src/monkeysphere-ssh-proxycommand



#!/usr/bin/env bash

# monkeysphere-ssh-proxycommand: MonkeySphere ssh ProxyCommand hook
#
# The monkeysphere scripts are written by:
# Jameson Rollins <jrollins@fifthhorseman.net>
#
# They are Copyright 2008, and are all released under the GPL, version 3
# or later.

# This is meant to be run as an ssh ProxyCommand to initiate a
# monkeysphere known_hosts update before an ssh connection to host is
# established.  Can be added to ~/.ssh/config as follows:
#  ProxyCommand monkeysphere-ssh-proxycommand %h %p

########################################################################
PGRM=$(basename $0)

SYSSHAREDIR=${MONKEYSPHERE_SYSSHAREDIR:-"/usr/share/monkeysphere"}
export SYSSHAREDIR
. "${SYSSHAREDIR}/common" || exit 1

########################################################################
# FUNCTIONS
########################################################################

usage() {
    cat <<EOF >&2
usage: ssh -o ProxyCommand="$(basename $0) %h %p" ...
EOF
}

log() {
    echo "$@" >&2
}

output_no_valid_key() {
    local sshKeyOffered
    local userID
    local type
    local validity
    local keyid
    local uidfpr
    local usage
    local sshKeyGPG
    local tmpkey
    local sshFingerprint
    local gpgSigOut

    userID="ssh://${HOSTP}"

    log "-------------------- Monkeysphere warning -------------------"
    log "Monkeysphere found OpenPGP keys for this hostname, but none had full validity."

    # retrieve the actual ssh key
    sshKeyOffered=$(ssh-keyscan -t rsa -p "$PORT" "$HOST" 2>/dev/null | awk '{ print $2, $3 }')
    # FIXME: should we do any checks for failed keyscans, eg. host not
    # found?

    # get the gpg info for userid
    gpgOut=$(gpg --list-key --fixed-list-mode --with-colon \
    --with-fingerprint --with-fingerprint \
    ="$userID" 2>/dev/null)

    # find all 'pub' and 'sub' lines in the gpg output, which each
    # represent a retrieved key for the user ID
    echo "$gpgOut" | cut -d: -f1,2,5,10,12 | \
    while IFS=: read -r type validity keyid uidfpr usage ; do
    case $type in
        'pub'|'sub')
        # get the ssh key of the gpg key
        sshKeyGPG=$(gpg2ssh "$keyid")

        # if one of keys found matches the one offered by the
        # host, then output info
        if [ "$sshKeyGPG" = "$sshKeyOffered" ] ; then
            log "An OpenPGP key matching the ssh key offered by the host was found:"
            log

            # do some crazy "Here Strings" redirection to get the key to
            # ssh-keygen, since it doesn't read from stdin cleanly
            sshFingerprint=$(ssh-keygen -l -f /dev/stdin \
            <<<$(echo "$sshKeyGPG") | \
            awk '{ print $2 }')

            # get the sigs for the matching key
            gpgSigOut=$(gpg --check-sigs \
            --list-options show-uid-validity \
            "$keyid")

            # output the sigs, but only those on the user ID
            # we are looking for
            echo "$gpgSigOut" | awk '
{
if (match($0,"^pub")) { print; }
if (match($0,"^uid")) { ok=0; }
if (match($0,"^uid.*'$userID'$")) { ok=1; print; }
if (ok) { if (match($0,"^sig")) { print; } }
}
' >&2
            log

            # output the other user IDs for reference
            if (echo "$gpgSigOut" | grep "^uid" | grep -v -q "$userID") ; then
            log "Other user IDs on this key:"
            echo "$gpgSigOut" | grep "^uid" | grep -v "$userID" >&2
            log
            fi

            # output ssh fingerprint
            log "RSA key fingerprint is ${sshFingerprint}."

            # this whole process is in a "while read"
            # subshell.  the only way to get information out
            # of the subshell is to change the return code.
            # therefore we return 1 here to indicate that a
            # matching gpg key was found for the ssh key
            # offered by the host
            return 1
        fi
        ;;
    esac
    done

    # if no key match was made (and the "while read" subshell returned
    # 1) output how many keys were found
    if (($? != 1)) ; then
    log "None of the found keys matched the key offered by the host."
    log "Run the following command for more info about the found keys:"
    log "gpg --check-sigs --list-options show-uid-validity =${userID}"
    # FIXME: should we do anything extra here if the retrieved
    # host key is actually in the known_hosts file and the ssh
    # connection will succeed?  Should the user be warned?
    # prompted?
    fi

    log "-------------------- ssh continues below --------------------"
}

########################################################################

# export the monkeysphere log level
export MONKEYSPHERE_LOG_LEVEL

if [ "$1" = '--no-connect' ] ; then
    NO_CONNECT='true'
    shift 1
fi

HOST="$1"
PORT="$2"

if [ -z "$HOST" ] ; then
    log "Host not specified."
    usage
    exit 255
fi
if [ -z "$PORT" ] ; then
    PORT=22
fi

# set the host URI
if [ "$PORT" != '22' ] ; then
    HOSTP="${HOST}:${PORT}"
else
    HOSTP="${HOST}"
fi
URI="ssh://${HOSTP}"

# specify keyserver checking.  the behavior of this proxy command is
# intentionally different than that of running monkeyesphere normally,
# and keyserver checking is intentionally done under certain
# circumstances.  This can be overridden by setting the
# MONKEYSPHERE_CHECK_KEYSERVER environment variable.

# if the host is in the gpg keyring...
if gpg --list-key ="${URI}" 2>&1 >/dev/null ; then
    # do not check the keyserver
    CHECK_KEYSERVER="false"

# if the host is NOT in the keyring...
else
    # if the host key is found in the known_hosts file...
    # FIXME: this only works for default known_hosts location
    hostKey=$(ssh-keygen -F "$HOST" 2>/dev/null)

    if [ "$hostKey" ] ; then
    # do not check the keyserver
    # FIXME: more nuanced checking should be done here to properly
    # take into consideration hosts that join monkeysphere by
    # converting an existing and known ssh key
    CHECK_KEYSERVER="false"

    # if the host key is not found in the known_hosts file...
    else
    # check the keyserver
    CHECK_KEYSERVER="true"
    fi
fi
# set and export the variable for use by monkeysphere
MONKEYSPHERE_CHECK_KEYSERVER=${MONKEYSPHERE_CHECK_KEYSERVER:="$CHECK_KEYSERVER"}
export MONKEYSPHERE_CHECK_KEYSERVER

# update the known_hosts file for the host
monkeysphere update-known_hosts "$HOSTP"

# output on depending on the return of the update-known_hosts
# subcommand, which is (ultimately) the return code of the
# update_known_hosts function in common
case $? in
    0)
    # acceptable host key found so continue to ssh
    true
    ;;
    1)
    # no hosts at all found so also continue (drop through to
    # regular ssh host verification)
    true
    ;;
    2)
    # at least one *bad* host key (and no good host keys) was
    # found, so output some usefull information
    output_no_valid_key
    ;;
    *)
    # anything else drop through
    true
    ;;
esac

# FIXME: what about the case where monkeysphere successfully finds a
# valid key for the host and adds it to the known_hosts file, but a
# different non-monkeysphere key for the host already exists in the
# known_hosts, and it is this non-ms key that is offered by the host?
# monkeysphere will succeed, and the ssh connection will succeed, and
# the user will be left with the impression that they are dealing with
# a OpenPGP/PKI host key when in fact they are not.  should we use
# ssh-keyscan to compare the keys first?

# exec a netcat passthrough to host for the ssh connection
if [ -z "$NO_CONNECT" ] ; then
    if (which nc 2>/dev/null >/dev/null); then
    exec nc "$HOST" "$PORT"
    elif (which socat 2>/dev/null >/dev/null); then
    exec socat STDIO "TCP:$HOST:$PORT"
    else
    echo "Neither netcat nor socat found -- could not complete monkeysphere-ssh-proxycommand connection to $HOST:$PORT" >&2
    exit 255
    fi
fi