At least two places in passt fill in UDP over IPv4 checksums, although
since UDP checksums are optional with IPv4 that just amounts to storing
a 0 (in tap_ip_send()) or leaving a 0 from an earlier initialization (in
dhcp()). For consistency, add a helper for this "calculation".
Just for the heck of it, add the option (compile time disabled for now) to
calculate real UDP checksums.
Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
checksum.c | 33 +++++++++++++++++++++++++++++++++
checksum.h | 3 +++
dhcp.c | 2 +-
tap.c | 2 +-
4 files changed, 38 insertions(+), 2 deletions(-)
diff --git a/checksum.c b/checksum.c
index 0849fb1..72f1cfb 100644
--- a/checksum.c
+++ b/checksum.c
@@ -56,6 +56,11 @@
#include <linux/icmp.h>
#include <linux/icmpv6.h>
+/* Checksums are optional for UDP over IPv4, so we usually just set
+ * them to 0. Change this 1 to calculate real UDP over IPv4 checksums
+ */
+#define UDP4_REAL_CHECKSUMS 0
+
/**
* sum_16b() - Calculate sum of 16-bit words
* @buf: Input buffer
@@ -109,6 +114,34 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
}
+/**
+ * csum_udp4() - Calculate checksum for a UDP over IPv4 packet
+ * @udp4hr: UDP header, initialized apart from checksum
+ * @saddr: IPv4 source address
+ * @daddr: IPv4 destination address
+ * @payload: ICMPv4 packet payload
+ * @len: Length of @payload (not including UDP)
+ */
+void csum_udp4(struct udphdr *udp4hr,
+ in_addr_t saddr, in_addr_t daddr,
+ const void *payload, size_t len)
+{
+ /* UDP checksums are optional, so don't bother */
+ udp4hr->check = 0;
+
+ if (UDP4_REAL_CHECKSUMS) {
+ /* UNTESTED: if we did want real UDPv4 checksums, this
+ * is roughly what we'd need */
+ uint32_t psum = csum_fold(htonl(saddr))
+ + csum_fold(htonl(daddr))
+ + htons(len + sizeof(*udp4hr))
+ + htons(IPPROTO_UDP);
+ /* Add in partial checksum for the UDP header alone */
+ psum += sum_16b(udp4hr, sizeof(*udp4hr));
+ udp4hr->check = csum_unaligned(payload, len, psum);
+ }
+}
+
/**
* csum_icmp4() - Calculate checksum for an ICMPv4 packet
* @icmp4hr: ICMPv4 header, initialized apart from checksum
diff --git a/checksum.h b/checksum.h
index 1b9f48e..a9502b9 100644
--- a/checksum.h
+++ b/checksum.h
@@ -13,6 +13,9 @@ struct icmp6hdr;
uint32_t sum_16b(const void *buf, size_t len);
uint16_t csum_fold(uint32_t sum);
uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init);
+void csum_udp4(struct udphdr *udp4hr,
+ in_addr_t saddr, in_addr_t daddr,
+ const void *payload, size_t len);
void csum_icmp4(struct icmphdr *ih, const void *payload, size_t len);
void csum_udp6(struct udphdr *udp6hr,
const struct in6_addr *saddr,
diff --git a/dhcp.c b/dhcp.c
index 7f0cc0b..8dcf645 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -364,9 +364,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
opt_set_dns_search(c, sizeof(m->o));
uh->len = htons(len = offsetof(struct msg, o) + fill(m) + sizeof(*uh));
- uh->check = 0;
uh->source = htons(67);
uh->dest = htons(68);
+ csum_udp4(uh, c->ip4.gw, c->ip4.addr, uh + 1, len - sizeof(*uh));
iph->tot_len = htons(len += sizeof(*iph));
iph->daddr = c->ip4.addr;
diff --git a/tap.c b/tap.c
index 9c197cb..58fc1de 100644
--- a/tap.c
+++ b/tap.c
@@ -145,7 +145,7 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src,
uint8_t proto,
} else if (iph->protocol == IPPROTO_UDP) {
struct udphdr *uh = (struct udphdr *)(iph + 1);
- uh->check = 0;
+ csum_udp4(uh, iph->saddr, iph->daddr, uh + 1, len - sizeof(*uh));
} else if (iph->protocol == IPPROTO_ICMP) {
struct icmphdr *ih = (struct icmphdr *)(iph + 1);
csum_icmp4(ih, ih + 1, len - sizeof(*ih));
--
2.37.3