Squid & binding to specific addresses

I am puzzled by comments in the default squid.conf which say

# Usage: tcp_incoming_address 10.20.30.40
# udp_outgoing_address fully.qualified.domain.name
#
# These tags have replaced 'bind_address' and 'outbound_address'
# to provide more control for multihomed hosts.
#
# tcp_incoming_address is used for the HTTP socket which accepts
# connections from clients and other caches.
# tcp_outgoing_address is used for connections made to remote
# servers and other caches.
# udp_incoming_address is used for the ICP socket receiving packets
# from other caches.
# udp_outgoing_address is used for ICP packets sent out to other
# caches.
#
# The defaults behaviour is to not bind to any specific address.
#
# NOTE, udp_incoming_address and udp_outgoing_address can not have
# the same value since they both use port 3130.

If the last remark is correct, how are you supposed to configure Squid to
use a specific interface, only, when it claims you must use different
addresses for the incoming and outgoing UDP addresses? And what about a
system with one interface and a single address? A basic squid configuration
does not require a system with (at least) two assigned IP addresses, so in
that configuration it *must* be using the same IP address for all four uses!

Empirically, it looks like configuring all four addresses the same "works",
except that the cachemgr server stats shows ICP_MISS and/or ICP_DENIED in
all cases, looking suspiciously like it may be the consequence of
configuring the same incoming and outgoing address for UDP. [Do ftpget and
dnsserver behave as intended and use whichever of the configured addresses
are appropriate? I haven't checked that yet...]

My requirement is to configure Squid so that all its incoming and outgoing
traffic uses an alias IP address configured as a second address on the
system's ethernet controller, so that both IP address and Squid can be moved
almost trivially to another system (without requiring DNS changes) in the
event that the primary system is unavailable. It must *not* bind to the
system's primary IP address. How do I do that...?

                                John Line

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