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

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 25 Nov 2024 12:11:34 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> 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.

Sure, added as in4addr_broadcast.

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

-- 
Stefano