libiptc: adding DNAT rule with destination IP - c

Question

I am trying to make a c program to create port forwarding rule using libiptc.
static int insert_forward_rule(const char *table, const char *chain, unsigned int src, unsigned int dest, const char * ports, const char *target1, char * proto) {
struct xtc_handle *h;
struct ipt_entry *en = NULL;
struct ipt_entry * e;
struct ipt_entry_match * match_proto, * match_limit;
struct ipt_entry_target * target;
unsigned int size_ipt_entry, size_ipt_entry_match, size_ipt_entry_target, total_length;
size_ipt_entry = IPT_ALIGN(sizeof (struct ipt_entry));
size_ipt_entry_match = IPT_ALIGN(sizeof (struct ipt_entry_match)) + sizeof (struct ipt_tcp) + sizeof (int);
size_ipt_entry_target = IPT_ALIGN(sizeof (struct ipt_entry_target) + sizeof (IPTC_LABEL_ACCEPT));
total_length = 216; //size_ipt_entry + size_ipt_entry_match + size_ipt_entry_target + IPT_ALIGN(sizeof (struct ipt_natinfo)); //size_ipt_entry + 48 + 40
//printf("size of ipt ebtry=%u,match=%u,target=%u,total=%u\n", size_ipt_entry, size_ipt_entry_match, size_ipt_entry_target, total_length);
//memory allocation for all structs that represent the netfilter rule we want to insert
e = calloc(1, total_length);
if (e == NULL) {
printf("malloc failure");
exit(1);
}
int size = 160;
//offsets to the other bits:
//target struct begining
e->target_offset = size; //size_ipt_entry + size_ipt_entry_match; //+ size_ipt_tcp + size_rateinfo + size_physdevinfo;
//next "e" struct, end of the current one
e->next_offset = total_length;
e->nfcache = NFC_IP_DST_PT;
if (dest) {
e->ip.dst.s_addr = dest;
e->ip.dmsk.s_addr = 0xFFFFFFFF;
}
match_limit = (struct ipt_entry_match *) (e->elems); //+ match_proto->u.match_size
match_limit->u.user.match_size = size_ipt_entry_match; //size_ipt_entry_match*3; //+ size_rateinfo;
strncpy(match_limit->u.user.name, "tcp", sizeof (match_limit->u.user.name)); //set name of the module, we will use in this match
struct ipt_tcp *info = (struct ipt_tcp *) match_limit->data;
unsigned int i = 0;
if (strchr(ports, ':') != NULL) {
char aPort[2][10];
sscanf(ports, "%[^:], %[^:]", aPort[0], aPort[1]);
info->dpts[0] = atoi(aPort[0]);
info->dpts[1] = atoi(aPort[1]);
} else {
info->dpts[0] = info->dpts[1] = atoi(ports);
}
info->spts[0] = 0;
info->spts[1] = 0xFFFF;
e->ip.proto = IPPROTO_TCP;
printf("Target offset=%d,next offset=%d\n", e->target_offset, e->next_offset);
target = (struct ipt_entry_target *) ((void*) e + e->target_offset); // + size_ipt_entry_match); //+ size_ipt_tcp + size_rateinfo + size_physdevinfo
size = 56;
target->u.user.target_size = size_ipt_entry_target + IPT_ALIGN(sizeof (struct nf_nat_range)); // size_ipt_entry_target;
target->u.user.revision = 0;
strncpy(target->u.user.name, target1, sizeof (target->u.user.name));
struct ipt_natinfo *inf = (struct ipt_natinfo*) target->data;
struct nf_nat_range range;
memset(&range, 0, sizeof (range));
range.flags |= IP_NAT_RANGE_MAP_IPS;
range.min_ip = range.max_ip = src;
inf->mr.rangesize++;
inf->mr.range[inf->mr.rangesize] = range;
inf->t.u.target_size = XT_ALIGN(sizeof (*inf) + inf->mr.rangesize * sizeof (struct nf_nat_range));
printf("after s_addr\n\n");
//All the functions, mentioned below could be found in "Querying libiptc HOWTO" manual
h = iptc_init(table);
printf("h=%d\n", h);
if (!h) {
printf("Error initializing: %s\n", iptc_strerror(errno));
return 0;
}
printf("target_size=%d,match=%d,chain=%s,table=%s,target=%s\n", target->u.user.target_size, match_limit->u.user.match_size, chain, table, target1);
int x = iptc_append_entry(chain, e, h);
if (!x) {
printf("Error append_entry: %s\n", iptc_strerror(errno));
goto ERROR;
}
printf("%d,chain=%s,target=%s\n", target->u.user.target_size, chain, table);
int y = iptc_commit(h);
if (!y) {
printf("Error no=%d,commit: %s\n", errno, iptc_strerror(errno));
goto ERROR;
}
iptc_free(h);
if (e != NULL) {
free(e);
e = NULL;
}
return 1;
ERROR:
iptc_free(h);
if (e != NULL) {
free(e);
e = NULL;
}
return 0;
}
To call this function
inet_pton(AF_INET, "20.10.10.254", &a);
inet_pton(AF_INET, "20.10.10.100", &b);
insert_forward_rule("nat", "port_forward", b, a, "1111", "DNAT", "tcp");
Every time i run this code i am getting error as invalid arguments(code 22).If i change the position of rangesize to below iptc_init() then it gave an error code 94(socket operation on non-socket)
I tried struct nf_nat_range structure directly but then its not saving the destination ip in rules instead of that its showing random.
Rule which i want to make is
iptables -t nat -A port_forward -j DNAT -d 192.168.10.202 -p udp --dport 8080:8084 --to 20.10.10.112
what it is creating with structure nf_nat_range is
Chain port_forward (0 references)
pkts bytes target prot opt in out source destination
0 0 DNAT tcp -- * * 0.0.0.0/0 20.10.10.254 tcp dpt:1111 to: random
0 0 DNAT tcp -- * * 0.0.0.0/0 20.10.10.254 tcp dpt:1111 to: random
0 0 DNAT tcp -- * * 0.0.0.0/0 20.10.10.254 tcp dpt:1111 to: random

Answer 1

These are the functions which helped me to Add or Delete DNAT and SNAT rules using libiptc. I uploaded this to my GitHub account.
void iptc_add_rule(const char *table, const char *chain, const char *protocol, const char *iniface, const char *outiface, const char *src, const char *dest, const char *srcports, const char *destports, const char *target, const char *dnat_to, const int append) {
iptc_handle_t handle;
struct ipt_entry *chain_entry;
struct ipt_entry_match *entry_match = NULL;
struct ipt_entry_target *entry_target;
ipt_chainlabel labelit;
long match_size;
int result = 0;
chain_entry = (struct ipt_entry *) calloc(1, sizeof (*chain_entry));
if (src) {
chain_entry->ip.src.s_addr = inet_addr(src);
chain_entry->ip.smsk.s_addr = inet_addr("255.255.255.255");
}
if (dest) {
chain_entry->ip.dst.s_addr = inet_addr(dest);
chain_entry->ip.dmsk.s_addr = inet_addr("255.255.255.255");
}
if (iniface) strncpy(chain_entry->ip.iniface, iniface, IFNAMSIZ);
if (outiface) strncpy(chain_entry->ip.outiface, outiface, IFNAMSIZ);
if (strcmp(protocol, "TCP") == 0) {
chain_entry->ip.proto = IPPROTO_TCP;
entry_match = get_tcp_match(srcports, destports, &chain_entry->nfcache);
} else if (strcmp(protocol, "UDP") == 0) {
chain_entry->ip.proto = IPPROTO_UDP;
entry_match = get_udp_match(srcports, destports, &chain_entry->nfcache);
} else {
printf("Unsupported protocol: %s", protocol);
return;
}
if (strcmp(target, "") == 0
|| strcmp(target, IPTC_LABEL_ACCEPT) == 0
|| strcmp(target, IPTC_LABEL_DROP) == 0
|| strcmp(target, IPTC_LABEL_QUEUE) == 0
|| strcmp(target, IPTC_LABEL_RETURN) == 0) {
size_t size;
size = IPT_ALIGN(sizeof (struct ipt_entry_target)) + IPT_ALIGN(sizeof (int));
entry_target = (struct ipt_entry_target *) calloc(1, size);
entry_target->u.user.target_size = size;
strncpy(entry_target->u.user.name, target, IPT_FUNCTION_MAXNAMELEN);
} else if (strcmp(target, "DNAT") == 0) {
entry_target = get_dnat_target(dnat_to, &chain_entry->nfcache);
printf("dnat\n");
} else if (strcmp(target, "SNAT") == 0) {
entry_target = get_snat_target(dnat_to, &chain_entry->nfcache);
printf("snat\n");
}
if (entry_match)
match_size = entry_match->u.match_size;
else
match_size = 0;
struct ipt_entry *tmp_ipt = chain_entry;
chain_entry = (struct ipt_entry *) realloc(chain_entry, sizeof (*chain_entry) + match_size + entry_target->u.target_size);
if (chain_entry == NULL) {
free(tmp_ipt);
}
memcpy(chain_entry->elems + match_size, entry_target, entry_target->u.target_size);
chain_entry->target_offset = sizeof (*chain_entry) + match_size;
chain_entry->next_offset = sizeof (*chain_entry) + match_size + entry_target->u.target_size;
printf("target->offset=%d,next_offset=%d,target_size=%d\n", chain_entry->target_offset, chain_entry->next_offset, entry_target->u.user.target_size);
if (entry_match) {
memcpy(chain_entry->elems, entry_match, match_size);
printf("%d\n", __LINE__);
}
printf("%d\n", __LINE__);
handle = iptc_init(table);
if (!handle) {
printf("libiptc error: Can't initialize table %s, %s", table, iptc_strerror(errno));
free(chain_entry);
free(entry_target);
if (entry_match) free(entry_match);
return;
}
strncpy(labelit, chain, sizeof (ipt_chainlabel));
printf("%d\n", __LINE__);
result = iptc_is_chain(chain, handle);
if (!result) {
printf("libiptc error: Chain %s does not exist!", chain);
free(chain_entry);
free(entry_target);
if (entry_match) free(entry_match);
return;
}
printf("%d,labeit=%s\n", __LINE__, labelit);
if (append)
result = iptc_append_entry(labelit, chain_entry, handle);
else
result = iptc_insert_entry(labelit, chain_entry, 0, handle);
printf("%d\n", __LINE__);
if (!result) {
printf("libiptc error: Can't add, %s", iptc_strerror(errno));
free(chain_entry);
free(entry_target);
if (entry_match) free(entry_match);
return;
}
printf("%d\n", __LINE__);
result = iptc_commit(handle);
if (!result) {
printf("libiptc error: Commit error, %s", iptc_strerror(errno));
free(chain_entry);
free(entry_target);
if (entry_match) free(entry_match);
return;
} else
printf("added new rule to block successfully");
if (entry_match) free(entry_match);
free(entry_target);
free(chain_entry);
}
void iptc_delete_rule(const char *table, const char *chain, const char *protocol, const char *iniface, const char *outiface, const char *src, const char *dest, const char *srcports, const char *destports, const char *target, const char *dnat_to) {
iptc_handle_t handle;
const struct ipt_entry *e;
ipt_chainlabel labelit;
int i, result;
unsigned long int s_src, s_dest;
if (src) s_src = inet_addr(src);
if (dest) s_dest = inet_addr(dest);
handle = iptc_init(table);
if (!handle) {
printf("libiptc error: Can't initialize table %s, %s", table, iptc_strerror(errno));
return;
}
strncpy(labelit, chain, sizeof (ipt_chainlabel));
result = iptc_is_chain(chain, handle);
if (!result) {
printf("libiptc error: Chain %s does not exist!", chain);
return;
}
for (e = iptc_first_rule(chain, handle), i = 0; e; e = iptc_next_rule(e, handle), i++) {
if (src && e->ip.src.s_addr != s_src) continue;
else if (dest && e->ip.dst.s_addr != s_dest) continue;
else if (iniface && strcmp(e->ip.iniface, iniface) != 0) continue;
else if (outiface && strcmp(e->ip.outiface, outiface) != 0) continue;
else if (protocol && strcmp(protocol, "TCP") == 0 && e->ip.proto != IPPROTO_TCP) continue;
else if (protocol && strcmp(protocol, "UDP") == 0 && e->ip.proto != IPPROTO_UDP) continue;
else if ((srcports || destports) && IPT_MATCH_ITERATE_MY(e, matchcmp, srcports, destports) == 0) continue;
else if (target && strcmp(target, iptc_get_target(e, handle)) != 0) continue;
else if (dnat_to && strcmp(target, "DNAT") == 0) {
struct ipt_entry_target *t;
struct ip_nat_multi_range *mr;
struct ip_nat_range *r, range;
t = (struct ipt_entry_target *) (e + e->target_offset);
mr = (struct ip_nat_multi_range *) ((void *) &t->data);
if (mr->rangesize != 1) continue; // we have only single dnat_to target now
r = mr->range;
parse_range(dnat_to, &range);
if (r->flags == range.flags
&& r->min_ip == range.min_ip
&& r->max_ip == range.max_ip
&& r->min.all == range.min.all
&& r->max.all == range.max.all) {
break;
}
} else break;
}
if (!e) return;
result = iptc_delete_num_entry(chain, i, handle);
if (!result) {
printf("libiptc error: Delete error, %s", iptc_strerror(errno));
return;
}
result = iptc_commit(handle);
if (!result) {
printf("libiptc error: Commit error, %s", iptc_strerror(errno));
return;
} else
printf("deleted rule from block successfully");
}
int matchcmp(const struct ipt_entry_match *match, const char *srcports, const char *destports) {
u_int16_t temp[2];
if (strcmp(match->u.user.name, "tcp") == 0) {
struct ipt_tcp *tcpinfo = (struct ipt_tcp *) match->data;
if (srcports) {
parse_ports(srcports, temp);
if (temp[0] != tcpinfo->spts[0] || temp[1] != tcpinfo->spts[1]) return 0;
}
if (destports) {
parse_ports(destports, temp);
if (temp[0] != tcpinfo->dpts[0] || temp[1] != tcpinfo->dpts[1]) return 0;
}
return 1;
} else if (strcmp(match->u.user.name, "udp") == 0) {
struct ipt_udp *udpinfo = (struct ipt_udp *) match->data;
if (srcports) {
parse_ports(srcports, temp);
if (temp[0] != udpinfo->spts[0] || temp[1] != udpinfo->spts[1]) return 0;
}
if (destports) {
parse_ports(destports, temp);
if (temp[0] != udpinfo->dpts[0] || temp[1] != udpinfo->dpts[1]) return 0;
}
return 1;
} else return 0;
}
/* These functions are used to create structs */
struct ipt_entry_match *get_tcp_match(const char *sports, const char *dports, unsigned int *nfcache) {
struct ipt_entry_match *match;
struct ipt_tcp *tcpinfo;
size_t size;
size = IPT_ALIGN(sizeof (*match)) + IPT_ALIGN(sizeof (*tcpinfo));
match = (struct ipt_entry_match *) calloc(1, size);
match->u.match_size = size;
strncpy(match->u.user.name, "tcp", IPT_FUNCTION_MAXNAMELEN);
tcpinfo = (struct ipt_tcp *) match->data;
tcpinfo->spts[1] = tcpinfo->dpts[1] = 0xFFFF;
printf("sports=%s,dports=%s\n", sports, dports);
if (sports) {
*nfcache |= NFC_IP_SRC_PT;
parse_ports(sports, tcpinfo->spts);
printf("%d\n", __LINE__);
}
if (dports) {
*nfcache |= NFC_IP_DST_PT;
parse_ports(dports, tcpinfo->dpts);
printf("%d\n", __LINE__);
}
return match;
}
struct ipt_entry_match *get_udp_match(const char *sports, const char *dports, unsigned int *nfcache) {
struct ipt_entry_match *match;
struct ipt_udp *udpinfo;
size_t size;
size = IPT_ALIGN(sizeof (*match)) + IPT_ALIGN(sizeof (*udpinfo));
match = (struct ipt_entry_match *) calloc(1, size);
match->u.match_size = size;
strncpy(match->u.user.name, "udp", IPT_FUNCTION_MAXNAMELEN);
udpinfo = (struct ipt_udp *) match->data;
udpinfo->spts[1] = udpinfo->dpts[1] = 0xFFFF;
printf("sports=%s,dports=%s\n", sports, dports);
if (sports) {
*nfcache |= NFC_IP_SRC_PT;
parse_ports(sports, udpinfo->spts);
printf("%d\n", __LINE__);
}
if (dports) {
*nfcache |= NFC_IP_DST_PT;
parse_ports(dports, udpinfo->dpts);
printf("%d\n", __LINE__);
}
return match;
}
struct ipt_entry_target *get_dnat_target(const char *input, unsigned int *nfcache) {
struct ipt_entry_target *target;
struct ipt_natinfo *info;
struct ip_nat_range range;
char *buffer;
size_t size;
/* Can't cache this */
*nfcache |= NFC_UNKNOWN;
buffer = strdup(input);
size = IPT_ALIGN(sizeof (*target)) + IPT_ALIGN(sizeof (struct ip_nat_multi_range));
target = (struct ipt_entry_target *) calloc(1, size);
target->u.target_size = size;
strncpy(target->u.user.name, "DNAT", IPT_FUNCTION_MAXNAMELEN);
info = (struct ipt_natinfo *) target;
printf("buffer range=%s\n", buffer);
parse_range(buffer, &range);
target = &(append_range(info, &range)->t);
printf("range=%d\n", range.flags);
printf("%d\n", __LINE__);
free(buffer);
return target;
}
struct ipt_entry_target *get_snat_target(const char *input, unsigned int *nfcache) {
struct ipt_entry_target *target;
struct ipt_natinfo *info;
struct ip_nat_range range;
char *buffer;
size_t size;
/* Can't cache this */
*nfcache |= NFC_UNKNOWN;
buffer = strdup(input);
size = IPT_ALIGN(sizeof (*target)) + IPT_ALIGN(sizeof (struct ip_nat_multi_range));
target = (struct ipt_entry_target *) calloc(1, size);
target->u.target_size = size;
strncpy(target->u.user.name, "SNAT", IPT_FUNCTION_MAXNAMELEN);
info = (struct ipt_natinfo *) target;
printf("buffer range=%s\n", buffer);
parse_range(buffer, &range);
target = &(append_range(info, &range)->t);
printf("range=%d\n", range.flags);
printf("%d\n", __LINE__);
free(buffer);
return target;
}
/* Copied and modified from libipt_tcp.c and libipt_udp.c */
static u_int16_t parse_port(const char *port) {
unsigned int portnum;
if ((portnum = service_to_port(port)) != -1) {
return (u_int16_t) portnum;
} else {
return atoi(port);
}
}
static void parse_ports(const char *portstring, u_int16_t *ports) {
char *buffer;
char *cp;
buffer = strdup(portstring);
if ((cp = strchr(buffer, ':')) == NULL)
ports[0] = ports[1] = parse_port(buffer);
else {
*cp = '\0';
cp++;
ports[0] = buffer[0] ? parse_port(buffer) : 0;
ports[1] = cp[0] ? parse_port(cp) : 0xFFFF;
}
free(buffer);
}
static int service_to_port(const char *name) {
struct servent *service;
if ((service = getservbyname(name, "tcp")) != NULL)
return ntohs((unsigned short) service->s_port);
return -1;
}
/* Copied and modified from libipt_DNAT.c */
static void parse_range(const char *input, struct ip_nat_range *range) {
char *colon, *dash, *buffer;
in_addr_t ip;
buffer = strdup(input);
memset(range, 0, sizeof (*range));
colon = strchr(buffer, ':');
if (colon) {
int port;
range->flags |= IP_NAT_RANGE_PROTO_SPECIFIED;
port = atoi(colon + 1);
dash = strchr(colon, '-');
if (!dash) {
range->min.all
= range->max.all
= htons(port);
} else {
int maxport;
maxport = atoi(dash + 1);
range->min.all = htons(port);
range->max.all = htons(maxport);
}
/* Starts with a colon? No IP info...*/
if (colon == buffer) {
free(buffer);
return;
}
*colon = '\0';
}
range->flags |= IP_NAT_RANGE_MAP_IPS;
dash = strchr(buffer, '-');
if (colon && dash && dash > colon)
dash = NULL;
if (dash)
*dash = '\0';
ip = inet_addr(buffer);
range->min_ip = ip;
if (dash) {
ip = inet_addr(dash + 1);
range->max_ip = ip;
} else
range->max_ip = range->min_ip;
free(buffer);
return;
}
static struct ipt_natinfo *append_range(struct ipt_natinfo *info, const struct ip_nat_range *range) {
unsigned int size;
/* One ip_nat_range already included in ip_nat_multi_range */
size = IPT_ALIGN(sizeof (*info) + info->mr.rangesize * sizeof (*range));
info = (struct ipt_natinfo *) realloc(info, size);
info->t.u.target_size = size;
info->mr.range[info->mr.rangesize] = *range;
info->mr.rangesize++;
printf("range size=%d\n", info->mr.rangesize);
return info;
}