3 files changed, 19 insertions, 3 deletions
diff --git a/checksum.c b/checksum.c
index 78c6960..f35c948 100644
--- a/checksum.c
+++ b/checksum.c
@@ -52,6 +52,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <linux/icmp.h>
 #include <linux/icmpv6.h>
 
 /**
@@ -108,6 +109,21 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
 }
 
 /**
+ * csum_icmp4() - Calculate and set checksum for an ICMP packet
+ * @icmp4hr:	ICMP header, initialised apart from checksum
+ * @payload:	ICMP packet payload
+ * @len:	Length of @payload (not including ICMP header)
+ */
+void csum_icmp4(struct icmphdr *icmp4hr, const void *payload, size_t len)
+{
+	/* Partial checksum for ICMP header alone */
+	uint32_t psum = sum_16b(icmp4hr, sizeof(*icmp4hr));
+
+	icmp4hr->checksum = 0;
+	icmp4hr->checksum = csum_unaligned(payload, len, psum);
+}
+
+/**
  * csum_icmp6() - Calculate and set checksum for an ICMPv6 packet
  * @icmp6hr:	ICMPv6 header, initialised apart from checksum
  * @saddr:	IPv6 source address
diff --git a/checksum.h b/checksum.h
index d7daabf..bf0620f 100644
--- a/checksum.h
+++ b/checksum.h
@@ -6,11 +6,13 @@
 #ifndef CHECKSUM_H
 #define CHECKSUM_H
 
+struct icmphdr;
 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_icmp4(struct icmphdr *ih, const void *payload, size_t len);
 void csum_icmp6(struct icmp6hdr *icmp6hr,
 		const struct in6_addr *saddr, const struct in6_addr *daddr,
 		const void *payload, size_t len);
diff --git a/tap.c b/tap.c
index aafc92b..f082901 100644
--- a/tap.c
+++ b/tap.c
@@ -148,9 +148,7 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 			uh->check = 0;
 		} else if (iph->protocol == IPPROTO_ICMP) {
 			struct icmphdr *ih = (struct icmphdr *)(iph + 1);
-
-			ih->checksum = 0;
-			ih->checksum = csum_unaligned(ih, len, 0);
+			csum_icmp4(ih, ih + 1, len - sizeof(*ih));
 		}
 
 		if (tap_send(c, buf, len + sizeof(*iph) + sizeof(*eh), 1) < 0)