Proxy problem: "imap-login: Error: proxy(USERNAME): connect(10.x.x.178, 993) failed: Cannot assign requested address (after 0 secs, local=10.x.x.104)"

[Ralf Hildebrandt]

After fixing the "duplicate compression" problem, we're now
encountering 

"imap-login: Error: proxy(USERNAME): connect(10.x.x.178, 993) failed: Cannot assign requested address (after 0 secs, local=10.x.x.100)"

in the logs. We already tried raising the ulimit, the max number of
open files. Once we reach about 25k connections, we're getting the
error above... for all local addresses. It seems as if the system
cannot create any more outgoing connections.

We already optimized:

---- snip ----
net.ipv4.tcp_fin_timeout=5
# down from 30s

net.ipv4.tcp_tw_reuse=1
net.ipv4.tcp_tw_recycle=1
# http://redis4you.com/articles.php?id=012&name=Redis+high+traffic+connection+issue

net.ipv4.ip_local_port_range=10000 65000
# http://www.fromdual.com/huge-amount-of-time-wait-connections

net.ipv4.netfilter.ip_conntrack_max=524288
---- snip ----

But still we get (once the load is rising beyond some point):

# fgrep "Cannot assign requested address" /var/log/dovecot/dovecot.log |awk '{print $NF}' | sort | uniq -c | sort -n
    142 local=10.x.x.100)
    147 local=10.x.x.107)
    148 local=10.x.x.106)
    151 local=10.x.x.104)
    151 local=10.x.x.109)
    152 local=10.x.x.105)
    156 local=10.x.x.110)
    162 local=10.x.x.102)
    165 local=10.x.x.101)
    178 local=10.x.x.103)
    189 local=10.x.x.108)

We're using multiple local addresses when proxying to the backends

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