Libraries

Dug Song's libdnet - dumb network library:

Provides a simplified, portable interface to several low-level
networking routines, including network address manipulation, kernel
arp(4) cache and route(4) table lookup and manipulation, network
firewalling, network interface lookup and manipulation, and raw IP
packet and Ethernet frame transmission.

TCP Packet creation example:
tcp = (struct tcp_hdr *)(pkt + IP_HDR_LEN);
tcp_pack_hdr(tcp, con->th_dport, con->th_sport, ...);
iplen = IP_HDR_LEN + (tcp->th_off << 2) + len;
ip_pack_hdr(pkt, 0, iplen, id, df, Honeyd_ttl, IP_PROTO_TCP, ...);
memcpy(pkt + IP_HDR_LEN + (tcp->th_off << 2), payload, len);

ip_checksum(pkt, iplen);
ip_send(Honeyd_ip, pkt, iplen);