
CLI strings to binary wire format.  Parsed options are stored in
struct opts[] and injected into DHCP replies.  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>
---
v5:
  - Rename enum values: INT8/INT16/INT32/SINT32 → UINT8/UINT16/UINT32/INT32
  - Change int len to size_t len in dhcp_opt_parse(), remove int casts
  - Make dhcp_opt_parse() static, add dhcp_set_opt() to store options 
    directly in opts[] at startup instead of double-parsing via ctx->dhcp_opts
  - Add OPT_USER to enum opt_state to protect user options from being
    overwritten by built-in defaults
  - Add dhcp_opt_to_str() for conf_print() display
  - Remove dhcp_opts[] from struct ctx in passt.h
  - Remove conf_dhcp_option() wrapper from conf.c

v4:
  - Renamed custom_opts to dhcp_opts, 256 entries indexed by option
    code, removed MAX_CUSTOM_DHCP_OPTS and count field.
  - Changed str buffer from 256 to 255 bytes.
  - Moved function to conf.c as static conf_dhcp_option(), renamed
    from dhcp_add_option().
  - Made dhcp_opt_parse() non-static, declared in dhcp.h
  - Dropped val/len from ctx struct; conf_dhcp_option() validates
    with temp buffer, dhcp() parses str directly into opts[] at
    reply time.
  - Replaced strtok_r() + 256-byte buffer with strcspn() +
    INET_ADDRSTRLEN buffer.
  - Added DHCP_OPT_SINT32 for option 2 (Time Offset), uses strtol()
    per RFC 2132 Section 8.2.
  - All errors in dhcp_opt_parse() return -1, removed die() calls;
    caller handles error message consistently.
  - Removed redundant !slen check in DHCP_OPT_STR case.
  - Omitted explicit array size for dhcp_opt_types[], arraydded bounds
    check before lookup.
  - Added errno = 0 + errno check for strtoul() in case 34.
  - Fixed usage text: "Set DHCP option CODE to VAL".
  - Improved man page: added format description and examples

v3:
  - Replaced DHCP_OPT_INTEGER with separate DHCP_OPT_INT8/INT16/INT32
    enums, removed dhcp_opt_int_width[] array.
  - Shared logic between DHCP_OPT_IPV4 and DHCP_OPT_IPV4_LIST — parse
    both as list, error if >1 in single case.
  - Added errno = 0 before strtoul() and check after.
  - Fixed range check: 1ULL << (width * 8) for all widths including
    width==4.
  - strncpy → memcpy for DHCP_OPT_STR.
  - Moved enum to dhcp.c since not used in other files.
  - Removed options 55, 61 (client-only), 119 (DNS compression, use
    --dhcp-search instead), 33 (IP pairs not supported).
  - DHCP_OPT_PARSE_BUF 1024 → char tmp[256].
  - Upgraded dhcp_add_option() to call dhcp_opt_parse() and populate
    val[]/len.
  - Aligned array entries for readability.
  - Added tab after @DHCP_OPT_IPV4_LIST: in kerneldoc.
  - Reject empty value strings before parsing
  - Reject leading/trailing/consecutive commas in IP list values.

v2:
  - Replaced struct lookup table + dhcp_opt_type_lookup() function with flat dhcp_opt_types[256] array indexed by code.
  - Consolidated DHCP_OPT_UINT8/UINT16/UINT32 into single DHCP_OPT_INTEGER with dhcp_opt_int_width[256] table.
  - Dropped DHCP_OPT_ROUTES / option 121 entirely.
  - Added kerneldoc for enum dhcp_opt_type values.
  - Removed curly braces from switch cases, declarations before switch.
  - Added newlines before return statements.
  - Changed IP list delimiter from space to comma (--dhcp-opt 6,1.1.1.1,8.8.8.8).
  - Defined DHCP_OPT_PARSE_BUF constant for bare 1024.
  - Added len and val[255] fields to struct here (moved from patch 1).
  - Added kerneldoc for @custom_opts.len and @custom_opts.val.
  - Wired dhcp_opt_parse() into case 32 (--dhcp-boot) to populate val/len.

---
 conf.c  |  25 ++++-
 dhcp.c  | 338 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
 dhcp.h  |   2 +
 passt.1 |  42 +++++++
 4 files changed, 381 insertions(+), 26 deletions(-)

diff --git a/conf.c b/conf.c
index 5b6cc2b..e7f3ea0 100644
--- a/conf.c
+++ b/conf.c
@@ -47,6 +47,7 @@
 #include "lineread.h"
 #include "isolation.h"
 #include "log.h"
+#include "dhcp.h"
 #include "vhost_user.h"
 #include "epoll_ctl.h"
 #include "conf.h"
@@ -615,7 +616,8 @@ 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"
+		"  --dhcp-opt CODE,VAL	Set DHCP option CODE to VAL\n");
 	if (strstr(name, "pasta"))
 		FPRINTF(f, "    default: don't use any search list\n");
 	else
@@ -843,6 +845,9 @@ static void conf_print(const struct ctx *c)
 			info("    router: %s",
 			     inet_ntop(AF_INET, &c->ip4.guest_gw,
 				       buf, sizeof(buf)));
+			for (i = 1; i < 255; i++)
+				if (dhcp_opt_to_str(i, buf, sizeof(buf)))
+					info("    option %u: %s", i, buf);
 		}
 
 		for (i = 0; i < ARRAY_SIZE(c->ip4.dns); i++) {
@@ -1316,6 +1321,7 @@ 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 },
+		{"dhcp-opt", required_argument,		NULL,		34 },
 		{ 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:";
@@ -1330,10 +1336,13 @@ void conf(struct ctx *c, int argc, char **argv)
 	unsigned int ifi4 = 0, ifi6 = 0;
 	bool opt_a_is_prefix = false;
 	const char *logfile = NULL;
+	unsigned long optcode;
 	char *runas = NULL;
 	size_t logsize = 0;
+	const char *comma;
 	uint8_t opt_n = 0;
 	int name, ret;
+	char *end;
 	uid_t uid;
 	gid_t gid;
 
@@ -1557,6 +1566,20 @@ void conf(struct ctx *c, int argc, char **argv)
 		case 32:
 			c->chroot_fallback = true;
 			break;
+		case 34:
+			comma = strchr(optarg, ',');
+			if (!comma)
+				die("--dhcp-opt requires CODE,VALUE format");
+
+			errno = 0;
+			optcode = strtoul(optarg, &end, 0);
+			if (end != comma || errno ||
+			    optcode < 1 || optcode > 254)
+				die("DHCP option code must be 1-254: %s",
+				    optarg);
+
+			dhcp_set_opt(optcode, comma + 1);
+			break;
 		case 'd':
 			c->debug = 1;
 			c->quiet = 0;
diff --git a/dhcp.c b/dhcp.c
index 39f7952..cc910ee 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -23,6 +23,7 @@
 #include <unistd.h>
 #include <string.h>
 #include <limits.h>
+#include <errno.h>
 
 #include "util.h"
 #include "ip.h"
@@ -37,10 +38,12 @@
  * enum opt_state - DHCP option state
  * @OPT_UNSET:		Option not configured
  * @OPT_DEFAULT:	Option set from host config
+ * @OPT_USER:		Option set via --dhcp-opt command line
  */
 enum opt_state {
 	OPT_UNSET,
 	OPT_DEFAULT,
+	OPT_USER,
 };
 
 /**
@@ -50,7 +53,7 @@ enum opt_state {
  * @s:		Option payload from server
  * @clen:	Length of option received from client, -1 if not received
  * @c:		Option payload from client
- * @state:	Option state (unset or default)
+ * @state:	Option state (unset, default, or user)
  */
 struct opt {
 	int sent;
@@ -88,19 +91,20 @@ void dhcp_init(void)
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(opts); i++)
-		opts[i].state = OPT_UNSET;
-
-	/* Mask */
-	opts[1]  = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
-	/* Router */
-	opts[3]  = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
-	/* Lease time */
-	opts[51] = (struct opt) { 0, 4, { 0xff, 0xff, 0xff, 0xff },
+		if (opts[i].state != OPT_USER)
+			opts[i].state = OPT_UNSET;
+
+	if (opts[1].state != OPT_USER)			/* Mask */
+		opts[1]  = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
+	if (opts[3].state != OPT_USER)			/* Router */
+		opts[3]  = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
+	if (opts[51].state != OPT_USER)			/* Lease time */
+		opts[51] = (struct opt) { 0, 4, { 0xff, 0xff, 0xff, 0xff },
 				 0, { 0 }, OPT_DEFAULT, };
-	/* Type */
-	opts[53] = (struct opt) { 0, 1, {  0 }, 0, { 0 }, OPT_DEFAULT, };
-	/* Server ID */
-	opts[54] = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
+	if (opts[53].state != OPT_USER)			/* Type */
+		opts[53] = (struct opt) { 0, 1, {  0 }, 0, { 0 }, OPT_DEFAULT, };
+	if (opts[54].state != OPT_USER)			/* Server ID */
+		opts[54] = (struct opt) { 0, 4, {  0 }, 0, { 0 }, OPT_DEFAULT, };
 }
 
 /**
@@ -145,6 +149,280 @@ struct msg {
 	uint8_t o[OPT_MAX + 1 /* End option */ ];
 } __attribute__((__packed__));
 
+/**
+ * enum dhcp_opt_type - DHCP option value types per RFC 2132
+ * @DHCP_OPT_NONE:	Unsupported or unknown option
+ * @DHCP_OPT_STR:	Variable-length string
+ * @DHCP_OPT_IPV4:	Single IPv4 address
+ * @DHCP_OPT_IPV4_LIST:	Multiple IPv4 addresses, comma-separated
+ * @DHCP_OPT_UINT8:	Unsigned 8-bit integer
+ * @DHCP_OPT_UINT16:	Unsigned 16-bit integer
+ * @DHCP_OPT_UINT32:	Unsigned 32-bit integer
+ * @DHCP_OPT_INT32:	Signed 32-bit integer
+ */
+enum dhcp_opt_type {
+	DHCP_OPT_NONE,
+	DHCP_OPT_STR,
+	DHCP_OPT_IPV4,
+	DHCP_OPT_IPV4_LIST,
+	DHCP_OPT_UINT8,
+	DHCP_OPT_UINT16,
+	DHCP_OPT_UINT32,
+	DHCP_OPT_INT32,
+};
+
+/**
+ * dhcp_opt_types - Maps option code to RFC 2132 value type, indexed by code
+ */
+static const enum dhcp_opt_type dhcp_opt_types[] = {
+	[1]   = DHCP_OPT_IPV4,		/* Subnet Mask */
+	[2]   = DHCP_OPT_INT32,		/* Time Offset */
+	[3]   = DHCP_OPT_IPV4_LIST,	/* Router */
+	[4]   = DHCP_OPT_IPV4_LIST,	/* Time Server */
+	[5]   = DHCP_OPT_IPV4_LIST,	/* Name Server */
+	[6]   = DHCP_OPT_IPV4_LIST,	/* Domain Name Server */
+	[7]   = DHCP_OPT_IPV4_LIST,	/* Log Server */
+	[8]   = DHCP_OPT_IPV4_LIST,	/* Cookie Server */
+	[9]   = DHCP_OPT_IPV4_LIST,	/* LPR Server */
+	[10]  = DHCP_OPT_IPV4_LIST,	/* Impress Server */
+	[11]  = DHCP_OPT_IPV4_LIST,	/* Resource Location Server */
+	[12]  = DHCP_OPT_STR,		/* Host Name */
+	[13]  = DHCP_OPT_UINT16,	/* Boot File Size */
+	[15]  = DHCP_OPT_STR,		/* Domain Name */
+	[16]  = DHCP_OPT_IPV4,		/* Swap Server */
+	[17]  = DHCP_OPT_STR,		/* Root Path */
+	[19]  = DHCP_OPT_UINT8,		/* IP Forwarding */
+	[23]  = DHCP_OPT_UINT8,		/* Default IP TTL */
+	[26]  = DHCP_OPT_UINT16,	/* Interface MTU */
+	[28]  = DHCP_OPT_IPV4,		/* Broadcast Address */
+	[37]  = DHCP_OPT_UINT8,		/* TCP Default TTL */
+	[38]  = DHCP_OPT_UINT32,	/* TCP Keepalive Interval */
+	[40]  = DHCP_OPT_STR,		/* NIS Domain Name */
+	[41]  = DHCP_OPT_IPV4_LIST,	/* NIS Servers */
+	[42]  = DHCP_OPT_IPV4_LIST,	/* NTP Servers */
+	[44]  = DHCP_OPT_IPV4_LIST,	/* NetBIOS Name Server */
+	[50]  = DHCP_OPT_IPV4,		/* Requested IP Address */
+	[51]  = DHCP_OPT_UINT32,	/* IP Address Lease Time */
+	[53]  = DHCP_OPT_UINT8,		/* DHCP Message Type */
+	[54]  = DHCP_OPT_IPV4,		/* Server Identifier */
+	[57]  = DHCP_OPT_UINT16,	/* Max DHCP Message Size */
+	[58]  = DHCP_OPT_UINT32,	/* Renewal (T1) Time */
+	[59]  = DHCP_OPT_UINT32,	/* Rebinding (T2) Time */
+	[60]  = DHCP_OPT_STR,		/* Vendor Class Identifier */
+	[66]  = DHCP_OPT_STR,		/* TFTP Server Name */
+	[67]  = DHCP_OPT_STR,		/* Bootfile Name */
+	[252] = DHCP_OPT_STR,		/* WPAD URL */
+};
+
+/**
+ * dhcp_opt_parse() - Parse a DHCP option value
+ * @code:	DHCP 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 dhcp_opt_parse(uint8_t code, const char *str,
+		   uint8_t *buf, size_t buf_len)
+{
+	enum dhcp_opt_type type;
+	unsigned long val;
+	unsigned int i;
+	uint8_t width;
+	size_t slen;
+	size_t len;
+	char *end;
+
+	if (code >= ARRAY_SIZE(dhcp_opt_types))
+		return -1;
+
+	type = dhcp_opt_types[code];
+
+	if (!*str)
+		return -1;
+
+	switch (type) {
+	case DHCP_OPT_NONE:
+		return -1;
+	case DHCP_OPT_IPV4:
+	case DHCP_OPT_IPV4_LIST:
+		len = 0;
+
+		while (*str) {
+			char ipbuf[INET_ADDRSTRLEN];
+			size_t chunk;
+
+			chunk = strcspn(str, ",");
+
+			if (!chunk || chunk >= sizeof(ipbuf))
+				return -1;
+
+			memcpy(ipbuf, str, chunk);
+			ipbuf[chunk] = '\0';
+
+			if (len + sizeof(struct in_addr) > buf_len)
+				return -1;
+
+			if (inet_pton(AF_INET, ipbuf, buf + len) != 1)
+				return -1;
+
+			len += sizeof(struct in_addr);
+
+			if (type == DHCP_OPT_IPV4) {
+				if (str[chunk] == ',')
+					return -1;
+				break;
+			}
+
+			str += chunk + (str[chunk] == ',');
+		}
+
+		if (!len)
+			return -1;
+
+		return len;
+	case DHCP_OPT_UINT8:
+	case DHCP_OPT_UINT16:
+	case DHCP_OPT_UINT32:
+	case DHCP_OPT_INT32:
+		if (type == DHCP_OPT_UINT8)
+			width = 1;
+		else if (type == DHCP_OPT_UINT16)
+			width = 2;
+		else
+			width = 4;
+
+		if (buf_len < width)
+			return -1;
+
+		errno = 0;
+		if (type == DHCP_OPT_INT32) {
+			long sval;
+
+			sval = strtol(str, &end, 0);
+			if (*end || errno ||
+			    sval < INT32_MIN || sval > INT32_MAX)
+				return -1;
+			val = (uint32_t)sval;
+		} else {
+			val = strtoul(str, &end, 0);
+			if (*end || errno ||
+			    val >= (1ULL << (width * 8)))
+				return -1;
+		}
+
+		for (i = width; i > 0; i--) {
+			buf[i - 1] = val & 0xff;
+			val >>= 8;
+		}
+
+		return width;
+	case DHCP_OPT_STR:
+		slen = strlen(str);
+
+		if (slen >= buf_len)
+			return -1;
+
+		memcpy(buf, str, slen);
+
+		return slen;
+	}
+
+	return -1;
+}
+
+/**
+ * dhcp_set_opt() - Parse and store a user-specified DHCP option
+ * @code:	DHCP option code
+ * @val_str:	Value string from command line
+ */
+void dhcp_set_opt(uint8_t code, const char *val_str)
+{
+	int ret;
+
+	ret = dhcp_opt_parse(code, val_str, opts[code].s, sizeof(opts[code].s));
+	if (ret < 0)
+		die("Invalid value for DHCP option %u: %s", code, val_str);
+
+	opts[code].slen = ret;
+	opts[code].state = OPT_USER;
+}
+
+/**
+ * dhcp_opt_to_str() - Render a binary DHCP option value to a printable string
+ * @code:	DHCP option code
+ * @buf:	Output string buffer
+ * @buf_len:	Size of output buffer
+ *
+ * Return: pointer to @buf if option is user-set, NULL otherwise
+ */
+const char *dhcp_opt_to_str(uint8_t code, char *buf, size_t buf_len)
+{
+	enum dhcp_opt_type type;
+	int off = 0;
+	unsigned int i;
+
+	if (opts[code].state != OPT_USER || opts[code].slen < 0)
+		return NULL;
+
+	if (code >= ARRAY_SIZE(dhcp_opt_types))
+		return NULL;
+
+	type = dhcp_opt_types[code];
+
+	switch (type) {
+	case DHCP_OPT_IPV4:
+	case DHCP_OPT_IPV4_LIST:
+		for (i = 0; i + 4 <= (unsigned int)opts[code].slen; i += 4) {
+			if (off)
+				off += snprintf(buf + off, buf_len - off, ",");
+			inet_ntop(AF_INET, opts[code].s + i,
+				  buf + off, buf_len - off);
+			off = strlen(buf);
+		}
+		return buf;
+	case DHCP_OPT_UINT8:
+		if (snprintf(buf, buf_len, "%u",
+			     opts[code].s[0]) >= (int)buf_len)
+			return NULL;
+		return buf;
+	case DHCP_OPT_UINT16:
+		if (snprintf(buf, buf_len, "%u",
+			     ((unsigned)opts[code].s[0] << 8) |
+			      opts[code].s[1]) >= (int)buf_len)
+			return NULL;
+		return buf;
+	case DHCP_OPT_UINT32:
+		if (snprintf(buf, buf_len, "%u",
+			     ((unsigned)opts[code].s[0] << 24) |
+			     ((unsigned)opts[code].s[1] << 16) |
+			     ((unsigned)opts[code].s[2] << 8)  |
+			      opts[code].s[3]) >= (int)buf_len)
+			return NULL;
+		return buf;
+	case DHCP_OPT_INT32:
+		if (snprintf(buf, buf_len, "%d",
+			     (int)(((unsigned)opts[code].s[0] << 24) |
+				   ((unsigned)opts[code].s[1] << 16) |
+				   ((unsigned)opts[code].s[2] << 8)  |
+				    opts[code].s[3])) >= (int)buf_len)
+			return NULL;
+		return buf;
+	case DHCP_OPT_STR:
+		if ((size_t)opts[code].slen < buf_len) {
+			memcpy(buf, opts[code].s, opts[code].slen);
+			buf[opts[code].slen] = '\0';
+		} else {
+			memcpy(buf, opts[code].s, buf_len - 1);
+			buf[buf_len - 1] = '\0';
+		}
+		return buf;
+	default:
+		return NULL;
+	}
+}
+
 /**
  * fill_one() - Fill a single option into a buffer
  * @buf:	Buffer to write option
@@ -482,14 +760,19 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
 	info("    from %s", eth_ntop(m->chaddr, macstr, sizeof(macstr)));
 
 	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
-	memcpy(opts[1].s,  &mask,                sizeof(mask));
-	memcpy(opts[3].s,  &c->ip4.guest_gw,     sizeof(c->ip4.guest_gw));
-	memcpy(opts[54].s, &c->ip4.our_tap_addr, sizeof(c->ip4.our_tap_addr));
+	if (opts[1].state != OPT_USER)
+		memcpy(opts[1].s, &mask, sizeof(mask));
+	if (opts[3].state != OPT_USER)
+		memcpy(opts[3].s, &c->ip4.guest_gw, sizeof(c->ip4.guest_gw));
+	if (opts[54].state != OPT_USER)
+		memcpy(opts[54].s, &c->ip4.our_tap_addr,
+		       sizeof(c->ip4.our_tap_addr));
 
 	/* If the gateway is not on the assigned subnet, send an option 121
 	 * (Classless Static Routing) adding a dummy route to it.
 	 */
-	if ((c->ip4.addr.s_addr & mask.s_addr)
+	if (opts[121].state != OPT_USER &&
+	    (c->ip4.addr.s_addr & mask.s_addr)
 	    != (c->ip4.guest_gw.s_addr & mask.s_addr)) {
 		/* a.b.c.d/32:0.0.0.0, 0:a.b.c.d */
 		opts[121].slen = 14;
@@ -501,27 +784,32 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
 		       &c->ip4.guest_gw, sizeof(c->ip4.guest_gw));
 	}
 
-	if (c->mtu) {
+	if (opts[26].state != OPT_USER && c->mtu) {
 		opts[26].slen = 2;
 		opts[26].state = OPT_DEFAULT;
 		opts[26].s[0] = c->mtu / 256;
 		opts[26].s[1] = c->mtu % 256;
 	}
 
-	for (i = 0, opts[6].slen = 0;
-	     !c->no_dhcp_dns && i < ARRAY_SIZE(c->ip4.dns); i++) {
-		if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns[i]))
-			break;
-		((struct in_addr *)opts[6].s)[i] = c->ip4.dns[i];
-		opts[6].slen += sizeof(uint32_t);
+	if (opts[6].state != OPT_USER) {
+		for (i = 0, opts[6].slen = 0;
+		     !c->no_dhcp_dns && i < ARRAY_SIZE(c->ip4.dns); i++) {
+			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns[i]))
+				break;
+			((struct in_addr *)opts[6].s)[i] = c->ip4.dns[i];
+			opts[6].slen += sizeof(uint32_t);
+		}
+		if (!opts[6].slen)
+			opts[6].slen = -1;
 	}
+
 	if (opts[6].slen)
 		opts[6].state = OPT_DEFAULT;
 	else
 		opts[6].state = OPT_UNSET;
 
 	opt_len = strlen(c->hostname);
-	if (opt_len > 0) {
+	if (opts[12].state != OPT_USER && opt_len > 0) {
 		opts[12].slen = opt_len;
 		opts[12].state = OPT_DEFAULT;
 		memcpy(opts[12].s, &c->hostname, opt_len);
diff --git a/dhcp.h b/dhcp.h
index cd50c99..80abda6 100644
--- a/dhcp.h
+++ b/dhcp.h
@@ -8,5 +8,7 @@
 
 int dhcp(const struct ctx *c, struct iov_tail *data);
 void dhcp_init(void);
+void dhcp_set_opt(uint8_t code, const char *val_str);
+const char *dhcp_opt_to_str(uint8_t code, char *buf, size_t buf_len);
 
 #endif /* DHCP_H */
diff --git a/passt.1 b/passt.1
index 995590a..215a289 100644
--- a/passt.1
+++ b/passt.1
@@ -440,6 +440,48 @@ 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 \-\-dhcp-opt " " \fICODE\fR,\fIVALUE\fR
+Set DHCP option \fICODE\fR (1\-254) to \fIVALUE\fR. The value format depends
+on the option type and is determined automatically from the option code.
+Multiple IPv4 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. Options set with
+\fB\-\-dhcp-opt\fR override built-in values.
+.PP
+Examples:
+.nf
+  \-\-dhcp-opt 6,8.8.8.8,4.4.4.4
+  \-\-dhcp-opt 12,myhostname
+.fi
+.PP
+Only the following option codes are supported (unsupported codes cause an error):
+.RS
+.TP
+.B IPv4 address options
+1 (Subnet Mask), 16 (Swap Server), 28 (Broadcast Address), 50 (Requested IP),
+54 (Server Identifier)
+.TP
+.B IPv4 address list options (comma-separated)
+3 (Router), 4 (Time Server), 5 (Name Server), 6 (DNS), 7 (Log Server),
+8 (Cookie Server), 9 (LPR Server), 10 (Impress Server),
+11 (Resource Location Server), 41 (NIS Servers),
+42 (NTP Servers), 44 (NetBIOS Name Server)
+.TP
+.B Integer options
+2 (Time Offset, 32-bit), 13 (Boot File Size, 16-bit), 19 (IP Forwarding, 8-bit),
+23 (Default IP TTL, 8-bit), 26 (Interface MTU, 16-bit),
+37 (TCP Default TTL, 8-bit), 38 (TCP Keepalive Interval, 32-bit),
+51 (IP Address Lease Time, 32-bit),
+53 (DHCP Message Type, 8-bit), 57 (Max DHCP Message Size, 16-bit),
+58 (Renewal Time, 32-bit), 59 (Rebinding Time, 32-bit)
+.TP
+.B String options
+12 (Host Name), 15 (Domain Name), 17 (Root Path), 40 (NIS Domain Name),
+60 (Vendor Class Identifier), 66 (TFTP Server Name),
+67 (Bootfile Name), 252 (WPAD URL)
+.RE
+
 .TP
 .BR \-t ", " \-\-tcp-ports " " \fIspec
 Configure TCP port forwarding to guest or namespace. \fIspec\fR can be either:
-- 
2.54.0