screen(2) System Calls Manual screen(2)
NAME
screen - Gateway packet screening facility
SYNOPSIS
#include <sys/types.h> #include <net/gw_screen.h>
int mode; struct screen_data sdata; struct screen_stats sstats;
ioctl(s, SIOCSCREENON, (caddr_t)&mode); ioctl(s, SIOCSCREEN, (caddr_t)&data); ioctl(s, SIOCSCREENSTATS, (caddr_t)&sstats);
PARAMETERS
The interface to the gateway screen facility is a set of ioctl requests.
All these requests are meant to be used on a file descriptor created by the socket() system call. The mode parameter, passed by refer-
ence, can be SCREENMODE_OFF, SCREENMODE_ON, or SCREENMODE_NOCHANGE. Upon completion of the system call, the mode parameter contains the
previous value of the screen mode. Unprivileged users may only use the SCREENMODE_NOCHANGE request. This is the most important request and
is described below. Only the super-user may make this request. Returns, by reference using the sstats parameter, statistics in this
structure:
struct screen_stats {
u_long ss_packets; /* total packets screened */
u_long ss_nobuffer; /* dropped, buffer was full */
u_long ss_accept; /* total accepted */
u_long ss_reject; /* total rejected */
u_long ss_badsync; /* dropped, user was out of sync */
u_long ss_stale; /* dropped, too old */ };
DESCRIPTION
The gateway screen facility allows a user-level process to decide which network packets should be forwarded by the kernel (when the system
is acting as a gateway). When the screen mode is set to "off," all packets are forwarded normally; when the screen mode is set to "on," all
packets that would be forwarded must be approved through the use of this facility.
Use of SIOCSCREEN
The SIOCSCREEN request is used in the main loop of the user-level daemon. Each time it is called, it returns (by reference using the sdata
parameter) a screen_data structure containing a prefix of a packet (normally containing the packet headers) and some additional informa-
tion:
struct screen_data_hdr {
short sdh_count; /* length of entire record */
short sdh_dlen; /* bytes of packet header */
u_int sdh_xid; /* transaction ID */
struct timeval sdh_arrival; /* time packet arrived */
short sdh_family; /* address family */
int sdh_action; /* disposition for packet */
#define SCREEN_ACCEPT 0x0001 /* Accept this packet */ #define SCREEN_DROP 0x0000 /* Do not accept this packet */ #define SCREEN_NOTIFY
0x0002 /* Notify sender of failure */ #define SCREEN_NONOTIFY 0x0000 /* Do not notify sender */ };
struct screen_data {
struct screen_data_hdr sd_hdr;
char sd_data[SCREEN_DATALEN]; /* sd_dlen bytes of packet header */ };
#define sd_count sd_hdr.sdh_count #define sd_dlen sd_hdr.sdh_dlen #define sd_xid sd_hdr.sdh_xid #define sd_action
sd_hdr.sdh_action #define sd_arrival sd_hdr.sdh_arrival #define sd_family sd_hdr.sdh_family
The sd_family field indicates the protocol family (for example, AF_INET) under which the packet is being handled; there is no protocol-spe-
cific code in the kernel implementation of the gateway screen. Either the sd_family field should be initialized to a specific family
before the request is invoked (indicating that the user process is willing to handle requests for this family only), or it should be set to
AF_UNSPEC (indicating that the user process is willing to handle all protocols).
The user-level process examines the packet headers and decides whether or not the packet should be forwarded. It communicates this deci-
sion to the kernel by filling in the sd_action field in the screen_data structure with either SCREEN_ACCEPT, SCREEN_DROP, or SCREEN_DROP
bit-wise ORed with SCREEN_NOTIFY; the last choice causes the gateway to drop the packet but send an error packet to the source host (if
this is supported in the protocol family). The process then passes that structure back to the kernel in another invocation of the SIOC-
SCREEN request. That ioctl call then blocks until a new packet is available, at which point the cycle repeats.
Note that two actions are being carried out through one system call, and that each cycle starts mid-way through a system call. Thus, the
first time a daemon uses this ioctl request, it has to pass in a no-op decision to complete the first (half) cycle. The kernel matches
incoming decisions with pending packets by comparing both the transaction id (sd_xid) field, and the user's process id (so one process can-
not provide decisions on packets presented to a different process). Decisions must be supplied in first-in, first-out order; decisions
supplied in the wrong order may result in packets being dropped.
RETURN VALUES
If an error has occurred, a value of -1 is returned and errno is set to indicate the error.
ERRORS
In addition to those error codes described for ioctl(), the SIOCSCREEN request can also return: If the screen mode is set to SCREEN-
MODE_OFF, the SIOCSCREEN request is meaningless. If an operation reserved for the superuser is attempted by a non-superuser.
SEE ALSO
Functions: ioctl(2)
Daemons: screenmode(8), screend(8), screenstat(8)
screen(2)