The `s6-ucspitlsd` program

s6-ucspitlsd is a server-side program that establishes communication channels according to the UCSPI-TLS protocol, then execs into an application. Later, if the application sends a command, a TLS tunnel will be started and the application will be able to use it instead of communicating with the network via cleartext.

The point of this protocol, and this program, is to make it easy to implement commands like SMTP's STARTTLS without embedding a TLS stack in the server itself.

Interface

     s6-ucspitlsd [ -S | -s ] [ -Y | -y ] [ -Z | -z ] [ -v verbosity ] [ -K kimeout ] [ -- ] prog...

s6-ucspitlsd expects to have an open TCP connection it can talk to on its stdin and stdout.
It forks and establishes communication channels between the parent and the child. The parent executes into prog...; the child remains and waits for a command.
At any time, prog... can send a command to its control socket provided by s6-ucspitlsd, following the UCSPI-TLS protocol. Then the s6-ucspitlsd child will exec into a s6-tlsd-io process that will initialize the TLS connection, perform the handshake (expecting a TLS client on the other side of the network) and maintain a TLS tunnel. prog can use that TLS tunnel instead of talking directly to the network.

Exit codes

100: wrong usage.
111: system call failed.

Normally the parent s6-ucspitlsd process execs into prog... and the child process execs into s6-tlsd-io. If the parent dies or closes its control socket before sending a command to start TLS, the child exits 0.

Environment variables

Read

s6-ucspitlsd does not expect to have any particular environment variables, but it spawns a s6-tlsd-io program that does. So it should pay attention to the following variables:

CERTFILE and KEYFILE
(if the -Y or -y option has been given) CADIR or CAFILE
TLS_UID and TLS_GID

Written

By default, prog... is run with all these variables unset: CADIR, CAFILE, KEYFILE, CERTFILE, TLS_UID and TLS_GID. They're passed to the s6-tlsd-io child but not to prog.... The -Z option prevents that behaviour.

However, prog... is run with the following additional environment variables, following the UCSPI-TLS protocol:

SSLCTLFD contains the file descriptor number of the control socket.
SSLREADFD contains the file descriptor number of the pipe used to read data from the TLS tunnel after it has been activated.
SSLWRITEFD contains the file descriptor number of the pipe used to write data to the TLS tunnel after it has been activated.

Since prog is exec'ed before the TLS handshake takes place, it cannot get information about the TLS connection via environment variables. However, if it starts the TLS connection via a Y command (as opposed to a y command), it will receive this information as a string sent over the control socket.

Options

-v verbosity : Be more or less verbose. Default for verbosity is 1. 0 is quiet, 2 is verbose, more than 2 is debug output. This option currently has no effect.
-Z : do not clean the environment of the variables used by s6-tlsc-io before execing prog....
-z : clean the environment of the variables used by s6-tlsd-io before execing prog.... This is the default.
-S : send a close_notify alert and break the connection when prog sends EOF.
-s : transmit EOF by half-closing the TCP connection without using close_notify. This is the default.
-Y : Require an optional client certificate.
-y : Require a mandatory client certificate. The default, with neither the -Y nor the -y option, is not to require a client certificate at all.
-K kimeout : close the connection if the handshake takes more than kimeout milliseconds to complete. The default is 0, which means infinite timeout: let the handshake complete at its own pace, no matter how slow.

Notes

It only makes sense to run s6-ucspitlsd if its application program prog knows and understands the UCSPI-TLS protocol for opportunistic TLS. If it does not, you will not be able to secure your connection, and what you need is a regular immediate TLS program instead, which means s6-tlsd.

The s6-ucspitlsd program