testing: Configure curve25519-sha256 as key exchange for SSH

With Debian bookworm, the PQC KE sntrup761x25519-sha512 is negotiated, by
default.  This increases the overhead significantly, in particular, the
size of the KE message, which wouldn't get through IPsec tunnels without
MSS clamping.

diff --git a/testing/hosts/default/etc/ssh/sshd_config b/testing/hosts/default/etc/ssh/sshd_config
index f4ced3e37420bc93440d07fbda44836f710a05b5..700c7caaf7f64008207e584c2a91ba25782c35eb 100644 (file)
--- a/testing/hosts/default/etc/ssh/sshd_config
+++ b/testing/hosts/default/etc/ssh/sshd_config
@@ -1,6 +1,7 @@
 Port 22
 Protocol 2
 Ciphers aes128-gcm@openssh.com
+KexAlgorithms curve25519-sha256
 HostKey /etc/ssh/ssh_host_rsa_key
 HostKey /etc/ssh/ssh_host_ecdsa_key
 PermitRootLogin yes