Josh McAllister provided the following sample ipsec.conf, which can be used to connect m0n0wall with FreeS/WAN in a site to site IPsec configuration.
# /etc/ipsec.conf - FreeS/WAN IPsec configuration file version 2.0 # conforms to second version of ipsec.conf specification config setup interfaces=%defaultroute klipsdebug=none plutodebug=none uniqueids=yes # defaults for subsequent connection descriptions conn %default # How persistent to be in (re)keying negotiations (0 means very). keyingtries=0 #compress=yes conn block auto=ignore conn private auto=ignore conn private-or-clear auto=ignore conn clear-or-private auto=ignore conn clear auto=ignore conn packetdefault auto=ignore conn josh type=tunnel left=ip.add.of.m0n0 leftsubnet=m0n0.side.subnet/24 leftnexthop=%defaultroute right=ip.add.of.freeswan rightsubnet=freeswan.side.subnet/24 rightnexthop=%defaultroute authby=secret auth=esp esp=3des-md5-96 pfs=no auto=start m0n0-side: Phase1 Neg. mode = main Enc. Alg = 3DES Hash Alg = MD5 DH key grp = 5 Phase2 Protocol = ESP Uncheck all Enc. Alg. Except 3des Hash alg = md5 PFS key group = off