From 0f51cf5a216853cc5256eed26db2753a2fe92496 Mon Sep 17 00:00:00 2001
Message-Id: <0f51cf5a216853cc5256eed26db2753a2fe92496.1714725172.git.mdw@distorted.org.uk>
From: Mark Wooding <mdw@chiark.greenend.org.uk>
Date: Fri, 24 Apr 2015 10:11:23 +0100
Subject: [PATCH 1/1] contrib/knock.in: Can now be called from an ordinary
 shell.
Organization: Straylight/Edgeware

From: Mark Wooding <mdw@distorted.org.uk>

The script can now pick up information from environment variables rather
than the forced command, which makes the `tripe' user much more sane.
This also fits a little better with `sshsvc-mkauthkeys'.
---
 contrib/knock.in | 37 +++++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 16 deletions(-)

diff --git a/contrib/knock.in b/contrib/knock.in
index 28d8e532..b1754d93 100755
--- a/contrib/knock.in
+++ b/contrib/knock.in
@@ -3,21 +3,29 @@
 set -e
 
 ### This script performs the passive side of a dynamic association.  It is
-### intended to be set as the `tripe' user's shell, and invoked via ssh(1).
-### Specifically, for each dynamic peer, add a line to `.ssh/authorized_keys'
-### of the form
+### intended to be set as the forced command in an `.ssh/authorized_keys'
+### file.  Specifically, for each dynamic peer, add a line to
+### `.ssh/authorized_keys' of the form
 ###
-###	command="PEER" ssh-rsa ...
+###	environment="TRIPE_USER=PEER" ssh-rsa ...
 ###
 ### There's an additional wrinkle.  Suppose that the passive TrIPE endpoint
 ### is behind a NAT, and the SSH gateway is on a different machine.  The
 ### gateway should have its own `tripe' user, and this script should again be
 ### its shell.  On the gateway, add a `.ssh/authorized_keys' entry
 ###
-###	command="tripe@SERVER:PEER" ssh-rsa ...
+###	environment="TRIPE_USER=tripe@SERVER:PEER" ssh-rsa ...
 ###
 ### for the dynamic endpoint.  On the passive endpoint itself, you need an
-### entry for the gateway's `tripe' user's key, with no command.
+### entry for the gateway's `tripe' user's key, with `TRIPE_GATEWAY' set to
+### any value, like
+###
+###	environment="TRIPE_GATEWAY=t" ssh-rsa ...
+###
+### For backwards compatibility, it can also be set as the `tripe' user's
+### shell, with the `[tripe@SERVER:]PEER' indicator set as the forced
+### command.  If there are no forced command or `TRIPE_*' environment
+### variables then it is assumed that a gateway is calling.
 
 : ${prefix=@prefix@} ${exec_prefix=@exec_prefix@}
 : ${bindir=@bindir@}
@@ -26,28 +34,25 @@ set -e
 export TRIPEDIR TRIPESOCK
 
 ## Make sure we're being called properly, and figure out the peer identity.
-case "$#,$1" in
-  2,-c) ;;
+case "${TRIPE_USER+t},${TRIPE_GATEWAY+t},$#,$1" in
+  t,,0,) set -- "$TRIPE_USER" ;;
+  ,t,0,) set -- $SSH_ORIGINAL_COMMAND; unset SSH_ORIGINAL_COMMAND ;;
+  ,,2,-c) ;;
   *)
-    echo >&2 "usage: $0 -c '[SERVER:]PEER [hello|goodbye]'"
+    echo >&2 "usage: $0 -c [SERVER:]PEER [ACTION]"
     exit 1
     ;;
 esac
 
-## SSH has smushed all of our arguments together, so let's split them apart
-## again.
-set -- $2
-
 ## Examine the peer identifier and work out how to proceed.
 case "$#,$1" in
-  0,*) echo >&2 "$0: missing peer identifier"; exit 1 ;;
+  0,*) echo >&2 "missing peer identifier"; exit 1 ;;
   *:*) mode=proxy server=${1%:*} user=${1##*:} ;;
   *) mode=local user=$1 ;;
 esac
 shift
 
-## If there's no action then check to see whether SSH has hidden one
-## somewhere.  Make sure the command looks sensible.
+## Fetch the optional command from where SSH stashed it.
 case "$#" in 0) set -- $SSH_ORIGINAL_COMMAND ;; esac
 case "$#,$1" in
   0, | 1,hello) act=hello ;;
-- 
[mdw]