[Libguestfs] [v2v PATCH 2/2] input-xen: replace "enable LEGACY crypto" advice with targeted ssh options

[Laszlo Ersek]

- "KexAlgorithms": the Fedora 35 ssh binary, using the DEFAULT
  crypto-policy, cannot log in to RHEL5 sshd without relaxing
  "KexAlgorithms". The server offers three algorithms:
  "diffie-hellman-group-exchange-sha1", "diffie-hellman-group14-sha1",
  "diffie-hellman-group1-sha1"; and according to RFC 9142,
  "diffie-hellman-group14-sha1" is the least deprecated from those. (The
  RFC marks it as MAY be implemented, and marks the other two as SHOULD
  NOT be implemented.) Recommend "diffie-hellman-group14-sha1".

- "MACs": the Fedora 35 ssh binary, using the FUTURE crypto-policy, cannot
  log in to RHEL5 sshd without relaxing "MACs". The server offers
  "hmac-md5", "hmac-sha1", "hmac-ripemd160", "hmac-ripemd160 at openssh.com",
  "hmac-sha1-96", "hmac-md5-96". After eliminating the MD5-based algos
  (MD5 is considered completely broken), and the one based on truncated
  SHA1, we're left with "hmac-sha1", "hmac-ripemd160", and
  "hmac-ripemd160 at openssh.com". RIPEMD-160 is generally trusted, but it is
  compiled out of the Fedora 35 "ssh" client binary. Therefore only
  "hmac-sha1" remains.

- "HostKeyAlgorithms", "PubkeyAcceptedAlgorithms": these options control
  the usage of public key algorithms, for authenticating the server to the
  client, and vice versa, respectively. RHEL5 sshd only supports "ssh-rsa"
  and "ssh-dss", and from those, "ssh-rsa" is more commonly used (for
  example, "ssh-keygen" defaults to creating "ssh-rsa" keys). Recommend
  "ssh-rsa".

- "PubkeyAcceptedKeyTypes": this is the old ("legacy") name for
  "PubkeyAcceptedAlgorithms". As of this writing, the latest upstream
  release of libssh (also packaged in Fedora 35 -- libssh-0.9.6-1.fc35)
  does not recognize the new "PubkeyAcceptedAlgorithms" option name, only
  the original "PubkeyAcceptedKeyTypes". openssh-8.7p1-3.fc35 recognizes
  both option variants. Include "PubkeyAcceptedKeyTypes" in the
  recommendation along with "PubkeyAcceptedAlgorithms", for backward and
  forward compatbility.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2062360
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---
 docs/virt-v2v-input-xen.pod | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/docs/virt-v2v-input-xen.pod b/docs/virt-v2v-input-xen.pod
index afc4a019f13d..789853b4d194 100644
--- a/docs/virt-v2v-input-xen.pod
+++ b/docs/virt-v2v-input-xen.pod
@@ -37,12 +37,22 @@ option is incomplete.  Some operations remain that still require the
 user to enter the password manually.  Therefore ssh-agent is recommended
 over the I<-ip> option.  See L<https://bugzilla.redhat.com/1854275>.
 
-With some modern ssh implementations, legacy crypto policies required
-to interoperate with RHEL 5 sshd are disabled.  To enable them you may
-need to run this command on the conversion server (ie. ssh client),
-but read L<update-crypto-policies(8)> first:
+With some modern ssh implementations, legacy crypto algorithms required
+to interoperate with RHEL 5 sshd are disabled.  To enable them, you may
+need to add the following C<Host> stanza to your F<~/.ssh/config>:
 
- # update-crypto-policies --set LEGACY
+ Host xen.example.com
+   KexAlgorithms            +diffie-hellman-group14-sha1
+   MACs                     +hmac-sha1
+   HostKeyAlgorithms        +ssh-rsa
+   PubkeyAcceptedKeyTypes   +ssh-rsa
+   PubkeyAcceptedAlgorithms +ssh-rsa
+
+(C<PubkeyAcceptedKeyTypes> and C<PubkeyAcceptedAlgorithms> have
+identical meaning; the former is the old option name, the latter is the
+new one. Virt-v2v uses both C<libssh> and C<ssh> when converting a guest
+from Xen, and on some operating systems, C<libssh> and C<ssh> may not
+both accept the same option variant.)
 
 =head2 Test libvirt connection to remote Xen host
 
-- 
2.19.1.3.g30247aa5d201