usocks
======
SOCKS5 proxy for Linux 6.1 or newer on io_uring. CONNECT to IPv4, IPv6
and domain names; one event loop thread per worker process, libc only.
Domain names are resolved by getaddrinfo() in 16 threads per worker. A
lookup that waits 10 s for a thread fails untried; one in progress takes
as long as the system resolver allows, which options timeout:1 attempts:2
in /etc/resolv.conf shortens. Half-close and data sent before the reply
are relayed. The handshake and each connect attempt time out after 10 s;
relayed connections have no idle timeout and rely on TCP keepalive. Each
connection slot reserves 130 KB of buffers, resident once used and kept
for later connections; past the limit, clients wait in the listen
backlog. Connections that fail before relaying log one line to stderr.

A worker relays on one CPU core. With -w, workers share the port through
SO_REUSEPORT and nothing else: the kernel spreads new connections among
them by address hash, -m applies to each, and when one exits, all exit.

Destinations are not filtered: clients reach whatever the host can,
including its loopback services, so listening beyond loopback without -u
makes an open proxy. RFC 1929 sends the password in plaintext. -p is
visible in ps until usocks blanks it at startup; USOCKS_PASS is not. A
unix socket takes its mode from the umask; a stale socket at its path is
replaced, a live one is not. Container seccomp profiles often block
io_uring.


Build
-----
    make
    make install

Needs a C99 compiler and Linux headers (Alpine: build-base linux-headers).
PREFIX defaults to /usr/local; DESTDIR is honored. For a static binary, use
make LDFLAGS=-static, preferably with musl.


Usage
-----
    usocks [-l addr:port|path] [-m maxconn] [-w workers] [-u user -p pass]

    -l  listen on addr:port, [addr6]:port or a unix socket path
        (default 127.0.0.1:1080)
    -m  connection limit per worker, 1 to 16384 (default 1024)
    -w  worker processes, 1 to 64 (default 1); not with a unix socket
    -u  require RFC 1929 authentication with this username
    -p  password for -u

USOCKS_LISTEN, USOCKS_MAXCONN, USOCKS_WORKERS, USOCKS_USER and USOCKS_PASS
set the same values. Flags take precedence.


Testing
-------
    make && ./test

Needs python3; takes about 15 s.