ETHERS(3) FreeBSD Library Functions Manual ETHERS(3)


ethers, ether_line, ether_aton, ether_aton_r, ether_ntoa, ether_ntoa_r, ether_ntohost, ether_hosttonEthernet address conversion and lookup routines


Standard C Library (libc, -lc)


#include < sys/types.h>
#include < sys/socket.h>
#include < net/ethernet.h>

ether_line( const char *l, struct ether_addr *e, char *hostname);

struct ether_addr *
ether_aton( const char *a);

struct ether_addr *
ether_aton_r( const char *a, struct ether_addr *e);

char *
ether_ntoa( const struct ether_addr *n);

char *
ether_ntoa_r( const struct ether_addr *n, char *buf);

ether_ntohost( char *hostname, const struct ether_addr *e);

ether_hostton( const char *hostname, struct ether_addr *e);


These functions operate on ethernet addresses using an ether_addr structure, which is defined in the header file < net/ethernet.h>:

 * The number of bytes in an ethernet (MAC) address. 
#define ETHER_ADDR_LEN  6 
 * Structure of a 48-bit Ethernet address. 
struct  ether_addr { 
        u_char octet[ETHER_ADDR_LEN]; 

The function ether_line() scans l, an ASCII string in ethers(5) format and sets e to the ethernet address specified in the string and h to the hostname. This function is used to parse lines from /etc/ethers into their component parts.

The ether_aton() and ether_aton_r() functions convert ASCII representation of ethernet addresses into ether_addr structures. Likewise, the ether_ntoa() and ether_ntoa_r() functions convert ethernet addresses specified as ether_addr structures into ASCII strings.

The ether_ntohost() and ether_hostton() functions map ethernet addresses to their corresponding hostnames as specified in the /etc/ethers database. The ether_ntohost() function converts from ethernet address to hostname, and ether_hostton() converts from hostname to ethernet address.


The ether_line() function returns zero on success and non-zero if it was unable to parse any part of the supplied line l. It returns the extracted ethernet address in the supplied ether_addr structure e and the hostname in the supplied string h.

On success, ether_ntoa() and ether_ntoa_r() functions return a pointer to a string containing an ASCII representation of an ethernet address. If it is unable to convert the supplied ether_addr structure, it returns a NULL pointer. ether_ntoa() stores the result in a static buffer; ether_ntoa_r() stores the result in a user-passed buffer.

Likewise, ether_aton() and ether_aton_r() return a pointer to an ether_addr structure on success and a NULL pointer on failure. ether_aton() stores the result in a static buffer; ether_aton_r() stores the result in a user-passed buffer.

The ether_ntohost() and ether_hostton() functions both return zero on success or non-zero if they were unable to find a match in the /etc/ethers database.


The user must ensure that the hostname strings passed to the ether_line(), ether_ntohost() and ether_hostton() functions are large enough to contain the returned hostnames.


If the /etc/ethers contains a line with a single + in it, the ether_ntohost() and ether_hostton() functions will attempt to consult the NIS ethers.byname and ethers.byaddr maps in addition to the data in the /etc/ethers file.


ethers(5), yp(8)


This particular implementation of the ethers library functions were written for and first appeared in FreeBSD 2.1. Thread-safe function variants first appeared in FreeBSD 7.0.


The ether_aton() and ether_ntoa() functions returns values that are stored in static memory areas which may be overwritten the next time they are called.

ether_ntoa_r() accepts a character buffer pointer, but not a buffer length. The caller must ensure adequate space is available in the buffer in order to avoid a buffer overflow.

October 30, 2007 FreeBSD