s6-networking: the s6-tlsserver program

The `s6-tlsserver` program

s6-tlsserver is an UCSPI server tool for TLS/SSL connections over INET domain sockets. It acts as a TCP super-server that listens to connections, accepts them, and for each connection, establishes a TLS transport over it, then executes into a program.

Interface

     s6-tlsserver [ options ] [ -- ] ip port prog...

s6-tlsserver rewrites itself into a command line involving:
- s6-tcpserver, which listens to TCP connections on IP address ip port port and forks a command line for every connection. Note that s6-tcpserver also rewrites itself into a more complex command line (the final long-lived process being s6-tcpserverd), so your end command line may look a lot longer in ps than what you originally wrote. This is normal and healthy.
- (if applicable) s6-tcpserver-access, which performs TCP access control and various operations on the TCP connection.
- s6-tlsd, which establishes a TLS transport (server-side) over a connection, via an s6-tlsd-io child process.
- (if applicable) s6-applyuidgid, which drops root privileges.
- prog..., your client program, which is run with the same pid as s6-tlsd.
It runs until it is killed by a signal.

prog is expected to read from its peer on its standard input and write to its peer on its standard output. Since there will be an s6-tlsd-io program between prog and the network to perform the SSL encryption/decryption, those descriptors will not be a network socket - they will be pipes.

Signals

s6-tlsserver reacts to the same signals as s6-tcpserverd, which is the long-lived process hanging around.

Environment variables

Read

The following variables should be set before invoking s6-tlsserver, because they will be used by every s6-tlsd invocation:

KEYFILE
CERTFILE
TLS_UID and TLS_GID (if you run s6-tlsserver as root)
CADIR (if you want client certificates)
CAFILE (if you want client certificates, alternative to CADIR)

Setting both KEYFILE and CERTFILE is mandatory.

Written

prog... is run with the following variables added to, or removed from, its environment by s6-tcpserverd and possibly by s6-tcpserver-access:

PROTO
TCPREMOTEIP
TCPREMOTEPORT
TCPCONNNUM
TCPLOCALIP
TCPLOCALPORT
TCPREMOTEHOST
TCPLOCALHOST
TCPREMOTEINFO

Depending on TCP access rules (if the -i or -x option has been given), it is possible that prog's environment undergoes more modifications. Also, since s6-tlsd is always run after s6-tcpserver-access, it is possible to set different TLS/SSL parameters (typically a different KEYFILE and CERTFILE) depending on the client connection, by writing the correct set of TCP access rules.

Unless the -Z option is given to s6-tlsserver, the CADIR, CAFILE, KEYFILE, CERTFILE, TLS_UID and TLS_GID variables will not appear in prog's environment.

Options

s6-tlsserver accepts a myriad of options, most of which are passed as is to the correct executable. Not giving any options will generally work, but unless you're running a very public server (such as a Web server) or base your access control on client certificates, you probably still want TCP access rules.

Options informing s6-tlsserver behaviour

-L : if this option is given, s6-tlsserver will add an invocation of proxy-server so that a PROXY line or binary block is read in cleartext before the TLS tunnel is established. If the proxy uses the v2 version of the PROXY protocol and transmits SSL information from the client connection, this information will be reflected in the SSL environment variables transmitted to prog.

Options passed as is to s6-tcpserver

-q, -Q, -v
-1
-c maxconn
-C localmaxconn
-b backlog

Options passed as is to s6-tcpserver-access

The verbosity level, if not default, as -v0 or -v2
-w, -W : be strict or tolerant with DNS or IDENT resolution errors
-d, -D : enable or disable Nagle's algorithm
-r, -R : enable or disable IDENT lookups
-p, -P : enable or disable paranoid DNS cross-checking
-H : disable DNS lookups
-h : consult /etc/hosts before DNS
-l localname : get the local name from the command line, don't look it up
-B banner : initial server-side banner
-t timeout : set a timeout for all the lookups
-i rulesdir, -x rulesfile : TCP access control

Options passed as is to s6-tlsd

-Z, -z : keep or remove the s6-tlsd-io-specific variables from the application's environment
-S, -s : use close_notify or EOF to signal the end of a TLS connection
-J, -j : exit nonzero with an error message when the peer fails to close_notify, or ignore it
-Y, -y : request an optional or a mandatory client certificate
-K kimeout : set a timeout for the TLS handshake
-k snilevel : support SNI-based certificate chains

Options passed to s6-applyuidgid

-u uid, -g gid, -G gidlist : set uid, gid, or supplementary group list
-U (passed as -Uz) : get the uid, gid and supplementary group list from the UID, GID and GIDLIST variables, and remove these variables from the application's environment

Example

As root:

env KEYFILE=/etc/ssl/private/mykey.der CERTFILE=/etc/ssl/public/mycert.pem \
TLS_UID=65534 TLS_GID=65534 \
s6-envuidgid www \
s6-tlsserver -U -- 1.2.3.4 443 httpd

This will start a server listening to 1.2.3.4 on TCP port 443, and for every connection, spawn the httpd program reading queries on stdin and replying on stdout, as user www, with a TLS layer protecting the connection, the TLS engine running as user nobody (65534:65534). The server is authenticated by the certificate in /etc/ssl/public/mycert.pem that it sends to the client, and the private key in /etc/ssl/private/mykey.der that it keeps to itself.

The s6-tlsserver program