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NetworkMananger-l2tp

If you wish to distribute NetworkManager-l2tp 1.8.6 binaries for a Linux distribution, please note that there is a GPL/OpenSSL license conflict with OpenSSL < 3.0.0 on Linux distibutions that do not consider OpenSSL (or LibreSSL) to be a "System Library". See release notes for further details:

NetworkManager-l2tp is a VPN plugin for NetworkManager 1.8 and later which provides support for L2TP and L2TP/IPsec (i.e. L2TP over IPsec) connections.

For L2TP support, it uses xl2tpd ( https://www.xelerance.com/software/xl2tpd/ )

For IPsec support, it uses either of the following :

For user authentication it supports either:

  • username/pasword credentials.
  • TLS certificates.

For machine authentication it supports either:

  • Pre-shared key (PSK).
  • TLS certificates.

For TLS user certificate support, the ppp package has to have the EAP-TLS patch for pppd applied to the ppp source code (which many Linux distributions already do) :

The configure script will attempt to determine if the EAP-TLS patch for pppd has been applied and will disable the build time TLS user certificate support if it can not detect it has been applied.

This VPN plugin auto detects the following TLS certificate and private key file formats by looking at the file contents and not the file extension :

  • PKCS#12 certificates.
  • X509 certificates (PEM or DER).
  • PKCS#8 private keys (PEM or DER)
  • traditional OpenSSL RSA, DSA and ECDSA private keys (PEM or DER).

For details on pre-built packages, known issues and build dependencies, please visit the Wiki :

Table of Contents

Building

./autogen.sh
./configure  # (see below)
make

The default ./configure settings aren't reasonable and should be explicitly overridden with ./configure arguments. In the configure examples below, you may need to change the --with-pppd-plugin-dir value to an appropriate directory that exists, similarly --with-nm-ipsec-nss-dir may need to be set to the Libreswan NSS database location if it is not located in /var/lib/ipsec/nss.

Debian >= 10 and Ubuntu >= 18.04 (AMD64, i.e. x86-64)

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib/x86_64-linux-gnu \
  --libexecdir=/usr/lib/NetworkManager \
  --runstatedir=/run \
  --with-nm-ipsec-nss-dir=/var/lib/ipsec/nss \
  --with-pppd-plugin-dir=/usr/lib/pppd/2.4.7

Fedora and Red Hat Enterprise Linux 8 (x86-64)

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib64 \
  --localstatedir=/var \
  --with-nm-ipsec-nss-dir=/var/lib/ipsec/nss \
  --with-pppd-plugin-dir=/usr/lib64/pppd/2.4.7

openSUSE (x86-64)

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib64 \
  --libexecdir=/usr/lib \
  --localstatedir=/var \
  --with-pppd-plugin-dir=/usr/lib64/pppd/2.4.7

VPN connection profile files

VPN connection profile files (along with other NetworkManager profile files) are stored under /etc/NetworkManager/system-connections/

Run-time generated files

The following files located under /var/run assume --localstatedir=/var or --runstatedir=/var/run were supplied to the configure script at build time.

  • /var/run/nm-l2tp-UUID/xl2tpd.conf
  • /var/run/nm-l2tp-UUID/xl2tpd-control
  • /var/run/nm-l2tp-UUID/xl2tpd.pid
  • /var/run/nm-l2tp-UUID/ppp-options
  • /var/run/nm-l2tp-UUID/ipsec.conf
  • /etc/ipsec.d/ipsec.nm-l2tp.secrets

where UUID is the NetworkManager UUID for the VPN connection.

If strongswan is being used, NetworkManager-l2tp will append the following line to /etc/ipsec.secrets at run-time if the line is missing:

include ipsec.d/ipsec.nm-l2tp.secrets

Password protecting the libreswan NSS database

The NSS database is used by NetworkManager-l2tp for machine certificate VPN connections using libreswan.

libreswan >= 4.0 default NSS database location is /var/lib/ipsec/nss/ and for all versions of libreswan on Debian/Ubuntu. Older libreswan versions often use /etc/ipsec.d/ such as on older version of RHEL/Fedora/CentOS.

The default libreswan package install for most Linux distributions uses an empty password. It is up to the administrator to decide on whether to use a password or not. However, a non-empty database password must be provided when running in FIPS mode.

See the following page on how to set the password for the libreswan NSS database and the syntax for the /var/lib/ipsec/nss/nsspassword file where the password is stored:

Debugging

For Systemd based Linux distributions logging goes to the Systemd journal which can be viewed by issuing the following :

journalctl --unit=NetworkManager

For non-Systemd based Linux distributions, view the appropriate system log file which is most likely located under /var/log/.

Increase Debugging Output

To increase debugging output, issue the following on the command line, it will also prevent the run-time generated config files from being deleted after the VPN connection is disconnected :

Debian and Ubuntu

sudo killall -TERM nm-l2tp-service
sudo /usr/lib/NetworkManager/nm-l2tp-service --debug

Fedora and Red Hat Enterprise Linux

sudo killall -TERM nm-l2tp-service
sudo /usr/libexec/nm-l2tp-service --debug

openSUSE

sudo killall -TERM nm-l2tp-service
sudo /usr/lib/nm-l2tp-service --debug

then start your VPN connection and reproduce the problem.

For Systemd based Linux distributions when increasing the debugging output by running nm-l2tp-service --debug on the command-line, do not use journalctl --unit=NetworkManager as you may not see all the logs, instead issue:

journalctl -b

Libreswan Custom Debugging

The Libreswan debugging can be cutomized by setting the PLUTODEBUG env variable which corresponds to the plutodebug ipsec.conf config section option. The syntax for PLUTODEBUG is a white-space separated list of the following format :

PLUTODEBUG="TYPE TYPE ... TYPE"

Where TYPE is a debug option from the list output by issuing the following on the command-line :

ipsec whack --debug list

Examples:

Debian and Ubuntu

sudo PLUTODEBUG="all proposal-parser" /usr/lib/NetworkManager/nm-l2tp-service --debug

Fedora and Red Hat Enterprise Linux

sudo PLUTODEBUG="all proposal-parser" /usr/libexec/nm-l2tp-service --debug

strongSwan Custom Debugging

The strongSwan debugging can be cutomized by setting the CHARONDEBUG env variable which corresponds to the charondebug ipsec.conf config section option. The syntax for CHARONDEBUG is a comma separated list of the following format :

CHARONDEBUG="TYPE LEVEL, TYPE LEVEL, ..., TYPE LEVEL"

where TYPE is: any|dmn|mgr|ike|chd|job|cfg|knl|net|asn|enc|tnc|imc|imv|pts|tls|esp|lib

and LEVEL is: -1|0|1|2|3|4

Examples:

Debian and Ubuntu

sudo CHARONDEBUG="knl 1, ike 2, esp 2, lib 1, cfg 3" /usr/lib/NetworkManager/nm-l2tp-service --debug

Fedora and Red Hat Enterprise Linux

sudo CHARONDEBUG="knl 1, ike 2, esp 2, lib 1, cfg 3" /usr/libexec/nm-l2tp-service --debug

openSUSE

sudo CHARONDEBUG="knl 1, ike 2, esp 2, lib 1, cfg 3" /usr/lib/nm-l2tp-service --debug

Issue with blacklisting of L2TP kernel modules

For compatibility with Microsoft L2TP servers (and with later kernel updates, other L2TP servers), L2TP kernel modules are required.

If you see the following error message, then chances are that the l2tp_ppp and l2tp_netlink kernel modules are blacklisted :

xl2tpd[1234]: L2TP kernel support not detected (try modprobing l2tp_ppp and pppol2tp)

modprobe l2tp_ppp (or modprobe pppol2tp for older kernels) can be used as a temporary workaround instead of the permanent blacklist removal as described below.

The following is an extract from "Enhanced security of auto-loading kernel modules in RHEL 8 " web page :

To enhance Red Hat Enterprise Linux against possible future security vulnerabilities in lesser-known components which system administrators typically do not protect against, a set of kernel modules have been moved to the kernel-modules-extra package and blacklisted by default so those components cannot be loaded by non-root users.

When a system requires use of one of these kernel modules, the system administrator must explicitly remove the module blacklist.

Although the above is for RHEL8, it is also applicable to Fedora >= 31, CentOS 8 and other derivatives.

The /etc/modprobe.d/l2tp_netlink-blacklist.conf file contains:

# Remove the blacklist by adding a comment # at the start of the line.
blacklist l2tp_netlink

The /etc/modprobe.d/l2tp_ppp-blacklist.conf file contains :

# Remove the blacklist by adding a comment # at the start of the line.
blacklist l2tp_ppp

To remove the blacklist of the L2TP modules by adding a # comment to the start of the blacklist lines can be achieved with:

sudo sed -e '/blacklist l2tp_netlink/s/^b/#b/g' -i /etc/modprobe.d/l2tp_netlink-blacklist.conf
sudo sed -e '/blacklist l2tp_ppp/s/^b/#b/g' -i /etc/modprobe.d/l2tp_ppp-blacklist.conf

Issue with not stopping system xl2tpd service

NetworkManager-l2tp starts its own instance of xl2tpd and if the system xl2tpd service is running, its own xl2tpd instance will not be able to use UDP port 1701, so will use an ephemeral port (i.e. random high port).

Although the use of an ephemeral port is considered acceptable in RFC3193, the L2TP/IPsec standard co-authored by Microsoft and Cisco, there are some L2TP/IPsec servers and/or firewalls that will have issues if an ephemeral port is used.

Stopping the system xl2tpd service should free UDP port 1701 and on systemd based Linux distributions, the xl2tpd service can be stopped with the following:

sudo systemctl stop xl2tpd

If stopping the xl2tpd service fixes your VPN connection issue, you can disable the xl2tpd service from starting at boot time with :

sudo systemctl disable xl2tpd

IPsec IKEv1 weak legacy algorithms and backwards compatibility

There is a general consensus that the following legacy algorithms are now considered weak or broken in regards to security and should be phased out and replaced with stronger algorithms.

Encryption Algorithms :

  • 3DES
  • Blowfish

Integrity Algorithms :

  • MD5
  • SHA1

Diffie Hellman Groups :

  • MODP768
  • MODP1024
  • MODP1536

The following strongSwan page has more details on which algorithms are considered broken:

Legacy algorithms that are considered weak or broken are regularly removed from the default set of allowed algorithms with newer releases of strongSwan and libreswan.

As of NetworkManager-l2tp version 1.2.16, it was decided to compromise for backwards compatibility by not using the strongSwan and libreswan default set of allowed algorithms, instead algorithms that are a merge of Windows 10 and macOS/iOS/iPadOS L2TP/IPsec clients' IKEv1 proposals are used instead. The weakest proposals that were not common to both Win10 and iOS were dropped, but all of the strongest ones were kept:

Phase 1 - Main Mode
{enc=AES_CBC_256 integ=HMAC_SHA2_256_128 group=MODP_2048}
{enc=AES_CBC_256 integ=HMAC_SHA2_256_128 group=MODP_1536}
{enc=AES_CBC_256 integ=HMAC_SHA2_256_128 group=MODP_1024} *
{enc=AES_CBC_256 integ=HMAC_SHA1_96 group=MODP_2048}
{enc=AES_CBC_256 integ=HMAC_SHA1_96 group=MODP_1536}
{enc=AES_CBC_256 integ=HMAC_SHA1_96 group=MODP_1024} *
{enc=AES_CBC_256 integ=HMAC_SHA1_96 group=ECP_384}
{enc=AES_CBC_128 integ=HMAC_SHA1_96 group=MODP_1024} *
{enc=AES_CBC_128 integ=HMAC_SHA1_96 group=ECP_256}
{enc=3DES_CBC integ=HMAC_SHA1_96 group=MODP_2048}
{enc=3DES_CBC integ=HMAC_SHA1_96 group=MODP_1024} *
Phase 2 - Quick Mode
{enc=AES_CBC_256 integ=HMAC_SHA1_96}
{enc=AES_CBC_128 integ=HMAC_SHA1_96}
{enc=3DES_CBC integ=HMAC_SHA1_96}

* Libreswan >= 3.30 is no longer built with DH2 (modp1024) support, so above proposals which have modp1024 have been excluded when libreswan is used, except if NetworkManager-l2tp is built with the --enable-libreswan-dh2 configure switch.

The above proposals are equivalent to setting the following phase 1 and 2 algorithms in the Advanced section of NetworkManager-l2tp's IPsec Options dialog box:

Phase 1 algorithms with libreswan :

aes256-sha2_256-modp2048,aes256-sha2_256-modp1536,aes256-sha2_256-modp1024,aes256-sha1-modp2048,aes256-sha1-modp1536,aes256-sha1-modp1024,aes256-sha1-ecp_384,aes128-sha1-modp1024,aes128-sha1-ecp_256,3des-sha1-modp2048,3des-sha1-modp1024

Phase 2 algorithms with libreswan :

aes256-sha1,aes128-sha1,3des-sha1

Phase 1 algorithms with strongSwan :

aes256-sha2_256-modp2048,aes256-sha2_256-modp1536,aes256-sha2_256-modp1024,aes256-sha1-modp2048,aes256-sha1-modp1536,aes256-sha1-modp1024,aes256-sha1-ecp384,aes128-sha1-modp1024,aes128-sha1-ecp256,3des-sha1-modp2048,3des-sha1-modp1024!

Phase 2 algorithms with strongSwan :

aes256-sha1,aes128-sha1,3des-sha1!

If you are not sure if you are using libreswan or strongSwan, issue the following on the command-line:

ipsec --version

If you are concerned about security and wish to use algorithms that are stronger than the proposals offered by Windows 10 and macOS/iOS/iPadOS L2TP/IPsec clients, user specified phase 1 (ike - Main Mode) and phase 2 (esp - Quick Mode) algorithms can be specified in the IPsec Options dialog box. Please see the libreswan or strongSwan ipsec.conf documentation for the ike and esp (aka phase2alg) syntax.

If you are not sure which IKEv1 Phase 1 algorithms your VPN server proposes, you can query the VPN server with the ike-scan.sh script located in the IPsec IKEv1 algorithms section of the Wiki :

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