Changes/Deprecate TCP wrappers

Deprecate TCP wrappers

Summary

TCP wrappers is a simple tool to block incoming connection on application level. This was very useful 20 years ago, when there were no firewalls in Linux. This is not the case for today and connection filtering should be done in network level or completely in application scope if it makes sense. After recent discussions I believe it is time to go for this package, if not completely, than at least as a dependency of modern daemons in system by default.

Owner

• Name: Jakub Jelen
• Email: jjelen@redhat.com
• Release notes ticket: #85

Current status

• Targeted release: Fedora 28
• Last updated: 2022-05-14
• Tracker bug: #1495181

Detailed Description

The last version of tcp_wrappers was released 20 years ago (although IPv6 support was added later). At that time it was very powerful tool to "block all traffic", but these days we can do the same thing using firewalls/iptables/nftables for all traffic on network level or use similar filtering at the application level.

One of the motivating factors for this change was the removal of TCP wrappers support from systemd and OpenSSH in 2014, based on the thread on fedora devel list [1]. Another thread was started during 2017 [2] which is trying to explain the reasons why we should do that with other constructive ideas.

Another factor which has driven the deprecation of this package is the lack of any upstream community around it. Although the threats on networking communications continually increase, the threat coverage of this package has remained the same over the last two decades, leading one to draw the inference that new threats are now being handled by different components.

Benefit to Fedora

Removing this package from Fedora will remove a package from default and minimal installations (removing dependency of daemons such as SSHD). It also makes the configuration straight-forward for new users (no shared files defining access rules, poorly reporting any errors to users.

Removing the dependency from all packages and retiring the package in single release will minimize users confusion and avoids opening sensitive services after the update.

Scope

• Proposal owners:
  • Deprecate tcp_wrappers in Fedora:
    • Remove dependency on other packages maintained and notify other maintainers to follow the same procedure.
    • Remove tcp_wrappers-devel subpackage to avoid new packages building against it before Mass Rebuild (31st January 2018).

• Other developers: Remove dependency of your software on tcp_wrappers. See Dependencies section for more information.

• Release engineering: #7029 (a check of an impact with Release Engineering is needed)
  • List of deliverables: Not affected

• Policies and guidelines: Update packaging guidelines to NOT RECOMMEND building against tcp_wrappers after the removal.

• Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

Updating from older versions might expose existing services "protected" by tcp_wrappers before (sshd). The removal needs to be explicitly mentioned in the migration guide/release notes so the users are able to configure different layer of security (firewalld, application configuration) if this was the only one they used.

How To Test

You should be able to run system (for example with OpenSSH) without tcp_wrappers package.

For example, OpenSSH nor OpenLDAP daemons should not be linked with libwrap. The following command should not return anything:

ldd /usr/sbin/sshd  | grep libwrap
ldd /usr/sbin/slapd  | grep libwrap

There should be no tcp_wrappers-devel package in the system:

rpm -q tcp_wrappers-devel

User Experience

Users should not notice any difference. System administrators should configure different layer of security, if tcp_wrapper was the only one they relied on.

Dependencies

Other packages should be rebuilt without support for tcp_wrappers (if possible). That should be at most tens lines of code change, configure option (if upstream still supports it) or dropping downstream patch.

The list of packaged still using tcp_wrappers, based on the dnf repoquery --whatrequires 'libwrap.so.0()(64bit)'|grep x86_64 and manual removal of duplicates and packages building against net-snmp (therefore indirectly depending on libwrap):

• 389-ds-base
• aeskulap
• apcupsd
• apt-cacher-ng
• audit
• bacula
• bacula2
• conserver
• ctk
• cyrus-imapd
• dcmtk
• dovecot
• exim
• flow-tools
• gsi-openssh
• net-snmp
• nfs-utils
• ngircd
• nrpe
• openldap
• openssh
• pptpd
• prelude-manager
• proftpd
• pulseaudio
• quota
• redir
• rpcbind
• rwhoisd
• sendmail
• slapi-nis
• socat
• sslh
• stunnel
• syslog-ng
• tftp
• up-imapproxy
• uwsgi
• vsftpd
• xinetd
• yaz

If you wish to maintain compatibility with old releases/you can adjust spec file in the similar way how ocserv does it:

%if 0%{?fedora} >= 28 || 0%{?rhel} > 7
%define use_libwrap 0
%else
%define use_libwrap 1
%endif
...
%if %{use_libwrap}
	--with-libwrap
%else
	--without-libwrap
%endif

Contingency Plan

• Contingency mechanism: tcp_wrappers package will not be retired, offending packages will still carry this dependency, but guidelines should be updated to not recommend building against this package
• Contingency deadline: Beta freeze (2018-03-06)
• Blocks release? No

Documentation

Migration to tcpd

After removing the libwrap dependency from openssh, it will stop using rules defines in /etc/hosts.deny. The functionality can be "added back" if needed to any socket-activated service. For example SSHD:

• Disable default SSHD service sshd.service - will be started afterwards by sshd.socket

systemctl disable sshd

• Create directory for override config /etc/systemd/system/sshd@.service.d/

mkdir -p /etc/systemd/system/sshd@.service.d/

• Create override config file for SSH service /etc/systemd/system/sshd@.service.d/override.conf

cat <<'END' >/etc/systemd/system/sshd@.service.d/override.conf
[Unit]
ConditionFileIsExecutable=/usr/sbin/tcpd

[Service]
ExecStart=
END

• Append related part from default file (content varies over distributions) by prepending @-/usr/sbin/tcpd

grep '^ExecStart=' /usr/lib/systemd/system/sshd@.service | sed 's#[^\/]*\(.*\)#ExecStart=@-/usr/sbin/tcpd \1#' >>/etc/systemd/system/sshd@.service.d/override.conf

Resulting in (example from EL8):

[Unit]
ConditionFileIsExecutable=/usr/sbin/tcpd

[Service]
ExecStart=
ExecStart=@-/usr/sbin/tcpd /usr/sbin/sshd -i $OPTIONS $CRYPTO_POLICY

• Permanently enable related required SELinux boolean

setsebool -P ssh_use_tcpd=1

• Reload systemctl

systemctl daemon-reload

• Enable and start sshd.socket

systemctl enable --now sshd.socket

• Verify that you can connect to new service

ssh localhost

Should still result in successful connect (depending on existing content of /etc/hosts.allow and /etc/hosts.deny

• Check whether it's honoring /etc/hosts.deny

cat <<END >>/etc/hosts.deny
sshd: 127.0.0.1 [::1]
END

• Verify that content in /etc/hosts.deny is honored and connection is blocked

ssh localhost
kex_exchange_identification: read: Connection reset by peer

Blocked connection should appear in log

journalctl -t sshd | grep refused
May 14 08:29:11 *** sshd[1698]: refused connect from ::1 (::1)

A similar approach can be used for other services with dropped tcp_wrappers dependency.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1904301

Migration to systemd eBPF-based filter

SystemD 235 implemented eBPF-based filter for services. This provides a new options IPAddressAllow and IPAddressDeny for units. It is not restricted to socket-activated services, but because it is enforced on the kernel level, it can seamlessly work with standard services.

One can simply allow access to sshd service only from IP address 192.168.0.42 by creating a drop-in unit file in

IPAddressAllow=192.168.0.42

To implement similar effects as tcp_wrappers do for multiple services, you can apply these rules for whole system in system.slice.

Migrate to application-specific filters

Most of the tools currently support its own way of filtering traffic. For example, OpenSSH Match blocks can be used to disable authentication from specific IP addresses or hostnames (with UseDNS yes) or mod_wrap2 for ProFTPD, which allows even more flexibility.

Release Notes

Fedora 28 removes support for tcp_wrappers (aka /etc/hosts.deny access files) by default. The preferred replacement is software firewalld/nftables rules or software specific access rules for more complex filtering. If your system security depends on tcp_wrappers rules, convert them to firewall, or set up tcpd to do the same job for you.