7
IPsec VPN
148
Using ike -snoop
VPN tunnel negotiation
When
setting
up
IPsec
tunnels,
problems
can
arise
because
the
initial
negotiation
fails
when
the
network
devices
at
either
end
of
a
VPN
tunnel
try
but
fail
to
agree
on
which
protocols
and
encryption
methods
will
be
used.
The
CLI
command
ike
‐
snoop
is
a
tool
that
can
be
used
to
identify
such
problems
by
showing
the
details
of
the
exchanges
that
occur
when
the
peers
at
either
end
of
the
IPsec
tunnel
try
to
agree
on
a
mutually
acceptable
set
of
algorithms.
The
algorithm
lists
they
exchange
are
often
referred
to
as
proposal
lists
.
Using ike -snoop
The
ike
‐
snoop
command
is
entered
via
a
CLI
console.
To
begin
monitoring
with
basic,
summarized
output,
the
full
CLI
command
is:
Device:/>
ike
‐
snoop
‐
brief
This
causes
diagnostic
output
to
be
continuously
sent
to
the
console
for
all
VPN
tunnel
IKE
negotiations
that
takes
place,
regardless
of
interface.
To
turn
off
monitoring,
the
command
is:
Device:/>
ike
‐
snoop
‐
off
Presented
below
is
some
typical
ike
‐
snoop
output
(the
formatting
has
been
changed
slightly
to
fit
the
page).
The
tunnel
negotiation
considered
is
based
on
pre
‐
shared
Keys.
A
negotiation
based
on
certificates
is
not
discussed
here
but
the
principles
are
the
same.
SNOOP:
IKEv2:
2010
‐
05
‐
31
12:30:09
172.22.53.18:500
<
‐
172.22.53.23:500
IKE_SA_INIT
request
0
[
SA
KE
No
N(NATD_S_IP)
N(NATD_D_IP)
]
SNOOP:
IKEv2:
2010
‐
05
‐
31
12:30:09
172.22.53.18:500
‐
>
172.22.53.23:500
IKE_SA_INIT
response
0
[
SA
KE
No
N(NATD_S_IP)
N(NATD_D_IP)
N(MULT_AUTH)
]
SNOOP:
IKEv2:
2010
‐
05
‐
31
12:30:09
172.22.53.18:500
<
‐
172.22.53.23:500
IKE_AUTH
request
1
[
IDi
AUTH
SA
TSi
TSr
N(INIT_CONTACT)
N(ESP_TFC_PAD_N)
N(NON_FIRST_FRAG)
]
SNOOP:
IKEv2:
2010
‐
05
‐
31
12:30:09
172.22.53.18:500
‐
>
172.22.53.23:500
IKE_AUTH
response
1
[
IDr
AUTH
SA
TSi
TSr
N(AUTH_LFT)
]
In
these
listings
the
direction
of
packet
movement
is
indicated
by
the
<
‐
and
‐
>
symbols.
The
IPv4
address
and
port
number
given
on
the
left
side
of
these
symbols
is
local.
The
address
and
port
number
given
on
the
right
side
is
remote.
Each
packet
movement
is
preceded
by
a
timestamp.