Simple library to add layer-3 and layer-4 checksums to an IP packet.
Originally designed to make using
pcap_inject a bit
easier.
Currently supports:
- IPv4
- IPv6
- TCP
- UDP
- ICMP
- ICMPv6
Quite a bit of the source was taken from OpenBSD's tcpdump.
There is currently one function in this library, packetchecksum_calculate().
It expects two arguments: a pointer to the start of the Layer-3 header and the
total length of the packet. Upon return the layer-3 and layer-4 checksum
fields will be populated.
Compiling into a project should be as simple as dropping the source and header files in with the rest of the project's source.
Example:
uint8_t packet[BUFLEN];
struct ether_header *eh;
struct ip *ih;
struct tcphdr *th;
pcap_t *p = magic_pcap_init();
int ret;
bzero(packet, sizeof(packet));
/* Roll a packet */
eh = packet;
/* set Ethernet header things here */
ih = packet + sizeof(struct ether_header);
/* Set IP header things here */
th = packet + sizeof(struct ether_header) + sizeof(struct ip);
/* Set TCP header things here */
/* Calculate checksums */
switch (packetchecksum_calculate(packet, sizeof(packet))) { /* <-- This */
case PACKETCHECKSUM_TOOSHORT:
errx(1, "packet buffer too small");
case PACKETCHECKSUM_INVALID:
errx(2, "packet contents invalid");
}
/* Send it out */
if (sizeof(packet) != (ret = pcap_inject(p, packet, sizeof(packet)))) {
if (-1 == ret)
errx(3, "pcap_inject: %s", pcap_geterr(p));
else
errx(4, "short write (%i < %i)", ret, sizeof(packet));
}A more complete example can be found in example.c.
While the IPv6 header itself doesn't have a checksum field, skipping past the optional headers isn't quite as straightforward as one would like. In general this should work pretty well except for the ESP optional header.
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