To Linux and beyond ! – Page 2 – Plaisirs et dÃ©sillusions du monde moderne

Let’s talk about SELKS

The slides of my lightning talk at SSTIC are available: Let’s talk about SELKS. The slides are in French and are intended to be humorous.

The presentation is about defensive security that needs to get sexier. And Suricata 2.0 with EVE logging combined with Elasticsearch and Kibana can really help to reach that target. If you want to try Suricata and Elasticsearch, you can download and test SELKS.

The talk also present a small tool named Deny On Monitoring which demonstrate how easy it is to extract information from Suricata EVE JSON logging.

Playing with python-git

Introduction

I’m currently working on Scirius, the web management interface for Suricata developed by Stamus Networks.
Scirius is able to fetch IDS signatures from external place and the backend is storing this element in a git tree. As Scirius is a Django application, this means we need to interact with git in Python.

Usually the documentation of Python modules is good and enough to develop. This is sadly not the case for GitPython. There is documentation but the overall quality it not excellent, at least for a non genuine Python developer, and there is some big part missing.

Doing a commit

Doing a commit is really simple once you have understand what to do.
You need to open the repository and work on his index which is the object you add file to commit to. In the following example, I want to add everything under the rules directory:

    repo = git.Repo(source_git_dir)
    index = repo.index
    index.add(["rules"])
    message =  'source version at %s' % (self.updated_date)
    index.commit(message)

Set value in the configuration of a repository

It is possible to edit the configuration of a git repository with GitPython. To do that you need to get the config and
to use the set_value function. For example, the following code snippet create a repository and set user.email and user.name for that repository:

    repo = git.Repo.init(source_git_dir)
    config = repo.config_writer()
    config.set_value("user", "email", "scirius@stamus-networks.com")
    config.set_value("user", "name", "Scirius")

OSError 25: Inappropriate ioctl for device

I’ve encountered this fabulous exception when trying to do a commit in Scirius. The problem is only showing up when running the application in wsfcgi mode. It is documented in Issue 39 on GitHub but there is no workaround proposed.

The error comes from the fact the function used to guess the identity of the user running the application is called even if value are set in the config.
And this function is failing when it is called outside of a real session. This function is in fact trying to get things from environment but these value
are not set when the application is started by init. To fix this, it is possible to force the USERNAME environment variable.

Here’s how it is implemented in Scirius:

+    os.environ['USERNAME'] = 'scirius'
    index.add(["rules"])
    message =  'source version at %s' % (self.updated_date)
    index.commit(message)

You can see the diff on GitHub

Slides of my coccigrep lightning talk at HES2014

I’ve gave a lightning talk about coccigrep at Hackito Ergo Sum to show how it can be used to search in code during audit or hacking party. Here are the slides: coccigrep: a semantic grep for the C language.

The slides of my talk Suricata 2.0, Netfilter and the PRC will soon be available on Stamus Networks website.

Speeding up scapy packets sending

Sending packets with scapy

I’m currently doing some code based on scapy. This code reads data from a possibly huge file and send a packet for each line in the file using the contained information.
So the code contains a simple loop and uses sendp because the frame must be sent at layer 2.

     def run(self):
         filedesc = open(self.filename, 'r')
         # loop on read line
         for line in filedesc:
             # Build and send packet
             sendp(pkt, iface = self.iface, verbose = verbose)
             # Inter packet treatment

Doing that the performance are a bit deceptive. For 18 packets, we’ve got:

    real    0m2.437s
    user    0m0.056s
    sys     0m0.012s

If we strace the code, the explanation is quite obvious:

socket(PF_PACKET, SOCK_RAW, 768)        = 4
setsockopt(4, SOL_SOCKET, SO_RCVBUF, [0], 4) = 0
select(5, [4], [], [], {0, 0})          = 0 (Timeout)
ioctl(4, SIOCGIFINDEX, {ifr_name="lo", ifr_index=1}) = 0
bind(4, {sa_family=AF_PACKET, proto=0x03, if1, pkttype=PACKET_HOST, addr(0)={0, }, 20) = 0
setsockopt(4, SOL_SOCKET, SO_RCVBUF, [1073741824], 4) = 0
setsockopt(4, SOL_SOCKET, SO_SNDBUF, [1073741824], 4) = 0
getsockname(4, {sa_family=AF_PACKET, proto=0x03, if1, pkttype=PACKET_HOST, addr(6)={772, 000000000000}, [18]) = 0
ioctl(4, SIOCGIFNAME, {ifr_index=1, ifr_name="lo"}) = 0
sendto(4, "\377\377\377\377\377\377\0\0\0\0\0\0\10\0E\0\0S}0@\0*\6\265\373\307;\224\24\300\250"..., 97, 0, NULL, 0) = 97
select(0, NULL, NULL, NULL, {0, 0})     = 0 (Timeout)
close(4)                                = 0
socket(PF_PACKET, SOCK_RAW, 768)        = 4
setsockopt(4, SOL_SOCKET, SO_RCVBUF, [0], 4) = 0
select(5, [4], [], [], {0, 0})          = 0 (Timeout)
ioctl(4, SIOCGIFINDEX, {ifr_name="lo", ifr_index=1}) = 0
bind(4, {sa_family=AF_PACKET, proto=0x03, if1, pkttype=PACKET_HOST, addr(0)={0, }, 20) = 0
setsockopt(4, SOL_SOCKET, SO_RCVBUF, [1073741824], 4) = 0
setsockopt(4, SOL_SOCKET, SO_SNDBUF, [1073741824], 4) = 0
getsockname(4, {sa_family=AF_PACKET, proto=0x03, if1, pkttype=PACKET_HOST, addr(6)={772, 000000000000}, [18]) = 0
ioctl(4, SIOCGIFNAME, {ifr_index=1, ifr_name="lo"}) = 0
sendto(4, "\377\377\377\377\377\377\0\0\0\0\0\0\10\0E\0\0004}1@\0*\6\266\31\307;\224\24\300\250"..., 66, 0, NULL, 0) = 66
select(0, NULL, NULL, NULL, {0, 0})     = 0 (Timeout)
close(4)                                = 0

For each packet, a new socket is opened and this takes age.

Speeding up the sending

To speed up the sending, one solution is to build a list of packets and to send that list via a sendp() call.

     def run(self):
         filedesc = open(self.filename, 'r')
         pkt_list = []
         # loop on read line
         for line in filedesc:
             # Build and send packet
             pkt_list.append(pkt)
         sendp(pkt_list, iface = self.iface, verbose = verbose)

This is not possible in our case due to the inter packet treatment we have to do.
So the best way is to reuse the socket. This can be done easily when you’ve read the documentation^W code:

@@ -27,6 +27,7 @@ class replay:
     def run(self):
         # open filename
         filedesc = open(self.filename, 'r')
+        s = conf.L2socket(iface=self.iface)
         # loop on read line
         for line in filedesc:
             # Build and send packet
-            sendp(pkt, iface = self.iface, verbose = verbose)
+            s.send(pkt)

The idea is to create a socket via the function used in sendp() and to use the send() function of the object to send packets.

With that modification, the performance are far better:

    real    0m0.108s
    user    0m0.064s
    sys     0m0.004s

I’m not a scapy expert so ping me if there is a better way to do this.

Suricata and Ulogd meet Logstash and Splunk

Some progress on the JSON side

Suricata 2.0-rc2 is out and it brings some progress on the JSON side. The logging of SSH protocol has been added:

and the format of timestamp has been updated to be ISO 8601 compliant and it is now named timestamp instead of time.

Ulogd, the Netfilter logging daemon has seen similar change as it is now also using a ISO 8601 compliant timestamp for the . This feature is available in git and will be part of ulogd 2.0.4.

Thanks to this format change, the integration with logstash or splunk is easier and more accurate.
This permit to fix one problem regarding the timestamp of an event inside of the event and logging manager. At least in logstash, the used date was the one of the parsing which was not really
accurate. It could even be a problem when logstash was parsing a file with old entries because the difference in timestamp could be huge.

It is now possible to update logstash configuration to have a correct parsing of the timestamp. After doing this the internal @timestamp and the timestamp of the event are synchronized as show on the following screenshot:

Logstash configuration

To configure logstash, you simply needs to tell him that the timestamp field in JSON message is a date. To do so, you need to add a filter:

      date {
        match => [ "timestamp", "ISO8601" ]
      }

A complete logstash.conf would then looks like:

input {
   file {
      path => [ "/usr/local/var/log/suricata/eve.json", "/var/log/ulogd.json" ]
      codec =>   json
      type => "json-log"
   }
}

filter {
   if [type] == "json-log" {
      date {
        match => [ "timestamp", "ISO8601" ]
      }
   }
}

output {
  stdout { codec => rubydebug }
  elasticsearch { embedded => true }
}

Splunk configuration

In splunk, auto detection of the file format is failing and it seems you need to define a type to parse JSON in
$SPLUNK_DIR/etc/system/local/props.conf:

[suricata]
KV_MODE = json
NO_BINARY_CHECK = 1
TRUNCATE = 0

Then you can simply declare the log file in $SPLUNK_DIR/etc/system/local/inputs.conf:

[monitor:///usr/local/var/log/suricata/eve.json]
sourcetype = suricata

[monitor:///var/log/ulogd.json]
sourcetype = suricata

you can now build search events and build dashboard based on Suricata or Netfilter packet logging:

Nftables and the Netfilter logging framework

Nftables logging

If nftables is bringing a lot of changes on user side, this is also true in the logging area.
There is now only one single keyword for logging: log and this target is using the Netfilter logging framework.
A corollary of that is that why you may not see any log messages even if a rule with log is matching because the Netfilter logging framework has to be configured.

Netfilter logging framework

The Netfilter logging framework is a generic way of logging used in Netfilter components. This framework is implemented in two different
kernel modules:

xt_LOG: printk based logging, outputting everything to syslog (same module as the one used for iptables LOG target). It can only log packets for IPv4 and IPv6
nfnetlink_log: netlink based logging requiring to setup ulogd2 to get the events (same module as the one used for iptables NFLOG target). It can log packet for any family.

To use one of the two modules, you need to load them with modprobe. It is possible to have both modules loaded and in this case, you can then setup logging on a per-protocol basis. The active configuration is available for reading in /proc:

# cat /proc/net/netfilter/nf_log 
 0 NONE (nfnetlink_log)
 1 NONE (nfnetlink_log)
 2 nfnetlink_log (nfnetlink_log,ipt_LOG)
 3 NONE (nfnetlink_log)
 4 NONE (nfnetlink_log)
 5 NONE (nfnetlink_log)
 6 NONE (nfnetlink_log)
 7 nfnetlink_log (nfnetlink_log)
 8 NONE (nfnetlink_log)
 9 NONE (nfnetlink_log)
10 nfnetlink_log (nfnetlink_log,ip6t_LOG)
11 NONE (nfnetlink_log)
12 NONE (nfnetlink_log)

The syntax is the following FAMILY ACTIVE_MODULE (AVAILABLE_MODULES). Here nfnetlink_log was loaded first and xt_LOG was loaded afterward (xt_LOG is aliased to ipt_LOG and ip6t_LOG).

Protocol family numbers can look a bit strange. It is in fact mapped on the socket family name that is used in underlying code.
The list is the following:

#define AF_UNSPEC	0
#define AF_UNIX		1	/* Unix domain sockets 		*/
#define AF_INET		2	/* Internet IP Protocol 	*/
#define AF_AX25		3	/* Amateur Radio AX.25 		*/
#define AF_IPX		4	/* Novell IPX 			*/
#define AF_APPLETALK	5	/* Appletalk DDP 		*/
#define	AF_NETROM	6	/* Amateur radio NetROM 	*/
#define AF_BRIDGE	7	/* Multiprotocol bridge 	*/
#define AF_AAL5		8	/* Reserved for Werner's ATM 	*/
#define AF_X25		9	/* Reserved for X.25 project 	*/
#define AF_INET6	10	/* IP version 6			*/
#define AF_MAX		12	/* For now.. */

To update the configuration, you need to write in the file corresponding to the family in /proc/sys/net/netfilter/nf_log/ directory.
For example, if you want to use ipt_LOG for IPv4 (2 in the list), you can do:

echo "ipt_LOG" >/proc/sys/net/netfilter/nf_log/2

This will active ipt_LOG for IPv4 logging:

# cat /proc/net/netfilter/nf_log 
 0 NONE (nfnetlink_log)
 1 NONE (nfnetlink_log)
 2 ipt_LOG (nfnetlink_log,ipt_LOG)
 3 NONE (nfnetlink_log)
 4 NONE (nfnetlink_log)
 5 NONE (nfnetlink_log)
 6 NONE (nfnetlink_log)
 7 nfnetlink_log (nfnetlink_log)
 8 NONE (nfnetlink_log)
 9 NONE (nfnetlink_log)
10 nfnetlink_log (nfnetlink_log,ip6t_LOG)
11 NONE (nfnetlink_log)
12 NONE (nfnetlink_log)

Netfilter framework is used internally by Netfilter for some logging. For example, the connection tracking is using it to send messages when invalid packets are seen. These messages are useful because they contain the reason of the reject. For example, one of the message is “nf_ct_tcp: ACK is under the lower bound (possible overly delayed ACK)”.
This logging messages are only sent if the logging of invalid packet is asked. This is done by doing:

echo "255"> /proc/sys/net/netfilter/nf_conntrack_log_invalid

More information on the magical 255 value are available in kernel documentation of nf_conntrack sysctl.
If nfnetlink_log module is used for the protocol, then the used group is 0. So if you want to activate these messages, it could be a good idea to use non 0 nfnetlink group in the log rules.
This way you will be able to differentiate the log sources in a software like ulogd.

Logging with Nftables

As mentioned before, logging is made via a log keyword. A typical log and accept rule will look like:

nft add rule filter input tcp dport 22 ct state new log prefix \"SSH for ever\" group 2 accept

This rule is accepting packet to port 22 in the state NEW and it is logging them with prefix SSH for ever on group 2.
Here the group is only used when the active logging kernel module is nfnetlink_log. The option has no effect if xt_LOG is used. In fact, when used with xt_LOG, the only available option is prefix (at least for nftables 0.099).

The available options when using nfnetlink_log module are the following (at least for nftables 0.099):

prefix: A prefix string to include in the log message, up to 64 characters long, useful for distinguishing messages in the logs.
group: The netlink group (0 – 2^16-1) to which packets are (only applicable for nfnetlink_log). The default value is 0.
snaplen: The number of bytes to be copied to userspace (only applicable for nfnetlink_log). nfnetlink_log instances may specify their own range, this option overrides it.
queue-threshold: Number of packets to queue inside the kernel before sending them to userspace (only applicable for nfnetlink_log). Higher values result in less overhead per packet, but increase delay until the packets reach userspace. The default value is 1.

Note: the description are extracted from iptables man pages.

If you want to do some easy testing with nftables, simply load xt_LOG module before nfnetlink_log. It will bind to IPv4 and IPv6 protocol and provide you logging. For more fancy stuff involving nfnetlink_log, you can have a look at Using ulogd and JSON output.

Happy logging to all!

Logging connection tracking event with ulogd

Motivation

I’ve recently met @aurelsec and we’ve discussed about the interest of logging connection tracking entries. This is indeed a undervalued information source in a network.

Quoting Wikipedia: “Connection tracking allows the kernel to keep track of all logical network connections or sessions, and thereby relate all of the packets which may make up that connection. NAT relies on this information to translate all related packets in the same way, and iptables can use this information to act as a stateful firewall.”

Connection tracking being linked with Network Address Translation has a direct impact: it stores both side of each connection. If we use conntrack tool from conntrack-tools to list connections:

# conntrack  -L
tcp      6 431999 ESTABLISHED src=192.168.1.129 dst=19.1.16.7 sport=53400 dport=443 src=19.1.16.7 dst=1.2.3.4 sport=443 dport=53500 [ASSURED] mark=0 use=1
...

We have the two sides of a connection:

Orig: here 192.168.1.129:53400 to 19.1.16.7:443. This is the packet information as seen by the firewall when it reaches him. There is no translation at all.
Reply: here 19.1.16.7:443 to 1.2.3.4:53500. This is how will look like a answer coming from the server. The destination has been changed to the public IP of the firewall (here 1.2.3.4). And there is also a change of the destination port to the one used by the firewall when doing the initial mapping. In fact, as multiple client could use the same port at the same time, the firewall may have to rewrite the initial source port.

So the connection tracking stores all NAT transformations. This information is important because this is the only way to know which IP in a private network is responsible of something in the outside world. For example, let’s suppose that 19.1.16.7 has been attacked by our internal client (here 192.168.1.129). If the admin of this server sees the attack, it will only see the 1.2.3.4 IP address and port source 53500. If an authority asks you for the IP address responsible in your internal network you have no instrument but the conntrack to know that this was in fact 192.168.1.129.

That’s why logging connection tracking event is one of the only effective way to store the information necessary to get back to the internal IP address in case of external query. Let’s now do this with ulogd 2.

Ulogd setup

Ulogd installation

Ulogd 2 is able to get information from the connection tracking and to log them in files or database.
If your distribution is not providing ulogd and if you don’t know how to install it, you can check this post Using ulogd and JSON output.
To be sure that you will be able to log connection tracking event, you need to have NFCT plugin to yes at the end of configure output.

Ulogd configuration:
  Input plugins:
    NFLOG plugin:			yes
    NFCT plugin:			yes

Kernel setup

All functionalities are standard since kernel 2.6.14. You only need to load the following module:

modprobe nf_conntrack_netlink

It is the one in charge of kernel and userspace information exchange regarding connection tracking.
It provides features to dump the conntrack table or modify entries in the conntrack. For example the conntrack tool mentioned before is using that communication method to get the listing of connection tracking entries. But the feature that interest us in ulogd is the event mode. For each event in the life of a connection, a message is sent to the userspace. Ulogd is able to listen to these messages and this gives it the ability to store all information on the life of the connection in connection tracking.

Depending on the protocol you have on your network, you may need to run on of the following:

modprobe nf_conntrack_ipv4
modprobe nf_conntrack_ipv6

Ulogd setup

Our first objective will simply be to log all NAT decisions to a syslog-like file on disk. In term of connection tracking, this means we will log all connection in the NEW state. This way we will get information about any packet going through the firewall with the associated NAT transformation.

If you install from sources, copy ulogd.conf at the root of ulogd sources to your config directory (usually /usr/local/etc/. And start your favorite editor on it.

Ulogd is doing logging based on stack definition. A stack is one chain of plugins starting from a input plugin, finishing with an output one, and with filter in the middle. In our case, we want to get packet from Netfilter conntrack and the corresponding plugin is NFCT. The first example of stack containing NFCT in the ulogd.conf file is the one we are interested in, so we uncomment it:

stack=ct1:NFCT,ip2str1:IP2STR,print1:PRINTFLOW,emu1:LOGEMU

We are not sure that the setup of input and output plugin will be correct. For now, let’s just check the output:

[emu1]
file="/var/log/ulogd_syslogemu.log"
sync=1

As you may have seen, emu1 is also used by packet logging. So it may be a good idea that we have our own output file for connection tracking event.
To do that, we update the stack:

stack=ct1:NFCT,ip2str1:IP2STR,print1:PRINTFLOW,emunfct1:LOGEMU

and create a new config below emu1:

[emunfct1]
file="/var/log/ulogd_nfct.log"
sync=1

We have changed file name and keep the sync option which permit to avoid the a delay in write due to buffering effect during write which can be very annoying when debugging a setup.

Now, we can test:

ulogd -v

In /var/log/ulogd_nfct.log, we see things like

Feb 22 10:50:36 ice-age2 [DESTROY] ORIG: SRC=61.174.51.209 DST=192.168.1.129 PROTO=TCP SPT=6000 DPT=22 PKTS=0 BYTES=0 , REPLY: SRC=192.168.1.129 DST=61.174.51.209 PROTO=TCP SPT=22 DPT=6000 PKTS=0 BYTES=0

So we only have destruction messages. This is not exactly what we wanted to have. We are interested in NEW message that will allow us to have a correct timing of the event. Reading ulogd.conf file, it seems there is no information about chossing the event types. But let’s ask to the NFCT input plugin its capabilities. To do that we use option -i of ulogd:

# ulogd -v -i /usr/local/lib/ulogd/ulogd_inpflow_NFCT.so 
Name: NFCT
Config options:
        Var: pollinterval (Integer, Default: 0)
        Var: hash_enable (Integer, Default: 1)
        Var: hash_buckets (Integer, Default: 8192)
        Var: hash_max_entries (Integer, Default: 32768)
        Var: event_mask (Integer, Default: 5)
        Var: netlink_socket_buffer_size (Integer, Default: 0)
        Var: netlink_socket_buffer_maxsize (Integer, Default: 0)
        Var: netlink_resync_timeout (Integer, Default: 60)
        Var: reliable (Integer, Default: 0)
        Var: accept_src_filter (String, Default: )
        Var: accept_dst_filter (String, Default: )
        Var: accept_proto_filter (String, Default: )
...

The listing start with the configuration keys. One of them is event_mask. This is a the one controlling which events are sent from kernel to userspace.
The value is a mask combining some of the following values:

NF_NETLINK_CONNTRACK_NEW: 0x00000001
NF_NETLINK_CONNTRACK_UPDATE: 0x00000002
NF_NETLINK_CONNTRACK_DESTROY: 0x00000004

So default value of 5 is to listen to NEW and DESTROY events.
Clever reader will then ask: why did we only see DESTROY messages in that case. This is because ulogd NFCT plugin is running by default in hash_enable mode. In this mode, one single message is output for each connection (at end) and a hash is maintained in the kernel to store the info (here initial timestamp of the connection).
Our setup don’t need this feature because we only want to get the NAT transformation so we switch the hash feature off and limit the events to NEW:

[ct1]
event_mask=0x00000001
hash_enable=0

We can now restart ulogd and check the log file:

Feb 22 11:59:34 ice-age2 [NEW] ORIG: SRC=2a01:e35:1394:5bd0:da50:b6ff:fe3c:4250 DST=2001:41d0:1:9598::1 PROTO=TCP SPT=51162 DPT=22 PKTS=0 BYTES=0 , REPLY: SRC=2001:41d0:1:9598::1 DST=2a01:e35:1394:5bd0:da50:b6ff:fe3c:4250 PROTO=TCP SPT=22 DPT=51162 PKTS=0 BYTES=0
Feb 22 11:59:43 ice-age2 [NEW] ORIG: SRC=192.168.1.129 DST=68.232.35.139 PROTO=TCP SPT=60846 DPT=443 PKTS=0 BYTES=0 , REPLY: SRC=68.232.35.139 DST=1.2.3.4 PROTO=TCP SPT=443 DPT=60946 PKTS=0 BYTES=0

This is exactly what we wanted, we have a trace of all NAT transformation.

Maintain an history of connection tracking

Objective

We want to log all information describing a connection so we have a trace of what is going on the firewall. This means we need at least:

IP information for orig and reply way
Timestamp of start and end of connection
Bandwidth used by the connection

Kernel setup

By default, recent kernel have a limited handling of connection tracking. Some useful fields are not stored for performance reason. This is the case of the accounting (number of packets and bytes) and the case of the timestamp of the connection creation. The advantage of getting accounting information is trivial as you get information on bandwidth usage. Regarding timestamp, the interest is on implementation side. It allows ulogd to get all information needed for describing a connection in one single message (the DESTROY one). And ulogd does not need anymore to maintain a hash table to get the info and propagate it at exit.

To activate both features, you have to do:

 echo "1"> /proc/sys/net/netfilter/nf_conntrack_acct
 echo "1"> /proc/sys/net/netfilter/nf_conntrack_timestamp

Ulogd setup

For following setup, you will need ulogd build from git or a ulogd at a version superior or equal to 2.0.4.

Let’s first use JSON output to get the information in a readable format. We need to define a stack:

stack=ct2:NFCT,ip2str1:IP2STR,jsonnfct1:JSON

On ct2 side, we don’t want to use the hash and we only want to get DESTROY message, so our configuration looks like:

[ct2]
hash_enable=0
event_mask=0x00000004

Regarding, jsonnfct1 we could have reused the default JSON configuration but for ease of testing we will dedicate a file to the NFCT logging:

[jsonnfct1]
sync=1
file="/var/log/ulogd_nfct.json"

After a ulogd restart, we’ve got this type of entries:

{"reply.ip.daddr.str": "2a01:e35:1394:5ad0:da50:e6ff:fe3c:1250", "oob.protocol": 0, "dvc": "Netfilter", "timestamp": "Sat Feb 22 12:27:04 2014", "orig.ip.protocol": 6, "reply.raw.pktcount": 20, "flow.end.sec": 1393068424, "orig.l4.sport": 51384, "orig.l4.dport": 22, "orig.raw.pktlen": 5600, "ct.id": 1384991512, "orig.raw.pktcount": 23, "reply.raw.pktlen": 4328, "reply.ip.protocol": 6, "reply.l4.sport": 22, "reply.l4.dport": 51384, "ct.mark": 0, "ct.event": 4, "flow.start.sec": 1393068302, "flow.start.usec": 637516, "flow.end.usec": 403240, "reply.ip.saddr.str": "2001:41d0:1:9598::1", "oob.family": 10, "src_ip": "2a01:e35:1394:5ad0:da50:e6ff:fe3c:1250", "dest_ip": "2001:41d0:1:9598::1"}

The fields we wanted are here:

flow.start.* keys store the timestamp of flow start
flow.end.* keys store the end of the connection
*.raw.pkt* keys store the accounting information

You can then add this file to the file parsed by logstash. For that if you can use information from Using ulogd and JSON output and modify the input section:

input {
   file { 
      path => [ "/var/log/ulogd.json", "/var/log/ulogd_nfct.json"]
      codec =>   json 
   }
}

One interesting information in a connection tracking entry is the duration. But the field is not available in ulogd JSON output and it is not possible to do mathematical operations in Kibana. A solution to get the information is to add a filter in logstash.conf to compute the duration:

filter {
  if [type] == "json-log" {
    ruby {
      code => "if event['ct.id']; event['flow.duration.sec']=(event['flow.end.sec'].to_i - event['flow.start.sec'].to_i); end"
    }
  }
}

A thing to notice to understand the obtained duration is that a connection is dying following contextual timeout. For example, in the case of a TCP connection, even after a FIN packet there’s a timeout applied. So a short connection will at least be of the duration of the timeout.

An other logging method is PostgreSQL. The stack to use is almost the same as JSON one but use, as you may have guess, the PGSQL plugin:

stack=ct2:NFCT,ip2str1:IP2STR,pgsql2:PGSQL

The configuration of the PostgreSQL plugin is easy based on the setup available in the configuration:

[pgsql2]
db="nulog"
host="localhost"
user="nupik"
table="ulog2_ct"
#schema="public"
pass="changeme"
procedure="INSERT_CT"

I’m not the one who will explain how to connect to a PostgreSQL database and create a ulogd2 database. See Pollux post for that: ulogd2: the new userspace logging daemon for netfilter/iptables (part 2)

Other setup are possible. For example, you can maintain a copy of the connection tracking table in the database and also keep the history.
To do that you need to use the INSERT_OR_REPLACE_CT procedure and a connection tracking INPUT plugin not using the hash table but getting NEW and DESTROY events:

stack=ct2:NFCT,ip2str1:IP2STR,pgsql2:PGSQL

[ct2]
hash_enable=0

[pgsql2]
db="nulog"
host="localhost"
user="nupik"
table="ulog2_ct"
#schema="public"
pass="changeme"
procedure="INSERT_OR_REPLACE_CT"

Connection will be inserted in the table when getting the NEW event and the connection entry in the database will be updated when the DESTROY message will be received.

Suricata and Nftables

Iptables and suricata as IPS

Building a Suricata ruleset with iptables has always been a complicated task when trying to combined the rules that are necessary for the IPS with the firewall rules. Suricata has always used Netfilter advanced features allowing some more or less tricky methods to be used.

For the one not familiar with IPS using Netfilter, here’s a few starting points:

IPS receives the packet coming from kernel via rules using the NFQUEUE target
The IPS must received all packets of a given flow to be able to handle detection cleanly
The NFQUEUE target is a terminal target: when the IPS verdicts a packet, it is or accepted (and leave current chain)

So the ruleset needs to send all packets to the IPS. A basic ruleset for an IPS could thus looks like:

iptables -A FORWARD -j NFQUEUE

With such a ruleset, all packets going through the box are sent to the IPS.

If now you want to combine this with your ruleset, then usually your first try is to add rules to the filter chain:

iptables -A FORWARD -j NFQUEUE
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED -j ACCEPT
# your firewall rules here

But this will not work because of point 2: All packets sent via NFQUEUE to the IPS are or blocked or if accepted, leave the FORWARD chain directly and are going for evaluation to the next chain (mangle POSTROUTING in our case).
With such a ruleset, the result is that there is no firewall but an IPS in place.

As mentioned before there is some existing solutions (see Building a Suricata ruleset for extensive information). The simplest one is to dedicated one another chain such as mangle to IPS:

iptables -A FORWARD -t mangle -j NFQUEUE
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED -j ACCEPT
# your firewall rules here

No conflict here but you have to be sure nothing in your system will use the the mangle table or you will have the same problem as the one seen previously in the filter chain. So there was no universal and simple solution to implement an IPS and a firewall ruleset with iptables.

IPS the easy way with Nftables

In Nftables, chains are defined by the user using nft command line. The user can specify:

The hook: the place in packet life where the chain will be set. See this diagram for more info.
- prerouting: chain will be placed before packet are routed
- input: chain will receive packets going to the box
- forward: chain will receive packets routed by the box
- postrouting: chain will receive packets after routing and before sending packets
- output: chain will receive packet sent by the host
The chain type: define the objective of the chain
- filter: chain will filter packet
- nat: chain will only contains NAT rules
- route: chain is containing rule that may change the route (previously now as mangle)
The priority: define the evaluation order of the different chains of a given hook. It is an integer that can be freely specified. But it also permits to place chain before or after some internal operation such as connection tracking.

In our case, we want to act on forwarded packets. And we want to have a chain for filtering followed by a chain for IPS. So the setup is simple of chain is simple

nft -i
nft> add table filter
nft> add chain filter firewall { type filter hook forward priority 0;}
nft> add chain filter IPS { type filter hook forward priority 10;}

With this setup, a packet will reach the firewall chain first where it will be filtered. If the packet is blocked, it will be destroy inside of the kernel. It the packet is accepted it will then jump to the next chain following order of increasing priority. In our case, the packet reaches the IPS chain.

Now, that we’ve got our chains we can add filtering rules, for example:

nft add rule filter firewall ct state established accept
nft add rule filter firewall tcp dport ssh counter accept
nft add rule filter firewall tcp dport 443 accept
nft add rule filter firewall counter log drop

And for our Suricata IPS, that’s just trivial:

nft add rule filter IPS queue

A bit more details

The queue target in nftables

The complete support for the queue target will be available in Linux 3.14. The syntax looks as follow:

nft add rule filter output queue num 3 total 2 options fanout

This rule sends matching packets to 2 load-balanced queues (total 2) starting at 3 (num 3).
fanout is one of the two queue options:

fanout: When used together with total load balancing, this will use the CPU ID as an index to map packets to the queues. The idea is that you can improve perfor mance if there’s a queue per CPU. This requires total with a value superior to 1 to be specified.
bypass: By default, if no userspace program is listening on an Netfilter queue,then all packets that are to be queued are dropped. When this option is used, the queue rule behaves like ACCEPT instead, and the packet will move on to the next table.

For a complete description of queueing mechanism in Netfilter see Using NFQUEUE and libnetfilter_queue.

If you want to test this before Linux 3.14 release, you can get nft sources from nftables git and use next-3.14 branch.

Chain priority

For reference, here are the priority values of some important internal operations and of iptables static chains:

NF_IP_PRI_CONNTRACK_DEFRAG (-400): priority of defragmentation
NF_IP_PRI_RAW (-300): traditional priority of the raw table placed before connection tracking operation
NF_IP_PRI_SELINUX_FIRST (-225): SELinux operations
NF_IP_PRI_CONNTRACK (-200): Connection tracking operations
NF_IP_PRI_MANGLE (-150): mangle operation
NF_IP_PRI_NAT_DST (-100): destination NAT
NF_IP_PRI_FILTER (0): filtering operation, the filter table
NF_IP_PRI_SECURITY (50): Place of security table where secmark can be set for example
NF_IP_PRI_NAT_SRC (100): source NAT
NF_IP_PRI_SELINUX_LAST (225): SELInux at packet exit
NF_IP_PRI_CONNTRACK_HELPER (300): connection tracking at exit

For example, one can create in nftables an equivalent of the raw PREROUTING chain of iptables by doing:

# nft -i
nft> add chain filter pre_raw { type filter hook prerouting priority -300;}

Using ulogd and JSON output

Ulogd and JSON output

In February 2014, I’ve commited a new output plugin to ulogd, the userspace logging daemon for Netfilter. This is a JSON output plugin which output logs into a file in JSON format. The interest of the JSON format is that it is easily parsed by software just as logstash. And once data are understood by logstash, you can get some nice and useful dashboard in Kibana:

This post explains how to configure ulogd and iptables to do packet logging and differentiate accepted and blocked packets. If you want to see how cool is the result, just check my post: Investigation on an attack tool used in China.

Installation

At the time of this writing, the JSON output plugin for ulogd is only available in the git tree. Ulogd 2.0.4 will contain the feature.

If you need to get the source, you can do:

git clone git://git.netfilter.org/ulogd2

Then the build is standard:

./autogen.sh
./configure
make
sudo make install

Please note that at the end of the configure, you must see:

Ulogd configuration:
  Input plugins:
    NFLOG plugin:			yes
...
    NFACCT plugin:			yes
  Output plugins:
    PCAP plugin:			yes
...
    JSON plugin:			yes

If the JSON plugin is not build, you need to install libjansson devel files on your system and rerun configure.

Configuration

Ulogd configuration

All the edits are made in the ulogd.conf file. With default configure option the file is in /usr/local/etc/.

First, you need to activate the JSON plugin:

plugin="/home/eric/builds/ulogd/lib/ulogd/ulogd_output_JSON.so"

Then we define two stacks for logging. It will be used to differentiate accepted packets from dropped packets:

stack=log2:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,mac2str1:HWHDR,json1:JSON
stack=log3:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,mac2str1:HWHDR,json1:JSON

The first stack will be used to log accepted packet, so we the numeric_label to 1 in set in [log2].
In [log3], we use a numeric_label of 0.

[log2]
group=1 # Group has to be different from the one use in log1
numeric_label=1

[log3]
group=2 # Group has to be different from the one use in log1/log2
numeric_label=0 # you can label the log info based on the packet verdict

The last thing to edit is the configuration of the JSON instance:

[json1]
sync=1
device="My awesome FW"
boolean_label=1

Here we say we want log and write on disk configuration (via sync) and we named our device My awesome FW.
Last value boolean_label is the most tricky. It this configuration variable is set to 1, the numeric_label will
be used to decide if a packet has been accepted or blocked. It this variable is set non null, then the packet is seen as allowed.
If not, then it is seen as blocked.

Sample Iptables rules

In this example, packets to port 22 are logged and accepted and thus are logged in nflog-group 1. Packet in the default drop rule are sent to group 2 because they are dropped.

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT ! -i lo -p tcp -m tcp --dport 22 --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j NFLOG --nflog-prefix  "SSH Attempt" --nflog-group 1
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -j ACCEPT
iptables -A INPUT -j NFLOG --nflog-prefix  "Input IPv4 Default DROP" --nflog-group 2

There is no difference in IPv6, we just use nflog-group 1 and 2 with the same purpose:

ip6tables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
ip6tables -A INPUT ! -i lo -p tcp -m tcp --dport 22 --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j NFLOG --nflog-prefix  "SSH Attempt" --nflog-group 1
ip6tables -A INPUT ! -i lo -p ipv6-icmp -m icmp6 --icmpv6-type 128 -m state --state NEW -j NFLOG --nflog-prefix  "Input ICMPv6" --nflog-group 1
ip6tables -A INPUT -p ipv6-icmp -j ACCEPT
ip6tables -A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -j ACCEPT
ip6tables -A INPUT -i lo -j ACCEPT
ip6tables -A INPUT -j NFLOG --nflog-prefix  "Input IPv6 Default DROP" --nflog-group 2

Logstash configuration

Logstash configuration is simple. You must simply declare the ulogd.json file as input and optionaly you can activate geoip on the src_ip key:

input {
   file { 
      path => [ "/var/log/ulogd.json"]
      codec =>   json 
   }
}

filter {
  if [src_ip]  {
    geoip {
      source => "src_ip"
      target => "geoip"
      add_field => [ "[geoip][coordinates]", "%{[geoip][longitude]}" ]
      add_field => [ "[geoip][coordinates]", "%{[geoip][latitude]}"  ]
    }
    mutate {
      convert => [ "[geoip][coordinates]", "float" ]
    }
  }
}

output { 
  stdout { codec => rubydebug }
  elasticsearch { embedded => true }
}

Usage

To start ulogd in daemon mode, simply run:

ulogd -d

You can download logstash from their website and start it with the following command line:

java -jar logstash-1.3.3-flatjar.jar agent -f etc/logstash.conf --log log/logstash-indexer.out -- web

Once done, just point your browser to localhost:9292 and enjoy nice and interesting graphs.

Investigation on an attack tool used in China

Log analysis experiment

I’ve been playing lately with logstash using data from the ulogd JSON output plugin and the Suricata full JSON output as well as standard system logs.

Ulogd is getting Netfilter firewall logs from Linux kernel and is writing them in JSON format. Suricata is doing the same with alert and other traces. Logstash is getting both log as well as sytem log. This allows to create some dashboard with information coming from multiple sources. If you want to know how to configure ulogd for JSON output check this post. For suricata, you can have a look at this one.

Ulogd output is really new and I was experimenting with it in Kibana. When adding some custom graphs, I’ve observed some strange things and decided to investigate.

Displaying TCP window

TCP window size at the start of the connection is not defined in the RFC. So every OSes have choozen their own default value. It was thus looking interesting to display TCP window to be able to find some strange behavior. With the new ulogd JSON plugin, the window size information is available in the tcp.window key. So, after doing a query on tcp.syn:1 to only get TCP syn packet, I was able to graph the TCP window size of SYN packets.

Most of the TCP window sizes are well-known and correspond to standard operating systems:

65535 is or MacOSX or some MS Windows OS.
14600 is used by some Linux.

The first uncommon value is 16384. Graph are clickable on Kibana, so I was at one click of some interesting information.

First information when looking at dashboard after selection TCP syn packet with a window size of 16384 was the fact, it was only ssh scanning:

Second information is the fact that, according to geoip, all IPs are chinese:

A SSH scanning software

When looking at the details of the attempt made on my IP, there was something interesting:

For all hosts, all requests are done with the same source port (6000). This is not possible to do that with a standard ssh client where the source port is by default choosen by the operating system. So or we have a custom standard software that perform a bind operation to port 6000 at socket creation. This is possible and one advantage would be to be easily authorized through a firewall if the country had one. Or we could have a software developped with low level (RAW) sockets for performance reason. This would allow a faster scanning of the internet by skipping OS TCP connection handling. There is a lot of posts regarding the usage of port 6000 as source for some scanning but I did not find any really interesting information in them.

On suricata side, most of the source IPs are referenced in ET compromised rules:

Analysing my SSH logs, I did not see any trace of ssh bruteforce coming from source port 6000. But when selecting an IP, I’ve got trace of brute force from at least one of the IP:

These attackers seems to really love the root account. In fact, I did not manage to find any trace of attempts for user different than root for IP address that are using the port 6000.

Getting back to my ulogd dashboard, I’ve displayed more info about the used scanning sequence:

The host scans the box using a scanner using raw socket, then it attacks with a few minutes later with SSH bruteforce tool. The bruteforce tool has a TCP window size at start of 65535. It indicates that a separated software is used for scanning. So we should have an queueing mechanism between the scanner and the bruteforce tool. This may explains the duration between the scan and the bruteforce. Regarding TCP window size value, 65535 seems to indicate a Windows server (which is coherent with TTL value).

Looking at the scanner traffic

Capturing a sample traffic did not give to much information. This is a scanner sending a SYN and cleanly sending a reset when it got the SYN, ACK:

14:27:54.982273 IP (tos 0x0, ttl 103, id 256, offset 0, flags [none], proto TCP (6), length 40)
    218.2.22.118.6000 > 192.168.1.19.22: Flags [S], cksum 0xa525 (correct), seq 9764864, win 16384, length 0
14:27:54.982314 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 44)
    192.168.1.19.22 > 218.2.22.118.6000: Flags [S.], cksum 0xeee2 (correct), seq 2707606274, ack 9764865, win 29200, options [mss 1460], length 0
14:27:55.340992 IP (tos 0x0, ttl 111, id 14032, offset 0, flags [none], proto TCP (6), length 40)
    218.2.22.118.6000 > 192.168.1.19.22: Flags [R], cksum 0xe48c (correct), seq 9764865, win 0, length 0

But it seems the RST packet after the SYN, ACK is not well crafted:

More info on SSH bruteforce tool

Knowing the the behavior was scanning from 6000 and starting a normal scanning, I’ve focused the Suricata dashboard on one IP to see if I had some more information:

One single IP in the list of the scanning host is triggering multiple alerts. The event table confirmed this:

Studying the geographical repartition of the Libssh alert, it appears there is used in other countries than China:

So, libssh is not a discriminatory element of the attacks.

Conclusion

A custom attack tool has been been deployed on some Chinese IPs. This is a combination of a SSH scanner based on RAW socket and a SSH bruteforce tool. It tries to gain access to the root account of system via the ssh service. On an organisational level, it is possible there is a Chinese initiative trying to get the low-hanging fruit (system with ssh root account protected by password) or maybe it is just a some organization using some compromised Chinese IPs to try to get control other more boxes.