From 712c1520f8ebfecf3bc35d9aca21d73d7188f0c2 Mon Sep 17 00:00:00 2001
From: rockfish-vyos <321823801+rockfish-vyos@users.noreply.github.com>
Date: Mon, 31 Aug 2026 17:57:40 +0000
Subject: ipsec: T9254: do not emit ESN transforms in IKE proposals

get_esp_ike_cipher() renders both IKE_SA proposals and CHILD_SA
proposals. It unconditionally appended the -esn / -noesn variants,
which are only meaningful for ESP: ESN is Transform Type 5 per
RFC 7296 section 3.3.2 and is defined for CHILD_SAs only.

Emitting ESN in an IKE_SA proposal breaks interoperability with
implementations that reject the malformed payload without replying
at all. Observed against a Cisco FTD: the IKE_SA_INIT is silently
discarded, no NO_PROPOSAL_CHOSEN and no INVALID_KE_PAYLOAD, so the
tunnel never comes up and nothing on the initiator side explains it.

Add an explicit esn parameter defaulting to True so ESP behaviour is
unchanged, and pass esn=False at the four IKE call sites: peer.j2,
profile.j2, remote_access.j2 and l2tp.j2. The five esp_proposals call
sites keep ESN.

ike_group could have served as the discriminator, since IKE call sites
do not pass it, but that coupling is incidental: the argument only
exists to resolve PFS.

Smoke test assertions on the proposals lines are updated accordingly;
the esp_proposals assertions are left untouched.
---
 python/vyos/template.py | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

(limited to 'python')

diff --git a/python/vyos/template.py b/python/vyos/template.py
index dbbc4df84..1233710f2 100755
--- a/python/vyos/template.py
+++ b/python/vyos/template.py
@@ -483,7 +483,16 @@ def get_first_ike_dh_group(ike_group):
     return 'dh-group2' # Fallback on dh-group2
 
 @register_filter('get_esp_ike_cipher')
-def get_esp_ike_cipher(group_config, ike_group=None):
+def get_esp_ike_cipher(group_config, ike_group=None, esn=True):
+    """Render strongSwan proposal strings.
+
+    esn=True  : ESP/CHILD_SA proposals, where ESN transforms are meaningful
+    esn=False : IKE_SA proposals. ESN is a CHILD_SA transform (RFC 7296
+                section 3.3.2, Transform Type 5) and has no meaning in an
+                IKE_SA proposal. Emitting it there breaks interoperability
+                with implementations that reject the malformed payload
+                without replying at all (observed with Cisco FTD, T9254).
+    """
     pfs_lut = {
         'dh-group1'  : 'modp768',
         'dh-group2'  : 'modp1024',
@@ -530,10 +539,12 @@ def get_esp_ike_cipher(group_config, ike_group=None):
                     group = get_first_ike_dh_group(ike_group)
                 tmp += '-' + pfs_lut[group]
 
-            # For 'optional' and 'disabled' we need two values as
-            # proposal without '-esn'/'-noesn' is incompatible with
-            # proposals with any of them.
-            if 'esn' in proposal:
+            # ESP/CHILD_SA only. For 'optional' and 'disabled' we need two
+            # values as a proposal without '-esn'/'-noesn' is incompatible
+            # with proposals carrying any of them. This pairing is meaningless
+            # for an IKE_SA, which has no ESN transform at all - see the esn
+            # parameter above.
+            if esn and 'esn' in proposal:
                 if proposal['esn'] == 'required':
                     tmp += '-esn'
                 elif proposal['esn'] == 'optional':
-- 
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