vs conntrack changes TCP ports mid-stream

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



I'm forwarding the report from https://bugzilla.netfilter.org/show_bug.cgi?id=1669 here, since it was pointed out there, that this list would be more appropriate.

When using an ipvs service in combination with SNAT and a NOTRACK rule, specific circumstances can lead to TCP ports of packets being changed mid-stream, which results in successful connections that no data can be effectively sent over.

Consider the following example:

```
root@router:~# sysctl net.ipv4.vs.conntrack
net.ipv4.vs.conntrack = 1

root@router:~# iptables -t raw -L -n -v
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination 24 1296 CT tcp -- enp1s0 * 0.0.0.0/0 10.0.0.1 tcp dpt:1234 NOTRACK

root@router:~# iptables -t nat -L -n -v
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination 4 240 SNAT all -- * * 10.0.0.0/24 10.0.1.0/24 to:10.0.1.1

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination

root@router:~# ipvsadm -L -n
IP Virtual Server version 1.2.1 (size=4096)
Prot LocalAddress:Port Scheduler Flags
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
TCP  10.0.0.1:1234 rr
  -> 10.0.1.2:1234                Masq    1      0          0
  -> 10.0.1.3:1234                Masq    1      0          0
```

The reals servers are running

```
socat TCP4-LISTEN:1234,fork 'EXEC:sh -c echo${IFS}hello;read${IFS}r${IFS}L;sleep${IFS}1'
```

We dump the network traffic between the router and client on the ipvs router as follows:

```
root@router:~# tcpdump -pXXni enp1s0 icmp or tcp -w /tmp/ipvs_port_reuse.pcap tcpdump: listening on enp1s0, link-type EN10MB (Ethernet), capture size 262144 bytes
^C16 packets captured
16 packets received by filter
0 packets dropped by kernel
```

While the capture is running, we run the following commands on a client to trigger the buggy behavior:

```
root@debian:~# netcat -p 4321 -v 10.0.01 1234
Connection to 10.0.01 1234 port [tcp/*] succeeded!
hello
^C
root@debian:~# sleep 60
root@debian:~# netcat -p 4321 -v 10.0.01 1234
Connection to 10.0.01 1234 port [tcp/*] succeeded!
^C
root@debian:~#
```

We can see that on the first connection attempt we successfully receive a reply with payload from the server and then terminate the connection with Ctrl+C. Then we wait 60 seconds, which is necessary for the previous connection to move out of the TIME_WAIT state. Afterwards we open another connection, reusing the same src port as on the first connection and don't receive a reply from the server. The captured traffic shows, that after the three-way handshake for the second TCP connection, packets from the router to the clients use another server port than the one used for the initiation of the connection.

Regards
Sven

Attachment: ipvs_port_reuse.pcap
Description: application/vnd.tcpdump.pcap


[Index of Archives]     [Linux Filesystem Devel]     [Linux NFS]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite News]     [Linux SCSI]     [X.Org]

  Powered by Linux