Re: [PATCH v2 1/2] dhcpv6: Add --dhcpv6-opt with option type table and value parser

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jun 18, 2026 at 05:35:28PM +0530, Anshu Kumari wrote:
> 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>
> ---
> 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   |  60 +++++++++++++++-
>  dhcpv6.c | 209 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  dhcpv6.h |   2 +
>  passt.1  |  31 +++++++++
>  passt.h  |  12 ++++
>  5 files changed, 313 insertions(+), 1 deletion(-)
> 
> diff --git a/conf.c b/conf.c
> index cd05adf..3981c1b 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"
> @@ -616,7 +617,8 @@ 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
> @@ -884,6 +886,10 @@ 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 = 0; i < c->dhcpv6_opts_count; i++)
> +			info("    v6 option %u: %s",
> +			     c->dhcpv6_opts[i].code,
> +			     c->dhcpv6_opts[i].str);
>  
>  dns6:
>  		for (i = 0; i < ARRAY_SIZE(c->ip6.dns); i++) {
> @@ -1150,6 +1156,41 @@ static void conf_sock_listen(const struct ctx *c)
>  		die_perror("Couldn't add configuration socket to epoll");
>  }
>  
> +/**
> + * conf_dhcpv6_option() - Set value for a DHCPv6 option in configuration
> + * @c:		Execution context
> + * @code:	DHCPv6 option code
> + * @val_str:	Value string from command line
> + */
> +static void conf_dhcpv6_option(struct ctx *c, uint16_t code,
> +			       const char *val_str)
> +{
> +	uint8_t tmp[255];
> +	int idx;
> +
> +	if (dhcpv6_opt_parse(code, val_str, tmp, sizeof(tmp)) < 0)
> +		die("Invalid value for DHCPv6 option %u: %s", code, val_str);

As for DHCPv4, I wonder if we can avoid double parsing each option.  I
realise the data structures in dhcpv6.c are different from those in
dhcp.c, so it might not reasonable here, even if it is there.

> +	for (idx = 0; idx < c->dhcpv6_opts_count; idx++) {
> +		if (c->dhcpv6_opts[idx].code == code)
> +			break;
> +	}
> +
> +	if (idx == c->dhcpv6_opts_count) {
> +		if (c->dhcpv6_opts_count >= MAX_DHCPV6_OPTS)
> +			die("Too many --dhcpv6-opt entries (max %d)",
> +			    MAX_DHCPV6_OPTS);
> +		c->dhcpv6_opts_count++;
> +	}
> +
> +	c->dhcpv6_opts[idx].code = code;
> +
> +	if (snprintf_check(c->dhcpv6_opts[idx].str,
> +			   sizeof(c->dhcpv6_opts[0].str),
> +			   "%s", val_str))
> +		die("DHCPv6 option value too long: %s", val_str);
> +}
> +
>  /**
>   * conf() - Process command-line arguments and set configuration
>   * @c:		Execution context
> @@ -1233,6 +1274,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  		{"migrate-no-linger", no_argument,	NULL,		30 },
>  		{"stats", required_argument,		NULL,		31 },
>  		{"conf-path",	required_argument,	NULL,		'c' },
> +		{"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:";
> @@ -1248,10 +1290,13 @@ void conf(struct ctx *c, int argc, char **argv)
>  	uint8_t prefix_len_from_opt = 0;
>  	unsigned int ifi4 = 0, ifi6 = 0;
>  	const char *logfile = NULL;
> +	unsigned long v6optcode;
>  	char *runas = NULL;
>  	size_t logsize = 0;
> +	const char *comma;
>  	long fd_tap_opt;
>  	int name, ret;
> +	char *end;
>  	uid_t uid;
>  	gid_t gid;
>  
> @@ -1467,6 +1512,19 @@ void conf(struct ctx *c, int argc, char **argv)
>  				die("Can't display statistics if not running in foreground");
>  			c->stats = strtol(optarg, NULL, 0);
>  			break;
> +		case 35:
> +			comma = strchr(optarg, ',');
> +			if (!comma)
> +				die("--dhcpv6-opt requires CODE,VALUE format");
> +
> +			errno = 0;
> +			v6optcode = strtoul(optarg, &end, 0);
> +			if (end != comma || errno || v6optcode < 1)
> +				die("Invalid DHCPv6 option code: %s",
> +				    optarg);
> +
> +			conf_dhcpv6_option(c, v6optcode, comma + 1);
> +			break;
>  		case 'd':
>  			c->debug = 1;
>  			c->quiet = 0;
> diff --git a/dhcpv6.c b/dhcpv6.c
> index 97c04e2..1e1dd0d 100644
> --- a/dhcpv6.c
> +++ b/dhcpv6.c
> @@ -25,6 +25,7 @@
>  #include <string.h>
>  #include <time.h>
>  #include <limits.h>
> +#include <errno.h>
>  
>  #include "packet.h"
>  #include "util.h"
> @@ -278,6 +279,214 @@ 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_INT8:		Unsigned 8-bit integer
> + * @DHCPV6_OPT_INT16:		Unsigned 16-bit integer
> + * @DHCPV6_OPT_INT32:		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_INT8,
> +	DHCPV6_OPT_INT16,
> +	DHCPV6_OPT_INT32,
> +	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, 17, 32, 82, 83
> + * RFC 5970 Options: 59, 60
> + * RFC 4075 Options: 31
> + */
> +static const enum dhcpv6_opt_type dhcpv6_opt_types[] = {
> +	[7]   = DHCPV6_OPT_INT8,		/* Preference */
> +	[15]  = DHCPV6_OPT_LEN_STR_LIST,	/* User Class */
> +	[16]  = DHCPV6_OPT_VENDOR_CLASS,	/* Vendor Class */
> +	[17]  = DHCPV6_OPT_VENDOR_CLASS,	/* Vendor Opts */
> +	[31]  = DHCPV6_OPT_IPV6_LIST,		/* SNTP Servers */
> +	[32]  = DHCPV6_OPT_INT32,		/* Information Refresh Time */
> +	[59]  = DHCPV6_OPT_STR,		/* Boot File URL */
> +	[60]  = DHCPV6_OPT_LEN_STR_LIST,	/* Boot File Params */
> +	[82]  = DHCPV6_OPT_INT32,		/* SOL_MAX_RT */
> +	[83]  = DHCPV6_OPT_INT32,		/* 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
> + */
> +int dhcpv6_opt_parse(uint16_t code, const char *str,
> +		     uint8_t *buf, size_t buf_len)
> +{
> +	enum dhcpv6_opt_type type;
> +	unsigned long val;
> +	unsigned int i;
> +	uint8_t width;
> +	size_t slen;
> +	char *end;
> +	int len;
> +
> +	if (!*str)
> +		return -1;
> +
> +	if (code >= ARRAY_SIZE(dhcpv6_opt_types))
> +		return -1;
> +
> +	type = dhcpv6_opt_types[code];
> +
> +	switch (type) {

A number of these types result in identical parsing to DHCPv4.  Could
we have a unified parser function?  Obviously things like IPv4 address
and IPv6 address would need different types, but we should be able to
share at least the integer and string code.

> +	case DHCPV6_OPT_NONE:
> +		return -1;
> +	case DHCPV6_OPT_IPV6:
> +	case DHCPV6_OPT_IPV6_LIST:
> +		len = 0;
> +
> +		while (*str) {
> +			char chunk[INET6_ADDRSTRLEN];
> +			size_t clen;
> +
> +			clen = strcspn(str, ",");
> +			if (!clen || clen >= sizeof(chunk))
> +				return -1;
> +
> +			if (len + (int)sizeof(struct in6_addr) > (int)buf_len)
> +				return -1;
> +
> +			memcpy(chunk, str, clen);
> +			chunk[clen] = '\0';
> +
> +			if (inet_pton(AF_INET6, chunk, buf + len) != 1)
> +				return -1;
> +
> +			len += sizeof(struct in6_addr);
> +
> +			if (type == DHCPV6_OPT_IPV6) {
> +				if (str[clen] == ',')
> +					return -1;
> +				break;
> +			}
> +
> +			str += clen + (str[clen] == ',');
> +		}
> +
> +		if (!len)
> +			return -1;
> +
> +		return len;
> +	case DHCPV6_OPT_INT8:
> +	case DHCPV6_OPT_INT16:
> +	case DHCPV6_OPT_INT32:
> +		if (type == DHCPV6_OPT_INT8)
> +			width = 1;
> +		else if (type == DHCPV6_OPT_INT16)
> +			width = 2;
> +		else
> +			width = 4;
> +
> +		if (buf_len < width)
> +			return -1;
> +
> +		errno = 0;
> +		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 DHCPV6_OPT_STR:
> +		slen = strlen(str);
> +
> +		if (slen >= buf_len)
> +			return -1;
> +
> +		memcpy(buf, str, slen);
> +
> +		return slen;
> +	case DHCPV6_OPT_VENDOR_CLASS: {
> +		const char *colon;
> +		uint32_t ent;
> +
> +		colon = strchr(str, ':');
> +		if (!colon)
> +			return -1;
> +
> +		errno = 0;
> +		val = strtoul(str, &end, 0);
> +		if (end != colon || errno || val > UINT32_MAX)
> +			return -1;
> +
> +		slen = strlen(colon + 1);
> +		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));
> +
> +		buf[4] = slen >> 8;
> +		buf[5] = slen & 0xff;
> +
> +		memcpy(buf + sizeof(uint32_t) + sizeof(uint16_t),
> +		       colon + 1, 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;
> +
> +			buf[len]     = slen >> 8;
> +			buf[len + 1] = slen & 0xff;
> +			len += sizeof(uint16_t);
> +
> +			memcpy(buf + len, str, slen);
> +			len += slen;
> +
> +			str += slen;
> +			if (*str == ',')
> +				str++;
> +		}
> +
> +		if (!len)
> +			return -1;
> +
> +		return len;
> +	}
> +
> +	return -1;
> +}
> +
>  /**
>   * 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 c706dfd..2da1c76 100644
> --- a/dhcpv6.h
> +++ b/dhcpv6.h
> @@ -9,5 +9,7 @@
>  int dhcpv6(struct ctx *c, struct iov_tail *data,
>  	   struct in6_addr *saddr, struct in6_addr *daddr);
>  void dhcpv6_init(const struct ctx *c);
> +int dhcpv6_opt_parse(uint16_t code, const char *str,
> +		     uint8_t *buf, size_t buf_len);
>  
>  #endif /* DHCPV6_H */
> diff --git a/passt.1 b/passt.1
> index 908fd4a..0f771cb 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -430,6 +430,37 @@ 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), 17 (Vendor-specific Info, 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 one of:
> diff --git a/passt.h b/passt.h
> index 3a07294..8f4ddef 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -182,6 +182,10 @@ struct ip6_ctx {
>   * @dns_search:		DNS search list
>   * @hostname:		Guest hostname
>   * @fqdn:		Guest FQDN
> + * @dhcpv6_opts:	User-specified DHCPv6 options from --dhcpv6-opt
> + * @dhcpv6_opts.code:	DHCPv6 option code
> + * @dhcpv6_opts.str:	String value from command line
> + * @dhcpv6_opts_count:	Number of entries in @dhcpv6_opts
>   * @ifi6:		Template interface for IPv6, -1: none, 0: IPv6 disabled
>   * @ip6:		IPv6 configuration
>   * @pasta_ifn:		Name of namespace interface for pasta
> @@ -264,6 +268,14 @@ struct ctx {
>  	char hostname[PASST_MAXDNAME];
>  	char fqdn[PASST_MAXDNAME];
>  
> +#define MAX_DHCPV6_OPTS	32
> +
> +	struct {
> +		uint16_t code;
> +		char str[255];
> +	} dhcpv6_opts[MAX_DHCPV6_OPTS];
> +	int dhcpv6_opts_count;
> +
>  	int ifi6;
>  	struct ip6_ctx ip6;
>  
> -- 
> 2.54.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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