Shorewall Traffic Accounting

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2009/06/05


Table of Contents

Accounting Basics
Accounting with Bridges
Integrating Shorewall Accounting with Collectd

Caution

This article applies to Shorewall 4.0 and later. If you are running a version of Shorewall earlier than Shorewall 4.0.0 then please see the documentation for that release.

Accounting Basics

Shorewall accounting rules are described in the file /etc/shorewall/accounting. By default, the accounting rules are placed in a chain called “accounting” and can thus be displayed using “shorewall[-lite] show -x accounting”. All traffic passing into, out of, or through the firewall traverses the accounting chain including traffic that will later be rejected by interface options such as “tcpflags” and “maclist”.

The columns in the accounting file are as follows:

  • ACTION - What to do when a match is found. Possible values are:

    • COUNT- Simply count the match and continue trying to match the packet with the following accounting rules

    • DONE- Count the match and don't attempt to match any following accounting rules.

    • <chain> - The name of a chain; Shorewall will create the chain automatically if it doesn't already exist. A jump to this chain will be generated from the chain specified by the CHAIN column. If the name of the chain is followed by “:COUNT” then a COUNT rule matching this entry will automatically be added to <chain>. Chain names must start with a letter, must be composed of letters and digits, and may contain underscores (“_”) and periods (“.”). Beginning with Shorewall version 1.4.8, chain names may also contain embedded dashes (“-”) and are not required to start with a letter.

    • COMMENT - (Shorewall-perl only) - The remainder of the line is treated as a comment which is attached to subsequent rules until another COMMENT line is found or until the end of the file is reached. To stop adding comments to rules, use a line with only the word COMMENT.

  • CHAIN - The name of the chain where the accounting rule is to be added. If empty or “-” then the “accounting” chain is assumed (see below for exceptions).

  • SOURCE - Packet Source. The name of an interface, an address (host or net), or an interface name followed by “:” and a host or net address.

  • DESTINATION - Packet Destination. Format the same as the SOURCE column.

  • PROTOCOL - A protocol name (from /etc/protocols), a protocol number or “ipp2p”. For “ipp2p”, your kernel and iptables must have ipp2p match support from Netfilter Patch_o_matic_ng.

  • DEST PORT - Destination Port number. Service name from /etc/services or port number. May only be specified if the protocol is TCP or UDP (6 or 17). If the PROTOCOL is “ipp2p”, then this column is interpreted as an ipp2p option without the leading “--” (default “ipp2p”). For a list of value ipp2p options, as root type iptables -m ipp2p --help.

  • SOURCE PORT- Source Port number. Service name from /etc/services or port number. May only be specified if the protocol is TCP or UDP (6 or 17).

  • USER/GROUP - This column may only be non-empty if the CHAIN is OUTPUT. The column may contain:

    [!][<user name or number>][:<group name or number>][+<program name>]

    When this column is non-empty, the rule applies only if the program generating the output is running under the effective <user> and/or <group> specified (or is NOT running under that id if “!” is given).

    Examples:

    joe #program must be run by joe
    :kids #program must be run by a member of the “kids” group.
    !:kids #program must not be run by a member of the “kids” group
    +upnpd #program named upnpd (This feature was removed from Netfilter in kernel version 2.6.14).
  • MARK - Only count packets with particular mark values.

    [!]<value>[/<mask>][:C]

    Defines a test on the existing packet or connection mark. The rule will match only if the test returns true.

    If you don’t want to define a test but need to specify anything in the following columns, place a “-” in this field.

    ! — Inverts the test (not equal)
    <value> — Value of the packet or connection mark.
    <mask> — A mask to be applied to the mark before testing.
    :C — Designates a connection mark. If omitted, the packet mark’s value is tested. This option is only supported by Shorewall-perl.

In all columns except ACTION and CHAIN, the values “-”, “any” and “all” are treated as wild-cards.

The accounting rules are evaluated in the Netfilter “filter” table. This is the same environment where the “rules” file rules are evaluated and in this environment, DNAT has already occurred in inbound packets and SNAT has not yet occurred on outbound packets.

Accounting rules are not stateful -- each rule only handles traffic in one direction. For example, if eth0 is your Internet interface, and you have a web server in your DMZ connected to eth1, then to count HTTP traffic in both directions requires two rules:

        #ACTION CHAIN   SOURCE  DESTINATION     PROTOCOL        DEST            SOURCE
        #                                                       PORT            PORT
        DONE    -       eth0    eth1            tcp             80
        DONE    -       eth1    eth0            tcp             -               80

Associating a counter with a chain allows for nice reporting. For example:

        #ACTION         CHAIN   SOURCE  DESTINATION     PROTOCOL        DEST            SOURCE
        #                                                               PORT            PORT
        web:COUNT       -       eth0    eth1            tcp             80
        web:COUNT       -       eth1    eth0            tcp             -               80
        web:COUNT       -       eth0    eth1            tcp             443
        web:COUNT       -       eth1    eth0            tcp             -               443
        DONE            web

Now shorewall show web (or shorewall-lite show web for Shorewall Lite users) will give you a breakdown of your web traffic:

     [root@gateway shorewall]# shorewall show web
     Shorewall-1.4.6-20030821 Chain web at gateway.shorewall.net - Wed Aug 20 09:48:56 PDT 2003
     
     Counters reset Wed Aug 20 09:48:00 PDT 2003

     Chain web (4 references)
     pkts bytes target     prot opt in     out     source               destination
       11  1335            tcp  --  eth0   eth1    0.0.0.0/0            0.0.0.0/0          tcp dpt:80
       18  1962            tcp  --  eth1   eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:80
        0     0            tcp  --  eth0   eth1    0.0.0.0/0            0.0.0.0/0          tcp dpt:443
        0     0            tcp  --  eth1   eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:443
       29  3297 RETURN     all  --  *      *       0.0.0.0/0            0.0.0.0/0
       [root@gateway shorewall]#

Here is a slightly different example:

        #ACTION         CHAIN   SOURCE  DESTINATION     PROTOCOL        DEST            SOURCE
        #                                                               PORT            PORT
        web             -       eth0    eth1            tcp             80
        web             -       eth1    eth0            tcp             -               80
        web             -       eth0    eth1            tcp             443
        web             -       eth1    eth0            tcp             -               443
        COUNT           web     eth0    eth1
        COUNT           web     eth1    eth0

Now shorewall show web (or shorewall-lite show web for Shorewall Lite users) simply gives you a breakdown by input and output:

     [root@gateway shorewall]# shorewall show accounting web
     Shorewall-1.4.6-20030821 Chains accounting web at gateway.shorewall.net - Wed Aug 20 10:27:21 PDT 2003

     Counters reset Wed Aug 20 10:24:33 PDT 2003

     Chain accounting (3 references)
         pkts bytes target     prot opt in     out     source               destination
         8767  727K web        tcp  --  eth0   eth1    0.0.0.0/0            0.0.0.0/0          tcp dpt:80
            0     0 web        tcp  --  eth0   eth1    0.0.0.0/0            0.0.0.0/0          tcp dpt:443
        11506   13M web        tcp  --  eth1   eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:80
            0     0 web        tcp  --  eth1   eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:443

     Chain web (4 references)
         pkts bytes target     prot opt in     out     source               destination
         8767  727K            all  --  eth0   eth1    0.0.0.0/0            0.0.0.0/0
        11506   13M            all  --  eth1   eth0    0.0.0.0/0            0.0.0.0/0
     [root@gateway shorewall]#

Here's how the same example would be constructed on an HTTP server with only one interface (eth0).

Caution

READ THE ABOVE CAREFULLY -- IT SAYS SERVER. If you want to account for web browsing, you have to reverse the rules below.

        #ACTION         CHAIN   SOURCE  DESTINATION     PROTOCOL        DEST            SOURCE
        #                                                               PORT            PORT
        web             -       eth0    -               tcp             80
        web             -       -       eth0            tcp             -               80
        web             -       eth0    -               tcp             443
        web             -       -       eth0            tcp             -               443
        COUNT           web     eth0
        COUNT           web     -       eth0

Note that with only one interface, only the SOURCE (for input rules) or the DESTINATION (for output rules) is specified in each rule.

Here's the output:

     [root@mail shorewall]# shorewall show accounting web Shorewall-1.4.7
     Chains accounting web at mail.shorewall.net - Sun Oct 12 10:27:21 PDT 2003

     Counters reset Sat Oct 11 08:12:57 PDT 2003

     Chain accounting (3 references)
      pkts bytes target     prot opt in     out     source               destination
      8767  727K web        tcp  --  eth0   *       0.0.0.0/0            0.0.0.0/0          tcp dpt:80
     11506   13M web        tcp  --  *      eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:80
         0     0 web        tcp  --  eth0   *       0.0.0.0/0            0.0.0.0/0          tcp dpt:443
         0     0 web        tcp  --  *      eth0    0.0.0.0/0            0.0.0.0/0          tcp spt:443

     Chain web (4 references)
      pkts bytes target     prot opt in     out     source               destination
      8767  727K            all  --  eth0   *       0.0.0.0/0            0.0.0.0/0
     11506   13M            all  --  *      eth0    0.0.0.0/0            0.0.0.0/0
     [root@mail shorewall]#

For an example of integrating Shorewall Accounting with MRTG, see http://www.nightbrawler.com/code/shorewall-stats/.

Accounting with Bridges

The structure of the accounting rules changes slightly when there are bridges defined in the Shorewall configuration. Because of the restrictions imposed by Netfilter in kernel 2.6.21 and later, output accounting rules must be segregated from forwarding and input rules. To accomplish this separation, Shorewall-perl creates two accounting chains:

  • accounting - for input and forwarded traffic.

  • accountout - for output traffic.

If the CHAIN column contains “-”, then:

  • If the SOURCE column in a rule includes the name of the firewall zone (e.g., $FW), then the default chain to insert the rule into is accountout only.

  • Otherwise, if the DEST in the rule is any or all or 0.0.0.0/0, then the rule is added to both accounting and accountout.

  • Otherwise, the rule is added to accounting only.

Integrating Shorewall Accounting with Collectd

Sergiusz Pawlowicz has written a nice article that shows how to integrate Shorewall Accounting with collectd to produce nice graphs of traffic activity. The article may be found at http://collectd.org/wiki/index.php/Plugin:IPTables.