From 40e162e58dff93bb941340d832bca8e6ce368600 Mon Sep 17 00:00:00 2001
From: Elliott Hughes <enh@google.com>
Date: Thu, 20 Dec 2018 11:25:26 -0800
Subject: Fix UDP checksum verification error.
From RFC 768, if UDP packet checksum computation yields a result of
zero, change it to hex 0xFFFF. The current udhcpc checksum verification
would yield false positive for this case. A better way is to compute the
checksum with the original checksum field and the result should be zero
for good udp packet.
Signed-off-by: Yangchun Fu <yangchun@google.com>
---
toys/pending/dhcp.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)
diff --git a/toys/pending/dhcp.c b/toys/pending/dhcp.c
index b99bb4c0..25f9b913 100644
--- a/toys/pending/dhcp.c
+++ b/toys/pending/dhcp.c
@@ -650,7 +650,6 @@ static int mode_app(void)
static int read_raw(void)
{
dhcp_raw_t packet;
- uint16_t check;
int bytes = 0;
memset(&packet, 0, sizeof(packet));
@@ -676,18 +675,19 @@ static int read_raw(void)
dbg("\tUnrelated/bogus packet, ignoring\n");
return -2;
}
- // verify IP checksum
- check = packet.iph.check;
- packet.iph.check = 0;
- if (check != dhcp_checksum(&packet.iph, sizeof(packet.iph))) {
+ // Verify IP checksum.
+ if (dhcp_checksum(&packet.iph, sizeof(packet.iph)) != 0) {
dbg("\tBad IP header checksum, ignoring\n");
return -2;
}
+ // Verify UDP checksum. From RFC 768, the UDP checksum is done over the IPv4
+ // pseudo header, the UDP header and the UDP data. The IPv4 pseudo header
+ // includes saddr, daddr, protocol, and UDP length. The IP header has to be
+ // modified for this.
memset(&packet.iph, 0, ((size_t) &((struct iphdr *)0)->protocol));
+ packet.iph.check = 0;
packet.iph.tot_len = packet.udph.len;
- check = packet.udph.check;
- packet.udph.check = 0;
- if (check && check != dhcp_checksum(&packet, bytes)) {
+ if (packet.udph.check != 0 && dhcp_checksum(&packet, bytes) != 0) {
dbg("\tPacket with bad UDP checksum received, ignoring\n");
return -2;
}
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