.\" SPDX-License-Identifier: AGPL-3.0-or-later .\" Copyright (c) 2020-2022 Red Hat GmbH .\" Author: Stefano Brivio .TH passt 1 .SH NAME .B passt \- Unprivileged user-mode network connectivity for virtual machines .br .B pasta \- Unprivileged user-mode network connectivity for network namespaces .SH SYNOPSIS .B passt [\fIOPTION\fR]... .br .B pasta [\fIOPTION\fR]... [\fIPID\fR|\fIPATH\fR|\fINAME\fR] .SH DESCRIPTION .SS passt .B passt (\fIP\fRlug \fIA\fR \fIS\fRimple \fIS\fRocket \fIT\fRransport) provides full, quasi-native network connectivity to virtual machines in user-mode without requiring any capabilities or privileges. The data plane implements a translation layer between a Layer-2 virtual network interface and native Layer-4 (TCP, UDP, ping) sockets on the host, giving the illusion that application processes residing on the guest are running on the local host, from a networking perspective. Built-in ARP, DHCP, NDP, and DHCPv6 implementations are designed to provide the guest with a network configuration that tightly resembles the host native configuration. With the default options, guest and host share IP addresses, routes, and port bindings. Port forwarding and translation allow networking services running in the guest to be reachable from both local and remote hosts. Unlike \fBslirp4netns\fR(1), \fBpasst\fR doesn't implement a full TCP stack: the TCP translation layer has no stateful data buffering and operates by reflecting one peer's observed parameters (congestion window size, acknowledged data, etc.) to the corresponding peer. Currently, the only supported hypervisor is \fBqemu\fR(1), connecting to \fBpasst\fR by means of a UNIX domain socket. This configuration can be obtained via out-of-tree qemu patches, available at: \fIhttps://passt.top/passt/tree/contrib/qemu\fR or with the \fBqrap\fR(1) wrapper. .SS pasta .B pasta (\fIP\fRack \fIA\fR \fIS\fRubtle \fIT\fRap \fIA\fRbstraction) provides equivalent functionality to network namespaces, as the one offered by \fBpasst\fR for virtual machines. If PID, PATH or NAME are given, \fBpasta\fR associates to an existing user and network namespace. Otherwise, \fBpasta\fR creates a new user and network namespace, and spawns an interactive shell within this context. A \fItap\fR device within the network namespace is created to provide network connectivity. For local TCP and UDP traffic only, \fBpasta\fR also implements a bypass path directly mapping Layer-4 sockets between \fIinit\fR and target namespaces, for performance reasons. .SH OPTIONS .TP .BR \-d ", " \-\-debug Be verbose, don't run in background. .TP .BR \-\-trace Be extra verbose, show single packets, don't run in background. Implies \fB--debug\fR. .TP .BR \-q ", " \-\-quiet Don't print informational messages. .TP .BR \-f ", " \-\-foreground Don't run in background. This implies that the process is not moved to a detached PID namespace after starting, because the PID itself cannot change. Default is to fork into background. .TP .BR \-e ", " \-\-stderr Log to standard error too. Default is to log to system logger only, if started from an interactive terminal, and to both system logger and standard error otherwise. .TP .BR \-\-runas " " \fIUID\fR|\fIUID:GID\fR|\fILOGIN\fR|\fILOGIN:GROUP\fR If started as root, change to given UID and corresponding group if UID is given, or to given UID and given GID if both are given. Alternatively, login name, or login name and group name can be passed. Default is to change to user \fInobody\fR if started as root. .TP .BR \-h ", " \-\-help Display a help message and exit. .TP .BR \-p ", " \-\-pcap " " \fIfile Capture tap-facing (that is, guest-side or namespace-side) network packets to \fIfile\fR in \fBpcap\fR format. .TP .BR \-P ", " \-\-pid " " \fIfile Write own PID to \fIfile\fR once initialisation is done, before forking to background (if configured to do so). .TP .BR \-m ", " \-\-mtu " " \fImtu Assign \fImtu\fR via DHCP (option 26) and NDP (option type 5). By default, no MTU options will be sent. .TP .BR \-a ", " \-\-address " " \fIaddr Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option 5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3) for an IPv6 \fIaddr\fR. This option can be specified zero (for defaults) to two times (once for IPv4, once for IPv6). By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces with the first default route for the corresponding IP version. .TP .BR \-n ", " \-\-netmask " " \fImask Assign IPv4 netmask \fImask\fR, expressed as dot-decimal or number of bits, via DHCP (option 1). By default, the netmask associated to the host address matching the assigned one is used. If there's no matching address on the host, the netmask is determined according to the CIDR block of the assigned address (RFC 4632). .TP .BR \-M ", " \-\-mac-addr " " \fIaddr Use source MAC address \fIaddr\fR when communicating to the guest or to the target namespace. Default is to use the MAC address of the interface with the first IPv4 default route on the host. .TP .BR \-g ", " \-\-gateway " " \fIaddr Assign IPv4 \fIaddr\fR as default gateway via DHCP (option 3), or IPv6 \fIaddr\fR as source for NDP Router Advertisement and DHCPv6 messages. This option can be specified zero (for defaults) to two times (once for IPv4, once for IPv6). By default, IPv4 and IPv6 addresses are taken from the host interface with the first default route for the corresponding IP version. Note: these addresses are also used as source address for packets directed to the guest or to the target namespace having a loopback or local source address, to allow mapping of local traffic to guest and target namespace. See the \fBNOTES\fR below for more details about this mechanism. .TP .BR \-i ", " \-\-interface " " \fIname Use host interface \fIname\fR to derive addresses and routes. Default is to use the interfaces with the first default routes for each IP version. .TP .BR \-D ", " \-\-dns " " \fIaddr Use \fIaddr\fR (IPv4 or IPv6) for DHCP, DHCPv6, NDP or DNS forwarding, as configured (see options \fB--no-dhcp-dns\fR, \fB--dhcp-dns\fR, \fB--dns-forward\fR) instead of reading addresses from \fI/etc/resolv.conf\fR. This option can be specified multiple times. Specifying \fB-D none\fR disables usage of DNS addresses altogether. .TP .BR \-\-dns-forward " " \fIaddr Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the first configured DNS resolver (with corresponding IP version). Mapping is limited to UDP traffic directed to port 53, and DNS answers are translated back with a reverse mapping. This option can be specified zero to two times (once for IPv4, once for IPv6). .TP .BR \-S ", " \-\-search " " \fIlist Use space-separated \fIlist\fR for DHCP, DHCPv6, and NDP purposes, instead of reading entries from \fI/etc/resolv.conf\fR. See options \fB--no-dhcp-search\fR and \fB--dhcp-search\fR. \fB--search none\fR disables the DNS domain search list altogether (if you need to search a domain called "none" you can use \fB--search none.\fR). .TP .BR \-\-no-dhcp-dns " " \fIaddr In \fIpasst\fR mode, do not assign IPv4 addresses via DHCP (option 23) or IPv6 addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolvers. By default, all the configured addresses are passed. .TP .BR \-\-dhcp-dns " " \fIaddr In \fIpasta\fR mode, assign IPv4 addresses via DHCP (option 23) or IPv6 addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolvers. By default, configured addresses, if any, are not passed. .TP .BR \-\-no-dhcp-search " " \fIaddr In \fIpasst\fR mode, do not send the DNS domain search list addresses via DHCP (option 119), via NDP Router Advertisement (option type 31) and DHCPv6 (option 24). By default, the DNS domain search list resulting from configuration is passed. .TP .BR \-\-dhcp-search " " \fIaddr In \fIpasta\fR mode, send the DNS domain search list addresses via DHCP (option 119), via NDP Router Advertisement (option type 31) and DHCPv6 (option 24). By default, the DNS domain search list resulting from configuration is not passed. .TP .BR \-\-no-tcp Disable the TCP protocol handler. No TCP connections will be accepted host-side, and TCP packets coming from guest or target namespace will be silently dropped. .TP .BR \-\-no-udp Disable the UDP protocol handler. No UDP traffic coming from the host side will be forwarded, and UDP packets coming from guest or target namespace will be silently dropped. .TP .BR \-\-no-icmp Disable the ICMP/ICMPv6 echo handler. ICMP and ICMPv6 echo requests coming from guest or target namespace will be silently dropped. .TP .BR \-\-no-dhcp Disable the DHCP server. DHCP client requests coming from guest or target namespace will be silently dropped. .TP .BR \-\-no-ndp Disable NDP responses. NDP messages coming from guest or target namespace will be ignored. .TP .BR \-\-no-dhcpv6 Disable the DHCPv6 server. DHCPv6 client requests coming from guest or target namespace will be silently dropped. .TP .BR \-\-no-ra Disable Router Advertisements. Router Solicitations coming from guest or target namespace will be ignored. .TP .BR \-\-no-map-gw Don't remap TCP connections and untracked UDP traffic, with the gateway address as destination, to the host. .TP .BR \-4 ", " \-\-ipv4-only Enable IPv4-only operation. IPv6 traffic will be ignored. By default, IPv6 operation is enabled as long as at least an IPv6 default route and an interface address are configured on a given host interface. .TP .BR \-4 ", " \-\-ipv6-only Enable IPv6-only operation. IPv4 traffic will be ignored. By default, IPv4 operation is enabled as long as at least an IPv4 default route and an interface address are configured on a given host interface. .SS \fBpasst\fR-only options .TP .BR \-s ", " \-\-socket " " \fIpath Path for UNIX domain socket used by \fBqemu\fR(1) or \fBqrap\fR(1) to connect to \fBpasst\fR. Default is to probe a free socket, not accepting connections, starting from \fI/tmp/passt_1.socket\fR to \fI/tmp/passt_64.socket\fR. .TP .BR \-t ", " \-\-tcp-ports " " \fIspec Configure TCP port forwarding to guest. \fIspec\fR can be one of: .RS .TP .BR none Don't forward any ports .TP .BR all Forward all unbound, non-ephemeral ports, as permitted by current capabilities. For low (< 1024) ports, see \fBNOTES\fR. .TP .BR ports A comma-separated list of ports, optionally ranged with \fI-\fR, and, optionally, with target ports after \fI:\fR, if they differ. Specific addresses can be bound as well, separated by \fI/\fR. Within given ranges, selected ports and ranges can be excluded by an additional specification prefixed by \fI~\fR. Specifying excluded ranges only implies that all other ports are forwarded. Examples: .RS .TP -t 22 Forward local port 22 to 22 on the guest .TP -t 22:23 Forward local port 22 to port 23 on the guest .TP -t 22,25 Forward local ports 22 and 25 to ports 22 and 25 on the guest .TP -t 22-80 Forward local ports 22 to 80 to corresponding ports on the guest .TP -t 22-80-32:90 Forward local ports 22 to 80 to corresponding ports on the guest plus 10 .TP -t 192.0.2.1/22 Forward local port 22, bound to 192.0.2.1, to port 22 on the guest .TP -t 2000-5000,~3000-3010 Forward local ports 2000 to 5000, but not 3000 to 3010 .TP -t 192.0.2.1/20-30,~25 Forward local ports 20 to 24, and 26 to 30, bound to 192.0.2.1 .TP -t ~20000-20010 Forward all ports to the guest, except for the range from 20000 to 20010 .RE Default is \fBnone\fR. .RE .TP .BR \-u ", " \-\-udp-ports " " \fIspec Configure UDP port forwarding to guest. \fIspec\fR is as described for TCP above. Note: unless overridden, UDP ports with numbers corresponding to forwarded TCP port numbers are forwarded too, without, however, any port translation. IPv6 bound ports are also forwarded for IPv4. Default is \fBnone\fR. .SS \fBpasta\fR-only options .TP .BR \-I ", " \-\-ns-ifname " " \fIname Name of tap interface to be created in target namespace. By default, the same interface name as the external, routable interface is used. .TP .BR \-t ", " \-\-tcp-ports " " \fIspec Configure TCP port forwarding to namespace. \fIspec\fR can be one of: .RS .TP .BR none Don't forward any ports .TP .BR auto Dynamically forward ports bound in the namespace. The list of ports is periodically derived (every second) from listening sockets reported by \fI/proc/net/tcp\fR and \fI/proc/net/tcp6\fR, see \fBproc\fR(5). .TP .BR ports A comma-separated list of ports, optionally ranged with \fI-\fR, and, optionally, with target ports after \fI:\fR, if they differ. Specific addresses can be bound as well, separated by \fI/\fR. Within given ranges, selected ports and ranges can be excluded by an additional specification prefixed by \fI~\fR. Specifying excluded ranges only implies that all other ports are forwarded. Examples: .RS .TP -t 22 Forward local port 22 to 22 in the target namespace .TP -t 22:23 Forward local port 22 to port 23 in the target namespace .TP -t 22,25 Forward local ports 22 and 25 to ports 22 and 25 in the target namespace .TP -t 22-80 Forward local ports 22 to 80 to corresponding ports in the target namespace .TP -t 22-80-32:90 Forward local ports 22 to 80 to corresponding ports plus 10 in the target namespace .TP -t 192.0.2.1/22 Forward local port 22, bound to 192.0.2.1, to port 22 in the target namespace .TP -t 2000-5000,~3000-3010 Forward local ports 2000 to 5000, but not 3000 to 3010 .TP -t 192.0.2.1/20-30,~25 Forward local ports 20 to 24, and 26 to 30, bound to 192.0.2.1 .TP -t ~20000-20010 Forward all ports to the namespace, except for the range from 20000 to 20010 .RE IPv6 bound ports are also forwarded for IPv4. Default is \fBauto\fR. .RE .TP .BR \-u ", " \-\-udp-ports " " \fIspec Configure UDP port forwarding to namespace. \fIspec\fR is as described for TCP above, and the list of ports is derived from listening sockets reported by \fI/proc/net/udp\fR and \fI/proc/net/udp6\fR, see \fBproc\fR(5), when \fBpasta\fR starts (not periodically). Note: unless overridden, UDP ports with numbers corresponding to forwarded TCP port numbers are forwarded too, without, however, any port translation. IPv6 bound ports are also forwarded for IPv4. Default is \fBauto\fR. .TP .BR \-T ", " \-\-tcp-ns " " \fIspec Configure TCP port forwarding from target namespace to init namespace. \fIspec\fR is as described above for TCP. Default is \fBauto\fR. .TP .BR \-U ", " \-\-udp-ns " " \fIspec Configure UDP port forwarding from target namespace to init namespace. \fIspec\fR is as described above for UDP. Default is \fBauto\fR. .TP .BR \-\-userns " " \fIspec Target user namespace to join, as path or name (i.e. suffix for --nsrun-dir). If PID is given, without this option, the user namespace will be the one of the corresponding process. This option requires PID, PATH or NAME to be specified. .TP .BR \-\-netns-only Join only a target network namespace, not a user namespace, and don't create one for sandboxing purposes either. This is implied if PATH or NAME are given without \-\-userns. .TP .BR \-\-no-netns-quit If the target network namespace is bound to the filesystem (that is, if PATH or NAME are given as target), do not exit once the network namespace is deleted. .TP .BR \-\-nsrun-dir " " \fIpath Directory for nsfs mountpoints, used as path prefix for names of namespaces. The default path is shown with --help. .TP .BR \-\-config-net Configure networking in the namespace: set up addresses and routes as configured or sourced from the host, and bring up the tap interface. .TP .BR \-\-ns-mac-addr " " \fIaddr Configure MAC address \fIaddr\fR on the tap interface in the namespace. Default is to let the tap driver build a pseudorandom hardware address. .SH EXAMPLES .SS \fBpasta .BR "Create and use a new, connected, user and network namespace" .RS .nf $ iperf3 -s -D $ ./pasta Outbound interface: eth0, namespace interface: eth0 ARP: address: 28:16:ad:39:a9:ea DHCP: assign: 192.168.1.118 mask: 255.255.255.0 router: 192.168.1.1 NDP/DHCPv6: assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17 router: fe80::62e3:27ff:fe33:2b01 # # dhclient -4 --no-pid # dhclient -6 --no-pid # ip address show 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: mtu 65520 qdisc pfifo_fast state UNKNOWN group default qlen 1000 link/ether 5e:90:02:eb:b0:2a brd ff:ff:ff:ff:ff:ff inet 192.168.1.118/24 brd 192.168.1.255 scope global eth0 valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:5c90:2ff:feeb:b02a/64 scope global dynamic mngtmpaddr valid_lft 3591sec preferred_lft 3591sec inet6 fe80::5c90:2ff:feeb:b02a/64 scope link valid_lft forever preferred_lft forever # ip route show default via 192.168.1.1 dev eth0 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.118 # ip -6 route show 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17 dev eth0 proto kernel metric 256 pref medium 2a02:6d40:3ca5:2001::/64 dev eth0 proto kernel metric 256 expires 3584sec pref medium fe80::/64 dev eth0 proto kernel metric 256 pref medium default via fe80::62e3:27ff:fe33:2b01 dev eth0 proto ra metric 1024 expires 3584sec pref medium # iperf3 -c 127.0.0.1 -t1 Connecting to host 127.0.0.1, port 5201 [ 5] local 127.0.0.1 port 51938 connected to 127.0.0.1 port 5201 [ ID] Interval Transfer Bitrate Retr Cwnd [ 5] 0.00-1.00 sec 4.46 GBytes 38.3 Gbits/sec 0 3.93 MBytes - - - - - - - - - - - - - - - - - - - - - - - - - [ ID] Interval Transfer Bitrate Retr [ 5] 0.00-1.00 sec 4.46 GBytes 38.3 Gbits/sec 0 sender [ 5] 0.00-1.41 sec 4.45 GBytes 27.1 Gbits/sec receiver iperf Done. # iperf3 -c ::1 -t1 Connecting to host ::1, port 5201 [ 5] local ::1 port 50108 connected to ::1 port 5201 [ ID] Interval Transfer Bitrate Retr Cwnd [ 5] 0.00-1.00 sec 4.35 GBytes 37.4 Gbits/sec 0 4.99 MBytes - - - - - - - - - - - - - - - - - - - - - - - - - [ ID] Interval Transfer Bitrate Retr [ 5] 0.00-1.00 sec 4.35 GBytes 37.4 Gbits/sec 0 sender [ 5] 0.00-1.41 sec 4.35 GBytes 26.4 Gbits/sec receiver iperf Done. # ping -c1 -4 spaghetti.pizza PING spaghetti.pizza (172.67.192.217) 56(84) bytes of data. 64 bytes from 172.67.192.217: icmp_seq=1 ttl=255 time=37.3 ms --- spaghetti.pizza ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms # ping -c1 -6 spaghetti.pizza PING spaghetti.pizza(2606:4700:3034::6815:147a (2606:4700:3034::6815:147a)) 56 data bytes 64 bytes from 2606:4700:3034::6815:147a: icmp_seq=1 ttl=255 time=35.6 ms --- spaghetti.pizza ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 35.605/35.605/35.605/0.000 ms # logout $ .RE .fi .BR "Connect an existing user and network namespace" .RS .nf $ unshare -rUn # echo $$ 2446678 .fi .BR " [From another terminal]" .nf $ ./pasta 2446678 Outbound interface: eth0, namespace interface: eth0 ARP: address: 28:16:ad:39:a9:ea DHCP: assign: 192.168.1.118 mask: 255.255.255.0 router: 192.168.1.1 NDP/DHCPv6: assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17 router: fe80::62e3:27ff:fe33:2b01 .fi .BR " [Back to the original terminal]" .nf # dhclient -4 --no-pid # dhclient -6 --no-pid # ip address show 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: mtu 65520 qdisc pfifo_fast state UNKNOWN group default qlen 1000 link/ether fa:c1:2a:27:92:a9 brd ff:ff:ff:ff:ff:ff inet 192.168.1.118/24 brd 192.168.1.255 scope global eth0 valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:f8c1:2aff:fe27:92a9/64 scope global dynamic mngtmpaddr valid_lft 3594sec preferred_lft 3594sec inet6 fe80::f8c1:2aff:fe27:92a9/64 scope link valid_lft forever preferred_lft forever .fi .RE .SS \fBpasst .BR "Start and connect a guest with basic port forwarding" .RS .nf $ ./passt -f -t 2222:22 Outbound interface: eth0 ARP: address: 28:16:ad:39:a9:ea DHCP: assign: 192.168.1.118 mask: 255.255.255.0 router: 192.168.1.1 search: redhat.com NDP/DHCPv6: assign: 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17 router: fe80::62e3:27ff:fe33:2b01 search: redhat.com UNIX domain socket bound at /tmp/passt_1.socket You can now start qrap: ./qrap 5 qemu-system-x86_64 ... -net socket,fd=5 -net nic,model=virtio or directly qemu, patched with: qemu/0001-net-Allow-also-UNIX-domain-sockets-to-be-used-as-net.patch as follows: qemu-system-x86_64 ... -net socket,connect=/tmp/passt_1.socket -net nic,model=virtio .fi .BR " [From another terminal]" .nf $ ./qrap 5 qemu-system-x86_64 test.qcow2 -m 1024 -display none -nodefaults -nographic -net socket,fd=5 -net nic,model=virtio Connected to /tmp/passt_1.socket .fi .BR " [Back to the original terminal]" .nf passt: DHCP: ack to request passt: from 52:54:00:12:34:56 passt: NDP: received NS, sending NA passt: NDP: received RS, sending RA passt: DHCPv6: received SOLICIT, sending ADVERTISE passt: NDP: received NS, sending NA passt: DHCPv6: received REQUEST/RENEW/CONFIRM, sending REPLY passt: NDP: received NS, sending NA .fi .BR " [From yet another terminal]" .nf $ ssh -p 2222 root@localhost root@localhost's password: .fi .BR " [...]" .nf # ip address show 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: ens2: mtu 65520 qdisc pfifo_fast state UP group default qlen 1000 link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff inet 192.168.1.118/24 brd 192.168.1.255 scope global noprefixroute ens2 valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:b81d:fa4a:8cdd:cf17/128 scope global noprefixroute valid_lft forever preferred_lft forever inet6 2a02:6d40:3ca5:2001:b019:9ae2:a2fe:e6b4/64 scope global dynamic noprefixroute valid_lft 3588sec preferred_lft 3588sec inet6 fe80::1f98:d09f:9309:9e77/64 scope link noprefixroute valid_lft forever preferred_lft forever .fi .RE .SH NOTES .SS Handling of traffic with local destination and source addressses Both \fBpasst\fR and \fBpasta\fR can bind on ports with a local address, depending on the configuration. Local destination or source addresses need to be changed before packets are delivered to the guest or target namespace: most operating systems would drop packets received from non-loopback interfaces with local addresses, and it would also be impossible for guest or target namespace to route answers back. For convenience, and somewhat arbitrarily, the source address on these packets is translated to the address of the default IPv4 or IPv6 gateway -- this is known to be an existing, valid address on the same subnet. Loopback destination addresses are instead translated to the observed external address of the guest or target namespace. For IPv6 packets, if usage of a link-local address by guest or namespace has ever been observed, and the original destination address is also a link-local address, the observed link-local address is used. Otherwise, the observed global address is used. For both IPv4 and IPv6, if no addresses have been seen yet, the configured addresses will be used instead. For example, if \fBpasst\fR or \fBpasta\fR receive a connection from 127.0.0.1, with destination 127.0.0.10, and the default IPv4 gateway is 192.0.2.1, while the last observed source address from guest or namespace is 192.0.2.2, this will be translated to a connection from 192.0.2.1 to 192.0.2.2. Similarly, for traffic coming from guest or namespace, packets with destination address corresponding to the default gateway will have their destination address translated to a loopback address, if and only if a packet, in the opposite direction, with a loopback destination or source address, port-wise matching for UDP, or connection-wise for TCP, has been recently forwarded to guest or namespace. This behaviour can be disabled with \-\-no\-map\-gw. .SS Handling of local traffic in pasta Depending on the configuration, \fBpasta\fR can bind to local ports in the init namespace, in the target namespace, or both, and forward connections and packets to corresponding ports in the other namespace. To avoid unnecessary overhead, these connections and packets are not forwarded through the tap device connecting the namespaces: \fBpasta\fR creates a socket in the destination namespace, with matching Layer-4 protocol, and uses it to forward local data. For TCP, data is forwarded between the originating socket and the new socket using the \fBsplice\fR(2) system call, and for UDP, a pair of \fBrecvmmsg\fR(2) and \fBsendmmsg\fR(2) system calls deals with packet transfers. This bypass only applies to local connections and traffic, because it's not possible to bind sockets to foreign addresses. .SS Binding to low numbered ports (well-known or system ports, up to 1023) If the port forwarding configuration requires binding to port numbers lower than 1024, \fBpasst\fR and \fBpasta\fR will try to bind to them, but will fail if not running as root, or without the \fICAP_NET_BIND_SERVICE\fR Linux capability, see \fBservices\fR(5) and \fBcapabilities\fR(7). To grant the \fICAP_NET_BIND_SERVICE\fR capability to passt, you can issue, as root: .RS setcap 'cap_net_bind_service=+ep' $(which passt) .RE .SS ICMP/ICMPv6 Echo sockets ICMP and ICMPv6 Echo requests coming from guest or target namespace are handled using so-called "ping" sockets, introduced in Linux 2.6.30. To preserve the original identifier (see RFC 792, page 14, for ICMP, and RFC 4443, section 4.1, for ICMPv6), \fBpasst\fR and \fBpasta\fR try to bind these sockets using the observed source identifier as "port" -- that corresponds to Echo identifiers for "ping" sockets. As \fBbind\fR(2) failures were seen with particularly restrictive SELinux policies, a fall-back mechanism maps different identifiers to different sockets, and identifiers in replies will be mapped back to the original identifier of the request. However, if \fBbind\fR(2) fails and the fall-back mechanism is used, echo requests will be forwarded with different, albeit unique, identifiers. For ICMP and ICMPv6 Echo requests to work, the \fIping_group_range\fR parameter needs to include the PID of \fBpasst\fR or \fBpasta\fR, see \fBicmp\fR(7). .SS pasta and loopback interface As \fBpasta\fR connects to an existing namespace, or once it creates a new namespace, it will also ensure that the loopback interface, \fIlo\fR, is brought up. This is needed to bind ports using the loopback address in the namespace. .SS TCP sending window and \fITCP_INFO\fB before Linux 5.3 To synchronise the TCP sending window from host Layer-4 sockets to the TCP parameters announced in TCP segments sent over the Layer-2 interface, \fBpasst\fR and \fBpasta\fR routinely query the size of the sending window seen by the kernel on the corresponding socket using the \fITCP_INFO\fR socket option, see \fBtcp\fR(7). Before Linux 5.3, i.e. before Linux kernel commit 8f7baad7f035 ("tcp: Add snd_wnd to TCP_INFO"), the sending window (\fIsnd_wnd\fR field) is not available. If the sending window cannot be queried, it will always be announced as the current sending buffer size to guest or target namespace. This might affect throughput of TCP connections. .SH LIMITATIONS Currently, IGMP/MLD proxying (RFC 4605) and support for SCTP (RFC 4960) are not implemented. TCP Selective Acknowledgment (RFC 2018), as well as Protection Against Wrapped Sequences (PAWS) and Round-Trip Time Measurement (RTTM), both described by RFC 7232, are currently not implemented. .SH AUTHOR Stefano Brivio .SH REPORTING BUGS Please report issues on the bug tracker at https://passt.top/passt/bugs, or send a message to the passt-user@passt.top mailing list, see https://passt.top/passt/lists. .SH COPYRIGHT Copyright (c) 2020-2022 Red Hat GmbH. \fBpasst\fR and \fBpasta\fR are free software: you can redistribute them and/or modify them under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. .SH SEE ALSO \fBnamespaces\fR(7), \fBqemu\fR(1), \fBqrap\fR(1), \fBslirp4netns\fR(1). High-level documentation is available at https://passt.top/passt/about/.