Hello, World! using pcap_inject.

pcaphelloworld.c

/*
 * pcaphelloworld.c
 * Simple libpcap program
 * By J. Stuart McMurray
 * Created 20190527
 * Last Modified 20190527
 */

#include <arpa/inet.h>

#include <netinet/in.h>
#include <net/if.h>
#include <netinet/if_ether.h>

#include <err.h>
#include <pcap.h>
#include <string.h>

/* HELLO is the message to put in an ethernet frame. */
#define HELLO "Hello, World!"

/* Make sure we have a capture device */
#ifndef DEVICE
#error DEVICE is not defined
#endif /* #ifdef DEVICE */

__attribute__((constructor)) void
send_hello(void)
{
        char errbuf[PCAP_ERRBUF_SIZE + 1];
        pcap_t *p;
        uint8_t buf[sizeof(struct ether_header) + strlen(HELLO)];
        struct ether_header *ep;

        bzero(errbuf, sizeof(errbuf));
        bzero(buf, sizeof(buf));

        /* Roll a frame */
        ep = (struct ether_header *)buf;
        memcpy(ep->ether_shost, "\x73\x74\x75\x61\x72\x74",
                        sizeof(ep->ether_shost) <= 6 ?
                        sizeof(ep->ether_shost) : 6);
        memcpy(ep->ether_dhost, "\xff\xff\xff\xff\xff\xff",
                        sizeof(ep->ether_dhost) <= 6 ?
                        sizeof(ep->ether_dhost) : 6);
        ep->ether_type = htons(0x0804); /* CHAOS */
        memcpy(buf + sizeof(struct ether_header), HELLO, strlen(HELLO));

        /* Open the device */
        if (NULL == (p = pcap_open_live(DEVICE, 65535, 0, 10, errbuf))) {
                warn("pcap_open_live: %s", errbuf);
                return;
        }

        /* Send the message */
        if (sizeof(buf) != pcap_inject(p, buf, sizeof(buf))) {
                warn("pcap_inject: %s", pcap_geterr(p));
        }

        /* Clean up */
        pcap_close(p);
}

int
main(int argc, char **argv)
{
        /* send_hello will be called already, so no need to call it here. */
        return 0;
}

run.sh

#!/bin/sh

wget -nv https://www.tcpdump.org/release/libpcap-1.9.0.tar.gz
tar xzf libpcap-1.9.0.tar.gz && cd libpcap-1.9.0 && \
        ./configure >/dev/null && \
        make >/dev/null && cd ..

wget -nv -O pcaphelloworld.c https://git.io/fjRW9
more pcaphelloworld.c

cc -DDEVICE=\"eth0\" -Ilibpcap-1.9.0 \
        -O2 --pedantic -Wall \
        pcaphelloworld.c libpcap-1.9.0/libpcap.a -o pcaphelloworld 
ls -l pcaphelloworld && \
        file pcaphelloworld && \
        ldd pcaphelloworld

cc -DDEVICE=\"eth0\" -Ilibpcap-1.9.0 -Llibpcap-1.9.0 \
        -O2 --pedantic -Wall \
        pcaphelloworld.c -o libpcaphelloworld.so \
        -Wl,-Bstatic -lpcap -Wl,-Bdynamic -fPIC -shared
ls -l libpcaphelloworld.so && \
        file libpcaphelloworld.so && \
        ldd libpcaphelloworld.so

sudo tcpdump -elnni eth0 -X not ip and not ip6 &
sudo ./pcaphelloworld

echo 'print __libc_dlopen_mode("/home/stuart/libpcaphelloworld.so", 2)' | sudo gdb -p 1
sudo grep pcaphelloworld /proc/1/maps
echo 'print __libc_dlclose()' | sudo gdb -p 1