atoaddr.3

This a good VPN source

.TH IPSEC_ATOADDR 3 "11 June 2001"
.\" RCSID $Id: atoaddr.3,v 1.13 2004/04/09 18:00:38 mcr Exp $
.SH NAME
ipsec atoaddr, addrtoa \- convert Internet addresses to and from ASCII
.br
ipsec atosubnet, subnettoa \- convert subnet/mask ASCII form to and from addresses
.SH SYNOPSIS
.B "#include <freeswan.h>
.sp
.B "const char *atoaddr(const char *src, size_t srclen,"
.ti +1c
.B "struct in_addr *addr);"
.br
.B "size_t addrtoa(struct in_addr addr, int format,"
.ti +1c
.B "char *dst, size_t dstlen);"
.sp
.B "const char *atosubnet(const char *src, size_t srclen,"
.ti +1c
.B "struct in_addr *addr, struct in_addr *mask);"
.br
.B "size_t subnettoa(struct in_addr addr, struct in_addr mask,"
.ti +1c
.B "int format, char *dst, size_t dstlen);"
.SH DESCRIPTION
These functions are obsolete; see
.IR ipsec_ttoaddr (3)
for their replacements.
.PP
.I Atoaddr
converts an ASCII name or dotted-decimal address into a binary address
(in network byte order).
.I Addrtoa
does the reverse conversion, back to an ASCII dotted-decimal address.
.I Atosubnet
and
.I subnettoa
do likewise for the ``address/mask'' ASCII form used to write a
specification of a subnet.
.PP
An address is specified in ASCII as a
dotted-decimal address (e.g.
.BR 1.2.3.4 ),
an eight-digit network-order hexadecimal number with the usual C prefix (e.g.
.BR 0x01020304 ,
which is synonymous with
.BR 1.2.3.4 ),
an eight-digit host-order hexadecimal number with a
.B 0h
prefix (e.g.
.BR 0h01020304 ,
which is synonymous with
.B 1.2.3.4
on a big-endian host and
.B 4.3.2.1
on a little-endian host),
a DNS name to be looked up via
.IR gethostbyname (3),
or an old-style network name to be looked up via
.IR getnetbyname (3).
.PP
A dotted-decimal address may be incomplete, in which case
ASCII-to-binary conversion implicitly appends
as many instances of
.B .0
as necessary to bring it up to four components.
The components of a dotted-decimal address are always taken as
decimal, and leading zeros are ignored.
For example,
.B 10
is synonymous with
.BR 10.0.0.0 ,
and
.B 128.009.000.032
is synonymous with
.BR 128.9.0.32
(the latter example is verbatim from RFC 1166).
The result of
.I addrtoa
is always complete and does not contain leading zeros.
.PP
The letters in
a hexadecimal address may be uppercase or lowercase or any mixture thereof.
Use of hexadecimal addresses is
.B strongly
.BR discouraged ;
they are included only to save hassles when dealing with
the handful of perverted programs which already print 
network addresses in hexadecimal.
.PP
DNS names may be complete (optionally terminated with a ``.'')
or incomplete, and are looked up as specified by local system configuration
(see
.IR resolver (5)).
The
.I h_addr
value returned by
.IR gethostbyname (3)
is used,
so with current DNS implementations,
the result when the name corresponds to more than one address is
difficult to predict.
Name lookup resorts to
.IR getnetbyname (3)
only if
.IR gethostbyname (3)
fails.
.PP
A subnet specification is of the form \fInetwork\fB/\fImask\fR.
The
.I network
and
.I mask
can be any form acceptable to
.IR atoaddr .
In addition, the
.I mask
can be a decimal integer (leading zeros ignored) giving a bit count,
in which case
it stands for a mask with that number of high bits on and all others off
(e.g.,
.B 24
means
.BR 255.255.255.0 ).
In any case, the mask must be contiguous
(a sequence of high bits on and all remaining low bits off).
As a special case, the subnet specification
.B %default
is a synonym for
.BR 0.0.0.0/0 .
.PP
.I Atosubnet
ANDs the mask with the address before returning,
so that any non-network bits in the address are turned off
(e.g.,
.B 10.1.2.3/24
is synonymous with
.BR 10.1.2.0/24 ).
.I Subnettoa
generates the decimal-integer-bit-count
form of the mask,
with no leading zeros,
unless the mask is non-contiguous.
.PP
The
.I srclen
parameter of
.I atoaddr
and
.I atosubnet
specifies the length of the ASCII string pointed to by
.IR src ;
it is an error for there to be anything else
(e.g., a terminating NUL) within that length.
As a convenience for cases where an entire NUL-terminated string is
to be converted,
a
.I srclen
value of
.B 0
is taken to mean
.BR strlen(src) .
.PP
The
.I dstlen
parameter of
.I addrtoa
and
.I subnettoa
specifies the size of the
.I dst
parameter;
under no circumstances are more than
.I dstlen
bytes written to
.IR dst .
A result which will not fit is truncated.
.I Dstlen
can be zero, in which case
.I dst
need not be valid and no result is written,
but the return value is unaffected;
in all other cases, the (possibly truncated) result is NUL-terminated.
The
.I freeswan.h
header file defines constants,
.B ADDRTOA_BUF
and
.BR SUBNETTOA_BUF ,
which are the sizes of buffers just large enough for worst-case results.
.PP
The
.I format
parameter of
.I addrtoa
and
.I subnettoa
specifies what format is to be used for the conversion.
The value
.B 0
(not the ASCII character
.BR '0' ,
but a zero value)
specifies a reasonable default,
and is in fact the only format currently available.
This parameter is a hedge against future needs.
.PP
The ASCII-to-binary functions return NULL for success and
a pointer to a string-literal error message for failure;
see DIAGNOSTICS.
The binary-to-ASCII functions return
.B 0
for a failure, and otherwise
always return the size of buffer which would 
be needed to
accommodate the full conversion result, including terminating NUL;
it is the caller's responsibility to check this against the size of
the provided buffer to determine whether truncation has occurred.
.SH SEE ALSO
inet(3)
.SH DIAGNOSTICS
Fatal errors in
.I atoaddr
are:
empty input;
attempt to allocate temporary storage for a very long name failed;
name lookup failed;
syntax error in dotted-decimal form;
dotted-decimal component too large to fit in 8 bits.
.PP
Fatal errors in
.I atosubnet
are:
no
.B /
in
.IR src ;
.I atoaddr
error in conversion of
.I network
or
.IR mask ;
bit-count mask too big;
mask non-contiguous.
.PP
Fatal errors in
.I addrtoa
and
.I subnettoa
are:
unknown format.
.SH HISTORY
Written for the FreeS/WAN project by Henry Spencer.
.SH BUGS
The interpretation of incomplete dotted-decimal addresses
(e.g.
.B 10/24
means
.BR 10.0.0.0/24 )
differs from that of some older conversion
functions, e.g. those of
.IR inet (3).
The behavior of the older functions has never been
particularly consistent or particularly useful.
.PP
Ignoring leading zeros in dotted-decimal components and bit counts
is arguably the most useful behavior in this application,
but it might occasionally cause confusion with the historical use of leading 
zeros to denote octal numbers.
.PP
It is barely possible that somebody, somewhere,
might have a legitimate use for non-contiguous subnet masks.
.PP
.IR Getnetbyname (3)
is a historical dreg.
.PP
The restriction of ASCII-to-binary error reports to literal strings
(so that callers don't need to worry about freeing them or copying them)
does limit the precision of error reporting.
.PP
The ASCII-to-binary error-reporting convention lends itself
to slightly obscure code,
because many readers will not think of NULL as signifying success.
A good way to make it clearer is to write something like:
.PP
.RS
.nf
.B "const char *error;"
.sp
.B "error = atoaddr( /* ... */ );"
.B "if (error != NULL) {"
.B "        /* something went wrong */"
.fi
.RE