Laszlo Ersek
2022-Jul-11 07:01 UTC
[Libguestfs] [v2v PATCH 2/2] input-xen: replace "enable LEGACY crypto" advice with targeted ssh options
- "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
Richard W.M. Jones
2022-Jul-11 13:05 UTC
[Libguestfs] [v2v PATCH 2/2] input-xen: replace "enable LEGACY crypto" advice with targeted ssh options
On Mon, Jul 11, 2022 at 09:01:57AM +0200, Laszlo Ersek wrote:> - "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 hostFor the series: Reviewed-by: Richard W.M. Jones <rjones at redhat.com> Thanks, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW