From c0fbc7ef2ae2919bf6162b4149d341f448289836 Mon Sep 17 00:00:00 2001
From: Stefano Brivio <sbrivio@redhat.com>
Date: Mon, 25 Nov 2024 00:52:57 +0100
Subject: dhcp: Honour broadcast flag (RFC 2131, 4.1)
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>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
dhcp.c | 12 ++++++++++--
ip.h | 3 +++
2 files changed, 13 insertions(+), 2 deletions(-)
diff --git a/dhcp.c b/dhcp.c
index a16cde8..d8515aa 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -112,6 +112,8 @@ 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;
@@ -285,10 +287,10 @@ 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;
@@ -400,7 +402,13 @@ 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 = in4addr_broadcast;
+ else
+ dst = c->ip4.addr;
+
+ tap_udp4_send(c, c->ip4.our_tap_addr, 67, dst, 68, m, dlen);
return 1;
}
diff --git a/ip.h b/ip.h
index 0742612..1544dbf 100644
--- a/ip.h
+++ b/ip.h
@@ -101,4 +101,7 @@ static const struct in6_addr in6addr_ll_all_nodes = {
},
};
+/* IPv4 Limited Broadcast (RFC 919, Section 7), 255.255.255.255 */
+static const struct in_addr in4addr_broadcast = { 0xffffffff };
+
#endif /* IP_H */
--
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