SYNOPSIS

sredird option loglevel device [pollinginterval]

DESCRIPTION

sredird is:

• an RFC 2217 compliant serial port redirector
• maps a network port to a serial device: serial port parameters are configured using an extension to the telnet protocol
• runs under a UCSPI or other inetd style service such as systemd for process level isolation
• restricts process operations using seccomp(2), pledge(2), capsicum(4) or setrlimit(2)

sredird can be used as a minimal serial console server on a device like a raspberry pi zero w.

A picocom branch supports RFC 2217 (with the patch below).

This version of sredird is a fork of sredird 2.2.1-1.1 taken from Ubuntu 16.04 (there does not seem to be a canonical source repository for this project). sredird 2.2.1-1.1 is the last C version: later versions of sredird (2.2.1-2) switched to C++.

EXAMPLES

apt install daemontools

• unixexec
• hexlog
• tscat

Here is my setup:

• raspberry pi zero w acting as a console server for other raspberry pi's
• example of setup using unixexec
• TODO: show example xmppbot

/etc/udev/rules.d/10-usb-serial.rules

SUBSYSTEM=="tty", ATTRS{idProduct}=="6001", ATTRS{idVendor}=="0403", ATTRS{serial}=="FTG9GBNY", SYMLINK+="console@getpid"
SUBSYSTEM=="tty", ATTRS{idProduct}=="2008", ATTRS{idVendor}=="0557", SYMLINK+="console@switch"
SUBSYSTEM=="tty", ATTRS{idProduct}=="2303", ATTRS{idVendor}=="067b", ATTRS{version}==" 1.10", SYMLINK+="console@getsid"
SUBSYSTEM=="tty", ATTRS{idProduct}=="2303", ATTRS{idVendor}=="067b", ATTRS{version}==" 2.00", SYMLINK+="console@sigquit"

service run

• service/console@getpid/run

#!/bin/bash

umask 077

mkdir -p /tmp/sredird

exec 2>&1
exec unixexec /tmp/sredird/console@getpid \
  hexlog none \
  sredird -t 900 5 /dev/console@getpid

service run log

• service/console@getpid/log/run

#!/bin/bash

set -o errexit
set -o nounset
set -o pipefail

SERVICE="$(basename $(dirname $PWD))"
exec tscat -o 2 "$SERVICE"

USAGE

loglevel : numeric syslog level, see syslog(3)

device : serial device

pollinginterval : Poll interval is in milliseconds, default is 100, 0 means no polling

OPTIONS

-i, --cisco-compatibility : indicates Cisco IOS Bug compatibility

-t, --timeout seconds :set inactivity timeout

BUILDING

make

# selecting process restrictions
RESTRICT_PROCESS=seccomp make clean all

# rlimit
RESTRICT_PROCESS=rlimit make clean all

# disable process restrictions
RESTRICT_PROCESS=null make clean all

#### using musl
# sudo apt install musl-dev musl-tools

RESTRICT_PROCESS=rlimit ./musl-make clean all

## linux seccomp sandbox: requires kernel headers
export MUSL_INCLUDE=/tmp
git clone https://github.com/sabotage-linux/kernel-headers.git $MUSL_INCLUDE/kernel-headers
./musl-make clean all

ALTERNATIVES

PICOCOM

To use the picocom branch with RFC 2217 protocol support with sredird, apply the following patch and compile:

git clone -b rfc2217 https://github.com/npat-efault/picocom.git
cd picocom
patch -p1 < picocom.patch
make

diff --git a/Makefile b/Makefile
index 75f3fdb..f3c344f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 
-VERSION = 4.0a
+VERSION = 4.0a+0.2.1
 
 #CC ?= gcc
 CPPFLAGS += -DVERSION_STR=\"$(VERSION)\"
@@ -46,9 +46,9 @@ linenoise-1.0/linenoise.o : linenoise-1.0/linenoise.c linenoise-1.0/linenoise.h
 #CPPFLAGS += -DNO_CUSTOM_BAUD
 
 ## Comment these in to enable RFC2217 support
-#CPPFLAGS += -DUSE_RFC2217
-#OBJS += tn2217.o
-#tn2217.o : tn2217.c tn2217.h tncomport.h fdio.h termint.h term.h
+CPPFLAGS += -DUSE_RFC2217
+OBJS += tn2217.o
+tn2217.o : tn2217.c tn2217.h tncomport.h fdio.h termint.h term.h
 
 ## Comment this IN to remove help strings (saves ~ 4-6 Kb).
 #CPPFLAGS += -DNO_HELP
diff --git a/tn2217.c b/tn2217.c
index d54676a..f02b836 100644
--- a/tn2217.c
+++ b/tn2217.c
@@ -129,7 +129,7 @@ struct tn2217_state {
     struct termios termios;     /* Predicted remote com port geometry */
     int modem;                  /* Predicted remote com port signals */
 
-    unsigned char cmdbuf[32];   /* IAC command accumulator */
+    unsigned char cmdbuf[255];  /* IAC command accumulator */
     unsigned char cmdbuflen;
     unsigned int cmdiac : 1;    /* 1 iff last cmdbuf ch is incomplete IAC */