Re: [PATCH 3/3] dhcpv6: Inject custom options into DHCPv6 replies

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

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On Thu, Jun 04, 2026 at 04:21:50PM +0530, Anshu Kumari wrote:
> Add dhcpv6_custom_opts_fill() which appends user-specified custom
> options to DHCPv6 response packets, and call it from the main dhcpv6()
> handler after the built-in DNS and FQDN options.
> 
> Each custom option is written with its DHCPv6 option header (code +
> length) followed by the binary-encoded value from dhcpv6_opt_parse().
> Options that would exceed the IPv6 minimum MTU are skipped with a
> debug message.
> 
> Signed-off-by: Anshu Kumari <anskuma@redhat.com>

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

> ---
>  dhcpv6.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/dhcpv6.c b/dhcpv6.c
> index 85e2926..590c40d 100644
> --- a/dhcpv6.c
> +++ b/dhcpv6.c
> @@ -798,6 +798,41 @@ static size_t dhcpv6_client_fqdn_fill(const struct iov_tail *data,
>  	return offset + sizeof(struct opt_hdr) + opt_len;
>  }
>  
> +/**
> + * dhcpv6_custom_opts_fill() - Append user-specified custom options to reply
> + * @c:		Execution context
> + * @buf:	Response message buffer
> + * @offset:	Current offset in buffer
> + *
> + * Return: updated offset after appending custom options
> + */
> +static size_t dhcpv6_custom_opts_fill(const struct ctx *c,
> +				      char *buf, int offset)
> +{
> +	int i;
> +
> +	for (i = 0; i < c->custom_v6opts_count; i++) {
> +		struct opt_hdr *hdr;
> +		uint16_t len = c->custom_v6opts[i].len;
> +
> +		if ((size_t)offset + sizeof(struct opt_hdr) + len > OPT_MAX_SIZE) {
> +			debug("DHCPv6: custom option %u doesn't fit, skipping",
> +			      c->custom_v6opts[i].code);
> +			continue;
> +		}
> +
> +		hdr = (struct opt_hdr *)(buf + offset);
> +		hdr->t = htons(c->custom_v6opts[i].code);
> +		hdr->l = htons(len);
> +		offset += sizeof(struct opt_hdr);
> +
> +		memcpy(buf + offset, c->custom_v6opts[i].val, len);
> +		offset += len;
> +	}
> +
> +	return offset;
> +}
> +
>  /**
>   * dhcpv6() - Check if this is a DHCPv6 message, reply as needed
>   * @c:		Execution context
> @@ -936,6 +971,7 @@ 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_custom_opts_fill(c, (char *)&resp, n);
>  
>  	resp.hdr.xid = mh->xid;
>  
> -- 
> 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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