When a listening daemon is started (by means of the -bd command line option), the interfaces and ports on which it listens are controlled by the following options:

The default list separator in both cases is a colon, but this can be changed as described in section 6.19. When IPv6 addresses are involved, it is usually best to change the separator to avoid having to double all the colons. For example:

local_interfaces = <; 127.0.0.1 ; \
                      192.168.23.65 ; \
                      ::1 ; \
                      3ffe:ffff:836f::fe86:a061

There are two different formats for specifying a port along with an IP address in local_interfaces:

When a port is not specified, the value of daemon_smtp_ports is used. The default setting contains just one port:

daemon_smtp_ports = smtp

If more than one port is listed, each interface that does not have its own port specified listens on all of them. Ports that are listed in daemon_smtp_ports can be identified either by name (defined in /etc/services) or by number. However, when ports are given with individual IP addresses in local_interfaces, only numbers (not names) can be used.

The addresses 0.0.0.0 and ::0 are treated specially. They are interpreted as “all IPv4 interfaces” and “all IPv6 interfaces”, respectively. In each case, Exim tells the TCP/IP stack to “listen on all IPvx interfaces” instead of setting up separate listening sockets for each interface. The default value of local_interfaces is

local_interfaces = 0.0.0.0

when Exim is built without IPv6 support; otherwise it is:

local_interfaces = <; ::0 ; 0.0.0.0

Thus, by default, Exim listens on all available interfaces, on the SMTP port.

The -oX command line option can be used to override the values of daemon_smtp_ports and/or local_interfaces for a particular daemon instance. Another way of doing this would be to use macros and the -D option. However, -oX can be used by any admin user, whereas modification of the runtime configuration by -D is allowed only when the caller is root or exim.

The value of -oX is a list of items. The default colon separator can be changed in the usual way if required. If there are any items that do not contain dots or colons (that is, are not IP addresses), the value of daemon_smtp_ports is replaced by the list of those items. If there are any items that do contain dots or colons, the value of local_interfaces is replaced by those items. Thus, for example,

-oX 1225

overrides daemon_smtp_ports, but leaves local_interfaces unchanged, whereas

-oX 192.168.34.5.1125

overrides local_interfaces, leaving daemon_smtp_ports unchanged. (However, since local_interfaces now contains no items without ports, the value of daemon_smtp_ports is no longer relevant in this example.)

Exim supports the obsolete SSMTP protocol (also known as SMTPS) that was used before the STARTTLS command was standardized for SMTP. Some legacy clients still use this protocol. If the tls_on_connect_ports option is set to a list of port numbers, connections to those ports must use SSMTP. The most common use of this option is expected to be

tls_on_connect_ports = 465

because 465 is the usual port number used by the legacy clients. There is also a command line option -tls-on-connect, which forces all ports to behave in this way when a daemon is started.

Warning: Setting tls_on_connect_ports does not of itself cause the daemon to listen on those ports. You must still specify them in daemon_smtp_ports, local_interfaces, or the -oX option. (This is because tls_on_connect_ports applies to inetd connections as well as to connections via the daemon.)

IPv6 addresses have “scopes”, and a host with multiple hardware interfaces can, in principle, have the same link-local IPv6 address on different interfaces. Thus, additional information is needed, over and above the IP address, to distinguish individual interfaces. A convention of using a percent sign followed by something (often the interface name) has been adopted in some cases, leading to addresses like this:

fe80::202:b3ff:fe03:45c1%eth0

To accommodate this usage, a percent sign followed by an arbitrary string is allowed at the end of an IPv6 address. By default, Exim calls getaddrinfo() to convert a textual IPv6 address for actual use. This function recognizes the percent convention in operating systems that support it, and it processes the address appropriately. Unfortunately, some older libraries have problems with getaddrinfo(). If

IPV6_USE_INET_PTON=yes

is set in Local/Makefile (or an OS-dependent Makefile) when Exim is built, Exim uses inet_pton() to convert a textual IPv6 address for actual use, instead of getaddrinfo(). (Before version 4.14, it always used this function.) Of course, this means that the additional functionality of getaddrinfo() – recognizing scoped addresses – is lost.

The default case in an IPv6 environment is

daemon_smtp_ports = smtp
local_interfaces = <; ::0 ; 0.0.0.0

This specifies listening on the smtp port on all IPv6 and IPv4 interfaces. Either one or two sockets may be used, depending on the characteristics of the TCP/IP stack. (This is complicated and messy; for more information, read the comments in the daemon.c source file.)

To specify listening on ports 25 and 26 on all interfaces:

daemon_smtp_ports = 25 : 26

(leaving local_interfaces at the default setting) or, more explicitly:

local_interfaces = <; ::0.25     ; ::0.26 \
                      0.0.0.0.25 ; 0.0.0.0.26

To listen on the default port on all IPv4 interfaces, and on port 26 on the IPv4 loopback address only:

local_interfaces = 0.0.0.0 : 127.0.0.1.26

To specify listening on the default port on specific interfaces only:

local_interfaces = 192.168.34.67 : 192.168.34.67

Warning: such a setting excludes listening on the loopback interfaces.

The local_interfaces option is also used when Exim needs to determine whether or not an IP address refers to the local host. That is, the IP addresses of all the interfaces on which a daemon is listening are always treated as local.

For this usage, port numbers in local_interfaces are ignored. If either of the items 0.0.0.0 or ::0 are encountered, Exim gets a complete list of available interfaces from the operating system, and extracts the relevant (that is, IPv4 or IPv6) addresses to use for checking.

Some systems set up large numbers of virtual interfaces in order to provide many virtual web servers. In this situation, you may want to listen for email on only a few of the available interfaces, but nevertheless treat all interfaces as local when routing. You can do this by setting extra_local_interfaces to a list of IP addresses, possibly including the “all” wildcard values. These addresses are recognized as local, but are not used for listening. Consider this example:

local_interfaces = <; 127.0.0.1 ; ::1 ; \
                      192.168.53.235 ; \
                      3ffe:2101:12:1:a00:20ff:fe86:a061

extra_local_interfaces = <; ::0 ; 0.0.0.0

The daemon listens on the loopback interfaces and just one IPv4 and one IPv6 address, but all available interface addresses are treated as local when Exim is routing.

In some environments the local host name may be in an MX list, but with an IP address that is not assigned to any local interface. In other cases it may be desirable to treat other host names as if they referred to the local host. Both these cases can be handled by setting the hosts_treat_as_local option. This contains host names rather than IP addresses. When a host is referenced during routing, either via an MX record or directly, it is treated as the local host if its name matches hosts_treat_as_local, or if any of its IP addresses match local_interfaces or extra_local_interfaces.

Delivery to a remote host is handled by the smtp transport. By default, it allows the system’s TCP/IP functions to choose which interface to use (if there is more than one) when connecting to a remote host. However, the interface option can be set to specify which interface is used. See the description of the smtp transport in chapter 30 for more details.