SERVXCHECK(3)

NAME
servxcheck - Internet service access check

SYNOPSIS
#define _MINIX_SOURCE 1
#include </net/gen/netdb.h>
int servxcheck(ipaddr_t peer, const char *service,
	void (*logf)(int pass, const char *name));
char *servxfile(const char *file);

DESCRIPTION
Servxcheck() is used by programs like inetd to perform an access check on the host connected to the other end of the TCP channel that has IP address peer.

Servxcheck() translates the IP address to the associated host name if necessary, and checks if the host is granted access as guided by the file /etc/serv.access. (See serv.access(5).) The service name used to search the access file is passed by the caller as service. These names should be the same as the service names in /etc/services.

The caller should use the NWIOGTCPCONF ioctl() call to find out what the IP address of the remote end is. It is wise to bypass the servxcheck() call if the remote end happens to be the local machine (remaddr == locaddr), so that local connections aren't impeded by slow checks. Servxcheck() will itself allow connections from 127.0.0.1/8 immediately, so you don't have to check for that. Example of use:

    if (ioctl(fd, NWIOGTCPCONF, &tcpconf) < 0
    	|| tcpconf.nwtc_remaddr == tcpconf.nwtc_locaddr
    	|| servxcheck(tcpconf.nwtc_remaddr, service_name, NULL)
    ) {
    	serve();
    }

An attempt to connect to a service is logged if the access is denied. You can use the special checkword "log" to also log if access is granted. Logging will be done with syslog() at the warning level. A syntax error in the access file may be logged under the err level. The caller must use openlog() to set the appropriate logging facility. One may do one's own logging by supplying a logf function that will be called by servxcheck with a first argument that is true if access is granted, false if denied, and a second argument that is the name of the remote host whose access has been checked.

The default is to fail the check unless the access file says otherwise. Strange errors make the check succeed. (We do not want remote access to fail because of some system error.) Note that this function is not meant to check access to the system, that's what passwords and such are for, but only to limit access to those who are allowed to use the services the system offers.

Connections from a machine to itself are accepted immediately. No further checks, no logging.

Servxfile() may be used to specify a file other than the default /etc/serv.access. This is useful for programs started from inetd that want to handle the access check themselves, using a private access file. The return value of servxfile() is the pathname of the old access file. Only a pointer to the new path is saved, the caller must keep the string it points to intact.

FILES

/etc/serv.access
    Default access check file.

SEE ALSO
syslog(3), serv.access(5), services(5), inetd(8).

DIAGNOSTICS
Servxcheck() returns 0 if the access is denied, 1 if granted.

Typical syslog message:

    Jan 10 20:27:20 flotsam inetd[174]: service 'shell' granted to jetsam.cs.vu.nl

BUGS
IP and DNS based access checks will stop most crackers, but not the really determined ones. Luckily MINIX 3 is sufficiently strange to thwart the well known cracking schemes. But don't ever allow yourself to feel secure.

AUTHOR
Kees J. Bot <kjb@cs.vu.nl>