[PATCH v2] dhcp: Don't re-use request message for reply

[PATCH v2] dhcp: Don't re-use request message for reply

[Enrique Llorente]

The logic composing the DHCP reply message is reusing the request
message to compose it, future long options like FQDN may
exceed the request message limit making it go beyond the lower
bound.

This change creates a new reply message with a fixed options size of 308
and fills it in with proper fields from requests adding on top the generated
options, this way the reply lower bound does not depend on the request.

Signed-off-by: Enrique Llorente <ellorent@redhat.com>
---
 dhcp.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/dhcp.c b/dhcp.c
index d8515aa..2a23ed4 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -151,9 +151,6 @@ static int fill(struct msg *m)
 {
 	int i, o, offset = 0;
 
-	m->op = BOOTREPLY;
-	m->secs = 0;
-
 	for (o = 0; o < 255; o++)
 		opts[o].sent = 0;
 
@@ -291,8 +288,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	const struct ethhdr *eh;
 	const struct iphdr *iph;
 	const struct udphdr *uh;
+	struct msg const *m;
+	struct msg reply;
 	unsigned int i;
-	struct msg *m;
 
 	eh  = packet_get(p, 0, offset, sizeof(*eh),  NULL);
 	offset += sizeof(*eh);
@@ -321,6 +319,22 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	    m->op != BOOTREQUEST)
 		return -1;
 
+	reply.op		= BOOTREPLY;
+	reply.htype		= m->htype;
+	reply.hlen		= m->hlen;
+	reply.hops		= 0;
+	reply.xid		= m->xid;
+	reply.secs		= 0;
+	reply.flags		= m->flags;
+	reply.ciaddr		= m->ciaddr;
+	reply.yiaddr		= c->ip4.addr;
+	reply.siaddr		= 0;
+	reply.giaddr		= m->giaddr;
+	memcpy(&reply.chaddr,	m->chaddr,	sizeof(reply.chaddr));
+	memset(&reply.sname,	0,		sizeof(reply.sname));
+	memset(&reply.file,	0,		sizeof(reply.file));
+	reply.magic		= m->magic;
+
 	offset += offsetof(struct msg, o);
 
 	for (i = 0; i < ARRAY_SIZE(opts); i++)
@@ -364,7 +378,6 @@ int dhcp(const struct ctx *c, const struct pool *p)
 
 	info("    from %s", eth_ntop(m->chaddr, macstr, sizeof(macstr)));
 
-	m->yiaddr = c->ip4.addr;
 	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
 	memcpy(opts[1].s,  &mask,                sizeof(mask));
 	memcpy(opts[3].s,  &c->ip4.guest_gw,     sizeof(c->ip4.guest_gw));
@@ -401,14 +414,14 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	if (!c->no_dhcp_dns_search)
 		opt_set_dns_search(c, sizeof(m->o));
 
-	dlen = offsetof(struct msg, o) + fill(m);
+	dlen = offsetof(struct msg, o) + fill(&reply);
 
 	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);
+	tap_udp4_send(c, c->ip4.our_tap_addr, 67, dst, 68, &reply, dlen);
 
 	return 1;
 }
-- 
@@ -151,9 +151,6 @@ static int fill(struct msg *m)
 {
 	int i, o, offset = 0;
 
-	m->op = BOOTREPLY;
-	m->secs = 0;
-
 	for (o = 0; o < 255; o++)
 		opts[o].sent = 0;
 
@@ -291,8 +288,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	const struct ethhdr *eh;
 	const struct iphdr *iph;
 	const struct udphdr *uh;
+	struct msg const *m;
+	struct msg reply;
 	unsigned int i;
-	struct msg *m;
 
 	eh  = packet_get(p, 0, offset, sizeof(*eh),  NULL);
 	offset += sizeof(*eh);
@@ -321,6 +319,22 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	    m->op != BOOTREQUEST)
 		return -1;
 
+	reply.op		= BOOTREPLY;
+	reply.htype		= m->htype;
+	reply.hlen		= m->hlen;
+	reply.hops		= 0;
+	reply.xid		= m->xid;
+	reply.secs		= 0;
+	reply.flags		= m->flags;
+	reply.ciaddr		= m->ciaddr;
+	reply.yiaddr		= c->ip4.addr;
+	reply.siaddr		= 0;
+	reply.giaddr		= m->giaddr;
+	memcpy(&reply.chaddr,	m->chaddr,	sizeof(reply.chaddr));
+	memset(&reply.sname,	0,		sizeof(reply.sname));
+	memset(&reply.file,	0,		sizeof(reply.file));
+	reply.magic		= m->magic;
+
 	offset += offsetof(struct msg, o);
 
 	for (i = 0; i < ARRAY_SIZE(opts); i++)
@@ -364,7 +378,6 @@ int dhcp(const struct ctx *c, const struct pool *p)
 
 	info("    from %s", eth_ntop(m->chaddr, macstr, sizeof(macstr)));
 
-	m->yiaddr = c->ip4.addr;
 	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
 	memcpy(opts[1].s,  &mask,                sizeof(mask));
 	memcpy(opts[3].s,  &c->ip4.guest_gw,     sizeof(c->ip4.guest_gw));
@@ -401,14 +414,14 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	if (!c->no_dhcp_dns_search)
 		opt_set_dns_search(c, sizeof(m->o));
 
-	dlen = offsetof(struct msg, o) + fill(m);
+	dlen = offsetof(struct msg, o) + fill(&reply);
 
 	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);
+	tap_udp4_send(c, c->ip4.our_tap_addr, 67, dst, 68, &reply, dlen);
 
 	return 1;
 }
-- 
2.47.0

Re: [PATCH v2] dhcp: Don't re-use request message for reply

[Stefano Brivio]

On Tue,  4 Feb 2025 10:43:37 +0100
Enrique Llorente <ellorent@redhat.com> wrote:

> The logic composing the DHCP reply message is reusing the request
> message to compose it, future long options like FQDN may
> exceed the request message limit making it go beyond the lower
> bound.
> 
> This change creates a new reply message with a fixed options size of 308
> and fills it in with proper fields from requests adding on top the generated
> options, this way the reply lower bound does not depend on the request.
> 
> Signed-off-by: Enrique Llorente <ellorent@redhat.com>

Applied, thanks, and welcome to the git log!

-- 
Stefano