03. [??p] Extending the Linux firewall

Pro tip: since you may want to consult the man pages at some point, add this to your .bashrc or .zshrc:

# color schemes for man pages
man() {
    LESS_TERMCAP_mb=$'\e[1;34m'   \
    LESS_TERMCAP_md=$'\e[1;32m'   \
    LESS_TERMCAP_so=$'\e[1;33m'   \
    LESS_TERMCAP_us=$'\e[1;4;31m' \
    LESS_TERMCAP_me=$'\e[0m'      \
    LESS_TERMCAP_se=$'\e[0m'      \
    LESS_TERMCAP_ue=$'\e[0m'      \
    command man "$@"

Now, source your file to load the new command. man will color certain keywords appropriately.

# update your shell's environment with the man() wrapper
[student@host]$ source ~/.bashrc
# check out the manual page for iptables
[student@host]$ man iptables

iptables is a configuration tool for the kernel packet filter.

The system as a whole provides many functionalities that are grouped by tables: filter, nat, mangle, raw, security. If you want to alter a packet header, you place a rule in the mangle table. If you want to mask the private IP address of an internal host with the external IP address of the default gateway, you place a rule in the nat table. Depending on the table you choose, you will gain or lose access to some chains. If not specified, the default is the filter table.

Chains are basically lists of rules. The five built-in chains are PREROUTING, FORWARD, POSTROUTING, INPUT, OUTPUT. Each of these corresponds to certain locations in the network stack where packets trigger Netfilter hooks (here is the PREROUTING kernel hook as an example – not that hard to add one, right?) For a selected chain, the order in which the rules are evaluated is determined primarily by the priority of their tables and secondarily by the user's discretionary arrangement (i.e.: order in which rules are inserted).

A rule consists of two entities: a sequence of match criteria and a jump target.

The jump target represents an action to be taken. You are most likely familiar with the built-in actions such as ACCEPT or DROP. These actions decide the ultimate fate of the packet and are final (i.e.: rule iteration stops when these are invoked). However, there are also extended actions (see man iptables-extensions(8)) that are not terminal verdicts and can be used for various tasks such as auditing, forced checksum recalculation or removal of Explicit Congestion Notification (ECN) bits.

The match criteria of every rule are checked to determine if the jump target is applied. The way this is designed is very elegant: every type of feature (e.g.: layer 3 IP address vs layer 4 port) that you can check has a match callback function defined in the kernel. If you want, you can write your own such function in a Linux Kernel Module (LKM) and thus extend the functionality of iptables (Writing Netfilter Modules with code example). However, you will need to implement a userspace shared library counterpart. When you start an iptables process, it searches in /usr/lib/xtables/ and automatically loads certain shared libraries (note: this path can be overwritten or extended using the XTABLES_LIBDIR environment variable). Each library there must do three things:

  • define iptables flags for the new criteria that you want to include.
  • define help messages for when iptables --help is called (its help message is an amalgamation of each library's help snippet).
  • provide an initialization function for the structure containing the rule parameters; this structure will end up in the kernel's rule chain.

So when you want to test the efficiency of the iptables rule evaluation process, keep in mind that each rule may imply the invocation of multiple callbacks such as this.

[??p] Task A - Primer / Reminder

Before writing our own match module, here's a small task to freshen your memory on how to use iptables.

Write an iptables rule according to the following specifications:

  • chain: OUTPUT
  • match rule: TCP packets originating from ephemeral ports bound to a socket created by root
  • target: enable kernel logging of matched packets with the “TCP_LOG: ” prefix

How to test:

$ sudo curl www.google.com
$ sudo dmesg

multiport, owner modules

$ man 8 iptables-extensions

[??p] Task B - Writing an xtables module

xtables is the backbone of iptables and provides a protocol-agnostic infrastructure for adding match modules. We touched on this topic earlier in the exercise but now we're going to jump right in. Following this brief introduction will be three subsections detailing the data structures, the user space shared library and the kernel module. All these are partially implemented in the 02/ directory and you will have specific TODOs in the source files. By the end, you will have implemented a match filter for requested domains in DNS queries (i.e.: you can block DNS queries for “google.com” but not “kernel.org”, for example). Note that you can solve this task either in the VM, or on your localhost. We won't be doing anything dangerous, like overwriting kernel structures.

If you're not that familiar with DNS, refer to this primer (Sec. 1-3).

The header

We mentioned before that an iptables extension has two components. A kernel module implementing the verification of a rule and a user space library that is able to parse the user's rules (in iptables “syntax”). Considering that our module will be named xt_dns_name (the xt_ part conforming to the naming convention), what links these two elements is the include/xt_dns_name.h header. In it, the xt_dns_name_mtinfo structure will hold all information necessary to the kernel module to match a packet. This structure will be initialized in user space and transferred over to the xtables framework by iptables. Looking closer, we notice two fields. While name will hold the queried domain name, flags specifies what features are enabled for verification. Our module is simple and has only one match criteria, but note that iptables can invert a selection by specifying the ! symbol before it. So we can match a packet either on a queried domain name match, or a mismatch.

Click to display ⇲

Click to hide ⇱

#ifndef _XT_DNS_NAME_H
#define _XT_DNS_NAME_H
/* defines enabled properties to be checked */
enum {
    XT_DNS_NAME     = 1 << 0,   /* search for DNS name match    */
    XT_DNS_NAME_INV = 1 << 1,   /* search for DNS name mismatch */
/* rule match information */
struct xt_dns_name_mtinfo {
    __u8 flags;
    __u8 name[127];
#endif /* _XT_DNS_NAME_H */

The __u8 type is equivalent to uint8_t and is a typedef of u8. The kernel defines u8, u16, etc. because C types have different sizes, depending on the underlying CPU architecture. __u8 is used here in stead of u8 to indicate that the header is shared with userspace. Also, the reason why the kernel doesn't simply use uint8_t is because u8 predates stdint.h.

The user space iptables plugin

The plugin consists of a shared library compiled from plugin/libxt_dns_name.c. The source is broken down into four sections:

  • API: These are prototypes of functions that must be made available to iptables to invoke when it needs certain things done. Here is a description of each function:
    • dns_name_mt_help(): When you invoke iptables --help, each plugin will print its own help message. This is our contribution.
    • dns_name_mt_init(): Called before the argument parsing begins. Will zero out our xt_dns_name_mtinfo structure.
    • dns_name_mt_parse(): Based on optarg, this function will be invoked for each argument that has something to do with our plugin. It will update the xt_dns_name_mtinfo structure accordingly on each pass.
    • dns_name_mt_check(): Final check before sending the structure in kernel space. Will verify that all required arguments were provided.
    • dns_name_mt_print(): When invoking iptables -L, this function will print out a rule's match criteria.
    • dns_name_mt_save(): Given a xt_dns_name_mtinfo structure, this function will print out the CLI arguments that would generate such a function. Called upon by iptables-save.
  • MODULE SPECIFICATION STRUCTURES: Here we declare two structures:
    • dns_name_mt_opts : A structure defining the CLI arguments that we accept (see man 3 getopt for details).
    • dns_name_mt_reg : A structure containing function pointers for specific tasks that the module needs to accomplish. We initialize it with the function described in the API section. This kind of structure is usually called a virtual table (or vtable).
  • IPTABLES MODULE CALLBACKS: this is where we actually implement the functions that we added to the vtable. Here, you will search for TODOs.
  • LIBRARY MANAGEMENT FUNCTIONS: When loaded, each library (i.e.: a .so file) can have a number of constructors defined. These constructors are functions called upon by the loader once the library was mapped in virtual memory. Our constructor will invoke xtables_register_match() in order to register the vtable with iptables, letting it know that it has yet another plugin at its disposal.

Click to display ⇲

Click to hide ⇱

#include <stdio.h>
#include <stdint.h>
#include <getopt.h>
#include <string.h>
#include <xtables.h>
#include "xt_dns_name.h"
 ************************************ API *************************************
static void dns_name_mt_help(void);
static void dns_name_mt_init(struct xt_entry_match *match);
static int  dns_name_mt_parse(int c, char **argv, int invert,
                              unsigned int *flags, const void *entry,
                              struct xt_entry_match **match);
static void dns_name_mt_check(unsigned int flags);
static void dns_name_mt_print(const void *entry,
                              const struct xt_entry_match *match, int numeric);
static void dns_name_mt_save(const void *entry,
                             const struct xt_entry_match *match);
 *********************** MODULE SPECIFICATION STRUCTURES **********************
/* module specific options */
const struct option dns_name_mt_opts[] = {
    { .name="domain", .has_arg=required_argument, .val='1' },
    { NULL },
/* module userspace extension vtable */
static struct xtables_match dns_name_mt_reg = {
    .version        = XTABLES_VERSION,
    .name           = "dns_name",
    .revision       = 0,
    .family         = NFPROTO_IPV4,
    .size           = XT_ALIGN(sizeof(struct xt_dns_name_mtinfo)),
    .userspacesize  = XT_ALIGN(sizeof(struct xt_dns_name_mtinfo)),
    .help           = dns_name_mt_help,
    .init           = dns_name_mt_init,
    .parse          = dns_name_mt_parse,
    .final_check    = dns_name_mt_check,
    .print          = dns_name_mt_print,
    .save           = dns_name_mt_save,
    .extra_opts     = dns_name_mt_opts,
 ************************* IPTABLES MODULE CALLBACKS **************************
/* dns_name_mt_help - prints help message for this module
static void
    printf("dns_name match options\n"
           "[!] --domain <string>\t\tQueried domain name.\n");
/* dns_name_mt_init - initializes our data struct fields before parsing
 *  @match : pointer to our data struct
static void
dns_name_mt_init(struct xt_entry_match *match)
    /* match is an internal structure; we are only interested in data */
    struct xt_dns_name_mtinfo *info = (void *)match->data;
    /* zero out structure */
    memset(info, 0, sizeof(*info));
/* dns_name_mt_parse - called for each module-specific argument
 *  @c      : option id (see .val in dns_name_mt_opts)
 *  @argv   : argv (simple as that)
 *  @invert : 1 if user specified "!" before argument
 *  @flags  : for parser's discretionary use
 *  @entry  : ptr to an ipt_entry struct (don't care)
 *  @match  : contains pointer to our data struct (data field)
 *  @return : true if option was parsed, false otherwise
static int
dns_name_mt_parse(int c, char **argv, int invert, unsigned int *flags,
                  const void *entry, struct xt_entry_match **match)
    /* get reference to our xt_dns_name_mtinfo instance */
    struct xt_dns_name_mtinfo *info = (void *)(*match)->data;
    /* option-specific parsing */
    switch (c) {
        case '1':       /* --domain */
            /* check for multiple occurrences */
            if (*flags & XT_DNS_NAME)
                xtables_error(PARAMETER_PROBLEM, "xt_dns_name: "
                    "use \"--domain\" only once!");
            /* update parser flags and match criteria flags */
            *flags      |= XT_DNS_NAME;
            info->flags |= XT_DNS_NAME;
            /* check for match rule inversion */
            if (invert)
                info->flags |= XT_DNS_NAME_INV;
            /* initalize info->name                                      *
             * NOTE: argument is in global variable <optarg>             *
             * NOTE: convert the "." characters according to QNAME specs */
            /* TODO 1: initialize info->name */
            return true;
    /* unknown option */
    return false;
/* dns_name_mt_check - verify that all required options were processed
 *  @flags : the persistent <flags> argument from dns_name_mt_parse()
static void
dns_name_mt_check(unsigned int flags)
    if (!(flags & XT_DNS_NAME))
        xtables_error(PARAMETER_PROBLEM, "xt_dns_name: "
            "make sure to specify the \"--domain\" argument!");
/* dns_name_mt_print - print the match criteria fields for `iptables -L`
 *  @entry   : internal stuff (don't care)
 *  @match   : contains pointer to our data struct (data field)
 *  @numeric : do not resolve IP addresses to host names if true (don't care)
static void
dns_name_mt_print(const void *entry, const struct xt_entry_match *match,
                  int numeric)
    const struct xt_dns_name_mtinfo *info = (void *) match->data;
    /* check for match rule reversal */
    if (info->flags & XT_DNS_NAME_INV)
        printf("! ");
    /* print domain name                                  *
     * NOTE: replace length of labels with "." characters *
     * NOTE: do NOT print a "\n" character                */
    /* TODO 2: print info->name */
/* dns_name_mt_save - print out arguments that generate this rule
 *  @entry : internal stuff (don't care)
 *  @match : contains pointer to our data struct (data field)
static void
dns_name_mt_save(const void *entry, const struct xt_entry_match *match)
    const struct xt_dns_name_mtinfo *info = (void *) match->data;
    /* check for match rule reversal */
    if (info->flags & XT_DNS_NAME_INV)
        printf("! ");
    /* print "--domain" and the argument */
    printf("--domain ");
    /* TODO 3: copy paste TODO 2 here */
 ************************ LIBRARY MANAGEMENT FUNCTIONS ************************
/* _init - iptables library constructor
 * NOTE: the '_init' symbol is expanded as a macro by iptables
static void

When solving some of the TODOs and consulting the DNS format specification from before, it would be useful to have a DNS capture available to you in wireshark.

The kernel space xtables module

The kernel module source is organized similarly to the user space plugin. The dns_name_mt_reg structure acts as a vtable but also includes information about permissible chains and layer 3 protocols that work with our implementation. Specifically, any rule that makes use of this module can be inserted only in the OUTPUT chain, meaning that we can only catch requests originating from our localhost. Moreover, we implement support only for IPv4, not for IPv6. As we can see, this structure is used on module initialization, in dns_name_mt_init(), to register our module with the xtables framework via xt_register_match().

dns_name_check() and dns_name_mt() implement the functionalities required of our module. The former performs checks on each newly inserted rule, or at least on the part that pertains to this module. In other words, it must make sure that a valid domain name (i.e.: ”.”s replaced with length of following label, etc.) was inserted, for example. The latter function is called upon to verify if a packet matches a certain rule. Its first argument does not represent the packet itself, but a socket buffer structure (see also this, and possibly this) that contains this information, in addition to much, much more. We made sure to provide you with pointers to our xt_dns_name_mtinfo structure, but also to the beginning of the IPv4 header. However, it is up to you to implement this logic and obtain a working match module.

Click to display ⇲

Click to hide ⇱

#include <linux/kernel.h>
#include <linux/netfilter/x_tables.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/module.h>
#include "xt_dns_name.h"
MODULE_DESCRIPTION("Xtables: DNS query QNAME matching");
#define MOD_TAG "xt_dns_name: "
 ************************************ API *************************************
static int  dns_name_check(const struct xt_mtchk_param *par);
static bool dns_name_mt(const struct sk_buff *skb, struct xt_action_param *par);
 *********************** MODULE SPECIFICATION STRUCTURES **********************
/* registration information */
static struct xt_match dns_name_mt_reg __read_mostly = {
    .name       = "dns_name",
    .revision   = 0,
    .family     = NFPROTO_IPV4,
    .matchsize  = sizeof(struct xt_dns_name_mtinfo),
    .checkentry = dns_name_check,
    .match      = dns_name_mt,
    .hooks      = 1 << NF_INET_LOCAL_OUT,
    .me         = THIS_MODULE,
 ************************** XTABLES MODULE CALLBACKS **************************
/* dns_name_check - checks rule validity
 *  @par : parameters for match extensions
 *  @return : 0 if everything is ok, !0 otherwise
static int
dns_name_check(const struct xt_mtchk_param *par)
    const struct xt_dns_name_mtinfo *info = par->matchinfo;
    /* TODO 4: userspace is not to be trusted! check inserted rule */
    return 0;
/* dns_name_mt - performs packet match check
 *  @skb : packet buffer information
 *  @par : parameters for matches / targets
 *  @return : true if matched, false otherwise
static bool
dns_name_mt(const struct sk_buff *skb, struct xt_action_param *par)
    const struct xt_dns_name_mtinfo *info = par->matchinfo;
    struct iphdr                    *iph  = ip_hdr(skb);
    /* TODO 5: be 100% sure that the packet is a DNS request */
    /* TODO 6: match check on any & all QNAMEs in request */
    return false;
 *********************** MODULE ENTRY & EXIT CALLBACKS ************************
static int dns_name_mt_init(void)
    pr_info(MOD_TAG "loading xt_dns_name module");
    return xt_register_match(&dns_name_mt_reg);
static void dns_name_mt_exit(void)
    pr_info(MOD_TAG "unloading xt_dns_name module");

Testing your solution:

[student@host]$ sudo insmod xt_dns_name.ko
# depending on your distro, libxt_*.so may be installed in different places
[student@host]$ sudo XTABLES_LIBDIR="$(pwd):/usr/lib/xtables:/usr/lib/x86_64-linux-gnu/xtables" \
                  iptables                                                                      \
                    -m dns_name                                                                 \
                    -I OUTPUT                                                                   \
                    --domain 'fep.grid.pub.ro'                                                  \
                    -j DROP
[student@host]$ dig +short fep.grid.pub.ro @
[student@host]$ sudo iptables -F OUTPUT
[student@host]$ sudo rmmod xt_dns_name

Try running a wireshark instance and filter by “dns” to make sure no queries pass through.
Note that some distros come with a DNS cache preinstalled and might make your filtering rule redundant.

Make sure to add plenty of pr_info() in your match function to make debugging easier.

Remember that although your host is most likely little-endian, the Internet is big-endian. So when accessing data that is larger than 1 byte (e.g.: port number), use the htons() family of functions. They should be readily available to you in the kernel module.

isc/labs/kernel/tasks/03.txt · Last modified: 2021/12/02 16:11 by radu.mantu
CC Attribution-Share Alike 3.0 Unported
www.chimeric.de Valid CSS Driven by DokuWiki do yourself a favour and use a real browser - get firefox!! Recent changes RSS feed Valid XHTML 1.0