Without this patch, if we receive a SYN packet from the client while
the firewall is out-of-sync, we let it go through. Then, if we see
the SYN/ACK reply coming from the server, we destroy the conntrack
entry and drop the packet to trigger a new retransmission. Then,
the retransmision from the client is used to start a new clean
session.
This patch improves the current handling. Basically, if we see an
unexpected SYN packet, we annotate the TCP options. Then, if we
see the reply SYN/ACK, this means that the firewall was indeed
out-of-sync. Therefore, we set a clean new session from the existing
entry based on the annotated values.
This patch adds two new 8-bits fields that fit in a 16-bits gap of
the ip_ct_tcp structure.
This patch is particularly useful for conntrackd since the
asynchronous nature of the state-synchronization allows to have
backup nodes that are not perfect copies of the master. This helps
to improve the recovery under some worst-case scenarios.
I have tested this by creating lots of conntrack entries in wrong
state:
for ((i=1024;i<65535;i++)); do conntrack -I -p tcp -s 192.168.2.101 -d 192.168.2.2 --sport $i --dport 80 -t 800 --state ESTABLISHED -u ASSURED,SEEN_REPLY; done
Then, I make some TCP connections:
$ echo GET / | nc 192.168.2.2 80
The events show the result:
[UPDATE] tcp 6 60 SYN_RECV src=192.168.2.101 dst=192.168.2.2 sport=33220 dport=80 src=192.168.2.2 dst=192.168.2.101 sport=80 dport=33220 [ASSURED]
[UPDATE] tcp 6 432000 ESTABLISHED src=192.168.2.101 dst=192.168.2.2 sport=33220 dport=80 src=192.168.2.2 dst=192.168.2.101 sport=80 dport=33220 [ASSURED]
[UPDATE] tcp 6 120 FIN_WAIT src=192.168.2.101 dst=192.168.2.2 sport=33220 dport=80 src=192.168.2.2 dst=192.168.2.101 sport=80 dport=33220 [ASSURED]
[UPDATE] tcp 6 30 LAST_ACK src=192.168.2.101 dst=192.168.2.2 sport=33220 dport=80 src=192.168.2.2 dst=192.168.2.101 sport=80 dport=33220 [ASSURED]
[UPDATE] tcp 6 120 TIME_WAIT src=192.168.2.101 dst=192.168.2.2 sport=33220 dport=80 src=192.168.2.2 dst=192.168.2.101 sport=80 dport=33220 [ASSURED]
and tcpdump shows no retransmissions:
20:47:57.271951 IP 192.168.2.101.33221 > 192.168.2.2.www: S 435402517:435402517(0) win 5840 <mss 1460,sackOK,timestamp 4294961827 0,nop,wscale 6>
20:47:57.273538 IP 192.168.2.2.www > 192.168.2.101.33221: S 3509927945:3509927945(0) ack 435402518 win 5792 <mss 1460,sackOK,timestamp 235681024 4294961827,nop,wscale 4>
20:47:57.273608 IP 192.168.2.101.33221 > 192.168.2.2.www: . ack 3509927946 win 92 <nop,nop,timestamp 4294961827 235681024>
20:47:57.273693 IP 192.168.2.101.33221 > 192.168.2.2.www: P 435402518:435402524(6) ack 3509927946 win 92 <nop,nop,timestamp 4294961827 235681024>
20:47:57.275492 IP 192.168.2.2.www > 192.168.2.101.33221: . ack 435402524 win 362 <nop,nop,timestamp 235681024 4294961827>
20:47:57.276492 IP 192.168.2.2.www > 192.168.2.101.33221: P 3509927946:3509928082(136) ack 435402524 win 362 <nop,nop,timestamp 235681025 4294961827>
20:47:57.276515 IP 192.168.2.101.33221 > 192.168.2.2.www: . ack 3509928082 win 108 <nop,nop,timestamp 4294961828 235681025>
20:47:57.276521 IP 192.168.2.2.www > 192.168.2.101.33221: F 3509928082:3509928082(0) ack 435402524 win 362 <nop,nop,timestamp 235681025 4294961827>
20:47:57.277369 IP 192.168.2.101.33221 > 192.168.2.2.www: F 435402524:435402524(0) ack 3509928083 win 108 <nop,nop,timestamp 4294961828 235681025>
20:47:57.279491 IP 192.168.2.2.www > 192.168.2.101.33221: . ack 435402525 win 362 <nop,nop,timestamp 235681025 4294961828>
I also added a rule to log invalid packets, with no occurrences :-).
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
include/linux/netfilter/nf_conntrack_tcp.h | 3 ++
net/netfilter/nf_conntrack_proto_tcp.c | 51 +++++++++++++++++++++++-----
2 files changed, 44 insertions(+), 10 deletions(-)
diff --git a/include/linux/netfilter/nf_conntrack_tcp.h b/include/linux/netfilter/nf_conntrack_tcp.h
index f6d97f6..6e135f9 100644
--- a/include/linux/netfilter/nf_conntrack_tcp.h
+++ b/include/linux/netfilter/nf_conntrack_tcp.h
@@ -66,6 +66,9 @@ struct ip_ct_tcp {
u_int32_t last_ack; /* Last sequence number seen in opposite dir */
u_int32_t last_end; /* Last seq + len */
u_int16_t last_win; /* Last window advertisement seen in dir */
+ /* For SYN packets while we may be out-of-sync */
+ u_int8_t last_wscale; /* Last window scaling factor seen */
+ u_int8_t last_flags; /* Last flags set */
};
#endif /* __KERNEL__ */
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index ba2b769..37a8c74 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -896,23 +896,54 @@ static int tcp_packet(struct nf_conn *ct,
/* b) This SYN/ACK acknowledges a SYN that we earlier
* ignored as invalid. This means that the client and
* the server are both in sync, while the firewall is
- * not. We kill this session and block the SYN/ACK so
- * that the client cannot but retransmit its SYN and
- * thus initiate a clean new session.
+ * not. We get in sync from the previously annotated
+ * values.
*/
- spin_unlock_bh(&ct->lock);
- if (LOG_INVALID(net, IPPROTO_TCP))
- nf_log_packet(pf, 0, skb, NULL, NULL, NULL,
- "nf_ct_tcp: killing out of sync session ");
- nf_ct_kill(ct);
- return NF_DROP;
+ old_state = TCP_CONNTRACK_SYN_SENT;
+ new_state = TCP_CONNTRACK_SYN_RECV;
+ ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_end =
+ ct->proto.tcp.last_end;
+ ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_maxend =
+ ct->proto.tcp.last_end;
+ ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_maxwin =
+ ct->proto.tcp.last_win == 0 ?
+ 1 : ct->proto.tcp.last_win;
+ ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_scale =
+ ct->proto.tcp.last_wscale;
+ ct->proto.tcp.seen[ct->proto.tcp.last_dir].flags =
+ ct->proto.tcp.last_flags;
+ memset(&ct->proto.tcp.seen[dir], 0,
+ sizeof(struct ip_ct_tcp_state));
+ break;
}
ct->proto.tcp.last_index = index;
ct->proto.tcp.last_dir = dir;
ct->proto.tcp.last_seq = ntohl(th->seq);
ct->proto.tcp.last_end =
segment_seq_plus_len(ntohl(th->seq), skb->len, dataoff, th);
-
+ ct->proto.tcp.last_win = ntohs(th->window);
+
+ /* a) This is a SYN in ORIGINAL. The client and the server
+ * may be in sync but we are not. In that case, we annotate
+ * the TCP options and let the packet go through. If it is a
+ * valid SYN packet, the server will reply with a SYN/ACK, and
+ * then we'll get in sync. Otherwise, the server ignores it. */
+ if (index == TCP_SYN_SET && dir == IP_CT_DIR_ORIGINAL) {
+ struct ip_ct_tcp_state seen = {};
+
+ ct->proto.tcp.last_flags =
+ ct->proto.tcp.last_wscale = 0;
+ tcp_options(skb, dataoff, th, &seen);
+ if (seen.flags & IP_CT_TCP_FLAG_WINDOW_SCALE) {
+ ct->proto.tcp.last_flags |=
+ IP_CT_TCP_FLAG_WINDOW_SCALE;
+ ct->proto.tcp.last_wscale = seen.td_scale;
+ }
+ if (seen.flags & IP_CT_TCP_FLAG_SACK_PERM) {
+ ct->proto.tcp.last_flags |=
+ IP_CT_TCP_FLAG_SACK_PERM;
+ }
+ }
spin_unlock_bh(&ct->lock);
if (LOG_INVALID(net, IPPROTO_TCP))
nf_log_packet(pf, 0, skb, NULL, NULL, NULL,
--
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