systemd-analyze security dnscrypt-proxy.service shows no systemd sandboxing

[a1346054]

Output of the following commands:

./dnscrypt-proxy -version
2.1.4

./dnscrypt-proxy -check
[2023-07-08 10:54:22] [NOTICE] dnscrypt-proxy 2.1.4
[2023-07-08 10:54:23] [NOTICE] Source [relays] loaded
[2023-07-08 10:54:24] [NOTICE] Source [public-resolvers] loaded
[2023-07-08 10:54:24] [NOTICE] Configuration successfully checked

./dnscrypt-proxy -resolve example.com
Resolving [example.com] using 127.0.0.1 port 53

Resolver : 193.70.85.11 (radia.bortzmeyer.org.)

Canonical name: example.com.

IPv4 addresses: 93.184.216.34
IPv6 addresses: 2606:2800:220:1:248:1893:25c8:1946

Name servers : a.iana-servers.net., b.iana-servers.net.
DNSSEC signed : yes
Mail servers : 1 mail servers found

HTTPS alias : -
HTTPS info : -

Host info : -
TXT records : v=spf1 -all, wgyf8z8cgvm2qmxpnbnldrcltvk4xqfn

What is affected by this bug?

dnscrypt-proxy.service generated by dnscrypt-proxy -service install

When does this occur?

When you run systemd-analyze security dnscrypt-proxy.service

Where does it happen?

In /etc/systemd/system/dnscrypt-proxy.service

How do we replicate the issue?

Use linux distribution debian 12 (bookworm), which runs systemd as init.

Download dnscrypt-proxy-linux_x86_64-2.1.4.tar.gz from github, unpack, install using dnscrypt-proxy -service install

Execute:

$ systemd-analyze security dnscrypt-proxy.service

Notice this output:

  NAME                                                        DESCRIPTION                                                             EXPOSURE
✗ RootDirectory=/RootImage=                                   Service runs within the host's root directory                                0.1
  SupplementaryGroups=                                        Service runs as root, option does not matter                            
  RemoveIPC=                                                  Service runs as root, option does not apply                             
✗ User=/DynamicUser=                                          Service runs as root user                                                    0.4
✗ CapabilityBoundingSet=~CAP_SYS_TIME                         Service processes may change the system clock                                0.2
✗ NoNewPrivileges=                                            Service processes may acquire new privileges                                 0.2
✓ AmbientCapabilities=                                        Service process does not receive ambient capabilities                   
✗ PrivateDevices=                                             Service potentially has access to hardware devices                           0.2
✗ ProtectClock=                                               Service may write to the hardware clock or system clock                      0.2
✗ CapabilityBoundingSet=~CAP_SYS_PACCT                        Service may use acct()                                                       0.1
✗ CapabilityBoundingSet=~CAP_KILL                             Service may send UNIX signals to arbitrary processes                         0.1
✗ ProtectKernelLogs=                                          Service may read from or write to the kernel log ring buffer                 0.2
✗ CapabilityBoundingSet=~CAP_WAKE_ALARM                       Service may program timers that wake up the system                           0.1
✗ CapabilityBoundingSet=~CAP_(DAC_*|FOWNER|IPC_OWNER)         Service may override UNIX file/IPC permission checks                         0.2
✗ ProtectControlGroups=                                       Service may modify the control group file system                             0.2
✗ CapabilityBoundingSet=~CAP_LINUX_IMMUTABLE                  Service may mark files immutable                                             0.1
✗ CapabilityBoundingSet=~CAP_IPC_LOCK                         Service may lock memory into RAM                                             0.1
✗ ProtectKernelModules=                                       Service may load or read kernel modules                                      0.2
✗ CapabilityBoundingSet=~CAP_SYS_MODULE                       Service may load kernel modules                                              0.2
✗ CapabilityBoundingSet=~CAP_SYS_TTY_CONFIG                   Service may issue vhangup()                                                  0.1
✗ CapabilityBoundingSet=~CAP_SYS_BOOT                         Service may issue reboot()                                                   0.1
✗ CapabilityBoundingSet=~CAP_SYS_CHROOT                       Service may issue chroot()                                                   0.1
✗ PrivateMounts=                                              Service may install system mounts                                            0.2
✗ SystemCallArchitectures=                                    Service may execute system calls with all ABIs                               0.2
✗ CapabilityBoundingSet=~CAP_BLOCK_SUSPEND                    Service may establish wake locks                                             0.1
✗ MemoryDenyWriteExecute=                                     Service may create writable executable memory mappings                       0.1
✗ RestrictNamespaces=~user                                    Service may create user namespaces                                           0.3
✗ RestrictNamespaces=~pid                                     Service may create process namespaces                                        0.1
✗ RestrictNamespaces=~net                                     Service may create network namespaces                                        0.1
✗ RestrictNamespaces=~uts                                     Service may create hostname namespaces                                       0.1
✗ RestrictNamespaces=~mnt                                     Service may create file system namespaces                                    0.1
✗ CapabilityBoundingSet=~CAP_LEASE                            Service may create file leases                                               0.1
✗ CapabilityBoundingSet=~CAP_MKNOD                            Service may create device nodes                                              0.1
✗ RestrictNamespaces=~cgroup                                  Service may create cgroup namespaces                                         0.1
✗ RestrictSUIDSGID=                                           Service may create SUID/SGID files                                           0.2
✗ RestrictNamespaces=~ipc                                     Service may create IPC namespaces                                            0.1
✗ ProtectHostname=                                            Service may change system host/domainname                                    0.1
✗ CapabilityBoundingSet=~CAP_(CHOWN|FSETID|SETFCAP)           Service may change file ownership/access mode/capabilities unrestricted      0.2
✗ CapabilityBoundingSet=~CAP_SET(UID|GID|PCAP)                Service may change UID/GID identities/capabilities                           0.3
✗ LockPersonality=                                            Service may change ABI personality                                           0.1
✗ ProtectKernelTunables=                                      Service may alter kernel tunables                                            0.2
✗ RestrictAddressFamilies=~AF_PACKET                          Service may allocate packet sockets                                          0.2
✗ RestrictAddressFamilies=~AF_NETLINK                         Service may allocate netlink sockets                                         0.1
✗ RestrictAddressFamilies=~AF_UNIX                            Service may allocate local sockets                                           0.1
✗ RestrictAddressFamilies=~…                                  Service may allocate exotic sockets                                          0.3
✗ RestrictAddressFamilies=~AF_(INET|INET6)                    Service may allocate Internet sockets                                        0.3
✗ CapabilityBoundingSet=~CAP_MAC_*                            Service may adjust SMACK MAC                                                 0.1
✗ RestrictRealtime=                                           Service may acquire realtime scheduling                                      0.1
✗ CapabilityBoundingSet=~CAP_SYS_RAWIO                        Service has raw I/O access                                                   0.2
✗ CapabilityBoundingSet=~CAP_SYS_PTRACE                       Service has ptrace() debugging abilities                                     0.3
✗ CapabilityBoundingSet=~CAP_SYS_(NICE|RESOURCE)              Service has privileges to change resource use parameters                     0.1
✗ DeviceAllow=                                                Service has no device ACL                                                    0.2
✗ CapabilityBoundingSet=~CAP_NET_ADMIN                        Service has network configuration privileges                                 0.2
✗ ProtectSystem=                                              Service has full access to the OS file hierarchy                             0.2
✗ ProtectProc=                                                Service has full access to process tree (/proc hidepid=)                     0.2
✗ ProcSubset=                                                 Service has full access to non-process /proc files (/proc subset=)           0.1
✗ ProtectHome=                                                Service has full access to home directories                                  0.2
✗ CapabilityBoundingSet=~CAP_NET_(BIND_SERVICE|BROADCAST|RAW) Service has elevated networking privileges                                   0.1
✗ CapabilityBoundingSet=~CAP_AUDIT_*                          Service has audit subsystem access                                           0.1
✗ CapabilityBoundingSet=~CAP_SYS_ADMIN                        Service has administrator privileges                                         0.3
✗ PrivateNetwork=                                             Service has access to the host's network                                     0.5
✗ PrivateUsers=                                               Service has access to other users                                            0.2
✗ PrivateTmp=                                                 Service has access to other software's temporary files                       0.2
✗ CapabilityBoundingSet=~CAP_SYSLOG                           Service has access to kernel logging                                         0.1
✓ KeyringMode=                                                Service doesn't share key material with other services                  
✓ Delegate=                                                   Service does not maintain its own delegated control group subtree       
✗ SystemCallFilter=~@clock                                    Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@cpu-emulation                            Service does not filter system calls                                         0.1
✗ SystemCallFilter=~@debug                                    Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@module                                   Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@mount                                    Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@obsolete                                 Service does not filter system calls                                         0.1
✗ SystemCallFilter=~@privileged                               Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@raw-io                                   Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@reboot                                   Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@resources                                Service does not filter system calls                                         0.2
✗ SystemCallFilter=~@swap                                     Service does not filter system calls                                         0.2
✗ IPAddressDeny=                                              Service does not define an IP address allow list                             0.2
✓ NotifyAccess=                                               Service child processes cannot alter service state                      
✗ UMask=                                                      Files created by service are world-readable by default                       0.1

→ Overall exposure level for dnscrypt-proxy.service: 9.6 UNSAFE 😨

Expected behavior (i.e. solution)

Apply more systemd service sandboxing measures by improving the default configuration of the dnscrypt-proxy.service when it is installed using dnscrypt-proxy -service install

Other Comments

I know that the author of dnscrypt-proxy does not use linux or systemd, so I'm asking anyone else for help in fixing/improving this.

I also know that dnscrypt-proxy is well-written and already has some security measures to protect against vulnerabilities.

Please also ignore the systemd output "Overall exposure level for dnscrypt-proxy.service: 9.6 UNSAFE 😨", it's not an accurate representation of dnscrypt-proxy security.

[jedisct1]

Not a bug, so moving to discussions.

[curiosityseeker]

@a1346054 : I suggest that you check the service file used in Arch Linux. It provides tight sandboxing:

Overall exposure level for dnscrypt-proxy.service: 1.9 OK 🙂

And if that's not enough for you, you can additionally confine dnscrypt-proxy with AppArmor. A profile is available here. In that case you should read the installation and usage instructions for that project.

[a1346054]

I checked whether the Arch Linux systemd service file could be merged into dnscrypt-proxy, but discovered that dnscrypt-proxy does not contain an editable service file but instead uses kardianos/service to generate a service file during installation.

Unfortunately, that vendor does not have any functionality to define any of these hardening options.

This means that a different service file generator or solution would be needed to make a proper pull request to maintain a hardened systemd service file in dnscrypt-proxy repository. Does anyone have any ideas?