ttosa.3

This a good VPN source

.TH IPSEC_TTOSA 3 "26 Nov 2001"
.\" RCSID $Id: ttosa.3,v 1.12 2004/04/09 18:00:37 mcr Exp $
.SH NAME
ipsec ttosa, satot \- convert IPsec Security Association IDs to and from text
.br
ipsec initsaid \- initialize an SA ID
.SH SYNOPSIS
.B "#include <freeswan.h>
.sp
.B "typedef struct {"
.ti +1c
.B "ip_address dst;"
.ti +1c
.B "ipsec_spi_t spi;"
.ti +1c
.B "int proto;"
.br
.B "} ip_said;"
.sp
.B "const char *ttosa(const char *src, size_t srclen,"
.ti +1c
.B "ip_said *sa);
.br
.B "size_t satot(const ip_said *sa, int format,"
.ti +1c
.B "char *dst, size_t dstlen);"
.br
.B "void initsaid(const ip_address *addr, ipsec_spi_t spi,"
.ti +1c
.B "int proto, ip_said *dst);"
.SH DESCRIPTION
.I Ttosa
converts an ASCII Security Association (SA) specifier into an
.B ip_said
structure (containing
a destination-host address
in network byte order,
an SPI number in network byte order, and
a protocol code).
.I Satot
does the reverse conversion, back to a text SA specifier.
.I Initsaid
initializes an
.B ip_said
from separate items of information.
.PP
An SA is specified in text with a mail-like syntax, e.g.
.BR esp.5a7@1.2.3.4 .
An SA specifier contains
a protocol prefix (currently
.BR ah ,
.BR esp ,
.BR tun ,
.BR comp ,
or
.BR int ),
a single character indicating the address family
.RB ( .
for IPv4,
.B :
for IPv6),
an unsigned integer SPI number in hexadecimal (with no
.B 0x
prefix),
and an IP address.
The IP address can be any form accepted by
.IR ipsec_ttoaddr (3),
e.g. dotted-decimal IPv4 address,
colon-hex IPv6 address,
or DNS name.
.PP
As a special case, the SA specifier
.B %passthrough4
or
.B %passthrough6
signifies the special SA used to indicate that packets should be
passed through unaltered.
(At present, these are synonyms for
.B tun.0@0.0.0.0
and
.B tun:0@::
respectively,
but that is subject to change without notice.)
.B %passthrough
is a historical synonym for
.BR %passthrough4 . 
These forms are known to both
.I ttosa
and
.IR satot ,
so the internal representation is never visible.
.PP
Similarly, the SA specifiers
.BR %pass ,
.BR %drop ,
.BR %reject ,
.BR %hold ,
.BR %trap ,
and
.BR %trapsubnet
signify special ``magic'' SAs used to indicate that packets should be
passed, dropped, rejected (dropped with ICMP notification),
held,
and trapped (sent up to
.IR ipsec_pluto (8),
with either of two forms of
.B %hold
automatically installed)
respectively.
These forms too are known to both routines,
so the internal representation of the magic SAs should never be visible.
.PP
The
.B <freeswan.h>
header file supplies the
.B ip_said
structure, as well as a data type
.B ipsec_spi_t
which is an unsigned 32-bit integer.
(There is no consistency between kernel and user on what such a type
is called, hence the header hides the differences.)
.PP
The protocol code uses the same numbers that IP does.
For user convenience, given the difficulty in acquiring the exact set of
protocol names used by the kernel,
.B <freeswan.h>
defines the names
.BR SA_ESP ,
.BR SA_AH ,
.BR SA_IPIP ,
and
.BR SA_COMP
to have the same values as the kernel names
.BR IPPROTO_ESP ,
.BR IPPROTO_AH ,
.BR IPPROTO_IPIP ,
and
.BR IPPROTO_COMP .
.PP
.B <freeswan.h>
also defines
.BR SA_INT
to have the value
.BR 61
(reserved by IANA for ``any host internal protocol'')
and
.BR SPI_PASS ,
.BR SPI_DROP ,
.BR SPI_REJECT ,
.BR SPI_HOLD ,
and
.B SPI_TRAP
to have the values 256-260 (in \fIhost\fR byte order) respectively.
These are used in constructing the magic SAs
(which always have address
.BR 0.0.0.0 ).
.PP
If
.I satot
encounters an unknown protocol code, e.g. 77,
it yields output using a prefix
showing the code numerically, e.g. ``unk77''.
This form is
.I not
recognized by
.IR ttosa .
.PP
The
.I srclen
parameter of
.I ttosa
specifies the length of the 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 satot
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
.B <freeswan.h>
header file defines a constant,
.BR SATOT_BUF ,
which is the size of a buffer just large enough for worst-case results.
.PP
The
.I format
parameter of
.I satot
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
(currently
lowercase protocol prefix, lowercase hexadecimal SPI,
dotted-decimal or colon-hex address).
The value
.B 'f'
is similar except that the SPI is padded with
.BR 0 s
to a fixed 32-bit width, to ease aligning displayed tables.
.PP
.I Ttosa
returns
.B NULL
for success and
a pointer to a string-literal error message for failure;
see DIAGNOSTICS.
.I Satot
returns
.B 0
for a failure, and otherwise
always returns 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.
.PP
There is also, temporarily, support for some obsolete
forms of SA specifier which lack the address-family indicator.
.SH SEE ALSO
ipsec_ttoul(3), ipsec_ttoaddr(3), ipsec_samesaid(3), inet(3)
.SH DIAGNOSTICS
Fatal errors in
.I ttosa
are:
empty input;
input too small to be a legal SA specifier;
no
.B @
in input;
unknown protocol prefix;
conversion error in
.I ttoul
or
.IR ttoaddr .
.PP
Fatal errors in
.I satot
are:
unknown format.
.SH HISTORY
Written for the FreeS/WAN project by Henry Spencer.
.SH BUGS
The restriction of text-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 text-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 = ttosa( /* ... */ );"
.B "if (error != NULL) {"
.B "        /* something went wrong */"
.fi
.RE