passt/passt.1

.\" SPDX-License-Identifier: AGPL-3.0-or-later
.\" Copyright (c) 2020-2021 Red Hat GmbH
.\" Author: Stefano Brivio <sbrivio@redhat.com>
.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/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 \-q ", " \-\-quiet
Don't print informational messages.

.TP
.BR \-f ", " \-\-foreground
Don't run in background.
Default is to fork into background, if started from an interactive terminal.

.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 \-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.

If \fIfile\fR is not given, capture packets to

	\fB/tmp/passt_\fIISO8601-timestamp\fR_\fIinstance-number\fB.pcap\fR

in \fBpasst\fR mode and to

	\fB/tmp/pasta_\fIISO8601-timestamp\fR_\fIinstance-number\fB.pcap\fR

in \fBpasta\fR mode, where \fIinstance-number\fR is a progressive count of
other detected instances running on the same host.

.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 interface
with the first default route.

.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 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.

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 interface with the first default route.

.TP
.BR \-D ", " \-\-dns " " \fIaddr
Assign IPv4 \fIaddr\fR via DHCP (option 23) or IPv6 \fIaddr\fR via NDP Router
Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolver.
This option can be specified multiple times, and a single, empty option disables
DNS options altogether.
In \fBpasst\fR mode, default is to use addresses from \fI/etc/resolv.conf\fR,
and, in \fBpasta\fR mode, no addresses are sent by default.
.TP
.BR \-S ", " \-\-search " " \fIlist
Assign space-separated \fIlist\fR via DHCP (option 119), via NDP Router
Advertisement (option type 31) and DHCPv6 (option 24) as DNS domain search list.
A single, empty option disables sending the DNS domain search list.
In \fBpasst\fR mode, default is to use the search list from
\fI/etc/resolv.conf\fR, and, in \fBpasta\fR mode, no list is sent by default.

.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. 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
.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. 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
.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 or create only the network namespace, not a user namespace. This is implied
if PATH or NAME are given without \-\-userns.

.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
#
# udhcpc -i eth0
udhcpc: started, v1.30.1
udhcpc: sending discover
udhcpc: sending select for 192.168.1.118
udhcpc: lease of 192.168.1.118 obtained, lease time 4294967295
# dhclient -6
# ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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
# udhcpc -i eth0
udhcpc: started, v1.30.1
udhcpc: sending discover
udhcpc: sending select for 192.168.1.118
udhcpc: lease of 192.168.1.118 obtained, lease time 4294967295
# dhclient -6
# ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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 kvm ... -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:
    kvm ... -net socket,connect=/tmp/passt_1.socket -net nic,model=virtio

.fi
.BR "	[From another terminal]"
.nf
$ ./qrap 5 kvm 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: <LOOPBACK,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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 <sbrivio@redhat.com>

.SH REPORTING BUGS

No public bug tracker is available at this time. For the moment being, report
issues to Stefano Brivio <sbrivio@redhat.com>.

.SH COPYRIGHT

Copyright (c) 2020-2021 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/.