From c4832c7bbc3f7a4813347e871d7238651bf437d3 Mon Sep 17 00:00:00 2001 From: Pablo Neira Ayuso Date: Mon, 23 Nov 2009 10:34:39 +0100 Subject: [PATCH] netfilter: nf_ct_tcp: improve out-of-sync situation in TCP tracking 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 20:47:57.273538 IP 192.168.2.2.www > 192.168.2.101.33221: S 3509927945:3509927945(0) ack 435402518 win 5792 20:47:57.273608 IP 192.168.2.101.33221 > 192.168.2.2.www: . ack 3509927946 win 92 20:47:57.273693 IP 192.168.2.101.33221 > 192.168.2.2.www: P 435402518:435402524(6) ack 3509927946 win 92 20:47:57.275492 IP 192.168.2.2.www > 192.168.2.101.33221: . ack 435402524 win 362 20:47:57.276492 IP 192.168.2.2.www > 192.168.2.101.33221: P 3509927946:3509928082(136) ack 435402524 win 362 20:47:57.276515 IP 192.168.2.101.33221 > 192.168.2.2.www: . ack 3509928082 win 108 20:47:57.276521 IP 192.168.2.2.www > 192.168.2.101.33221: F 3509928082:3509928082(0) ack 435402524 win 362 20:47:57.277369 IP 192.168.2.101.33221 > 192.168.2.2.www: F 435402524:435402524(0) ack 3509928083 win 108 20:47:57.279491 IP 192.168.2.2.www > 192.168.2.101.33221: . ack 435402525 win 362 I also added a rule to log invalid packets, with no occurrences :-) . Signed-off-by: Pablo Neira Ayuso Acked-by: Jozsef Kadlecsik Signed-off-by: Patrick McHardy --- 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 4352feed237..ece22e94dcb 100644 --- a/include/linux/netfilter/nf_conntrack_tcp.h +++ b/include/linux/netfilter/nf_conntrack_tcp.h @@ -67,6 +67,9 @@ 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 97a82ba7537..9cc6b5cb06a 100644 --- a/net/netfilter/nf_conntrack_proto_tcp.c +++ b/net/netfilter/nf_conntrack_proto_tcp.c @@ -908,23 +908,54 @@ 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, -- 2.39.5