Hi,
Got an Exim4 mailserver that appears to be guilty of backscatter. I understand that this is usually down to poor router config. I'd like to fix it if poss. I attach the configuration, in case some kind person has the time for a look, otherwise, any vague tips would be useful, thanks!

The only custom bit I'm aware of in this config is the router 350/850 which does a DB lookup.

Also, I know it's backscattering, as a telnet session on port 25 shows that email to NoSuchUser @ domain.com is accepted...

Alex

-----

######################################################################
# ROUTERS CONFIGURATION #
# Specifies how addresses are handled #
######################################################################
# THE ORDER IN WHICH THE ROUTERS ARE DEFINED IS IMPORTANT! #
# An address is passed to each router in turn until it is accepted. #
######################################################################

begin routers


### router/100_exim4-config_domain_literal
#################################

# This router handles e-mail addresses in "domain literal" form like
# <user@[10.11.12.13]>. The RFCs require this facility, but it is disabled
# in the default config since it is seldomly used and frequently abused.
# Domain literal support also needs to be enabled in the main config,
# which is automatically done if you use the enable macro
# MAIN_ALLOW_DOMAIN_LITERALS.

.ifdef MAIN_ALLOW_DOMAIN_LITERALS
domain_literal:
debug_print = "R: domain_literal for $local_part@$domain"
driver = ipliteral
domains = ! +local_domains
transport = remote_smtp
.endif

# router/150_exim4-config_hubbed_hosts
#################################

# route specific domains manually.
#
# see exim4-config_files(5) and spec.txt chapter 20.3 through 20.7 for
# more detailed documentation.

hubbed_hosts:
debug_print = "R: hubbed_hosts for $domain"
driver = manualroute
domains = "${if exists{CONFDIR/hubbed_hosts}\
{partial-lsearch;CONFDIR/hubbed_hosts}\
fail}"
same_domain_copy_routing = yes
route_data = ${lookup{$domain}partial-lsearch{CONFDIR/hubbed_hosts}}
transport = remote_smtp

### router/200_exim4-config_primary
#################################
# This file holds the primary router, responsible for nonlocal mails

.ifdef DCconfig_internet
# configtype=internet
#
# deliver mail to the recipient if recipient domain is a domain we
# relay for. We do not ignore any target hosts here since delivering to
# a site local or even a link local address might be wanted here, and if
# such an address has found its way into the MX record of such a domain,
# the local admin is probably in a place where that broken MX record
# could be fixed.

dnslookup_relay_to_domains:
debug_print = "R: dnslookup_relay_to_domains for $local_part@$domain"
driver = dnslookup
domains = ! +local_domains : +relay_to_domains
transport = remote_smtp
same_domain_copy_routing = yes
no_more

# deliver mail directly to the recipient. This router is only reached
# for domains that we do not relay for. Since we most probably can't
# have broken MX records pointing to site local or link local IP
# addresses fixed, we ignore target hosts pointing to these addresses.

dnslookup:
debug_print = "R: dnslookup for $local_part@$domain"
driver = dnslookup
domains = ! +local_domains
transport = remote_smtp
same_domain_copy_routing = yes
# ignore private rfc1918 and APIPA addresses
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8 : 192.168.0.0/16 :\
172.16.0.0/12 : 10.0.0.0/8 : 169.254.0.0/16 :\
255.255.255.255
no_more

.endif


.ifdef DCconfig_local
# configtype=local
#
# Stand-alone system, so generate an error for mail to a non-local domain
nonlocal:
debug_print = "R: nonlocal for $local_part@$domain"
driver = redirect
domains = ! +local_domains
allow_fail
data = :fail: Mailing to remote domains not supported
no_more

.endif


.ifdef DCconfig_smarthost DCconfig_satellite
# configtype=smarthost or configtype=satellite
#
# Send all non-local mail to a single other machine (smarthost).
#
# This means _ALL_ non-local mail goes to the smarthost. This will most
# probably not do what you want for domains that are listed in
# relay_domains. The most typical use for relay_domains is to control
# relaying for incoming e-mail on secondary MX hosts. In that case,
# it doesn't make sense to send the mail to the smarthost since the
# smarthost will probably send the message right back here, causing a
# loop.
#
# If you want to use a smarthost while being secondary MX for some
# domains, you'll need to copy the dnslookup_relay_to_domains router
# here so that mail to relay_domains is handled separately.

smarthost:
debug_print = "R: smarthost for $local_part@$domain"
driver = manualroute
domains = ! +local_domains
transport = remote_smtp_smarthost
route_list = * DCsmarthost byname
host_find_failed = defer
same_domain_copy_routing = yes
no_more

.endif


# The "no_more" above means that all later routers are for
# domains in the local_domains list, i.e. just like Exim 3 directors.

### router/300_exim4-config_real_local
#################################

# This router allows reaching a local user while avoiding local
# processing. This can be used to inform a user of a broken .forward
# file, for example. The userforward router does this.

real_local:
debug_print = "R: real_local for $local_part@$domain"
driver = accept
domains = +local_domains
local_part_prefix = real-
check_local_user
transport = LOCAL_DELIVERY


### router/350-config_remote_aliases
#################################

# This router part of the XXX project
# This router handles aliasing using a mysql database
#

remote_aliases:
debug_print = "R: remote_aliases for $local_part@$domain"
driver = redirect
domains = +local_domains
allow_fail
allow_defer
data = ${lookup mysql{select rcpt from remote_aliases where alias='${local_part}@${domain}'}{$value}fail}

### router/400_exim4-config_system_aliases
#################################

# This router handles aliasing using a traditional /etc/aliases file.
#
##### NB You must ensure that /etc/aliases exists. It used to be the case
##### NB that every Unix had that file, because it was the Sendmail default.
##### NB These days, there are systems that don't have it. Your aliases
##### NB file should at least contain an alias for "postmaster".
#
# This router handles the local part in a case-insensitive way which
# satisfies the RFCs requirement that postmaster be reachable regardless
# of case. If you decide to handle /etc/aliases in a caseful way, you
# need to make arrangements for a caseless postmaster.
#
# Delivery to arbitrary directories, files, and piping to programs in
# /etc/aliases is disabled per default.
# If that is a problem for you, see
# /usr/share/doc/exim4-base/README.Debian.gz
# for explanation and some workarounds.

system_aliases:
debug_print = "R: system_aliases for $local_part@$domain"
driver = redirect
domains = +local_domains
allow_fail
allow_defer
data = ${lookup{$local_part}lsearch{/etc/aliases}}
.ifdef SYSTEM_ALIASES_USER
user = SYSTEM_ALIASES_USER
.endif
.ifdef SYSTEM_ALIASES_GROUP
group = SYSTEM_ALIASES_GROUP
.endif
.ifdef SYSTEM_ALIASES_FILE_TRANSPORT
file_transport = SYSTEM_ALIASES_FILE_TRANSPORT
.endif
.ifdef SYSTEM_ALIASES_PIPE_TRANSPORT
pipe_transport = SYSTEM_ALIASES_PIPE_TRANSPORT
.endif
.ifdef SYSTEM_ALIASES_DIRECTORY_TRANSPORT
directory_transport = SYSTEM_ALIASES_DIRECTORY_TRANSPORT
.endif

### router/500_exim4-config_hubuser
#################################

.ifdef DCconfig_satellite
# This router is only used for configtype=satellite.
# It takes care to route all mail targetted to <somelocaluser@this.machine>
# to the host where we read our mail
#
hub_user:
debug_print = "R: hub_user for $local_part@$domain"
driver = redirect
domains = +local_domains
data = ${local_part}@DCreadhost
check_local_user

# Grab the redirected mail and deliver it.
# This is a duplicate of the smarthost router, needed because
# DCreadhost might end up as part of +local_domains
hub_user_smarthost:
debug_print = "R: hub_user_smarthost for $local_part@$domain"
driver = manualroute
domains = DCreadhost
transport = remote_smtp_smarthost
route_list = * DCsmarthost byname
host_find_failed = defer
same_domain_copy_routing = yes
check_local_user
.endif



### router/600_exim4-config_userforward
#################################

# This router handles forwarding using traditional .forward files in users'
# home directories. It also allows mail filtering with a forward file
# starting with the string "# Exim filter" or "# Sieve filter".
#
# The no_verify setting means that this router is skipped when Exim is
# verifying addresses. Similarly, no_expn means that this router is skipped if
# Exim is processing an EXPN command.
#
# The check_ancestor option means that if the forward file generates an
# address that is an ancestor of the current one, the current one gets
# passed on instead. This covers the case where A is aliased to B and B
# has a .forward file pointing to A.
#
# The four transports specified at the end are those that are used when
# forwarding generates a direct delivery to a directory, or a file, or to a
# pipe, or sets up an auto-reply, respectively.
#
userforward:
debug_print = "R: userforward for $local_part@$domain"
driver = redirect
domains = +local_domains
check_local_user
file = $home/.forward
no_verify
no_expn
check_ancestor
allow_filter
forbid_smtp_code = true
directory_transport = address_directory
file_transport = address_file
pipe_transport = address_pipe
reply_transport = address_reply
skip_syntax_errors
syntax_errors_to = real-$local_part@$domain
syntax_errors_text = \
This is an automatically generated message. An error has\n\
been found in your .forward file. Details of the error are\n\
reported below. While this error persists, you will receive\n\
a copy of this message for every message that is addressed\n\
to you. If your .forward file is a filter file, or if it is\n\
a non-filter file containing no valid forwarding addresses,\n\
a copy of each incoming message will be put in your normal\n\
mailbox. If a non-filter file contains at least one valid\n\
forwarding address, forwarding to the valid addresses will\n\
happen, and those will be the only deliveries that occur.


procmail:
debug_print = "R: procmail for $local_part@$domain"
driver = accept
domains = +local_domains
check_local_user
transport = procmail_pipe
# emulate OR with "if exists"-expansion
require_files = ${local_part}:\
${if exists{/etc/procmailrc}\
{/etc/procmailrc}{${home}/.procmailrc}}:\
+/usr/bin/procmail
no_verify
no_expn


### router/800_exim4-config_maildrop
#################################

maildrop:
debug_print = "R: maildrop for $local_part@$domain"
driver = accept
domains = +local_domains
check_local_user
transport = maildrop_pipe
require_files = ${local_part}:${home}/.mailfilter:+/usr/bin/maildrop
no_verify
no_expn


### router/850_exim4-config_db_user
###################################
#
# This router attempts to deliver mail based on data in the database. We
# allow it to defer or fail since you may still want to deliver to a couple of real users.

db_user:
debug_print = "R: db-user for $local_part@$domain"
driver = redirect
domains = +local_domains
data = /var/mail/${lookup mysql{select maildir from aliases where alias = '${local_part}@${domain}'}{$value}fail}
directory_transport = db_maildir_home
user = mail
group = mail

### router/900_exim4-config_local_user
#################################

# This router matches local user mailboxes. If the router fails, the error
# message is "Unknown user".

local_user:
debug_print = "R: local_user for $local_part@$domain"
driver = accept
domains = +local_domains
check_local_user
local_parts = ! root
transport = LOCAL_DELIVERY
cannot_route_message = Unknown user

### router/mmm_mail4root
#################################
# deliver mail addressed to root to /var/mail/mail as user mail:mail
# if it was not redirected in /etc/aliases or by other means
# Exim cannot deliver as root since 4.24 (FIXED_NEVER_USERS)

mail4root:
debug_print = "R: mail4root for $local_part@$domain"
driver = redirect
domains = +local_domains
data = /var/mail/mail
file_transport = address_file
local_parts = root
user = mail
group = mail