Changing traffic policies of DHCPv4 Filtering STREAMS Module at run-time using Solaris MDB

Overview
What will you do, if you are running a mission critical DHCP server and you want to adjust some traffic policies of dhcpmod kernel module without process restart?

The answer is: by using Solaris MDB.

            Finding out where the traffic policies reside

            It is assumed that you have already heard about Solaris STREAMS or even more have read STREAMS Programming Guide.

The basic concept is that Solaris exports special DDI interfaces for working with queues (queue_t). We use reading queue parameter q_ptr which stores the address to user-specific pointer - a structure created by routine mstrmod_open() when the queue is opened. In the case of dhcpmod all traffic policy parameters resides as unsigned integers in that structure. You can consult the dhcpmod's sources for more details.

Firstly you should look for dhcpmod in system streams cache:

mdb -k

> ::walk stream_head_cache | ::stream

<output stripped>

| ^

v |

+-----------------------+-----------------------+

| 0xffffffff9f2ce648| 0xffffffff9f2ce550|

| dhcpmod | dhcpmod |

| | |

| cnt = 0t0 | cnt = 0t0 |

| flg = 0x00000822 | flg = 0x00000832|

+-----------------------+-----------------------+

| ^

<output stripped>

0xffffffff9f2ce550 is a pointer to reading queue of filtering instance 1.

We need to print more information about it:

> ffffffff9f2ce550::print queue_t

{

q_qinfo = mstrmod_rinit

q_first = 0

q_last = 0

q_next = 0xffffffffa1546008

q_link = 0

q_ptr = 0xfffffe86a09d7400

q_count = 0

q_flag = 0x832

q_minpsz = 0

q_maxpsz = 0xffffffffffffffff

q_hiwat = 0

q_lowat = 0

q_bandp = 0

q_lock = {

_opaque = [ 0 ]

}

q_stream = 0xffffffffa1558d90

q_syncq = 0xffffffff9f2ce740

q_nband = 0

q_wait = {

_opaque = 0

}

q_sync = {

_opaque = 0

}

q_nfsrv = 0xfffffe86f89d0aa8

q_nbsrv = 0

q_draining = 0

q_struiot = 0xffff

q_syncqmsgs = 0

q_mblkcnt = 0

q_sqhead = 0

q_sqtail = 0xffffffff91e1c600

q_sqflags = 0

q_rwcnt = 0

q_sqnext = 0

q_sqprev = 0

q_sqtstamp = 0x3a55c18

q_qtstamp = 0xbaddcafebaddcafe

q_spri = 0

q_fp = 0xfffffe8683fca0b0

}

The parameter q_flag is from struct module_info and is equal 0x832 (for module dhcpmod). About STREAMS structures you can consult <sys/stream.h> for more details.

The parameter q_qinfo = mstrmod_rinit is our reading queue's primitive which confirms we are in the right place. The parameter q_ptr (0xfffffe86a09d7400) is our structure.

Now we dump what first 4 paragraphs (in MDB terms) of structure contains:

> 0xfffffe86a09d7400::dump -eq -w 4

fffffe86a09d7400:

9e9e82b0 ffffffff 00000000 00000000

8f81835 00000001 00000001 00000001

0000000a 00000005 00000001 00000000

000000ff 00000000 48a36f90 fffffe82

Offset 18 is the runtime-dependent instance ID of our queue (you can verify it's value via kstat as discussed earlier). If it isn't what you've requested, you should iterate via ::walk stream_head_cache to the next instance until succeeded.

> 0xffffffe86a09d7400+18/B

0xffffffe86a09d7400: 1

Offset 20 contains allowed packets per minute parameter. Offset 30 shows discard by rate policy parameter.

Note that for 32-bit kernel offset values will be different. Specifically, instance ID offset will be 9, allowed packets per minute offset will be 10, discard by rate policy offset will be 15.

Now we can accomplish our tasks.

Task 1. Turn off discard by rate policy.

Verifying discard by rate policy's current setting:

> 0xffffffe86a09d7400+30/B

0xffffffe86a09d7400: ff

You see ff → 255

Writing a new value:

> 0xffffffe86a09d7400+30/W 0x0

0xffffffe86a09d7400: 0xff = 0x0

Task 2. Increasing allowed packets per minute value

Verifying allowed packets per minute value:

> 0xffffffe86a09d7400+20/B

0xffffffe86a09d7400: a

You see a → 10

To increase allowed packets per minute value from 10 to 20 (0x14), type:

> 0xffffffe86a09d7400+20/W 0x14

0xffffffe86a09d7400: 0xa = 0x14

Protecting DHCP server against DoS attacks by creating specialized Solaris kernel STREAMS module

Overview

This article describes how writing kernel STREAMS module could solve DoS various issues faced by user-space DHCP server application that acts in heavy traffic condition. This software was tested on ORACLE Solaris 10 x64 and sparc platform. You can download source code. It ought to be used in conjunction with Internet System Consortium DHCP server. Patches for ISC DHCP software can be downloaded.

Abstract

Hosts in IP networks must be configured before it can communicate between them. Historically this task is solved by manual configuration of each host or via DHCP protocol.

DHCP protocol provides hosts automatically with the most essential configuration (but not limited to!) which is obtained from one or more DHCP servers: IP address, IP network mask, IP of default gateway, cache DNS servers etc.

The well-known scenario is the following : DHCP-aware clients send requests message to the DHCP server. After receiving valid request DHCP server, that manages a pool of IP addresses and other client-related information, assigns IP configuration parameters which is accepted by client via DHCP reply. DHCP traffic uses UDP broadcast or unicast.

Typical DHCP server implementation (ex. ISC DHCP) is single-threaded process that handles incoming DHCP requests and provides the reply packets if needed. Since it uses UDP, it can be easy compromised by sending a large amount of traffic. This leads packets not to be processed and getting dropped on high input load. Moreover the existence of bad implemented DHCP clients in modern IP networks significantly increases the probability of such situation. It is impossible to control which kind of devices are connected to the network.

The common approach to the high load issue has at least two ways:

• More threads are processing more traffic

Solaris bundled DHCP server is multi-threaded application, and so load of ingress traffic is redistributed between its threads thus reducing the negative effects of attacks if not solving it completely.

As to single-threaded DHCP software products (such as ISC DHCP), significant effort on re-writing of DHCP server software is required in order to have multhread support. Also to process all traffic a vast part of it could be a mess, can't be considered a suitable option.

• Localize attacker sources and drop before it reach DHCP server
  Standard IP firewalls are of little use, because packets to be filtered all come from small number of DHCP relays. DHCP packets passed through DHCP relay has a common source - IP address of relay. In order to archive the goal more detail about DHCP will be discussed.

Known practices against DHCP DoS

Many modern ISPs often use DHCP relaying for serving many IP subnets on centralized DHCP servers. In that case DHCP relay agent software installed on low-end edge equipment assumes control on traffic retransmission of their network segment. Some vendors (but not all) have already implemented DHCP snooping feature which takes measures against some types of attacks including DHCP DoS. This feature has an disadvantage that it is error prone, because it is required all DSLAMs/FTTH switches configured. The configuration can be more burdensome if VLAN trunking is used, because it must be configured per each VLAN. However the worst disadvantage is that feature cannot prevent flood from an already authorized DHCP client which can send a huge amount of completely justified (and indispensable) DHCP client traffic.

Some vendors have implemented a feature which addresses flood issue. It is so called DHCP sending rate (or rate-limit) feature which places client port state in administratively down for a reasonable period of time if the number of DHCP packets originated from a single port exceeds configured limit. But this prevents that customer from functioning properly until the attack is finished. So technical support will be involved in problem solving even if the port is unblocked automatically after some time.

Discussed techniques help to stop excess of traffic at access equipment edge, but requires special implementation efforts on the hardware vendor side, which isn't suitable in multivendor environments.

Modern ISPs requires more robust, centralized, vendor independent and free of human factor solution.

Module dhcpmod topology scheme

It is proposed to stop traffic at DHCP server before it enters into DHCP server software. That option means to move a part of packet processing into the kernel.

For accessing traffic ISC DHCP under Solaris uses Data Link Provider Interface (DLPI). DLPI V2 enables a data link service user (DHCP) to access and uses any of a variety of conforming data link service providers. ISC DHCP uses Data Link Service (DLS) provider V2. It opens a Ethernet-capable device, creates a stream between DLS provider and DLS user and then registers the desired Physical Point of Attachment (PPA).

New software is a multi-threaded STREAMS module (named as dhcpmod).

Fig. 1 presents module insertion scheme.

For filtering DHCP traffic if you have Solaris zones with DHCP installed in, another technology was choosed – Solaris Packet Filtering HOOK. It will be discussed later in the upcoming article. Why we cannot use it for DLPI-enabled DHCP? Because DLPI acts at lower level than PF HOOK interception entry points and so packet discarding for DLPI-aware clients is impossible.

Processing DHCP packets as STREAMS mblk

STREAMS reading queue delivers it with IP header (8 bytes long), followed by UDP header (20 bytes long). Start of packet is pointed by b_rptr and end of packet respectively by b_wptr.

Module dhcpmod firstly checks for IP version and headers length. All packets are discarded silently if are not directed to UDP port 67 or contains positive values of IP fragmentation offset, or IP flag is non-zero and not equal to IP_DF (don't fragment). Bit alignment checks are included too.

DHCP packet is composed of two parts:

• BOOTP header
  which is constantly in size and has predetermined number of BOOTP parameters
• DHCP Options which are not limited by the length of the value. Each option has a specific opcode that represents the type. The length specifies the length of the value. The length field is followed by the value. The set of options are appended to the DHCP packet after the BOOTP part of the packet. The options can be present in any order.

Workflow within kernel module dhcpmod

The reading queue (RQ) is the core element. The basic logic is to extract the most interesting parameters such as source IP address (in DHCP relay environment it helps to distinguish between IP subnets), Client-Ethernet-Address which represents MAC address of DHCP client equipment, DHCP Option 82 parameter (usually used for binding of IP address to physical port on low-end). Last parameter contains so called Agent Remote ID and Agent Circuit ID (at least one of them).

These parameters approximates packet information by using CRC32 algorithm incorporated in Solaris kernel.

Next action is to store calculated combined parameter in a HASH table and packet rating counters as well.

Fig. 2 shows the workflow briefly

For periodic cleanups of HASH table, special callback routine is installed. It is scheduled twice a second via qtimeout(). HASH table size is auto-adjusted (step is the power of two) while it grows to keep its effectiveness.

Management policies in dhcpmod

The module is configured using custom IOCTLS passed to a writing queue (WR). Accepted IOCTL argument is a pointer to uint_t variable. All policies can be set for the current queue instance.

The following traffic policies are supported:

/*

* Ioctls.

*/

#define DHCPIOC ('D' << 8)

/* dropping rate per min */

#define DHCPIOCSDROPANYPPM (DHCPIOC|1)

/* dropping policy if no DHCP Option 82 info */

#define DHCPIOCSDROPPOLIFNORA (DHCPIOC|2)

/* dropping rate per min if no DHCP Option 82 info */ 

#define DHCPIOCSDROPNORAPPM (DHCPIOC|3) 

/* dropping policy for all packets if no DHCP Option 82 info */

#define DHCPIOCSDROPPOLALLNORA (DHCPIOC|4)

/* dropping policy by rate */ 

#define DHCPIOCSDROPPOLBYPPM (DHCPIOC|5) 

DHCPIOCSDROPANYPPM Allowed number of packets for discard packets by rate

DHCPIOCSDROPPOLIFNORA Turning off/on discard packets by rate policy if no DHCP Option 82 information

DHCPIOCSDROPNORAPPM Allowed number of packets if no DHCP Option 82 information

DHCPIOCSDROPPOLALLNORA Turning off/on discard all packets policy with no DHCP Option 82

DHCPIOCSDROPPOLBYPPM Turning off/on discard packets by rate policy

Conclusion

In this article an effort was presented towards implementing new DHCPv4 filtering software to run as a STREAMS module in the Solaris kernel. This software automatically prevents DoS on DHCP server without wasting CPU cycles of DHCP server process.

Hence this solution will add some processing delay. It is supposed that DHCP server application uses DLPI and some filtering work is already done in the kernel space: after attaching to PPA it filters only destination UDP port 67. So an inserting of new module will increase the processing delay imperceptibly.

Also this opens a new room for further enhancements: discarding packets without certain DHCP options etc.

Note that this solution is centralized, vendor independent and can be treated as a part of protecting solution for DHCP large network scheme. It was built for use in conjunction with DHCP snooping and DHCP Option 82 features.

DHCP traffic statistics datasheet exposed by DHCPv4 Filtering STREAMS Module via KSTAT infrastructure

          Overview

This software is a STREAMS module that subjects BOOTP packets arriving on its read queue according to special filtering algorithm and passes only those packets that the filter accepts on to its upstream consumer – DHCP server process. In such a way the majority of DOS/DDOS attacks and flood don't influence the resources of DHCP server software which lies in user space.

The ORACLE Solaris Operating Environment (OE) kernel provides a set of functions and data structures - named KSTAT - for device drivers and other kernel modules to export module-specific statistics to the outside world. Current software version of DHCPv4 filter supports KSTAT for extracting its statistics about the processed DHCP traffic.

KSTAT instance behavior

DHCPv4 filter software can provide filtering services to many concurrent DHCP server process. Each of them has a separate inbound entry accessible via kstat utility (class net, module dhcpmod).When the instance is unloaded, the entry still remains in the system making the statistical data available for further analysis. Instance state is denoted by field state which can be 0 if filter instance is uninitialized, 1 – if it is running, 2 – if it is stopped, 3 – if it is failed to start.

Statistic counters

Module dhcpmod maintains and reports the following statistics. All statistics are maintained as unsigned. The statistics are 64 bits unless otherwise noted.

buffer errors

Shows how many errors while packet processing took place. Should be always 0.

cache buckets

Shows how many buckets allocated for HASH tables. Large values indicates traffic bursts.

cache errors

Shows how many errors/miss-consistences allocated for HASH tables. Should be always 0.

cache expired

Denotes cache expiration events.

cache hits

Denotes cache hits events. Should be more then cache misses in factor of 10 at least. May be less for the first ten minutes since instance startup.

cache misses

Denotes cache misses (not found) events.

cache records

Shows actual HASH usage. Large values indicates large traffic bursts.

discarded packets

Denotes discard events since instance startup.

discarded packets per sec

Denotes actual discard events rate. The filter effectiveness depends on that value.

discarded rate limit packets

Denotes actual discard events rate limited. Continuously high values means flood presence.

failure packets

Shows how many packet failed to be processed by traffic filter and were passed in to upstream neighbor. Should be always 0.

fragmented IP packets

Number of fragmented IPv4 packets. Such traffic is discarded silently.

input packets

Number of packets received from NIC.

input packets per sec

Rate of packets received from NIC.

invalid BOOTP packets

Number of BOOTP packets that violates RFC 2132. Such traffic is discarded silently.

invalid IP packets

Number of broken IPv4 packets. Such traffic is discarded silently.

invalid UDP packets

Number of broken UDP packets or its check sums. Such traffic is discarded silently.

malformed packets

Total number of the packets that cannot be processed normal due to its corruption or standard non-conformance. Such traffic is discarded silently.

no memory errors

For internal use. Signals not enough memory in system. Should be always 0.

non-def BOOTP cookie packets

Number of broken BOOTP magic cookie packets. Such traffic is discarded silently.

non-def BOOTP type packets

Number of broken BOOTP hardware type packets. Such traffic is discarded silently.

non-def dest port packets

Number of invalid destination packets. Such traffic is discarded silently.

non-def src port packets

Number of invalid source packets. Such traffic is discarded silently.

non-support media type packets

Number of non-IEEE ETHERNET packets. Such traffic is discarded silently.

non-support msg type packets

Number of invalid DHCP message type packets. Such traffic is discarded silently.

overrun packets

Number of oversized packets. Such traffic is discarded silently.

packets without DHCP Option 82

Number of packets which don't include RFC 3046 DHCP Option 82. Such traffic is handled according to custom policies.

passed packets

Denotes successful filter passed through unaltered events since instance startup.

passed packets per sec

Denotes rate of successful filter pass-through events.

underrun packets

Number of oversized packets. Such traffic is discarded silently.

Real-world example of accessing traffic statistics

You can use the command-line tool /usr/bin/kstat interactively to print all or selected KSTAT information about DHCP traffic from the system.

$ kstat -c net -m dhcpmod

module: dhcpmod instance: 1

name: inbound class: net

buffer errors 0

cache buckets 16384

cache errors 0

cache expired 1978313

cache hits 7263887

cache misses 2012974

cache records 8381

crtime 522.985408215

discarded packets 3230542

discarded packets per sec 67

discarded rate limit packets 3229064

failure packets 861

fragmented IP packets 0

input packets 27526307

input packets per sec 268

invalid BOOTP packets 0

invalid IP packets 4

invalid UDP packets 0

malformed packets 617

no memory errors 0

non-def BOOTP cookie packets 2

non-def BOOTP type packets 0

non-def dest port packets 0

non-def src port packets 861

non-support media type packets 5

non-support msg type packets 606

overrun packets 0

packets without DHCP Option 82 323744

passed packets 6048319

passed packets per sec 44

snaptime 160039.377522581

state 1

underrun packets 0

module: dhcpmod instance: 2

name: inbound class: net

buffer errors 0

cache buckets 4

cache errors 0

cache expired 490

cache hits 155

cache misses 499

cache records 2

crtime 522.986099883

discarded packets 0

discarded packets per sec 0

discarded rate limit packets 0

failure packets 0

fragmented IP packets 0

input packets 658

input packets per sec 0

invalid BOOTP packets 0

invalid IP packets 0

invalid UDP packets 0

malformed packets 0

no memory errors 0

non-def BOOTP cookie packets 0

non-def BOOTP type packets 0

non-def dest port packets 0

non-def src port packets 0

non-support media type packets 0

non-support msg type packets 0

overrun packets 0

packets without DHCP Option 82 0

passed packets 658

passed packets per sec 0

snaptime 160039.379437423

state 1

underrun packets 0

For viewing one instance separately use -i key followed by instance ID.

$ kstat -c net -m dhcpmod -i 1