/* MERD - MacVTap Egress and Routing Daemon
*
* dhcp.c - Minimalistic DHCP server for MERD
*
* Author: Stefano Brivio <sbrivio@redhat.com>
* License: GPLv2
*
*/
#include <stdio.h>
#include <stddef.h>
#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <net/if.h>
#include <arpa/inet.h>
#include "merd.h"
#include "dhcp.h"
#include "util.h"
/**
* struct opt - DHCP option
* @force: Force sending, even if the client didn't request it
* @sent: Convenience flag, set while filling replies
* @slen: Length of option defined for server
* @s: Option payload from server
* @clen: Length of option received from client
* @c: Option payload from client
*/
struct opt {
int sent;
int slen;
unsigned char s[255];
int clen;
unsigned char c[255];
};
static struct opt opts[255] = {
[1] = { 0, 4, { 0 }, 0, { 0 }, }, /* Subnet mask */
[3] = { 0, 4, { 0 }, 0, { 0 }, }, /* Router */
[6] = { 0, 4, { 0 }, 0, { 0 }, }, /* DNS */
[51] = { 0, 4, { 60 }, 0, { 0 }, }, /* Lease time */
[53] = { 0, 1, { 0 }, 0, { 0 }, }, /* Message type */
#define DHCPDISCOVER 1
#define DHCPOFFER 2
#define DHCPREQUEST 3
#define DHCPDECLINE 4
#define DHCPACK 5
#define DHCPNAK 6
#define DHCPRELEASE 7
#define DHCPINFORM 8
#define DHCPFORCERENEW 9
[54] = { 0, 4, { 0 }, 0, { 0 }, }, /* Server ID */
};
/**
* struct msg - BOOTP/DHCP message
* @op: BOOTP message type
* @htype: Hardware address type
* @hlen: Hardware address length
* @hops: DHCP relay hops
* @xid: Transaction ID randomly chosen by client
* @secs: Seconds elapsed since beginning of acquisition or renewal
* @flags: DHCP message flags
* @ciaddr: Client IP address in BOUND, RENEW, REBINDING
* @yiaddr: IP address being offered or assigned
* @siaddr: Next server to use in bootstrap
* @giaddr: Relay agent IP address
* @chaddr: Client hardware address
* @sname: Server host name
* @file: Boot file name
* @magic: Magic cookie prefix before options
* @o: Options
*/
struct msg {
uint8_t op;
#define BOOTREQUEST 1
#define BOOTREPLY 2
uint8_t htype;
uint8_t hlen;
uint8_t hops;
uint32_t xid;
uint16_t secs;
uint16_t flags;
uint32_t ciaddr;
uint32_t yiaddr;
uint32_t siaddr;
uint32_t giaddr;
uint8_t chaddr[16];
uint8_t sname[64];
uint8_t file[128];
uint32_t magic;
uint8_t o[308];
} __attribute__((__packed__));
/**
* fill_one() - Fill a single option in message
* @m: Message to fill
* @o: Option number
* @offset: Current offset within options field, updated on insertion
*/
static void fill_one(struct msg *m, int o, int *offset)
{
m->o[*offset] = o;
m->o[*offset + 1] = opts[o].slen;
memcpy(&m->o[*offset + 2], opts[o].s, opts[o].slen);
opts[o].sent = 1;
*offset += 2 + opts[o].slen;
}
/**
* fill() - Fill requested and forced options in message
* @m: Message to fill
*
* Return: current size of options field
*/
static int fill(struct msg *m)
{
int i, o, offset = 0;
m->op = BOOTREPLY;
m->secs = 0;
for (o = 0; o < 255; o++)
opts[o].sent = 0;
for (i = 0; i < opts[55].clen; i++) {
o = opts[55].c[i];
if (opts[o].slen)
fill_one(m, o, &offset);
}
for (o = 0; o < 255; o++) {
if (opts[o].slen && !opts[o].sent)
fill_one(m, o, &offset);
}
m->o[offset++] = 255;
m->o[offset++] = 0;
if (offset < 62 /* RFC 951 */) {
memset(&m->o[offset], 0, 62 - offset);
offset = 62;
}
return offset;
}
/**
* dhcp() - Check if this is a DHCP message, reply as needed
* @c: Execution context
* @len: Total L2 packet length
* @eh: Packet buffer, Ethernet header
*
* Return: 0 if it's not a DHCP message, 1 if handled, -1 on failure
*/
int dhcp(struct ctx *c, unsigned len, struct ethhdr *eh)
{
struct iphdr *iph = (struct iphdr *)(eh + 1);
struct udphdr *uh = (struct udphdr *)((char *)iph + iph->ihl * 4);
struct msg *m = (struct msg *)(uh + 1);
unsigned int i, mlen = len - sizeof(*eh) - sizeof(*iph);
if (uh->dest != htons(67))
return 0;
if (mlen != ntohs(uh->len) || mlen < offsetof(struct msg, o) ||
m->op != BOOTREQUEST)
return -1;
for (i = 0; i < mlen - offsetof(struct msg, o); i += m->o[i + 1] + 2)
memcpy(&opts[m->o[i]].c, &m->o[i + 2], m->o[i + 1]);
if (opts[53].c[0] == DHCPDISCOVER) {
fprintf(stderr, "DHCP: offer to discover");
opts[53].s[0] = DHCPOFFER;
} else if (opts[53].c[0] == DHCPREQUEST) {
fprintf(stderr, "DHCP: ack to request");
opts[53].s[0] = DHCPACK;
} else {
return -1;
}
fprintf(stderr, " from %02x:%02x:%02x:%02x:%02x:%02x\n\n",
m->chaddr[0], m->chaddr[1], m->chaddr[2],
m->chaddr[3], m->chaddr[4], m->chaddr[5]);
m->yiaddr = c->addr4;
*(unsigned long *)opts[1].s = c->mask4;
*(unsigned long *)opts[3].s = c->gw4;
*(unsigned long *)opts[6].s = c->dns4;
*(unsigned long *)opts[54].s = c->gw4;
uh->len = htons(len = offsetof(struct msg, o) + fill(m));
uh->check = 0;
uh->source = htons(67);
uh->dest = htons(68);
iph->tot_len = htons(len += sizeof(*iph));
iph->daddr = c->addr4;
iph->saddr = c->gw4;
iph->check = 0;
iph->check = csum_ip4(iph, iph->ihl * 4);
len += sizeof(*eh);
memcpy(eh->h_dest, eh->h_source, ETH_ALEN);
memcpy(eh->h_source, c->mac, ETH_ALEN);
if (send(c->fd_unix, eh, len, 0) < 0)
perror("DHCP: send");
return 1;
}
