Calculating UDP Checksum - HOW?

[Eddie Kohler]

Hi Filip,

Well, you're right; we should have a SetUDPChecksum element, but we didn't.

I've checked one in to the anonymous CVS repository. The relevant source
files are attached; you can just drop them in to your elements/tcpudp
directory and run 'make elemlist'.

Eddie

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// -*- c-basic-offset: 4 -*-
#ifndef CLICK_SETUDPCHECKSUM_HH
#define CLICK_SETUDPCHECKSUM_HH

/*
 * =c
 * SetUDPChecksum()
 * =s UDP
 * sets UDP packets' checksums
 * =d
 * Input packets should be UDP in IP.
 *
 * Calculates the UDP checksum and sets the UDP header's checksum field. Uses
 * IP header fields to generate the pseudo-header.
 *
 * =a CheckUDPHeader, SetIPChecksum, CheckIPHeader, SetTCPChecksum */

#include <click/element.hh>
#include <click/glue.hh>

class SetUDPChecksum : public Element { public:
    
    SetUDPChecksum();
    ~SetUDPChecksum();
  
    const char *class_name() const	{ return "SetUDPChecksum"; }
    const char *processing() const	{ return AGNOSTIC; }
    SetUDPChecksum *clone() const	{ return new SetUDPChecksum; }

    Packet *simple_action(Packet *);

};

#endif
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// -*- c-basic-offset: 4 -*-
/*
 * setudpchecksum.{cc,hh} -- sets the UDP header checksum
 * Robert Morris, Eddie Kohler
 *
 * Copyright (c) 1999-2000 Massachusetts Institute of Technology
 * Copyright (c) 2002 International Computer Science Institute
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, subject to the conditions
 * listed in the Click LICENSE file. These conditions include: you must
 * preserve this copyright notice, and you cannot mention the copyright
 * holders in advertising related to the Software without their permission.
 * The Software is provided WITHOUT ANY WARRANTY, EXPRESS OR IMPLIED. This
 * notice is a summary of the Click LICENSE file; the license in that file is
 * legally binding.
 */

#include <click/config.h>
#include "setudpchecksum.hh"
#include <click/glue.hh>
#include <click/error.hh>
#include <clicknet/ip.h>
#include <clicknet/udp.h>

SetUDPChecksum::SetUDPChecksum()
    : Element(1, 1)
{
    MOD_INC_USE_COUNT;
}

SetUDPChecksum::~SetUDPChecksum()
{
    MOD_DEC_USE_COUNT;
}

Packet *
SetUDPChecksum::simple_action(Packet *p_in)
{
    WritablePacket *p = p_in->uniqueify();
    if (!p)
	return 0;
    
    // XXX check IP header/UDP protocol?
    click_ip *iph = p->ip_header();
    click_udp *udph = p->udp_header();
    int len = ntohs(udph->uh_ulen);
    if ((unsigned)len > p->length() - p->transport_header_offset()) {
	// packet data too short
	p->kill();
	return 0;
    }

    udph->uh_sum = 0;
    unsigned csum = ~click_in_cksum((unsigned char *)udph, len) & 0xFFFF;
#ifdef CLICK_LINUXMODULE
    udph->uh_sum = csum_tcpudp_magic(iph->ip_src.s_addr, iph->ip_dst.s_addr,
				     len, IP_PROTO_UDP, csum);
#else
    const uint16_t *words = (const uint16_t *)&iph->ip_src;
    csum += words[0];
    csum += words[1];
    csum += words[2];
    csum += words[3];
    csum += htons(IP_PROTO_UDP);
    csum += htons(len);
    while (csum >> 16)
	csum = (csum & 0xFFFF) + (csum >> 16);
    udph->uh_sum = ~csum & 0xFFFF;
#endif

    return p;
}

EXPORT_ELEMENT(SetUDPChecksum)
ELEMENT_MT_SAFE(SetUDPChecksum)