From 7885effce1b8cb087bb76c278537f6bbc3cf33ff Mon Sep 17 00:00:00 2001
From: "Andrey Jr. Melnikov" <temnota.am@gmail.com>
Date: Fri, 9 Oct 2015 12:55:43 +0300
Subject: [PATCH 1/2] Fix checksum generation

---
 src/socket4.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/socket4.c b/src/socket4.c
index f1bdf5e..ac760fb 100644
--- a/src/socket4.c
+++ b/src/socket4.c
@@ -96,6 +96,7 @@ int socket_sendto_ping_ipv4(int s, struct sockaddr *saddr, socklen_t saddr_len,
 
     icp->icmp_type = ICMP_ECHO;
     icp->icmp_code = 0;
+    icp->icmp_cksum = 0;
     icp->icmp_seq = htons(icmp_seq_nr);
     icp->icmp_id = htons(icmp_id_nr);
 

From 4e14f407aac419609556871a0ac965009460277e Mon Sep 17 00:00:00 2001
From: "Andrey Jr. Melnikov" <temnota.am@gmail.com>
Date: Fri, 9 Oct 2015 13:05:05 +0300
Subject: [PATCH 2/2] Use more readably cksum variant and move function into v4
 sockets code.

---
 src/fping.c   | 49 -------------------------------------------------
 src/socket4.c | 19 ++++++++++++++++++-
 2 files changed, 18 insertions(+), 50 deletions(-)

diff --git a/src/fping.c b/src/fping.c
index 4a6b5d9..9e9aa5f 100644
--- a/src/fping.c
+++ b/src/fping.c
@@ -316,7 +316,6 @@ char *na_cat( char *name, struct in_addr ipaddr );
 void crash_and_burn( char *message );
 void errno_crash_and_burn( char *message );
 char *get_host_by_address( struct in_addr in );
-int in_cksum( unsigned short *p, int n );
 void u_sleep( int u_sec );
 int recvfrom_wto( int s, char *buf, int len, struct sockaddr *saddr, socklen_t *saddr_len, long timo );
 void remove_job( HOST_ENTRY *h );
@@ -1799,54 +1798,6 @@ int handle_random_icmp(FPING_ICMPHDR *p, struct sockaddr *addr, socklen_t addr_l
 } /* handle_random_icmp() */
 
 
-/************************************************************
-
-  Function: in_cksum
-
-*************************************************************
-
-  Inputs:  unsigned short *p, int n
-
-  Returns:  int
-
-  Description:
-
-  Checksum routine for Internet Protocol family headers (C Version)
-  From ping examples in W.Richard Stevens "UNIX NETWORK PROGRAMMING" book.
-
-************************************************************/
-
-int in_cksum( unsigned short *p, int n )
-{
-    register unsigned short answer;
-    register long sum = 0;
-    unsigned short odd_byte = 0;
-
-    while( n > 1 )
-    {
-        sum += *p++;
-        n -= 2;
-    
-    }/* WHILE */
-
-
-    /* mop up an odd byte, if necessary */
-    if( n == 1 )
-    {
-        *( unsigned char* )( &odd_byte ) = *( unsigned char* )p;
-        sum += odd_byte;
-    
-    }/* IF */
-
-    sum = ( sum >> 16 ) + ( sum & 0xffff ); /* add hi 16 to low 16 */
-    sum += ( sum >> 16 );                   /* add carry */
-    answer = ~sum;                          /* ones-complement, truncate*/
-    
-    return ( answer );
-
-} /* in_cksum() */
-
-
 /************************************************************
 
   Function: add_name
diff --git a/src/socket4.c b/src/socket4.c
index ac760fb..12b7803 100644
--- a/src/socket4.c
+++ b/src/socket4.c
@@ -87,6 +87,23 @@ void socket_set_src_addr_ipv4(int s, FPING_INADDR src_addr)
         errno_crash_and_burn( "cannot bind source address" );
 }
 
+unsigned short calcsum(unsigned short *buffer, int length)
+{
+    unsigned long sum;
+
+    // initialize sum to zero and loop until length (in words) is 0
+    for (sum=0; length>1; length-=2) // sizeof() returns number of bytes, we're interested in number of words
+	sum += *buffer++;	// add 1 word of buffer to sum and proceed to the next
+
+    // we may have an extra byte
+    if (length==1)
+	sum += (char)*buffer;
+
+    sum = (sum >> 16) + (sum & 0xFFFF); // add high 16 to low 16
+    sum += (sum >> 16);			// add carry
+    return ~sum;
+}
+
 int socket_sendto_ping_ipv4(int s, struct sockaddr *saddr, socklen_t saddr_len, uint16_t icmp_seq_nr, uint16_t icmp_id_nr)
 {
     struct icmp *icp;
@@ -106,7 +123,7 @@ int socket_sendto_ping_ipv4(int s, struct sockaddr *saddr, socklen_t saddr_len,
         }
     }
 
-    icp->icmp_cksum = in_cksum((unsigned short*) icp, ping_pkt_size );
+    icp->icmp_cksum = calcsum((unsigned short *) icp, ping_pkt_size);
 
     n = sendto(s, icp, ping_pkt_size, 0, saddr, saddr_len);