Re: [PATCH 3/3] dhcp: Honour broadcast flag (RFC 2131, 4.1)

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

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On Mon, Nov 25, 2024 at 01:04:23AM +0100, Stefano Brivio wrote:
> It's widely considered a legacy option nowadays, and I've haven't seen
> clients setting it since Windows 95, but it's convenient for a minimal
> DHCP client not using raw IP sockets such as what I'm playing with for
> muvm.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  dhcp.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/dhcp.c b/dhcp.c
> index aa0ad96..f1416ee 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -107,6 +107,8 @@ struct msg {
>  	uint32_t xid;
>  	uint16_t secs;
>  	uint16_t flags;
> +#define FLAG_BROADCAST	htons_constant(0x8000)
> +
>  	uint32_t ciaddr;
>  	struct in_addr yiaddr;
>  	uint32_t siaddr;
> @@ -280,10 +282,10 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  {
>  	size_t mlen, dlen, offset = 0, opt_len, opt_off = 0;
>  	char macstr[ETH_ADDRSTRLEN];
> +	struct in_addr mask, dst;
>  	const struct ethhdr *eh;
>  	const struct iphdr *iph;
>  	const struct udphdr *uh;
> -	struct in_addr mask;
>  	unsigned int i;
>  	struct msg *m;
>  
> @@ -398,7 +400,13 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  		opt_set_dns_search(c, sizeof(m->o));
>  
>  	dlen = offsetof(struct msg, o) + fill(m);
> -	tap_udp4_send(c, c->ip4.our_tap_addr, 67, c->ip4.addr, 68, m, dlen);
> +
> +	if (m->flags & FLAG_BROADCAST)
> +		dst = (struct in_addr){ 0xffffffff };

It would be nice to add a symbolic constant ("in4addr_broadcast"?) to
ip.h for this.

> +	else
> +		dst = c->ip4.addr;
> +
> +	tap_udp4_send(c, c->ip4.our_tap_addr, 67, dst, 68, m, dlen);
>  
>  	return 1;
>  }

-- 
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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