Re: [PATCH v3 3/6] dhcp: Add option type table and value parser

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

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On Mon, Jun 01, 2026 at 01:07:53PM +0530, Anshu Kumari wrote:
> Add an RFC 2132 type lookup table mapping DHCP option codes to their
> expected value formats, and a dhcp_opt_parse() function that converts
> CLI string values into their binary wire representation.
> 
> Wire dhcp_opt_parse() into dhcp_add_option() so that values are
> validated and encoded at configuration time.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=192
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>
> ---
> 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.

Thanks for the detailed changelogs, by the way.  I know these are a
bunch of work to maintain, but they really help when reviewing.

> 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.
> ---
>  dhcp.c  | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  passt.h |   4 ++
>  2 files changed, 181 insertions(+), 3 deletions(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index c5fbf37..07a42b9 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"
> @@ -33,6 +34,170 @@
>  #include "log.h"
>  #include "dhcp.h"
>  
> +/**
> + * 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_INT8:	Unsigned 8-bit integer
> + * @DHCP_OPT_INT16:	Unsigned 16-bit integer
> + * @DHCP_OPT_INT32:	Unsigned 32-bit integer
> + */
> +enum dhcp_opt_type {
> +	DHCP_OPT_NONE,
> +	DHCP_OPT_STR,
> +	DHCP_OPT_IPV4,
> +	DHCP_OPT_IPV4_LIST,
> +	DHCP_OPT_INT8,
> +	DHCP_OPT_INT16,
> +	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[256] = {
> +	[1]   = DHCP_OPT_IPV4,		/* Subnet Mask */
> +	[2]   = DHCP_OPT_INT32,		/* Time Offset */
> +	[3]   = DHCP_OPT_IPV4_LIST,	/* Router */

I'm still a bit unsure if we want to allow user modification of the
options, like this one, which we already manage ourselves.

> +	[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_INT16,		/* Boot File Size */
> +	[15]  = DHCP_OPT_STR,		/* Domain Name */
> +	[16]  = DHCP_OPT_IPV4,		/* Swap Server */
> +	[17]  = DHCP_OPT_STR,		/* Root Path */
> +	[19]  = DHCP_OPT_INT8,		/* IP Forwarding */
> +	[23]  = DHCP_OPT_INT8,		/* Default IP TTL */
> +	[26]  = DHCP_OPT_INT16,		/* Interface MTU */
> +	[28]  = DHCP_OPT_IPV4,		/* Broadcast Address */
> +	[37]  = DHCP_OPT_INT8,		/* TCP Default TTL */
> +	[38]  = DHCP_OPT_INT32,		/* 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_INT32,		/* IP Address Lease Time */
> +	[53]  = DHCP_OPT_INT8,		/* DHCP Message Type */
> +	[54]  = DHCP_OPT_IPV4,		/* Server Identifier */
> +	[57]  = DHCP_OPT_INT16,		/* Max DHCP Message Size */
> +	[58]  = DHCP_OPT_INT32,		/* Renewal (T1) Time */
> +	[59]  = DHCP_OPT_INT32,		/* 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 = dhcp_opt_types[code];
> +	char *tok, *saveptr, *end;
> +	struct in_addr addr;
> +	unsigned long val;
> +	unsigned int i;
> +	uint8_t width;
> +	char tmp[256];
> +	size_t slen;
> +	int len;
> +
> +	if (!*str)
> +		die("Empty value for DHCP option %u", code);
> +
> +	switch (type) {
> +	case DHCP_OPT_NONE:
> +		die("Unsupported DHCP option: %u,"
> +		    " see passt(1) for supported codes", code);
> +	case DHCP_OPT_IPV4:
> +	case DHCP_OPT_IPV4_LIST:
> +		len = 0;
> +
> +		/* Reject empty, leading/trailing, or consecutive commas */
> +		if (!*str || *str == ',' || str[strlen(str) - 1] == ',' ||
> +		    strstr(str, ",,"))
> +			return -1;
> +
> +		if (snprintf_check(tmp, sizeof(tmp), "%s", str))
> +			return -1;

The arbitrary 256 byte buffer limit here isn't great.  The string
encoding of an IPv4 address can be nearly 4 times as long as the
binary encoding, so we could potentially hit this with a longish, but
not ridiculous address list.

> +		for (tok = strtok_r(tmp, ",", &saveptr); tok;
> +		     tok = strtok_r(NULL, ",", &saveptr)) {

One way to avoid that would be to avoid using strtok_r() which relies
in in-place modifying the input.  Instead you'd need to repeatedly
find the length of the next chunk with strchr() or strcpsn(), then
extract each one into a tmp of length INET_ADDRSTRLEN to call
inet_pton().  On the plus side, that should naturally deal with the
case of extraneous commas (it would show up as an empth entry), rather
than requiring an explicit check at the top.


> +			if (inet_pton(AF_INET, tok, &addr) != 1)
> +				return -1;
> +
> +			if (len + (int)sizeof(addr) > (int)buf_len)
> +				return -1;

You could make this check before the inet_pton(), then do the
conversion directly into buf, avoiding the addr temporary.

> +
> +			memcpy(buf + len, &addr, sizeof(addr));
> +			len += sizeof(addr);
> +
> +			if (type == DHCP_OPT_IPV4)
> +				break;
> +		}
> +
> +		if (type == DHCP_OPT_IPV4 && strtok_r(NULL, ",", &saveptr))
> +			return -1;
> +
> +		return len;
> +	case DHCP_OPT_INT8:
> +	case DHCP_OPT_INT16:
> +	case DHCP_OPT_INT32:
> +		if (type == DHCP_OPT_INT8)
> +			width = 1;
> +		else if (type == DHCP_OPT_INT16)
> +			width = 2;
> +		else
> +			width = 4;
> +
> +		errno = 0;
> +		val = strtoul(str, &end, 0);
> +
> +		if (*end || errno)
> +			return -1;
> +
> +		if (buf_len < width)
> +			return -1;
> +
> +		if (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 || slen >= buf_len)
> +			return -1;
> +
> +		memcpy(buf, str, slen);

Do you need to include the terminating \0 here?  If so you'll need
slen + 1.

> +
> +		return slen;
> +	}
> +
> +	return -1;
> +}
>  
>  /**
>   * dhcp_add_option() - Add or update a custom DHCP option
> @@ -40,14 +205,15 @@
>   * @code:	DHCP option code
>   * @val_str:	Value string from command line
>   *
> - * If @code was already added, the previous value is overwritten.
> - * Calls die() on any error.
> + * Parses @val_str according to the type registered for @code in
> + * dhcp_opt_types[]. If @code was already added, the previous value
> + * is overwritten. Calls die() on any error.
>   *
>   * Return: 0 on success
>   */
>  int dhcp_add_option(struct ctx *c, uint8_t code, const char *val_str)
>  {
> -	int idx;
> +	int idx, ret;
>  
>  	for (idx = 0; idx < c->custom_opts_count; idx++) {
>  		if (c->custom_opts[idx].code == code)
> @@ -61,7 +227,15 @@ int dhcp_add_option(struct ctx *c, uint8_t code, const char *val_str)
>  		c->custom_opts_count++;
>  	}
>  
> +	ret = dhcp_opt_parse(code, val_str,
> +			     c->custom_opts[idx].val,
> +			     sizeof(c->custom_opts[0].val));

Now that this parsing and adding code is all in dhcp.c, could we parse
the options directly into the existing opts[] global, rather than
requiring both the string and parsed forms in c->custom_opts?

> +	if (ret < 0)
> +		die("Invalid value for DHCP option %u: %s",
> +		    code, val_str);
> +
>  	c->custom_opts[idx].code = code;
> +	c->custom_opts[idx].len = ret;
>  
>  	if (snprintf_check(c->custom_opts[idx].str,
>  			   sizeof(c->custom_opts[0].str),
> diff --git a/passt.h b/passt.h
> index 3a0816f..751fee3 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -184,6 +184,8 @@ struct ip6_ctx {
>   * @fqdn:		Guest FQDN
>   * @custom_opts:	User-specified DHCP options from --dhcp-opt
>   * @custom_opts.code:	DHCP option code
> + * @custom_opts.len:	Length of binary value in @val
> + * @custom_opts.val:	Binary-encoded option value
>   * @custom_opts.str:	Original string value from command line
>   * @custom_opts_count:	Number of entries in @custom_opts
>   * @ifi6:		Template interface for IPv6, -1: none, 0: IPv6 disabled
> @@ -271,6 +273,8 @@ struct ctx {
>  
>  	struct {
>  		uint8_t code;
> +		uint8_t len;
> +		uint8_t val[255];
>  		char str[256];
>  	} custom_opts[MAX_CUSTOM_DHCP_OPTS];
>  	int custom_opts_count;
> -- 
> 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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