Re: [PATCH 1/3] conf: Add --dhcpv6-opt command-line option

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

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On Thu, Jun 04, 2026 at 04:21:48PM +0530, Anshu Kumari wrote:
> Add a --dhcpv6-opt CODE,VALUE option for injecting custom DHCPv6
> options into server replies. This patch adds the CLI parsing in conf.c,
> storage in struct ctx, and a simple dhcpv6_add_option() that stores the
> option code and raw string value.
> 
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  conf.c   | 26 +++++++++++++++++++++++++-
>  dhcpv6.c | 37 +++++++++++++++++++++++++++++++++++++
>  dhcpv6.h |  1 +
>  passt.1  |  6 ++++++
>  passt.h  | 12 ++++++++++++
>  5 files changed, 81 insertions(+), 1 deletion(-)
> 
> diff --git a/conf.c b/conf.c
> index 6f86940..ac7e6e5 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 by code\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->custom_v6opts_count; i++)
> +			info("    v6 option %u: %s",
> +			     c->custom_v6opts[i].code,
> +			     c->custom_v6opts[i].str);
>  
>  dns6:
>  		for (i = 0; i < ARRAY_SIZE(c->ip6.dns); i++) {
> @@ -1233,6 +1239,7 @@ 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,		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:";
> @@ -1248,10 +1255,13 @@ 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;
>  	
> @@ -1465,6 +1475,20 @@ 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 34:
> +			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 || v6optcode > 255)
> +				die("DHCPv6 option code must be 1-255: %s",
> +				    optarg);
> +
> +			dhcpv6_add_option(c, v6optcode, comma + 1);
> +			break;
>  		case 'd':
>  			c->debug = 1;
>  			c->quiet = 0;
> diff --git a/dhcpv6.c b/dhcpv6.c
> index 97c04e2..a0fb77c 100644
> --- a/dhcpv6.c
> +++ b/dhcpv6.c
> @@ -32,6 +32,43 @@
>  #include "tap.h"
>  #include "log.h"
>  
> +/**
> + * dhcpv6_add_option() - Add or update a custom DHCPv6 option
> + * @c:		Execution context
> + * @code:	DHCPv6 option code
> + * @val_str:	Value string from command line
> + *
> + * Stores the option code and raw string value. Binary encoding and
> + * injection into DHCPv6 replies are handled by later patches.
> + *
> + * Return: 0 on success
> + */
> +int dhcpv6_add_option(struct ctx *c, uint16_t code, const char *val_str)
> +{
> +	int idx;
> +
> +	for (idx = 0; idx < c->custom_v6opts_count; idx++) {
> +		if (c->custom_v6opts[idx].code == code)
> +			break;
> +	}
> +
> +	if (idx == c->custom_v6opts_count) {
> +		if (c->custom_v6opts_count >= MAX_CUSTOM_DHCPV6_OPTS)
> +			die("Too many --dhcpv6-opt entries (max %d)",
> +			    MAX_CUSTOM_DHCPV6_OPTS);
> +		c->custom_v6opts_count++;
> +	}
> +
> +	c->custom_v6opts[idx].code = code;
> +
> +	if (snprintf_check(c->custom_v6opts[idx].str,
> +			   sizeof(c->custom_v6opts[0].str),
> +			   "%s", val_str))
> +		die("DHCPv6 option value too long: %s", val_str);
> +
> +	return 0;
> +}
> +
>  /**
>   * struct opt_hdr - DHCPv6 option header
>   * @t:		Option type
> diff --git a/dhcpv6.h b/dhcpv6.h
> index c706dfd..c01bc36 100644
> --- a/dhcpv6.h
> +++ b/dhcpv6.h
> @@ -9,5 +9,6 @@
>  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_add_option(struct ctx *c, uint16_t code, const char *val_str);
>  
>  #endif /* DHCPV6_H */
> diff --git a/passt.1 b/passt.1
> index 908fd4a..9c25214 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -430,6 +430,12 @@ 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 a DHCPv6 option by numeric code. The value format is determined
> +automatically from the option code. This option can be specified multiple
> +times. If the same option code is given more than once, the last value wins.
> +
>  .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 1726965..91509df 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
> + * @custom_v6opts:	User-specified DHCPv6 options from --dhcpv6-opt
> + * @custom_v6opts.code:	DHCPv6 option code
> + * @custom_v6opts.str:	Original string value from command line
> + * @custom_v6opts_count:Number of entries in @custom_v6opts
>   * @ifi6:		Template interface for IPv6, -1: none, 0: IPv6 disabled
>   * @ip6:		IPv6 configuration
>   * @pasta_ifn:		Name of namespace interface for pasta
> @@ -263,6 +267,14 @@ struct ctx {
>  	char hostname[PASST_MAXDNAME];
>  	char fqdn[PASST_MAXDNAME];
>  
> +#define MAX_CUSTOM_DHCPV6_OPTS	32
> +
> +	struct {
> +		uint16_t code;
> +		char str[256];
> +	} custom_v6opts[MAX_CUSTOM_DHCPV6_OPTS];
> +	int custom_v6opts_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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