---
name: "tls_config_parse_protocols"
section: "3"
platform: "linux"
package: "libretls"
version: "3.8.1-3"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/tls_config_set_protocols"
---

# tls\_config\_parse\_protocols(3)

TLS protocol and cipher selection

Part of [tls\_config\_set\_protocols(3)](/tls_config_set_protocols/3.md).

## NAME

**tls\_config\_set\_protocols**, **tls\_config\_parse\_protocols**, **tls\_config\_set\_alpn**, **tls\_config\_set\_ciphers**, **tls\_config\_set\_dheparams**, **tls\_config\_set\_ecdhecurves**, **tls\_config\_prefer\_ciphers\_client**, **tls\_config\_prefer\_ciphers\_server** – TLS protocol and cipher selection

## SYNOPSIS

**#include \<tls.h\>**

*int*\

**tls\_config\_set\_protocols**(*struct tls\_config \*config*, *uint32\_t protocols*);

*int*\

**tls\_config\_parse\_protocols**(*uint32\_t \*protocols*, *const char \*protostr*);

*int*\

**tls\_config\_set\_alpn**(*struct tls\_config \*config*, *const char \*alpn*);

*int*\

**tls\_config\_set\_ciphers**(*struct tls\_config \*config*, *const char \*ciphers*);

*int*\

**tls\_config\_set\_dheparams**(*struct tls\_config \*config*, *const char \*params*);

*int*\

**tls\_config\_set\_ecdhecurves**(*struct tls\_config \*config*, *const char \*curves*);

*void*\

**tls\_config\_prefer\_ciphers\_client**(*struct tls\_config \*config*);

*void*\

**tls\_config\_prefer\_ciphers\_server**(*struct tls\_config \*config*);

## DESCRIPTION

These functions modify a configuration by setting parameters. The configuration options apply to both clients and servers, unless noted otherwise.

**tls\_config\_set\_protocols**() specifies which versions of the TLS protocol may be used. Possible values are the bitwise OR of:

-

  TLS\_PROTOCOL\_TLSv1\_2

-

  TLS\_PROTOCOL\_TLSv1\_3

Additionally, the values TLS\_PROTOCOL\_TLSv1 (TLSv1.2, TLSv1.3), TLS\_PROTOCOLS\_ALL (all supported protocols) and TLS\_PROTOCOLS\_DEFAULT (TLSv1.2 and TLSv1.3) may be used.

The **tls\_config\_parse\_protocols**() utility function parses a protocol string and returns the corresponding value via the *protocols* argument. This value can then be passed to the **tls\_config\_set\_protocols**() function. The protocol string is a comma or colon separated list of keywords. Valid keywords are:

- tlsv1.2

  - tlsv1.3

  - all

  (all supported protocols)

- default

  (an alias for secure)

- legacy

  (an alias for all)

- secure

  (currently TLSv1.2 and TLSv1.3)

If a value has a negative prefix (in the form of a leading exclamation mark) then it is removed from the list of available protocols, rather than being added to it.

**tls\_config\_set\_alpn**() sets the ALPN protocols that are supported. The alpn string is a comma separated list of protocols, in order of preference.

**tls\_config\_set\_ciphers**() sets the list of ciphers that may be used. Lists of ciphers are specified by name, and the permitted names are:

-

  secure (or alias default)

-

  compat

-

  legacy

-

  insecure (or alias all)

Alternatively, libssl cipher strings can be specified. See the CIPHERS section of [openssl(1)](/openssl/1.md) for further information.

**tls\_config\_set\_dheparams**() specifies the parameters that will be used during Diffie-Hellman Ephemeral (DHE) key exchange. Possible values are:

-

  none

-

  auto

-

  legacy

In auto mode, the key size for the ephemeral key is automatically selected based on the size of the private key being used for signing. In legacy mode, 1024 bit ephemeral keys are used. The default value is none, which disables DHE key exchange.

**tls\_config\_set\_ecdhecurves**() specifies the names of the elliptic curves that may be used during Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) key exchange. This is a comma separated list, given in order of preference. The special value of "default" will use the default curves (currently X25519, P-256 and P-384). This function replaces **tls\_config\_set\_ecdhecurve**(), which is deprecated.

**tls\_config\_prefer\_ciphers\_client**() prefers ciphers in the client's cipher list when selecting a cipher suite (server only). This is considered to be less secure than preferring the server's list.

**tls\_config\_prefer\_ciphers\_server**() prefers ciphers in the server's cipher list when selecting a cipher suite (server only). This is considered to be more secure than preferring the client's list and is the default.

## RETURN VALUES

These functions return 0 on success or -1 on error.

## SEE ALSO

[tls\_config\_ocsp\_require\_stapling(3)](/tls_config_ocsp_require_stapling/3.md), [tls\_config\_set\_session\_id(3)](/tls_config_set_session_id/3.md), [tls\_config\_verify(3)](/tls_config_verify/3.md), [tls\_init(3)](/tls_init/3.md), [tls\_load\_file(3)](/tls_load_file/3.md)

## HISTORY

**tls\_config\_set\_ciphers**() appeared in OpenBSD 5.6 and got its final name in OpenBSD 5.7.

**tls\_config\_set\_protocols**(), **tls\_config\_parse\_protocols**(), **tls\_config\_set\_dheparams**(), and **tls\_config\_set\_ecdhecurve**() appeared in OpenBSD 5.7, **tls\_config\_prefer\_ciphers\_client**() and **tls\_config\_prefer\_ciphers\_server**() in OpenBSD 5.9, and **tls\_config\_set\_alpn**() in OpenBSD 6.1.

## AUTHORS

Joel Sing ⟨*jsing@openbsd.org*⟩ with contributions from Ted Unangst ⟨*tedu@openbsd.org*⟩ (**tls\_config\_set\_ciphers**()) and Reyk Floeter ⟨*reyk@openbsd.org*⟩ (**tls\_config\_set\_ecdhecurve**())