From mboxrd@z Thu Jan  1 00:00:00 1970
From: john@jjdev.com (John de la Garza)
Date: Tue, 11 Feb 2014 13:19:19 -0800
Subject: Verify checksum of IP header and TCP header
In-Reply-To: <CALdM3teorONFAMKYseQK4iRkS+-Y=+EGzrLubKTgoxm-Jas5fw@mail.gmail.com>
References: <CALdM3teorONFAMKYseQK4iRkS+-Y=+EGzrLubKTgoxm-Jas5fw@mail.gmail.com>
Message-ID: <20140211211918.GA4872@vega.jjdev.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Tue, Feb 11, 2014 at 03:03:42PM -0500, Guibin(Bill) Tian wrote:
> Hi,
> I am looking for the code in linux kernel that verifies the checksum of TCP
> header and IP header for inbound packet. The packet supposes to be dropped
> if the checksum doesn't match.

Here is some code that checks the TCP checksum.

/*
 * Calculate(/check) TCP checksum
 */
static inline __sum16 tcp_v4_check(int len, __be32 saddr,
                                   __be32 daddr, __wsum base)
{
        return csum_tcpudp_magic(saddr,daddr,len,IPPROTO_TCP,base);
}

from: include/net/tcp.h


also, try searching for csum_error in net/ipv4/tcp_ipv4.c