How conntrack works in Netfilter(Part 2 - How to track FTP connections)
本博客Netfilter/IPtables系列文章均基于Linux2.6.30内核。
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hereitis.cu@gmail.com- Initialization
- nf_conntrack_ftp_init(net/netfilter/nf_conntrack_ftp.c)
static int __init nf_conntrack_ftp_init(void)
{
int i, j = -1, ret = 0;
char *tmpname;
ftp_buffer = kmalloc(65536, GFP_KERNEL);
if (!ftp_buffer)
return -ENOMEM;
if (ports_c == 0) // static unsigned int ports_c;
ports[ports_c++] = FTP_PORT; //static u_int16_t ports[MAX_PORTS];
/* FIXME should be configurable whether IPv4 and IPv6 FTP connections
are tracked or not - YK */
for (i = 0; i < ports_c; i++) {
ftp[i][0].tuple.src.l3num = PF_INET;// Will be used when hash tuple
ftp[i][1].tuple.src.l3num = PF_INET6;
for (j = 0; j < 2; j++) {
ftp[i][j].tuple.src.u.tcp.port = htons(ports[i]); // Will be used when hash tuple
ftp[i][j].tuple.dst.protonum = IPPROTO_TCP; // Will be used when hash tuple
ftp[i][j].expect_policy = &ftp_exp_policy;
ftp[i][j].me = THIS_MODULE;
ftp[i][j].help = help;
tmpname = &ftp_names[i][j][0];
if (ports[i] == FTP_PORT)
sprintf(tmpname, "ftp");
else
sprintf(tmpname, "ftp-%d", ports[i]);
ftp[i][j].name = tmpname;
pr_debug("nf_ct_ftp: registering helper for pf: %d "
"port: %d\n",
ftp[i][j].tuple.src.l3num, ports[i]);
ret = nf_conntrack_helper_register(&ftp[i][j]);
if (ret) {
printk("nf_ct_ftp: failed to register helper "
" for pf: %d port: %d\n",
ftp[i][j].tuple.src.l3num, ports[i]);
nf_conntrack_ftp_fini();
return ret;
}
}
}
return 0;
}
- nf_conntrack_helper_register(net/netfilter/nf_conntrack_helper.c)
int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
{
unsigned int h = helper_hash(&me->tuple);
BUG_ON(me->expect_policy == NULL);
BUG_ON(me->expect_class_max >= NF_CT_MAX_EXPECT_CLASSES);
BUG_ON(strlen(me->name) > NF_CT_HELPER_NAME_LEN - 1);
mutex_lock(&nf_ct_helper_mutex);
hlist_add_head_rcu(&me->hnode, &nf_ct_helper_hash[h]); // Global helper list
nf_ct_helper_count++;
mutex_unlock(&nf_ct_helper_mutex);
return 0;
}
- helper_hash(net/netfilter/nf_conntrack_helper.c)
/* Stupid hash, but collision free for the default registrations of the
* helpers currently in the kernel. */
static unsigned int helper_hash(const struct nf_conntrack_tuple *tuple)
{
return (((tuple->src.l3num << 8) | tuple->dst.protonum) ^
(__force __u16)tuple->src.u.all) % nf_ct_helper_hsize;
}
- How ftp helper registered
- How ftp helper works
- Big picture
- Main helper function(net/netfilter/nf_conntrack_ftp.c)
static int help(struct sk_buff *skb,
unsigned int protoff,
struct nf_conn *ct,
enum ip_conntrack_info ctinfo)
{
unsigned int dataoff, datalen;
const struct tcphdr *th;
struct tcphdr _tcph;
const char *fb_ptr;
int ret;
u32 seq;
int dir = CTINFO2DIR(ctinfo);
unsigned int uninitialized_var(matchlen), uninitialized_var(matchoff);
struct nf_ct_ftp_master *ct_ftp_info = &nfct_help(ct)->help.ct_ftp_info;
struct nf_conntrack_expect *exp;
union nf_inet_addr *daddr;
struct nf_conntrack_man cmd = {};
unsigned int i;
int found = 0, ends_in_nl;
typeof(nf_nat_ftp_hook) nf_nat_ftp;
/* Until there's been traffic both ways, don't look in packets. */
if (ctinfo != IP_CT_ESTABLISHED
&& ctinfo != IP_CT_ESTABLISHED+IP_CT_IS_REPLY) {
pr_debug("ftp: Conntrackinfo = %u\n", ctinfo);
return NF_ACCEPT;
}
th = skb_header_pointer(skb, protoff, sizeof(_tcph), &_tcph);
if (th == NULL)
return NF_ACCEPT;
dataoff = protoff + th->doff * 4;
/* No data? */
if (dataoff >= skb->len) {
pr_debug("ftp: dataoff(%u) >= skblen(%u)\n", dataoff,
skb->len);
return NF_ACCEPT;
}
datalen = skb->len - dataoff;
spin_lock_bh(&nf_ftp_lock);
fb_ptr = skb_header_pointer(skb, dataoff, datalen, ftp_buffer);
BUG_ON(fb_ptr == NULL);
ends_in_nl = (fb_ptr[datalen - 1] == '\n');
seq = ntohl(th->seq) + datalen;
/* Look up to see if we're just after a \n. */
if (!find_nl_seq(ntohl(th->seq), ct_ftp_info, dir)) {
/* Now if this ends in \n, update ftp info. */
pr_debug("nf_conntrack_ftp: wrong seq pos %s(%u) or %s(%u)\n",
ct_ftp_info->seq_aft_nl_num[dir] > 0 ? "" : "(UNSET)",
ct_ftp_info->seq_aft_nl[dir][0],
ct_ftp_info->seq_aft_nl_num[dir] > 1 ? "" : "(UNSET)",
ct_ftp_info->seq_aft_nl[dir][1]);
ret = NF_ACCEPT;
goto out_update_nl;
}
/* Initialize IP/IPv6 addr to expected address (it's not mentioned
in EPSV responses) */
cmd.l3num = nf_ct_l3num(ct);
memcpy(cmd.u3.all, &ct->tuplehash[dir].tuple.src.u3.all,
sizeof(cmd.u3.all));
for (i = 0; i < ARRAY_SIZE(search[dir]); i++) {
found = find_pattern(fb_ptr, datalen, // Try to find packet which trigger a ftp connection tracking
search[dir][i].pattern,
search[dir][i].plen,
search[dir][i].skip,
search[dir][i].term,
&matchoff, &matchlen,
&cmd,
search[dir][i].getnum);
if (found) break;
}
if (found == -1) {
/* We don't usually drop packets. After all, this is
connection tracking, not packet filtering.
However, it is necessary for accurate tracking in
this case. */
pr_debug("conntrack_ftp: partial %s %u+%u\n",
search[dir][i].pattern, ntohl(th->seq), datalen);
ret = NF_DROP;
goto out;
} else if (found == 0) { /* No match */
ret = NF_ACCEPT;
goto out_update_nl;
}
pr_debug("conntrack_ftp: match `%.*s' (%u bytes at %u)\n",
matchlen, fb_ptr + matchoff,
matchlen, ntohl(th->seq) + matchoff);
exp = nf_ct_expect_alloc(ct);
if (exp == NULL) {
ret = NF_DROP;
goto out;
}
/* We refer to the reverse direction ("!dir") tuples here,
* because we're expecting something in the other direction.
* Doesn't matter unless NAT is happening. */
daddr = &ct->tuplehash[!dir].tuple.dst.u3;
/* Update the ftp info */
if ((cmd.l3num == nf_ct_l3num(ct)) &&
memcmp(&cmd.u3.all, &ct->tuplehash[dir].tuple.src.u3.all,
sizeof(cmd.u3.all))) {
/* Enrico Scholz's passive FTP to partially RNAT'd ftp
server: it really wants us to connect to a
different IP address. Simply don't record it for
NAT. */
if (cmd.l3num == PF_INET) {
pr_debug("conntrack_ftp: NOT RECORDING: %pI4 != %pI4\n",
&cmd.u3.ip,
&ct->tuplehash[dir].tuple.src.u3.ip);
} else {
pr_debug("conntrack_ftp: NOT RECORDING: %pI6 != %pI6\n",
cmd.u3.ip6,
ct->tuplehash[dir].tuple.src.u3.ip6);
}
/* Thanks to Cristiano Lincoln Mattos
for reporting this potential
problem (DMZ machines opening holes to internal
networks, or the packet filter itself). */
if (!loose) {
ret = NF_ACCEPT;
goto out_put_expect;
}
daddr = &cmd.u3;
}
nf_ct_expect_init(exp, NF_CT_EXPECT_CLASS_DEFAULT, cmd.l3num, // Setup expectation for this ftp connection
&ct->tuplehash[!dir].tuple.src.u3, daddr,
IPPROTO_TCP, NULL, &cmd.u.tcp.port);
/* Now, NAT might want to mangle the packet, and register the
* (possibly changed) expectation itself. */
nf_nat_ftp = rcu_dereference(nf_nat_ftp_hook);
if (nf_nat_ftp && ct->status & IPS_NAT_MASK)
ret = nf_nat_ftp(skb, ctinfo, search[dir][i].ftptype,
matchoff, matchlen, exp);
else {
/* Can't expect this? Best to drop packet now. */
if (nf_ct_expect_related(exp) != 0)
ret = NF_DROP;
else
ret = NF_ACCEPT;
}
out_put_expect:
nf_ct_expect_put(exp);
out_update_nl:
/* Now if this ends in \n, update ftp info. Seq may have been
* adjusted by NAT code. */
if (ends_in_nl)
update_nl_seq(ct, seq, ct_ftp_info, dir, skb);
out:
spin_unlock_bh(&nf_ftp_lock);
return ret;
}
- Pattern search(net/netfilter/nf_conntrack_ftp.c)
- Structure
static struct ftp_search {
const char *pattern;
size_t plen;
char skip;
char term;
enum nf_ct_ftp_type ftptype;
int (*getnum)(const char *, size_t, struct nf_conntrack_man *, char);
} search[IP_CT_DIR_MAX][2] = { // 2 directions and 2 modes
[IP_CT_DIR_ORIGINAL] = {
{
.pattern = "PORT",
.plen = sizeof("PORT") - 1,
.skip = ' ',
.term = '\r',
.ftptype = NF_CT_FTP_PORT,
.getnum = try_rfc959,
},
{
.pattern = "EPRT",
.plen = sizeof("EPRT") - 1,
.skip = ' ',
.term = '\r',
.ftptype = NF_CT_FTP_EPRT,
.getnum = try_eprt,
},
},
[IP_CT_DIR_REPLY] = {
{
.pattern = "227 ",
.plen = sizeof("227 ") - 1,
.skip = '(',
.term = ')',
.ftptype = NF_CT_FTP_PASV,
.getnum = try_rfc959,
},
{
.pattern = "229 ",
.plen = sizeof("229 ") - 1,
.skip = '(',
.term = ')',
.ftptype = NF_CT_FTP_EPSV,
.getnum = try_epsv_response,
},
},
};
- try_rfc959(net/netfilter/nf_conntrack_ftp.c)
/* Returns 0, or length of numbers: 192,168,1,1,5,6 */
static int try_rfc959(const char *data, size_t dlen,
struct nf_conntrack_man *cmd, char term)
{
int length;
u_int32_t array[6];
length = try_number(data, dlen, array, 6, ',', term);
if (length == 0)
return 0;
cmd->u3.ip = htonl((array[0] << 24) | (array[1] << 16) |
(array[2] << 8) | array[3]);
cmd->u.tcp.port = htons((array[4] << 8) | array[5]);
return length;
}
- try_eprt(net/netfilter/nf_conntrack_ftp.c)
/* Returns 0, or length of numbers: |1|132.235.1.2|6275| or |2|3ffe::1|6275| */
static int try_eprt(const char *data, size_t dlen, struct nf_conntrack_man *cmd,
char term)
{
char delim;
int length;
/* First character is delimiter, then "1" for IPv4 or "2" for IPv6,
then delimiter again. */
if (dlen <= 3) {
pr_debug("EPRT: too short\n");
return 0;
}
delim = data[0];
if (isdigit(delim) || delim < 33 || delim > 126 || data[2] != delim) {
pr_debug("try_eprt: invalid delimitter.\n");
return 0;
}
if ((cmd->l3num == PF_INET && data[1] != '1') ||
(cmd->l3num == PF_INET6 && data[1] != '2')) {
pr_debug("EPRT: invalid protocol number.\n");
return 0;
}
pr_debug("EPRT: Got %c%c%c\n", delim, data[1], delim);
if (data[1] == '1') {
u_int32_t array[4];
/* Now we have IP address. */
length = try_number(data + 3, dlen - 3, array, 4, '.', delim);
if (length != 0)
cmd->u3.ip = htonl((array[0] << 24) | (array[1] << 16)
| (array[2] << 8) | array[3]);
} else {
/* Now we have IPv6 address. */
length = get_ipv6_addr(data + 3, dlen - 3,
(struct in6_addr *)cmd->u3.ip6, delim);
}
if (length == 0)
return 0;
pr_debug("EPRT: Got IP address!\n");
/* Start offset includes initial "|1|", and trailing delimiter */
return get_port(data, 3 + length + 1, dlen, delim, &cmd->u.tcp.port);
}
- try_epsv_response(net/netfilter/nf_conntrack_ftp.c)
/* Returns 0, or length of numbers: |||6446| */
static int try_epsv_response(const char *data, size_t dlen,
struct nf_conntrack_man *cmd, char term)
{
char delim;
/* Three delimiters. */
if (dlen <= 3) return 0;
delim = data[0];
if (isdigit(delim) || delim < 33 || delim > 126
|| data[1] != delim || data[2] != delim)
return 0;
return get_port(data, 3, dlen, delim, &cmd->u.tcp.port);
}
- nf_ct_expect_init(net/netfilter/nf_conntrack_expect.c)
void nf_ct_expect_init(struct nf_conntrack_expect *exp, unsigned int class,
u_int8_t family,
const union nf_inet_addr *saddr,
const union nf_inet_addr *daddr,
u_int8_t proto, const __be16 *src, const __be16 *dst)
{
int len;
if (family == AF_INET)
len = 4;
else
len = 16;
exp->flags = 0;
exp->class = class;
exp->expectfn = NULL;
exp->helper = NULL;
exp->tuple.src.l3num = family;
exp->tuple.dst.protonum = proto;
if (saddr) {
memcpy(&exp->tuple.src.u3, saddr, len);
if (sizeof(exp->tuple.src.u3) > len)
/* address needs to be cleared for nf_ct_tuple_equal */
memset((void *)&exp->tuple.src.u3 + len, 0x00,
sizeof(exp->tuple.src.u3) - len);
memset(&exp->mask.src.u3, 0xFF, len);
if (sizeof(exp->mask.src.u3) > len)
memset((void *)&exp->mask.src.u3 + len, 0x00,
sizeof(exp->mask.src.u3) - len);
} else {
memset(&exp->tuple.src.u3, 0x00, sizeof(exp->tuple.src.u3));
memset(&exp->mask.src.u3, 0x00, sizeof(exp->mask.src.u3));
}
if (src) {
exp->tuple.src.u.all = *src;
exp->mask.src.u.all = htons(0xFFFF);
} else {
exp->tuple.src.u.all = 0;
exp->mask.src.u.all = 0;
}
memcpy(&exp->tuple.dst.u3, daddr, len);
if (sizeof(exp->tuple.dst.u3) > len)
/* address needs to be cleared for nf_ct_tuple_equal */
memset((void *)&exp->tuple.dst.u3 + len, 0x00,
sizeof(exp->tuple.dst.u3) - len);
exp->tuple.dst.u.all = *dst;
}
EXPORT_SYMBOL_GPL(nf_ct_expect_init);
- Let us take a real ftp traffic as an example
- The partial real ftp traffic
No. Time Source Destination Protocol Info
1 0.000000 172.20.9.84 172.20.9.80 TCP eportcomm > ftp [SYN] Seq=0 Win=65535 Len=0 MSS=1460
Frame 1 (62 bytes on wire, 62 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 0, Len: 0
No. Time Source Destination Protocol Info
2 0.000135 172.20.9.80 172.20.9.84 TCP ftp > eportcomm [SYN, ACK] Seq=0 Ack=1 Win=5840 Len=0 MSS=1460
Frame 2 (62 bytes on wire, 62 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp (21), Dst Port: eportcomm (4666), Seq: 0, Ack: 1, Len: 0
No. Time Source Destination Protocol Info
3 0.000152 172.20.9.84 172.20.9.80 TCP eportcomm > ftp [ACK] Seq=1 Ack=1 Win=65535 Len=0
Frame 3 (54 bytes on wire, 54 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 1, Ack: 1, Len: 0
No. Time Source Destination Protocol Info
4 0.006313 172.20.9.80 172.20.9.84 FTP Response: 220 Welcome to Test's FTP service.
Frame 4 (91 bytes on wire, 91 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp (21), Dst Port: eportcomm (4666), Seq: 1, Ack: 1, Len: 37
File Transfer Protocol (FTP)
...........
No. Time Source Destination Protocol Info
12 8.471443 172.20.9.84 172.20.9.80 TCP eportcomm > ftp [ACK] Seq=32 Ack=95 Win=65441 Len=0
Frame 12 (54 bytes on wire, 54 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 32, Ack: 95, Len: 0
No. Time Source Destination Protocol Info
13 10.124938 172.20.9.84 172.20.9.80 FTP Request: PORT 172,20,9,84,18,67
Frame 13 (78 bytes on wire, 78 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 32, Ack: 95, Len: 24
File Transfer Protocol (FTP)
No. Time Source Destination Protocol Info
14 10.125258 172.20.9.80 172.20.9.84 FTP Response: 200 PORT command successful. Consider using PASV.
Frame 14 (105 bytes on wire, 105 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp (21), Dst Port: eportcomm (4666), Seq: 95, Ack: 56, Len: 51
File Transfer Protocol (FTP)
No. Time Source Destination Protocol Info
15 10.126652 172.20.9.84 172.20.9.80 FTP Request: NLST
Frame 15 (60 bytes on wire, 60 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 56, Ack: 146, Len: 6
File Transfer Protocol (FTP)
No. Time Source Destination Protocol Info
16 10.127158 172.20.9.80 172.20.9.84 TCP ftp-data > dhct-status [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=21742713 TSER=0 WS=5
Frame 16 (74 bytes on wire, 74 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp-data (20), Dst Port: dhct-status (4675), Seq: 0, Len: 0
No. Time Source Destination Protocol Info
17 10.127192 172.20.9.84 172.20.9.80 TCP dhct-status > ftp-data [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=1460 WS=0 TSV=0 TSER=0
Frame 17 (78 bytes on wire, 78 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: dhct-status (4675), Dst Port: ftp-data (20), Seq: 0, Ack: 1, Len: 0
No. Time Source Destination Protocol Info
18 10.127311 172.20.9.80 172.20.9.84 TCP ftp-data > dhct-status [ACK] Seq=1 Ack=1 Win=5856 Len=0 TSV=21742713 TSER=0
Frame 18 (66 bytes on wire, 66 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp-data (20), Dst Port: dhct-status (4675), Seq: 1, Ack: 1, Len: 0
No. Time Source Destination Protocol Info
19 10.127412 172.20.9.80 172.20.9.84 FTP Response: 150 Here comes the directory listing.
- Assume we are on 172.20.9.80(ftp server side), so hooks on point NF_INET_PRE_ROUTING and NF_INET_LOCAL_IN will be invoked.
- Big Picture
No. Time Source Destination Protocol Info
1 0.000000 172.20.9.84 172.20.9.80 TCP eportcomm > ftp [SYN] Seq=0 Win=65535 Len=0 MSS=1460
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 0, Len: 0
No. Time Source Destination Protocol Info
2 0.000135 172.20.9.80 172.20.9.84 TCP ftp> eportcomm [SYN, ACK] Seq=0 Ack=1 Win=5840 Len=0 MSS=1460
Frame 2 (62 bytes on wire, 62 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp (21), Dst Port: eportcomm (4666), Seq: 0, Ack: 1, Len: 0
- What we got from Frame 13
No. Time Source Destination Protocol Info
13 10.124938 172.20.9.84 172.20.9.80 FTP Request: PORT 172,20,9,84,18,67
Frame 13 (78 bytes on wire, 78 bytes captured)
Internet Protocol, Src: 172.20.9.84 (172.20.9.84), Dst: 172.20.9.80 (172.20.9.80)
Transmission Control Protocol, Src Port: eportcomm (4666), Dst Port: ftp (21), Seq: 32, Ack: 95, Len: 24
File Transfer Protocol (FTP)
- What we got from Frame 16
No. Time Source Destination Protocol Info
16 10.127158 172.20.9.80 172.20.9.84 TCP ftp-data > dhct-status [SYN] Seq=0 Win=5840 Len=0 MSS=1460TSV=21742713 TSER=0 WS=5
Frame 16 (74 bytes on wire, 74 bytes captured)
Internet Protocol, Src: 172.20.9.80 (172.20.9.80), Dst: 172.20.9.84 (172.20.9.84)
Transmission Control Protocol, Src Port: ftp-data (20), Dst Port: dhct-status (4675), Seq: 0, Len: 0
