const char *atosa(const char *src, size_t srclen,
struct sa_id *sa);
size_t satoa(struct sa_id sa, int format,
char *dst, size_t dstlen);
struct sa_id {
struct in_addr dst;
ipsec_spi_t spi;
int proto;
};
Atosa converts an ASCII Security Association (SA) specifier into an sa_id structure (containing a destination-host address in network byte order, an SPI number in network byte order, and a protocol code). Satoa does the reverse conversion, back to an ASCII SA specifier.
An SA is specified in ASCII with a mail-like syntax, e.g. esp507@1.2.3.4. An SA specifier contains a protocol prefix (currently ah, esp, or tun), an unsigned integer SPI number, and an IP address. The SPI number can be decimal or hexadecimal (with 0x prefix), as accepted by ipsec_atoul(3). The IP address can be any form accepted by ipsec_atoaddr(3), e.g. dotted-decimal address or DNS name.
As a special case, the SA specifier %passthrough signifies the special SA used to indicate that packets should be passed through unaltered. (At present, this is a synonym for tun0x0@0.0.0.0, but that is subject to change without notice.) This form is known to both atosa and satoa, so the internal form of %passthrough is never visible.
The <freeswan.h> header file supplies the sa_id structure, as well as a data type 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.)
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, <freeswan.h> defines the names SA_ESP, SA_AH, and SA_IPIP to have the same values as the kernel names IPPROTO_ESP, IPPROTO_AH, and IPPROTO_IPIP.
The srclen parameter of atosa specifies the length of the ASCII string pointed to by 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 srclen value of 0 is taken to mean strlen(src).
The dstlen parameter of satoa specifies the size of the dst parameter; under no circumstances are more than dstlen bytes written to dst. A result which will not fit is truncated. Dstlen can be zero, in which case 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 freeswan.h header file defines a constant, SATOA_BUF, which is the size of a buffer just large enough for worst-case results.
The format parameter of satoa specifies what format is to be used for the conversion. The value 0 (not the ASCII character '0', but a zero value) specifies a reasonable default (currently lowercase protocol prefix, lowercase hexadecimal SPI, dotted-decimal address). The value d causes the SPI to be generated in decimal instead.
Atosa returns NULL for success and a pointer to a string-literal error message for failure; see DIAGNOSTICS. Satoa returns 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.
Fatal errors in satoa are: unknown format; unknown protocol code.
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.
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:
const char *error;
error = atoaddr( /* ... */ );
if (error != NULL) {
/* something went wrong */