NetworkManager-l2tp is a VPN plugin for NetworkManager 1.20 and later which provides support for L2TP and L2TP/IPsec (i.e. L2TP over IPsec) connections.

For L2TP support, it uses either of the following :

• kl2tpd from Katalix's go-l2tp project ( https://github.com/katalix/go-l2tp )
• xl2tpd ( https://github.com/xelerance/xl2tpd )

For IPsec support, it uses either of the following :

• Libreswan ( https://libreswan.org )
• strongSwan ( https://www.strongswan.org )

For user authentication it supports either:

• username/password credentials.
• TLS certificates.

For machine authentication it supports either:

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

For TLS user certificate support, ppp >= 2.4.9 is required or the EAP-TLS patch for pppd needs to be applied to the ppp source code for older versions :

• https://www.nikhef.nl/~janjust/ppp/

The configure script will attempt to determine if pppd EAP-TLS support is available and will disable the build time TLS user certificate support if it can not be detected.

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 :

• https://github.com/nm-l2tp/NetworkManager-l2tp/wiki

./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, if you have pppd < 2.5.0 you may need to use --with-pppd-plugin-dir and set it 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. The --enable-libreswan-dh2 switch can be used with libreswan < 3.30 or libreswan packages built with USE_DH2=true i.e. have modp1024 support.

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib/x86_64-linux-gnu \
  --runstatedir=/run \
  --with-gtk4

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib/x86_64-linux-gnu \
  --libexecdir=/usr/lib/NetworkManager \
  --runstatedir=/run \
  --with-pppd-plugin-dir=/usr/lib/pppd/2.4.9

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib64 \
  --runstatedir=/run \
  --with-gtk4

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib64 \
  --localstatedir=/var \
  --enable-libreswan-dh2 \
  --with-nm-ipsec-nss-dir=/etc/ipsec.d \
  --with-pppd-plugin-dir=/usr/lib64/pppd/2.4.7

./configure \
  --disable-static --prefix=/usr \
  --sysconfdir=/etc --libdir=/usr/lib64 \
  --libexecdir=/usr/lib \
  --localstatedir=/var \
  --enable-libreswan-dh2 \
  --with-gtk4

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

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:

• https://libreswan.org/wiki/HOWTO:_Using_NSS_with_libreswan

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

journalctl --no-hostname _SYSTEMD_UNIT=NetworkManager.service + SYSLOG_IDENTIFIER=pppd

if using go-l2tp's kl2tpd, it is recommended to issue the following :

journalctl --no-hostname _SYSTEMD_UNIT=NetworkManager.service + _COMM=kl2tpd + SYSLOG_IDENTIFIER=pppd

For some versions of Fedora, libreswan logging also goes to /var/log/pluto.log.

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

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 :

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

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

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, you may need to issue the following to see more log output:

journalctl -b

The Libreswan debugging can be customized 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:

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

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

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

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:

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

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

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

On some later Linux distributions, Libreswan no longer supports IKEv1 packets by default, the following error occurs if this is the case :

failed to add IKEv1 connection: global ikev1-policy does not allow IKEv1 connections

To re-enable IKEv1, add ikev1-policy=accept to the config setup section of /etc/ipsec.conf

go-l2tp's kl2tpd requires l2tp_ppp and l2tp_netlink kernel modules which will fail to auto-load if the kernel modules are blacklisted.

If you are using xl2tpd and 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)

For xl2tpd compatibility with Microsoft L2TP servers (and some other L2TP servers), L2TP kernel modules are required.

sudo modprobe l2tp_ppp (or sudo modprobe pppol2tp for older kernels) can be used as a temporary workaround, but it is recommended to do a blacklist removal as described further which provides a permanent solution.

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

• https://access.redhat.com/articles/3760101

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

First some examples showing successful L2TP connections demonstrating source port and ephemeral port terminologies used by the subsequent issues.

The following example uses network diagnostic tools netstat and the newer ss to show a successful L2TP connection between a client with its local address (source address and port) and a server with its foreign/peer address and port, where the source port is 1701.

$ netstat -u -n
Proto Recv-Q Send-Q Local Address           Foreign Address         State
udp        0      0 10.184.42.84:1701      123.45.6.78:1701        ESTABLISHED

$ ss -u -n
Recv-Q   Send-Q         Local Address:Port        Peer Address:Port   Process
0        0               10.184.42.84:1701         123.45.6.78:1701

The following shows a successful L2TP connection where the source port is an ephemeral port (i.e. random high port), in this example it is 45575.

$ netstat -un
Proto Recv-Q Send-Q Local Address           Foreign Address         State
udp        0      0 10.184.42.84:45575     123.45.6.78:1701        ESTABLISHED

$ ss -u -n
Recv-Q   Send-Q         Local Address:Port        Peer Address:Port   Process
0        0               10.184.42.84:45575        123.45.6.78:1701

There are some L2TP/IPsec servers that will reject L2TP connections when an ephemeral source port is used (i.e. when UDP source port 1701 is not used), even though the use of an ephemeral port is considered acceptable in RFC3193, the L2TP/IPsec standard co-authored by Microsoft and Cisco.

When NetworkManager-l2tp tries to start its own instance of xl2tpd or kl2tpd, if UDP port 1701 is not free (e.g. system xl2tpd is listening on UDP port 1701), an ephemeral source port will be used.

The following netstat and ss command-lines can be used to check if there is system xl2tpd (or some other daemon) listening on UDP port 1701 :

$ sudo netstat -unlp | grep 1701
udp        0      0 0.0.0.0:1701            0.0.0.0:*                           4123/xl2tpd

$ sudo ss -unlp | grep 1701
UNCONN 0      0                               0.0.0.0:1701         0.0.0.0:*     users:(("xl2tpd",pid=4123,fd=3))

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.service

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.service

There are some cases where disabling a service doesn't stop it from being started at boot time. You can check if the xl2tp service is still running with the following :

systemctl disable xl2tpd.service

If it is still running, you can issue the following to ensure is isn't started at boot time:

sudo systemctl mask xl2tpd.service

Generally NAT-Traversal does not work for multiple L2TP clients behind the same NAT if the clients are all using UDP source port 1701, as the server is unable to differentiate between multiple L2TP connections coming from the same NAT.

For NetworkManager-l2tp the simplest workaround to allow the server to differentiate between multiple L2TP connections from the same NAT is to use an ephemeral source port. Either click the "Use L2TP ephemeral source port" checkbox in the settings, or enable and start the system xl2tpd.

Some L2TP/IPsec servers can be configured to use a connmark plugin (or similar) to differentiate between L2TP connections from the same NAT.

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:

• https://wiki.strongswan.org/projects/strongswan/wiki/IKEv1CipherSuites

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:

* 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 :

• https://github.com/nm-l2tp/NetworkManager-l2tp/wiki/Known-Issues