IPFIREWALL(4)

NAME
ipfirewall - IP packet filter and traffic accounting

SYNOPSIS
#include <sys/types.h>
#include <sys/queue.h>
#include <netinet/in.h>
#include <netinet/ip_fw.h>
int
setsockopt(raw_socket, IPPROTO_IP, ipfw option, struct ipfw, size);

DESCRIPTION
IPFirewall (sometimes referred to as "ipfw") is a system facility which allows filtering, redirecting, and other operations on IP packets travelling through network interfaces. Packets are matched by applying an ordered list of pattern rules against each packet until a match is found, at which point the corresponding action is taken. Rules are numbered from 1 to 65534; multiple rules may share the same number.

There is one rule that always exists, rule number 65535. This rule normally causes all packets to be dropped. Hence, any packet which does not match a lower numbered rule will be dropped. However, the kernel compile time option IPFIREWALL_DEFAULT_TO_ACCEPT allows the administrator to change this fixed rule to permit everything.

The buffer passed down via the socket-option call should contain a "struct ip_fw" that is initialized with the required parameters for the firewall command being invoked. This structure is consistently required for every firewall command, even though in some cases the majority of its fields will go unused. The reason for this is the API versioning that the firewall supports for the sake of backward compatibility. The version field of this structure should always be set to IP_FW_CURRENT_API_VERSION or an EINVAL error will be returned.

  Commands
The following socket options are used to manage the rule list:

IP_FW_ADD
    inserts the rule into the rule list

IP_FW_DEL
    deletes all rules having the matching rule number

IP_FW_GET
    returns the (first) rule having the matching rule number

IP_FW_ZERO
    zeros the statistics associated with all rules having the matching rule number. If the rule number is zero, all rules are zeroed.

IP_FW_FLUSH
    removes all rules (except 65535).

When the kernel security level is greater than 2, only IP_FW_GET is allowed.

  Rule Structure
Rules are described by the following structure:

    /* One ipfw rule */
    struct ip_fw {
        u_int32_t version;              /* Version of this structure.  Should always be */
                                        /* set to IP_FW_CURRENT_API_VERSION by clients. */
        void *context;                  /* Context that is usable by user processes to */
                                        /* identify this rule. */
        u_int64_t fw_pcnt,fw_bcnt;          /* Packet and byte counters */
        struct in_addr fw_src, fw_dst;      /* Source and destination IP addr */
        struct in_addr fw_smsk, fw_dmsk;    /* Mask for src and dest IP addr */
        u_short fw_number;                  /* Rule number */
        u_int fw_flg;                       /* Flags word */
    #define IP_FW_MAX_PORTS 10              /* A reasonable maximum */
            union {
            u_short fw_pts[IP_FW_MAX_PORTS];        /* Array of port numbers to match */
    #define IP_FW_ICMPTYPES_MAX     128
    #define IP_FW_ICMPTYPES_DIM     (IP_FW_ICMPTYPES_MAX / (sizeof(unsigned) * 8))
            unsigned fw_icmptypes[IP_FW_ICMPTYPES_DIM]; /* ICMP types bitmap */
            } fw_uar;
        u_int fw_ipflg;                     /* IP flags word */
        u_char fw_ipopt,fw_ipnopt;          /* IP options set/unset */
        u_char fw_tcpopt,fw_tcpnopt;        /* TCP options set/unset */
        u_char fw_tcpf,fw_tcpnf;            /* TCP flags set/unset */
        long timestamp;                     /* timestamp (tv_sec) of last match */
        union ip_fw_if fw_in_if, fw_out_if; /* Incoming and outgoing interfaces */
        union {
            u_short fu_divert_port;         /* Divert/tee port (options IPDIVERT) */
            u_short fu_pipe_nr;             /* queue number (option DUMMYNET) */
            u_short fu_skipto_rule;         /* SKIPTO command rule number */
            u_short fu_reject_code;         /* REJECT response code */
            struct sockaddr_in fu_fwd_ip;
        } fw_un;
        u_char fw_prot;                     /* IP protocol */
            /*
             * N'of src ports and # of dst ports in ports array (dst ports
             * follow src ports; max of 10 ports in all; count of 0 means
             * match all ports)
             */
        u_char fw_nports;
        void *pipe_ptr;                    /* flow_set ptr for dummynet pipe */
        void *next_rule_ptr ;              /* next rule in case of match */
        uid_t fw_uid;                       /* uid to match */
        int fw_logamount;                   /* amount to log */
        u_int64_t fw_loghighest;            /* highest number packet to log */
    };
    The ip_fw.h header also contains macros for setting the fw_ports field and various
    flags and constants for setting other fields.

  Rule Actions
Each rule has an action described by the IP_FW_F_COMMAND bits in the flags word:

IP_FW_F_DENY
    drop packet

IP_FW_F_REJECT
    drop packet; send rejection via ICMP or TCP

IP_FW_F_ACCEPT
    accept packet

IP_FW_F_COUNT
    increment counters; continue matching

IP_FW_F_DIVERT
    divert packet to a divert(4) socket

IP_FW_F_TEE
    copy packet to a divert(4) socket; continue

IP_FW_F_SKIPTO
    skip to rule number fu_skipto_rule

In the case of IP_FW_F_REJECT, if the fu_reject_code is a number from 0 to 255, then an ICMP unreachable packet is sent back to the original packet's source IP address, with the corresponding code. Otherwise, the value must be 256 and the protocol IPPROTO_TCP, in which case a TCP reset packet is sent instead.

With IP_FW_F_SKIPTO, all succeeding rules having rule number less than fu_skipto_rule are skipped.

  Kernel Options
Options in the kernel configuration file:

options IPFIREWALL
    enable ipfirewall

options IPFIREWALL_VERBOSE
    enable firewall logging

options IPFIREWALL_VERBOSE_LIMIT
    limit firewall logging

options IPDIVERT
    enable divert(4) sockets

When packets match a rule with the IP_FW_F_PRN bit set, and if IPFIREWALL_VERBOSE has been enabled,a message is written to /dev/klog with the LOG_SECURITY facility (see syslog(3)) for further logging by syslogd(8); IPFIREWALL_VERBOSE_LIMIT limits the maximum number of times each rule can cause a log message. These variables are also available via the sysctl(3) interface.

RETURN VALUES
The setsockopt() function returns 0 on success. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS
The setsockopt() function will fail if:

[EINVAL]
    The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided.

[EINVAL]
    A structural error in ip_fw structure occurred (n_src_p+n_dst_p too big, ports set for ALL/ICMP protocols etc.).

[EINVAL]
    The version field of the ip_fw structure was set to a value not supported by the currently-installed IPFirewall, or no ip_fw structure was passed to it at all.

[EINVAL]
    An invalid rule number was used.

SEE ALSO
setsockopt(2), divert(4), ip(4), ipfw(8), sysctl(8), syslogd(8)

BUGS
The ``tee'' rule is not yet implemented (currently it has no effect).

This man page still needs work.

HISTORY
The ipfw facility was initially written as package to BSDI by Daniel Boulet <danny@BouletFermat.ab.ca>. It has been heavily modified and ported to FreeBSD by Ugen J.S. Antsilevich <ugen@NetVision.net.il>.

Several enhancements added by Archie Cobbs <archie@FreeBSD.org>.